US20230025285A1 - Method for Generating Simulations of Fluid Interfaces for Improved Animation of Fluid Interactions - Google Patents

Abstract

A method for generating visual representations of interactions between two different materials is provided. The method can be performed using a computing device operated by a computer user or artist. The method includes modeling a primary material as a plurality of first particles and modeling a layer portion of a secondary material as a fluid volume. The secondary material can include a layer portion positioned between the plurality of first particles and an outer portion. At least one boundary condition might be assigned to a boundary positioned between the layer portion and the outer portion, the at least one boundary condition includes at least one pressure value. Values of motion parameters might be determined by applying the at least one boundary condition at the boundary and generating one or more visual representations of the primary material interacting with the secondary material based on the values of the motion parameters.

Description

CROSS-REFERENCES TO PRIORITY AND RELATED APPLICATIONS
• This application is a continuation of U.S. application Ser. No. 17/523,293, Nov. 10, 2021, which is a continuation of U.S. application Ser. No. 17/183,993, Feb. 24, 2021, which claims the benefit of U.S. Provisional Application No. 62/983,435 filed Feb. 28, 2020, all of which are incorporated by reference in their entirety as though fully set forth herein.
FIELD OF THE INVENTION
• The present disclosure generally relates to simulating interactions between different materials, and more particularly to efficient computational approaches for simulation of interactions between different materials.
BACKGROUND
• Visual representations of scenes intended to reflect real-world scenarios are common in animation and other fields. For example, a computer-generated imagery scene could be created by having an artist manually draw a sequence of frames to form a video sequence. For simple cartoons, for example, this is a feasible approach. However, as viewers have come to expect more complex visuals, there is a need for computer-driven imagery generation. Some of that computer-driven imagery generation might rely on simulation.
• Computer simulation that is used for imagery generation has been used to animate natural phenomena as well as natural movements of characters, such as by using a physics engine to output movements of an articulated character that are consistent with real-world physics and joint constraints. In some ways, this is often a simple problem—how to determine natural-looking movements of at most a few dozen attached body parts. For other simulations, such as those with flexible objects, fluids, and the like, the number of degrees of freedom of individual units is much greater and typically computer simulation requires a trade-off between realism, resolution, and an amount of computing resources available. Because of this trade-off, efficient computer simulation techniques can be important as they might allow for an increase in realism and/or resolution without requiring significant increases in computing resources. Simulation computations involving bubbles, waterfalls, and other fluid interactions can often involve such trade-offs.
• Fluid simulation is ubiquitous in computer graphics. When there is only a single fluid (or gas) of interest, practitioners typically use conventional single-phase fluid simulation tools to determine the fluid's motion. This means the area outside of the fluid is treated as a vacuum. But, multiple fluids are often present and cannot be adequately simulated using conventional single-phase fluid simulation tools. For instance, a waterfall looks significantly different when the water falls through vacuum instead of air. Similarly, an underwater air bubble would collapse if the bubble is represented as a vacuum, which is clearly not case for a real-world air bubble. In these examples, air needs to be accounted for and not modeled as being a vacuum, to achieve the proper look of the interaction between the air and water. As such, this typically involves a two-phase air-water coupled simulation. Unfortunately, such two-phase air-water coupled simulations are typically quite computationally expensive to perform.
• Therefore, there is a need for a more efficient approach to performing simulations of interactions between different materials, that can be applicable to, for example, two-phase air-water coupled simulations.
BRIEF DESCRIPTION OF THE DRAWINGS
• Various embodiments in accordance with the present disclosure will be described with reference to the drawings, in which:
• FIG.1 is a diagram of a data flow through a system when the system is generating values of motion parameters, which are used to create visual representations of a first material interacting with a second material.
• FIG.2 is a flowchart of the process of generating the values of the motion parameters.
• FIG.3 illustrates a primary material surrounded by a secondary material.
• FIG.4 illustrates a drag force exchange that may be used to couple the primary material and the secondary material together after a first set of equations has been solved for the primary material and a second set of equations has been solved separately for the secondary material.
• FIG.5 illustrates a method of calculating a new velocity field as a function of a previous velocity field, an aeration field, and a drag force field.
• FIG.6 illustrates an example visual content generation system as might be used to generate imagery in the form of still images and/or video sequences of images, according to various embodiments.
• FIG.7 is a block diagram illustrating an example computer system upon which computer systems of the systems illustrated inFIGS.1 and6 may be implemented.
DETAILED DESCRIPTION
• In the following description, various embodiments will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without the specific details. Furthermore, well-known features may be omitted or simplified in order not to obscure the embodiment being described.
• In a computer simulation involving three dimensions and having an output that is imagery (such as a still image or a sequence of video frames), often the virtual objects and material being simulated are represented relative to a three-dimensional (“3D”) grid in a virtual space with the grid being divided into voxels. Some elements might have subvoxel resolution.
• In typical computer simulations, it is difficult to achieve realistic looking sceneries that comprise moving objects, e.g., waterfalls and underwater bubbles. Waterfall simulations typically involved dragging water towards a prescribed, artistically driven air field. In such simulations, the air affects the water, but the water does not affect the air. Another approach represents the air as a single velocity field, and (partially) applies a divergence-free projection to the single velocity field. This approach gives the appearance that the water has affected the air and may create an interesting flow of air that in turn affects the water. However, it is unclear to what degree such solutions are physics based. Earlier simulation techniques for simulating underwater bubbles include, for example, R. Goldade and C. Batty,Constraint bubbles: Adding efficient zero-density bubbles to incompressible free surface flow,2017 adopt a particle-in-cell fluid simulator that represents each air pocket as a volume conserving void with fixed pressure. While such technique is capable of recreating realistic gargling water effects, it does not capture subtle bubble detail that makes it fully realistic. By way of another example, L. Boyd and R. Bridson,Multiflip for energetic two-phase fluid simulation, ACM Trans. Graph., 31(2), April 2012, use a Fluid Implicit Particle (“FLIP”) method to discretize both water and air and perform a two-phase incompressible solve.
• On the other hand, bubbles smaller than a grid voxel size are typically represented as a separate particle system. For example, D. Kim, O. Song, and H. Ko,A practical simulation of dispersed bubble flow, ACM Trans. Graph., 29(4), July 2010, passively advect those particles with the bulk fluid and use them to adjust effective density of water, leading to naturalistic buoyancy effects. They employ a stochastic solver for additional sub-voxel motion. By way of another example, S. Patkar, M. Aanjaneya, D. Karpman, and R. Fedkiw,A hybrid lagrangian-eulerian formulation for bubble generation and dynamics, In Proc. of the ACM SIGGRAPH/Eur. Symp. on Comp. Anim., SCA, pages 105-114, New York, N.Y., USA, 2013, ACM, use an Eulerian two-phase approach for simulating bubbles larger than the grid voxel size and passively advected particles for tracking bubbles smaller than the grid voxel size. Patkar et al. combine the two differently sized groups of bubbles in a single linear solve, which also handles compressibility.
• A typical method of simulating bubbles (e.g.,primary material302 ofFIG.3) moving deep under water (e.g., secondary material304) creates a Fluid Implicit Particle (“FLIP”) model of the water and represents the bubbles as constraints. An example of this approach is provided by SideFX Houdini software. This approach concentrates most of the computational resources on the water and preserves its volume. Unfortunately, tracking the bubbles and preserving their volume is a problem because they are not modeled as a full phase. This makes the bubble movement with respect to the water less realistic, which is unfortunate because the bubbles are more visual significant than the water.
• Another method of simulating bubbles uses a FLIP model to simulate both the bubbles (e.g., the primary material302) and the water (e.g., the secondary material304). An example of this approach is provided by L. Boyd and R. Bridson,Multiflip for energetic two-phase fluid simulation, ACM Trans. Graph., 31(2), April 2012. Using this approach, both the water and the bubbles are accurately represented. Unfortunately, this approach is computationally expensive because it fully represents all of the water.
• Via various embodiments, more efficient simulation approaches that can provide the same high level of realistic looking interactions between different materials are presented. These efficient simulations are performed without partaking computationally expensive approaches that incur in traditional approaches that fully take into account all of the water.
• In many of the examples described herein, inputs to a computer simulation system include parameters about the virtual material/object/fluid/etc. being simulated and an output of a computer simulation are the positions/mass/movement/etc. of the virtual material/object/fluid/etc. Such an output might be an input to an animation system, which can provide for rendering computer-generated imagery of the virtual material/object/fluid/etc. present in a scene in a virtual space. The computer-generated imagery might be still images, stereoscopic images, video sequences, and/or stereoscopic video sequences. In some cases, the computer simulation of virtual elements seeks to match what would happen with corresponding real-world elements, but in other cases, artistic or other inputs are used in the computer simulation to create effects that do not correspond to anything in the real-world, or at least anything in available physical environments. For example, in a given simulation, an operator of a simulation engine might provide an input that corresponds to gravity “turning off” for a short period of time, which can be simulated but has no real-world correspondence.
• The primary material may be modeled as a plurality of particles or objects that may, in some cases, be unconstrained relative to one another, such that each object can move independently of the others. This may occur for example with granular media such as droplets or bubbles, and may be thought of as a zero-dimensional constraint, or a constraint on zero degrees of freedom. A one-dimensional constraint, or constraint of a single degree of freedom, may occur for example with hair, wherein the hairs are free to move relative to one another along most of their lengths, but are fixed at one end relative to one another. A two-dimensional constraint or two-degree-of-freedom constraint may for example occur with cloth, wherein the objects of the porous medium are interwoven fibers that are free to move, bend, or fold in three dimensions but have fixed locations relative to one another within the topological plane of the cloth. A three-dimensional constraint or three-degree-of-freedom constraint can occur for example with a three-dimensional network such as a sponge, wherein the objects of the porous medium are fibers or other shapes that intertwine in three dimensions. A sponge may be capable of bending or flexing, but the objects making up the sponge may have fixed spatial relationships to one another within the topological volume of the sponge. In some cases, coupling or constraint between two fluid objects, or objects within a fluid, may occur through surface tension.
• FIG.1 is a diagram of a data flow through asystem100 when thesystem100 is configured to perform a process200 (seeFIG.2) that generates values ofmotion parameters110. Themotion parameters110 are used by ananimation creation system630, which is a component of an example visual content generation system600 (seeFIG.6), to create visual representations of interactions betweenfirst material112 andsecond material114. For example, thesystem100 may be used to simulate one or more bubbles of thefirst material112 positioned inside (e.g., floating within) thesecond material114. Thefirst material112 and thesecond material114 are different materials and each may represent a gas, a combination of gases (e.g., air), a liquid, another fluid, or a combination of fluids. Additionally, thefirst material112 and thesecond material114 may include solid particles held in suspension or floating therein.
• In some embodiments, thefirst material112 and thesecond material114 may be configured to remain separate, at least temporarily, when mixed together. By way of a non-limiting example, thefirst material112 may be air and thesecond material114 may be water, or vice versa. When one of thefirst material112 and thesecond material114 is a gas and the other is a liquid, thesystem100 may be characterized as simulating interactions between multiple phases of matter, namely gas and liquid phases. Thesystem100 may also be used to simulate thefirst material112 and thesecond material114 in the same phase. For example, one of thefirst material112 and thesecond material114 may be a polar fluid (e.g., water) and the other may be a non-polar fluid (e.g., oil).
• Referring toFIG.1, thesystem100 as shown includes amotion simulation system120 and at least oneclient computing device140 operated by at least onehuman artist142. Themotion simulation system120 may be implemented by software executing on one or more computer systems (e.g., each like acomputer system700 illustrated inFIG.7). Themotion simulation system120 is configured to receive data defining thefirst material112 and data defining thesecond material114, which are used to output the values of themotion parameters110. Themotion simulation system120 may be implemented as a fluid simulator (e.g., a particle-in-cell fluid simulator) configured to strongly couple thefirst material112 and thesecond material114 together by solving a set of equations for thefirst material112 and thesecond material114 at the same time. For example, themotion simulation system120 may be configured to perform a two-phase pressure solve on thefirst material112 and thesecond material114 at the same time. The values of themotion parameters110 may include the solution obtained for the set of equations. The two-phase pressure solve may be an incompressible two-phase Navier-Stokes solve on an Eulerian grid (also referred to as a two-phase incompressible ghost-fluid Eulerian solve). Alternatively, and/or additionally, themotion simulation system120 may be configured to weakly or iteratively couple thefirst material112 and thesecond material114 together by separately solving a first set of equations for thefirst material112 and a second set of equations for thesecond material114. The values of themotion parameters110 may include the solutions obtained for the first and second sets of equations. After one of the first and second sets of equations is solved, the solution may be supplied to the other set of equations. Further, as explained below, themotion simulation system120 may be configured to weakly or iteratively couple the solutions together (e.g., with adrag force404 illustrated inFIG.4).
• The values of themotion parameters110 may include the data defining thefirst material112 and the data defining thesecond material114. The values of themotion parameters110 may be generated based at least in part onparameter values144 that may include parameter values defined by the artist142 (e.g., using the client computing device140) and/or parameter values that are predetermined and stored in a data store. When the parameter values144 include user-defined parameter values, the motion of thefirst material112 and/or thesecond material114 may be characterized as being at least partially art directable.
• As described below, the visual content generation system600 (seeFIG.6) is configured to receive the values of themotion parameters110 as input, and output one or more static images and/or one or more animated videos. The static image(s) and/or the animated video(s) include one or more visual representations of thefirst material112 and/or thesecond material114. The visualcontent generation system600 may transmit the static image(s) and/or the animated video(s) to theclient computing device140 for display to theartist142. Theartist142 may use the static image(s) and/or the animated video(s) to view the visual representations of thefirst material112 and/or thesecond material114 and may make further adjustments to the parameter values144. Then, themotion simulation system120 may output new values of themotion parameters110, which the visualcontent generation system600 may use to output new versions of the static image(s) and/or the animated video(s) that may be viewed by theartist142 on theclient computing device140, or an external computing device (not shown). This process may be repeated until theartist142 is satisfied with the appearance of thefirst material112 and/or thesecond material114.
• As disclosed above, theclient computing device140 is configured to communicate with themotion simulation system120. For example, theartist142 may use theclient computing device140 to specify the parameter values144 to themotion simulation system120. Optionally, themotion simulation system120 may be configured to display thefirst material112 and/or thesecond material114 to theartist142 on theclient computing device140 so that theartist142 may adjust the parameter values144 as desired before the values of themotion parameters110 are input into the visual content generation system600 (seeFIG.6). As described above, theclient computing device140 is configured to receive the static image(s) and/or the animated video(s) from the visual content generation system600 (seeFIG.5) and display the static image(s) and/or the animated video(s) to theartist142 so that theartist142 may adjust the parameter values144. Theclient computing device140 may be implemented using thecomputer system700 illustrated inFIG.7.
• Referring toFIG.3, one of thefirst material112 and/or thesecond material114 is selected (e.g., by theartist142 and/or the motion simulation system120) as aprimary material302, making the other asecondary material304. InFIG.3, for ease of illustration, the primary material302 (e.g., air) is illustrated as forming a bubble (e.g., particle) inside the secondary material304 (e.g., water). Thesecondary material304 may be considered generally invisible with respect to theprimary material302. For example, when observing bubbles moving in water, the water may be generally invisible with respect to the bubbles but, the water does influence the motion of the bubbles. Similarly, when observing a waterfall (not shown), the air may be generally invisible with respect to the water but, the air does influence the motion of the water. Thus, while some processes might treat thesecondary material304 as being less visually significant than theprimary material302, thesecondary material304 is physically significant and affects the movement of theprimary material302.
• By way of a non-limiting example, the process200 (seeFIG.2) may be used to simulate a bubble of theprimary material302 at least partially submerged inside thesecondary material304. The process200 (seeFIG.2) may be less computationally expensive than traditional methods because theprocess200 does not model an entire volume of thesecondary material304 as a liquid or gas. In some embodiments, the process200 (seeFIG.2) models a band or alayer portion308 of thesecondary material304 as a liquid or gas and, in doing so, treats anouter portion306 of thesecondary material304 as if the dynamics of theouter portion306 are prescribed. Theouter portion306 includes a region of thesecondary material304 that is too far away from theprimary material302 to affect the movement of theprimary material302 or to be moved by theprimary material302. Thus, theouter portion306 may be conceptualized and/or modeled as having prescribed dynamics. In some embodiments, theouter portion306 can be modeled as if the interaction between theprimary material302 and thelayer portion308 does not have any effect on the outer portion206. Consequently, theouter portion306 does not move as a result of interaction with theprimary material302. In this instance, theouter portion306 of thesecondary material304 is not included in the simulation, thus enabling efficient simulation of the overall scene by not computing theouter portion306 in the simulation. On the other hand, thelayer portion308 that surrounds at least a portion of theprimary material302 affects the movement of theprimary material302, and thus included in the simulation because thelayer portion308 is directly affected by the movement of theprimary material302. However, the secondary material304 (e.g., water) may have a density that is much larger (e.g., 1000 or 10000 times) than the density of the primary material302 (e.g., air bubbles). Thus, thesecondary material304 may exert greater force on the primary material302 (e.g., pushing theprimary material302 around) than theprimary material302 may exert on thesecondary material304.
• The motion simulation system120 (seeFIG.1) may represent thesecondary material304 as a sparsely modeled outer volume of fluid and a closely modeledlayer portion308. Since the representation of the outer portionsecondary material304 is sparse (e.g., modeled with a zero or constant velocity), the entire volume of thesecondary material304 is not modeled as a gas, liquid, or other fluid (e.g., with FLIP or Affine Particle in Cell (“APIC”) particles). In other words, instead of modeling the entire surrounding volume per se (e.g., a pool) as a fluid, only thelayer portion308 and theprimary material302 need to be considered when solving for the movement of theprimary material302 and thesecondary material304. Thus, theprocess200 allows the motion simulation system120 (seeFIG.1) to solve a single set of equations including both thelayer portion308 and theprimary material302 at the same time to obtain the values of the motion parameters110 (seeFIG.1) more efficiently than prior art methods.
• FIG.2 is a flowchart of theprocess200 that may be executed by thesystem100 ofFIG.1 and used to generate the values of those of themotion parameters110 that govern the motion of theprimary material302 and thelayer portion308 of the secondary material304 (seeFIG.3). Referring toFIG.2, infirst block205, the motion simulation system120 (seeFIG.1) represents the primary material302 (e.g., air) as a plurality of first phase particles (e.g., FLIP or APIC particles). The first phase particles may be implemented as Lagrangian particles. Each of the first phase particles has an initial position (e.g., with respect to an Eulerian grid). By representing theprimary material302 with the first phase particles, the motion simulation system120 (seeFIG.1) may track the first phase particles, which provide satisfactory accuracy for tracking and ensure volume conservation.
• Inblock210, themotion simulation system120 identifies athickness324 of thelayer portion308. Both thelayer portion308 and thethickness324 are defined between first andsecond boundaries320 and322. Thefirst boundary320 is an interface between thelayer portion308 and theprimary material302. Thesecond boundary322 is an outer surface of thelayer portion308 and may be characterized as being an interface between thelayer portion308 and theouter portion306. In some embodiments, thethickness324 of thelayer portion308 can be proportional to the size (e.g., diameter) of the bubble or particle of theprimary material302. In some embodiments, thethickness324 can be about 0.1 to about 10000 times the size (or average size if there are a plurality of bubbles or particles) of the bubble or particle of theprimary material302. For example, thethickness324 can be about 0.1, 0.2, 0.5, 0.7, 0.8, 1, 2, 5, 10, 15, 20, 25, 50, 100, 200, 500, 1000, 2000, 5000, or 10000 times, inclusive of a range between any two sizes listed therein, of the size (or average size of bubbles or particles) of the bubble or particle of theprimary material302. In some embodiments, thethickness324 can be between about 0.1 and 10000 times, between about 10 and 1000 times, or between about 1 and 100 times, of the size (or average size of bubbles or particles) of the bubble or particle of theprimary material302. In various embodiments, athickness324 or volume of thelayer portion308 may depend on the density of thesecondary material304, the difference in the densities between theprimary material302 and thesecondary material304, the temperature, humidity, pressure, etc. of the environment, or the like.
• In accordance with various embodiments, the thicker that thelayer portion308 is, the closer the simulation results are to being physically accurate, with a thinner layer portion leading to dampening effects. Therefore, in some embodiments, thethickness324 of thelayer portion308 represents a trade-off and may be determined by the artist142 (seeFIG.1). In some embodiments, the parameter values144 (seeFIG.1) may include thethickness324.
• Then, in block215 (seeFIG.2), the motion simulation system120 (seeFIG.1) represents thelayer portion308 as a second phase representation. The second phase representation may be a sparse Eulerian volume. Thus, thesecondary material304 may be reduced to a sparse Eulerian volume including only thelayer portion308. The second phase representation may include a plurality of voxels (e.g., Eulerian voxels arranged in an Eulerian grid). One or more attribute (e.g., velocity) may be associated with each voxel. The motion simulation system120 (seeFIG.1) may disregard compressibility of theprimary material302 and thesecondary material304 for efficiency reasons. In other words, the motion simulation system120 (seeFIG.1) may model both of theprimary material302 and thesecondary material304 as incompressible.
• Inblock220, themotion simulation system120 assigns one or more boundary conditions to thefirst boundary320 and/or thesecond boundary322. For example, when themotion simulation system120 is simulating a bubble of the primary material302 (e.g., air) positioned inside the secondary material304 (e.g., water), themotion simulation system120 may assign a pressure boundary condition to each point along thesecond boundary322. Themotion simulation system120 uses thesecond boundary322 to enforce the pressure boundary condition(s), which model the prescribed dynamics of theouter portion306 on thesecond boundary322. For example, themotion simulation system120 may enforce a pressure boundary condition at each point along thesecond boundary322. Themotion simulation system120 may enforce different pressure boundary conditions at different points along thesecond boundary322. Alternatively, themotion simulation system120 may enforce the same pressure boundary condition at all of the points along thesecond boundary322. The pressure boundary condition(s) is/are assigned to thesecond boundary322 independently of the type of coupling (e.g., weak/iterative, strong, and the like) used by themotion simulation system120.
• The pressure boundary condition(s) may include hydrostatic pressure values. For example, the pressure boundary condition(s) may be implemented as a hydrostatic pressure field that samples a hydrostatic pressure value for each position in the simulation. The hydrostatic pressure values may be calculated usingEquation 1 below, in which a variable “h” represents an evaluation height, a variable “ρ_w” represents the density of thesecondary material304, and a variable “g” represents the acceleration of gravity.
• 
  p_hydrostatic(h)=ρ_wgh  (Eqn. 1)
• When themotion simulation system120 enforces the pressure boundary condition(s) (which may be hydrostatic), as opposed to modeling theouter portion306 as a solid, an apparent sliding effect of theprimary material302 may be reduced. By using the pressure boundary condition(s) (e.g., the hydrostatic pressure values), themotion simulation system120 might also avoid null-modes in a Poisson pressure solve when the Poisson pressure solve is used.
• By way of a non-limiting example, themotion simulation system120 may use the hydrostatic pressure values to produce convincing rising bubble effects. As the layer portion308 (e.g., water) moves around the primary material302 (e.g., the bubble), the hydrostatic pressure values at thesecond boundary322 push on theprimary material302 and the layer portion308 (e.g., pushing theprimary material302 and thelayer portion308 upwardly).
• The pressure boundary condition(s) may be characterized as representing the physical effects of the entireouter portion306 on theprimary material302 and thelayer portion308. In other words, the pressure boundary condition(s) act as an invisible force that affects (e.g., holds up, shapes, etc.) theprimary material302 and thelayer portion308.
• Traditional simulations may produce a pressure field for thesecondary material304. When such pressure field includes the primary material302 (e.g., bubbles) embedded in thesecondary material304, the pressure field may be used to determine the pressure boundary condition(s) in traditional simulations. For example, the hydrostatic pressure values along thesecond boundary322 may be calculated from those pressures outside thesecond boundary322.
• After themotion simulation system120 enforces the pressure boundary condition(s) on thesecond boundary322, theprimary material302 and thelayer portion308 form a closed system. Because the representation of thesecondary material304 is sparse, the motion simulation system120 (seeFIG.1) may focus computational resources on those components that are important to the simulation, namely, the primary material302 (e.g., bubbles) and thelayer portion308. This may allow thesystem100 to achieve never-before seen simulation detail.
• Inblock225, themotion simulation system120 obtains the values of themotion parameters110. To obtain the values of themotion parameters110 themotion simulation system120 strongly or weakly couples thelayer portion308 and theprimary material302 together. When the motion simulation system120 (seeFIG.1) solves a single set of equations including both thelayer portion308 and theprimary material302 at the same time to obtain the values of the motion parameters110 (seeFIG.1), the motion simulation system120 (seeFIG.1) strongly couples thelayer portion308 and theprimary material302 together. When the motion simulation system120 (seeFIG.1) uses strong coupling, thefirst boundary320 is treated in a standard two-phase Eulerian Navier-Stokes way. For example, the motion simulation system120 (seeFIG.1) may enforce pressure jump and velocity continuity conditions. In some implementations, strong coupling may be computationally expensive because of the simultaneous solve for thelayer portion308 and theprimary material302. Therefore, in some cases, themotion simulation system120 may weakly couple thelayer portion308 and theprimary material302 together, which is less accurate but also less computationally expensive.
• When the motion simulation system120 (seeFIG.1) uses weak coupling (e.g., see Waterfall embodiment described below), the primary andsecondary materials302 and304 are modeled separately. In other words, themotion simulation system120 alternates between solving for thelayer portion308 and solving for theprimary material302 separately. While alternating, themotion simulation system120 performs an explicit interactions exchange (e.g., adrag force exchange400 illustrated inFIG.4).
• The values of themotion parameters110 may include at least one velocity field, which indicates how theprimary material302 and thelayer portion308 move with respect to their current positions. For example, themotion simulation system120 may obtain a first velocity field for the primary material302 (represented by the first phase particles) and a second velocity field for the layer portion308 (represented by the second phase representation). Each velocity field may include a vector for each position in the simulation (e.g., the Eulerian grid) that indicates how the environment effects the motion (e.g., direction and speed) of a portion of the material currently in that position.
• Themotion simulation system120 may calculate new material states for the first phase particles as well as new material states for the second phase representation. The new material states of the first phase particles may include positions and attributes of the first phase particles. The new positions may be based at least in part on the current positions of the first phase particles and the first velocity field. At least some of the new positions may be modified (e.g., by the motion simulation system120), if necessary, using additional processing known in the art. The new material states of the second phase representation may include states of the Eulerian voxels (in the Eulerian grid) and may be based at least in part on the second velocity field. The new material states of the first phase particles and the second phase representation may be included in the values of themotion parameters110.
• Themotion simulation system120 may identify new locations for the first andsecond boundaries320 and322 (seeFIG.3) based at least in part on the new material states (e.g., new positions) for the first phase particles. Additionally, themotion simulation system120 may determine new pressure boundary condition(s) (e.g., an updated hydrostatic pressure field) based at least in part on the new location of thesecond boundary322. For example, themotion simulation system120 may calculate the new pressure boundary condition(s) usingEquation 1 above. The values of themotion parameters110 may include the new locations of the first andsecond boundaries320 and322 (seeFIG.3).
• Referring toFIG.3, the type of solve performed by themotion simulation system120 inblock225 may depend on the type of coupling needed to achieve a satisfactory visual result. For example, if the visual result that is achievable by weak coupling is satisfactory, themotion simulation system120 may perform separate solves for theprimary material302 and thelayer portion308 and couple these solutions together (e.g., as illustrated inFIG.4). On the other hand, if the visual result that is achievable with weak coupling is unsatisfactory, themotion simulation system120 may perform a two-phase pressure solve that solves for theprimary material302 and thelayer portion308 at the same time and strongly couples theprimary material302 and thelayer portion308 together. The two-phase pressure solve may include an incompressible two-phase Navier-Stokes solve on an Eulerian grid (also referred to as a two-phase incompressible ghost-fluid Eulerian solve). Methods of performing two-phase pressure solves are known in the art and need not be described in detail.
• As explained above, the primary material302 (e.g., an air phase) is represented as the first phase particles, which facilitates volume conservation and accurate tracking of the new locations of the first boundary320 (seeFIG.3) and/or the second boundary322 (seeFIG.3). Thus, unlike the method described in Boyd et al., themotion simulation system120 tracks the first phase particles and recovers the new locations of the first boundary320 (seeFIG.3) and/or the second boundary322 (seeFIG.3).
• Indecision block230, themotion simulation system120 determines whether the simulation has completed. The decision indecision block230 is “YES,” when themotion simulation system120 determines the simulation has completed. Otherwise, the decision indecision block230 is “NO.” By way of a non-limiting example, blocks220-235 may be repeated a desired number of iterations (e.g., five times). The number of iterations might be specified by an artist (e.g., the artist142) or operator in advance. For example, blocks220-235 may be repeated a number of times required to generate the values of themotion parameters110 needed to create a desired number of frames.
• When the decision indecision block230 is “NO,” themotion simulation system120 advances to block235 whereat themotion simulation system120 advances the simulation in time. Then, themotion simulation system120 returns to block220 and assigns the new pressure boundary condition(s) to the second boundary322 (seeFIG.3).
• When the decision indecision block230 is “YES,” inblock240, themotion simulation system120 forwards the values of themotion parameters110 to an animation creation system, such as the animation creation system630 (seeFIGS.1 and6), which is a component of the visual content generation system600 (seeFIG.6), which uses the values of themotion parameters110 to create visual representations of thefirst material112 and/or thesecond material114. Then, theprocess200 terminates.
• By way of a non-limiting example, theprocess200 may be used to simulate a waterfall. In this example, referring toFIG.3, theprimary material302 is water, water particles, droplets, or mists. Thesecondary material304 is air that surrounds the water, water particles, droplets, or mists.
• As described above, in block205 (seeFIG.2), the motion simulation system120 (seeFIG.1) represents the primary material302 (e.g., water) as the first phase particles (e.g., FLIP or APIC particles) each having an initial position (e.g., with respect to an Eulerian grid).
• Then, in block210 (seeFIG.2), themotion simulation system120 identifies thethickness324 of thelayer portion308.
• Next, in block215 (seeFIG.2), the motion simulation system120 (seeFIG.1) represents thelayer portion308 as the second phase representation. The second phase representation may be a sparse Eulerian volume. Thus, the motion simulation system120 (seeFIG.1) may represent thesecondary material304 as a sparse Eulerian volume of fluid that includes only thelayer portion308.
• In block220 (seeFIG.2), the motion simulation system120 (seeFIG.1) assigns boundary condition(s) to the first andsecond boundaries320 and322. When solving the first set of equations for theprimary material302, themotion simulation system120 applies a freesurface boundary condition406 on thefirst boundary320 and thedrag force404 in the vicinity of and/or on thesecond boundary322. Together, the freesurface boundary condition406 and thedrag force404 model thelayer portion308 as having one or more prescribed velocities. In other words, themotion simulation system120 assumes the velocity of thelayer portion308 is prescribed. As described below, themotion simulation system120 may define thedrag force404 based on the velocity of the layer portion308 (e.g., stored in a second velocity field) and aeration values (described below). When solving the second set of equations for thelayer portion308, themotion simulation system120 applies the pressure boundary condition(s) discussed above on thesecond boundary322 and one or more solid boundary conditions402 (seeFIG.4) on thefirst boundary320. InFIG.4, the pressure boundary condition(s) are illustrated as pressure boundary condition(s)408. Themotion simulation system120 assumes the velocity of theprimary material302 is prescribed (e.g., by using the most recently calculatedfirst velocity field504 illustrated inFIG.5). As described below, themotion simulation system120 determines the solid boundary condition(s)402 based on the velocity of the primary material302 (e.g., stored in the first velocity field504). The solid boundary condition(s)402 and the pressure boundary condition(s)408 may be characterized as being “hard constraints” on velocity and pressure, respectively. Themotion simulation system120 applies the solid boundary condition(s)402 and the pressure boundary condition(s)408 at the first andsecond boundaries320 and322, respectively, while solving the second set of equations.
• In block225 (seeFIG.2), themotion simulation system120 solves the first set of equations for theprimary material302 to obtain the first velocity field504 (seeFIG.5), and the second set of equations for thesecondary material304 to obtain the second velocity field (not shown), separately. For each position being simulated (e.g., each position of the Eulerian grid), the first velocity field504 (seeFIG.5) stores a velocity value (e.g., a vector) that indicates how the environment affects the motion (e.g., direction and speed) of the first phase particle, if any, currently in that position. For each position being simulated (e.g., each position of the Eulerian grid), the second velocity field (not shown) stores a velocity value (e.g., a vector) that indicates how the environment effects the motion (e.g., direction and speed) of a portion (e.g., a voxel) of the second phase representation, if any, currently in that position. After the first set of equations is solved, the solution may be supplied to the second set of equations and used to obtain the second velocity field (not shown). When themotion simulation system120 next solves the first set of equations to obtain a newfirst velocity field510, themotion simulation system120 performs thedrag force exchange400, which weakly couples theprimary material302 and thelayer portion308 together.
• For example, the motion simulation system120 (seeFIG.1) may begin by first solving the first set of equations for theprimary material302 to obtain the first velocity field504 (seeFIG.5). When the motion simulation system120 (seeFIG.1) solves the first set of equations, themotion simulation system120 applies the freesurface boundary condition406 at each point along thefirst boundary320 and thedrag force404 in the vicinity of or at each point along thesecond boundary322, which models the influence of the layer portion308 (e.g., air) on the primary material302 (e.g., water). A free surface is a fluid surface that is subject to a prescribed pressure condition. For example, the prescribed pressure condition may be zero if the secondary material304 (e.g., air) is significantly lighter than the primary material302 (e.g., water). Methods of calculating the freesurface boundary condition406 are well-known and will not described herein. Thedrag force404 may be computed based on the aeration values and drag force values, which are determined based at least in part on the relative velocities of theprimary material302 and thelayer portion308. The freesurface boundary condition406 and thedrag force404 model thelayer portion308 as having one or more prescribed velocities.
• Then, the motion simulation system120 (seeFIG.1) may solve the second set of equations for thelayer portion308 to obtain the second velocity field. When the motion simulation system120 (seeFIG.1) solves the second set of equations, themotion simulation system120 applies the pressure boundary condition(s)408 assigned to the second boundary322 (based on the prescribed dynamics of the outer portion306), and the solid boundary condition(s)402 assigned to thefirst boundary320. The solid boundary condition(s)402 exert(s) force on the layer portion308 (e.g., gas), and not on theprimary material302. This makes sense in view of the fact that water is much heavier than air and easily pushes the air around. Thus, themotion simulation system120 applies the solid boundary condition(s)402 to the layer portion308 (e.g., gas). Thelayer portion308 reacts to the solid boundary condition(s)402 by being pushed around by the primary material302 (e.g., fluid). The solid boundary condition(s)402 is based at least in part on theprimary material302. For example, the solid boundary condition(s)402 may be based at least in part on the first velocity field504 (seeFIG.5). Thus, in this step, because theprimary material302 significantly influences the layer portion308 (e.g., because water is much heavier than air), themotion simulation system120 treats theprimary material302 as a solid boundary with a prescribed velocity (e.g., thefirst velocity field504 illustrated inFIG.5) that was computed when theprimary material302 solved the first set of equations. As mentioned above, the motion simulation system120 (seeFIG.1) assigns the solid boundary condition(s)402 (seeFIG.4) to the first boundary320 (seeFIG.3).
• Thus, for each iteration, the motion simulation system120 (seeFIG.1) solves for theprimary material302, assuming the velocity of thelayer portion308 is prescribed, and solves for thelayer portion308, assuming the velocity of theprimary material302 is prescribed.
• Alternating the solves for theprimary material302 and thelayer portion308 is a weaker coupling scheme than the two-phase solver coupling scheme discussed above and may be configured to allow the amount of interaction between the air and the water to be at least partially artist directed. This weaker coupling scheme may achieve believable breakup of the water into wispy patterns but may not preserve the shape of bubbles underwater. Thus, depending upon the implementation details, thedrag force exchange400 may not be suitable for simulating bubbles submerged in a fluid (e.g., water).
• As mentioned above, the solid boundary condition(s)402 applies one or more prescribed velocities to the layer portion308 (e.g., gas). At the same time, the layer portion308 (e.g., gas) exerts thedrag force404 on the primary material302 (e.g., fluid). Thedrag force404 may be stored in an adjusted drag force field502 (seeFIG.5) having a value at each position being simulated (e.g., each position of the Eulerian grid).
• Thedrag force404 may be determined based at least in part on the material properties of the primary material302 (e.g., fluid) and a drag force applied by thelayer portion308 to the primary material302 (and calculated based at least in part on the second velocity field). Examples of such material properties include a velocity property, a position property, and an aeration property. The first velocity field504 (seeFIG.5) stores values of the velocity property. The velocity values in thefirst velocity field504 may be vectors indicating both a direction and rate of motion. Referring toFIG.5, themotion simulation system120 may create anaeration field506. For each position being simulated (e.g., each position of the Eulerian grid), theaeration field506 may store an aeration value that indicates how aerated theprimary material302 should be at that position. The aeration values may be determined using any method know in the art (e.g., using an aeration heuristic) that measures how aerated theprimary material302 should be at each position being simulated (e.g., each position of the Eulerian grid). Methods of determining the aeration values are known in the art and will not be described herein.
• Next, themotion simulation system120 may create a drag force field508 (seeFIG.5). For each position being simulated (e.g., each position of the Eulerian grid), the drag force field508 (seeFIG.5) stores a drag force value (e.g., a vector) that indicates how thelayer portion308 effects the motion (e.g., direction and speed) of the first phase particle, if any, currently in that position. Methods of determining the drag force values are known in the art and will not be described herein. For example, the drag force value may be computed based at least in part on the relative velocities of theprimary material302 and thelayer portion308.
• The drag force404 (seeFIG.4) may be determined as a function of theaeration field506 and thedrag force field508. As illustrated inFIG.4 by a dashedline410, the drag force comes from thesecondary material304 and, as illustrated by a dashedline412, the aeration property comes from theprimary material302. Themotion simulation system120 may obtain the adjusteddrag force field502 by multiplying each value in thedrag force field508 by the value in theaeration field506 obtained for the same position.
• Then, a newfirst velocity field510 may be determined as a function of the adjusteddrag force field502 and the previousfirst velocity field504. For example, themotion simulation system120 may obtain the newfirst velocity field510 by multiplying each value in thefirst velocity field504 by the value in the adjusteddrag force field502 obtained for the same position. The values in the newfirst velocity field510 may be vectors indicating both a direction and rate of motion. The values of themotion parameters110 may include the newfirst velocity field510, which indicates where the first phase particles representing theprimary material302 move and how quickly.
• Themotion simulation system120 may calculate new material states for the first phase particles as well as new material states for the second phase representation. The new material states of the first phase particles may include positions and attributes of the first phase particles. The new positions may be based at least in part on the current positions of the first phase particles and the newfirst velocity field510. At least some of these new positions may be modified (e.g., by the motion simulation system120), if necessary, using additional processing known in the art. The new material states of the second phase representation may include states of Eulerian voxels and may be based at least in part on the second velocity field (not shown). The new material states of the first phase particles and the second phase representation may be included in the values of themotion parameters110.
• Themotion simulation system120 may identify new locations for the first andsecond boundaries320 and322 (seeFIG.3) based at least in part on the new material states (e.g., new positions) of the first phase particles. Additionally, themotion simulation system120 may determine new pressure boundary condition(s) (e.g., an updated hydrostatic pressure field) based at least in part on the new location of thesecond boundary322. For example, themotion simulation system120 may calculate the new pressure boundary condition(s) usingEquation 1 above. The values of themotion parameters110 may include the new locations of the first andsecond boundaries320 and322 (seeFIG.3).
• As explained above, referring toFIG.3, the more aerated theprimary material302 is, the greater the drag force404 (seeFIG.4) is. Thus, the aeration property may be used to modulate how much drag force is applied to theprimary material302 by thelayer portion308 of thesecondary material304.
• Optionally, the values of thefirst velocity field504, theaeration field506, thedrag force field508, and/or the newfirst velocity field510 may be modified (e.g., multiplied) by one or more additional values. Examples of such the additional value(s) include density of theprimary material302, artistic or artist-controlled parameters, and/or the like.
• Indecision block230, themotion simulation system120 determines whether the simulation has completed. The decision indecision block230 is “YES,” when themotion simulation system120 determines the simulation has completed. Otherwise, the decision indecision block230 is “NO.” By way of a non-limiting example, blocks220-235 may be repeated a desired number of iterations (e.g., five times). The number of iterations might be specified by an artist (e.g., the artist142) or operator in advance. For example, blocks220-235 may be repeated a number of times required to generate the values of themotion parameters110 needed to create a desired number of frames.
• When the decision indecision block230 is “NO,” themotion simulation system120 advances to block235 whereat themotion simulation system120 advances the simulation in time. Then, themotion simulation system120 returns to block220 and assigns new boundary condition(s) to the first andsecond boundaries320 and322.
• When the decision indecision block230 is “YES,” inblock240, themotion simulation system120 forwards the values of themotion parameters110 to the animation creation system630 (seeFIGS.1 and6) component of the visual content generation system600 (seeFIG.6), which uses the values of themotion parameters110 to create visual representations of a waterfall including thefirst material112 and/or thesecond material114. Then, theprocess200 terminates.
• For example,FIG.6 illustrates the example visualcontent generation system600 as might be used to generate imagery in the form of still images and/or video sequences of images. Visualcontent generation system600 might generate imagery of live action scenes, computer generated scenes, or a combination thereof. In a practical system, users are provided with tools that allow them to specify, at high levels and low levels where necessary, what is to go into that imagery. For example, a user might be an animation artist (likeartist142 illustrated inFIG.1) and might use visualcontent generation system600 to capture interaction between two human actors performing live on a sound stage and replace one of the human actors with a computer-generated anthropomorphic non-human being that behaves in ways that mimic the replaced human actor's movements and mannerisms, and then add in a third computer-generated character and background scene elements that are computer-generated, all in order to tell a desired story or generate desired imagery.
• Still images that are output by visualcontent generation system600 might be represented in computer memory as pixel arrays, such as a two-dimensional array of pixel color values, each associated with a pixel having a position in a two-dimensional image array. Pixel color values might be represented by three or more (or fewer) color values per pixel, such as a red value, a green value, and a blue value (e.g., in RGB format). Dimensions of such a two-dimensional array of pixel color values might correspond to a preferred and/or standard display scheme, such as 1920-pixel columns by 1280-pixel rows or 4096-pixel columns by 2160-pixel rows, or some other resolution. Images might or might not be stored in a compressed format, but either way, a desired image may be represented as a two-dimensional array of pixel color values. In another variation, images are represented by a pair of stereo images for three-dimensional presentations and in other variations, an image output, or a portion thereof, might represent three-dimensional imagery instead of just two-dimensional views. In yet other embodiments, pixel values are data structures and a pixel value is associated with a pixel and can be a scalar value, a vector, or another data structure associated with a corresponding pixel. That pixel value might include color values, or not, and might include depth values, alpha values, weight values, object identifiers or other pixel value components.
• A stored video sequence might include a plurality of images such as the still images described above, but where each image of the plurality of images has a place in a timing sequence and the stored video sequence is arranged so that when each image is displayed in order, at a time indicated by the timing sequence, the display presents what appears to be moving and/or changing imagery. In one representation, each image of the plurality of images is a video frame having a specified frame number that corresponds to an amount of time that would elapse from when a video sequence begins playing until that specified frame is displayed. A frame rate might be used to describe how many frames of the stored video sequence are displayed per unit time. Example video sequences might include 24 frames per second (24 FPS), 50 FPS, 140 FPS, or other frame rates. In some embodiments, frames are interlaced or otherwise presented for display, but for clarity of description, in some examples, it is assumed that a video frame has one specified display time, but other variations might be contemplated.
• One method of creating a video sequence is to simply use a video camera to record a live action scene, i.e., events that physically occur and can be recorded by a video camera. The events being recorded can be events to be interpreted as viewed (such as seeing two human actors talk to each other) and/or can include events to be interpreted differently due to clever camera operations (such as moving actors about a stage to make one appear larger than the other despite the actors actually being of similar build, or using miniature objects with other miniature objects so as to be interpreted as a scene containing life-sized objects).
• Creating video sequences for story-telling or other purposes often calls for scenes that cannot be created with live actors, such as a talking tree, an anthropomorphic object, space battles, and the like. Such video sequences might be generated computationally rather than capturing light from live scenes. In some instances, an entirety of a video sequence might be generated computationally, as in the case of a computer-animated feature film. In some video sequences, it is desirable to have some computer-generated imagery and some live action, perhaps with some careful merging of the two.
• While computer-generated imagery might be creatable by manually specifying each color value for each pixel in each frame, this is likely too tedious to be practical. As a result, a creator uses various tools to specify the imagery at a higher level. As an example, an artist (e.g.,artist142 illustrated inFIG.1) might specify the positions in a scene space, such as a three-dimensional coordinate system, of objects and/or lighting, as well as a camera viewpoint, and a camera view plane. From that, a rendering engine could take all of those as inputs, and compute each of the pixel color values in each of the frames. In another example, an artist specifies position and movement of an articulated object having some specified texture rather than specifying the color of each pixel representing that articulated object in each frame.
• In a specific example, a rendering engine performs ray tracing wherein a pixel color value is determined by computing which objects lie along a ray traced in the scene space from the camera viewpoint through a point or portion of the camera view plane that corresponds to that pixel. For example, a camera view plane might be represented as a rectangle having a position in the scene space that is divided into a grid corresponding to the pixels of the ultimate image to be generated, and if a ray defined by the camera viewpoint in the scene space and a given pixel in that grid first intersects a solid, opaque, blue object, that given pixel is assigned the color blue. Of course, for modern computer-generated imagery, determining pixel colors—and thereby generating imagery—can be more complicated, as there are lighting issues, reflections, interpolations, and other considerations.
• As illustrated inFIG.6, a liveaction capture system602 captures a live scene that plays out on astage604. Liveaction capture system602 is described herein in greater detail, but might include computer processing capabilities, image processing capabilities, one or more processors, program code storage for storing program instructions executable by the one or more processors, as well as user input devices and user output devices, not all of which are shown.
• In a specific live action capture system, cameras606(1) and606(2) capture the scene, while in some systems, there might be other sensor(s)608 that capture information from the live scene (e.g., infrared cameras, infrared sensors, motion capture (“mo-cap”) detectors, etc.). Onstage604, there might be human actors, animal actors, inanimate objects, background objects, and possibly an object such as agreen screen610 that is designed to be captured in a live scene recording in such a way that it is easily overlaid with computer-generated imagery.Stage604 might also contain objects that serve as fiducials, such as fiducials612(1)-(3), that might be used post-capture to determine where an object was during capture. A live action scene might be illuminated by one or more lights, such as anoverhead light614.
• During or following the capture of a live action scene, liveaction capture system602 might output live action footage to a liveaction footage storage620. A liveaction processing system622 might process live action footage to generate data about that live action footage and store that data into a liveaction metadata storage624. Liveaction processing system622 might include computer processing capabilities, image processing capabilities, one or more processors, program code storage for storing program instructions executable by the one or more processors, as well as user input devices and user output devices, not all of which are shown. Liveaction processing system622 might process live action footage to determine boundaries of objects in a frame or multiple frames, determine locations of objects in a live action scene, where a camera was relative to some action, distances between moving objects and fiducials, etc. Where elements have sensors attached to them or are detected, the metadata might include location, color, and intensity ofoverhead light614, as that might be useful in post-processing to match computer-generated lighting on objects that are computer-generated and overlaid on the live action footage. Liveaction processing system622 might operate autonomously, perhaps based on predetermined program instructions, to generate and output the live action metadata upon receiving and inputting the live action footage. The live action footage can be camera-captured data as well as data from other sensors.
• Ananimation creation system630 is another part of visualcontent generation system600.Animation creation system630 might include computer processing capabilities, image processing capabilities, one or more processors, program code storage for storing program instructions executable by the one or more processors, as well as user input devices and user output devices, not all of which are shown.Animation creation system630 might be used by animation artists, managers, and others to specify details, perhaps programmatically and/or interactively, of imagery to be generated. From user input and data from a database or other data source, indicated as adata store632,animation creation system630 might generate and output data representing objects (e.g., a horse, a human, a ball, a teapot, a cloud, a light source, a texture, etc.) to anobject storage634, generate and output data representing a scene into ascene description storage636, and/or generate and output data representing animation sequences to ananimation sequence storage638.
• Scene data might indicate locations of objects and other visual elements, values of their parameters, lighting, camera location, camera view plane, and other details that arendering engine650 might use to render CGI imagery. For example, scene data might include the locations of several articulated characters, background objects, lighting, etc. specified in a two-dimensional space, three-dimensional space, or other dimensional space (such as a 2.5-dimensional space, three-quarter dimensions, pseudo-3D spaces, etc.) along with locations of a camera viewpoint and view place from which to render imagery. For example, scene data might indicate that there is to be a red, fuzzy, talking dog in the right half of a video and a stationary tree in the left half of the video, all illuminated by a bright point light source that is above and behind the camera viewpoint. In some cases, the camera viewpoint is not explicit, but can be determined from a viewing frustum. In the case of imagery that is to be rendered to a rectangular view, the frustum would be a truncated pyramid. Other shapes for a rendered view are possible and the camera view plane could be different for different shapes.
• Animation creation system630 might be interactive, allowing a user to read in animation sequences, scene descriptions, object details, etc. and edit those, possibly returning them to storage to update or replace existing data. As an example, an operator might read in objects from object storage into abaking processor642 that would transform those objects into simpler forms and return those to objectstorage634 as new or different objects. For example, an operator might read in an object that has dozens of specified parameters (movable joints, color options, textures, etc.), select some values for those parameters and then save a baked object that is a simplified object with now fixed values for those parameters.
• Rather than requiring user specification of each detail of a scene, data fromdata store632 might be used to drive object presentation. For example, if an artist is creating an animation of a spaceship passing over the surface of the Earth, instead of manually drawing or specifying a coastline, the artist might specify thatanimation creation system630 is to read data fromdata store632 in a file containing coordinates of Earth coastlines and generate background elements of a scene using that coastline data.
• Animation sequence data might be in the form of time series of data for control points of an object that has attributes that are controllable. For example, an object might be a humanoid character with limbs and joints that are movable in manners similar to typical human movements. An artist can specify an animation sequence at a high level, such as “the left hand moves from location (X1, Y1, Z1) to (X2, Y2, Z2) over time T1 to T2”, at a lower level (e.g., “move the elbow joint 2.5 degrees per frame”) or even at a very high level (e.g., “character A should move, consistent with the laws of physics that are given for this scene, from point P1 to point P2 along a specified path”).
• Animation sequences in an animated scene might be specified by what happens in a live action scene. Ananimation driver generator644 might read in live action metadata, such as data representing movements and positions of body parts of a live actor during a live action scene.Animation driver generator644 might generate corresponding animation parameters to be stored inanimation sequence storage638 for use in animating a CGI object. This can be useful where a live action scene of a human actor is captured while wearing mo-cap fiducials (e.g., high-contrast markers outside actor clothing, high-visibility paint on actor skin, face, etc.) and the movement of those fiducials is determined by liveaction processing system622.Animation driver generator644 might convert that movement data into specifications of how joints of an articulated CGI character are to move over time.
• Arendering engine650 can read in animation sequences, scene descriptions, and object details, as well as rendering engine control inputs, such as a resolution selection and a set of rendering parameters. Resolution selection might be useful for an operator to control a trade-off between speed of rendering and clarity of detail, as speed might be more important than clarity for a movie maker to test some interaction or direction, while clarity might be more important than speed for a movie maker to generate data that will be used for final prints of feature films to be distributed.Rendering engine650 might include computer processing capabilities, image processing capabilities, one or more processors, program code storage for storing program instructions executable by the one or more processors, as well as user input devices and user output devices, not all of which are shown.
• Visualcontent generation system600 can also include amerging system660 that merges live footage with animated content. The live footage might be obtained and input by reading from liveaction footage storage620 to obtain live action footage, by reading from liveaction metadata storage624 to obtain details such as presumed segmentation in captured images segmenting objects in a live action scene from their background (perhaps aided by the fact thatgreen screen610 was part of the live action scene), and by obtaining CGI imagery fromrendering engine650.
• A mergingsystem660 might also read data from rulesets for merging/combiningstorage662. A very simple example of a rule in a ruleset might be “obtain a full image including a two-dimensional pixel array from live footage, obtain a full image including a two-dimensional pixel array fromrendering engine650, and output an image where each pixel is a corresponding pixel fromrendering engine650 when the corresponding pixel in the live footage is a specific color of green, otherwise output a pixel value from the corresponding pixel in the live footage.”
• Mergingsystem660 might include computer processing capabilities, image processing capabilities, one or more processors, program code storage for storing program instructions executable by the one or more processors, as well as user input devices and user output devices, not all of which are shown. Mergingsystem660 might operate autonomously, following programming instructions, or might have a user interface or programmatic interface over which an operator can control a merging process. In some embodiments, an operator can specify parameter values to use in a merging process and/or might specify specific tweaks to be made to an output of mergingsystem660, such as modifying boundaries of segmented objects, inserting blurs to smooth out imperfections, or adding other effects. Based on its inputs, mergingsystem660 can output an image to be stored in astatic image storage670 and/or a sequence of images in the form of video to be stored in an animated/combinedvideo storage672.
• Thus, as described, visualcontent generation system600 can be used to generate video that combines live action with computer-generated animation using various components and tools, some of which are described in more detail herein. While visualcontent generation system600 might be useful for such combinations, with suitable settings, it can be used for outputting entirely live action footage or entirely CGI sequences. The code may also be provided and/or carried by a transitory computer readable medium, e.g., a transmission medium such as in the form of a signal transmitted over a network.
• According to one embodiment, the techniques described herein are implemented by one or more generalized computing systems programmed to perform the techniques pursuant to program instructions in firmware, memory, other storage, or a combination. Special-purpose computing devices may be used, such as desktop computer systems, portable computer systems, handheld devices, networking devices or any other device that incorporates hard-wired and/or program logic to implement the techniques.
• For example,FIG.7 is a block diagram that illustrates acomputer system700 upon which the computer systems of the systems described herein and/or visual content generation system600 (seeFIG.6) may be implemented.Computer system700 includes abus702 or other communication mechanism for communicating information, and aprocessor704 coupled withbus702 for processing information.Processor704 may be, for example, a general-purpose microprocessor.
• Computer system700 also includes amain memory706, such as a random-access memory (RAM) or other dynamic storage device, coupled tobus702 for storing information and instructions to be executed byprocessor704.Main memory706 may also be used for storing temporary variables or other intermediate information during execution of instructions to be executed byprocessor704. Such instructions, when stored in non-transitory storage media accessible toprocessor704, rendercomputer system700 into a special-purpose machine that is customized to perform the operations specified in the instructions.
• Computer system700 further includes a read only memory (ROM)708 or other static storage device coupled tobus702 for storing static information and instructions forprocessor704. Astorage device710, such as a magnetic disk or optical disk, is provided and coupled tobus702 for storing information and instructions.
• Computer system700 may be coupled viabus702 to adisplay712, such as a computer monitor, for displaying information to a computer user. Aninput device714, including alphanumeric and other keys, is coupled tobus702 for communicating information and command selections toprocessor704. Another type of user input device is acursor control716, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections toprocessor704 and for controlling cursor movement ondisplay712. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane.
• Computer system700 may implement the techniques described herein using customized hard-wired logic, one or more ASICs or FPGAs, firmware and/or program logic which in combination with the computer system causes orprograms computer system700 to be a special-purpose machine. According to one embodiment, the techniques herein are performed bycomputer system700 in response toprocessor704 executing one or more sequences of one or more instructions contained inmain memory706. Such instructions may be read intomain memory706 from another storage medium, such asstorage device710. Execution of the sequences of instructions contained inmain memory706 causesprocessor704 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions.
• The term “storage media” as used herein refers to any non-transitory media that store data and/or instructions that cause a machine to operation in a specific fashion. Such storage media may include non-volatile media and/or volatile media. Non-volatile media includes, for example, optical or magnetic disks, such asstorage device710. Volatile media includes dynamic memory, such asmain memory706. Common forms of storage media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, NVRAM, any other memory chip or cartridge.
• Storage media is distinct from but may be used in conjunction with transmission media. Transmission media participates in transferring information between storage media. For example, transmission media includes coaxial cables, copper wire, and fiber optics, including the wires that includebus702. Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications.
• Various forms of media may be involved in carrying one or more sequences of one or more instructions toprocessor704 for execution. For example, the instructions may initially be carried on a magnetic disk or solid-state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a network connection. A modem or network interface local tocomputer system700 can receive the data.Bus702 carries the data tomain memory706, from whichprocessor704 retrieves and executes the instructions. The instructions received bymain memory706 may optionally be stored onstorage device710 either before or after execution byprocessor704.
• Computer system700 also includes acommunication interface718 coupled tobus702.Communication interface718 provides a two-way data communication coupling to anetwork link720 that is connected to alocal network722. For example,communication interface718 may be a network card, a modem, a cable modem, or a satellite modem to provide a data communication connection to a corresponding type of telephone line or communications line. Wireless links may also be implemented. In any such implementation,communication interface718 sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.
• Network link720 typically provides data communication through one or more networks to other data devices. For example,network link720 may provide a connection throughlocal network722 to ahost computer724 or to data equipment operated by an Internet Service Provider (ISP)726.ISP726 in turn provides data communication services through the world-wide packet data communication network now commonly referred to as the “Internet”728.Local network722 andInternet728 both use electrical, electromagnetic, or optical signals that carry digital data streams. The signals through the various networks and the signals onnetwork link720 and throughcommunication interface718, which carry the digital data to and fromcomputer system700, are example forms of transmission media.
• Computer system700 can send messages and receive data, including program code, through the network(s),network link720, andcommunication interface718. In the Internet example, aserver730 might transmit a requested code for an application program through theInternet728,ISP726,local network722, andcommunication interface718. The received code may be executed byprocessor704 as it is received, and/or stored instorage device710, or other non-volatile storage for later execution.
• Operations of processes described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. Processes described herein (or variations and/or combinations thereof) may be performed under the control of one or more computer systems configured with executable instructions and may be implemented as code (e.g., executable instructions, one or more computer programs or one or more applications) executing collectively on one or more processors, by hardware or combinations thereof. The code may be stored on a computer-readable storage medium, for example, in the form of a computer program comprising a plurality of instructions executable by one or more processors. The computer-readable storage medium may be non-transitory. The code may also be provided carried by a transitory computer readable medium e.g., a transmission medium such as in the form of a signal transmitted over a network.
• Conjunctive language, such as phrases of the form “at least one of A, B, and C,” or “at least one of A, B and C,” unless specifically stated otherwise or otherwise clearly contradicted by context, is otherwise understood with the context as used in general to present that an item, term, etc., may be either A or B or C, or any nonempty subset of the set of A and B and C. For instance, in the illustrative example of a set having three members, the conjunctive phrases “at least one of A, B, and C” and “at least one of A, B and C” refer to any of the following sets: {A}, {B}, {C}, {A, B}, {A, C}, {B, C}, {A, B, C}. Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of A, at least one of B and at least one of C each to be present.
• The use of examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
• In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The sole and exclusive indicator of the scope of the invention, and what is intended by the applicants to be the scope of the invention, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction.
• Further embodiments can be envisioned to one of ordinary skill in the art after reading this disclosure. In other embodiments, combinations or sub-combinations of the above-disclosed invention can be advantageously made. The example arrangements of components are shown for purposes of illustration and combinations, additions, re-arrangements, and the like are contemplated in alternative embodiments of the present invention. Thus, while the invention has been described with respect to exemplary embodiments, one skilled in the art will recognize that numerous modifications are possible.
• For example, the processes described herein may be implemented using hardware components, software components, and/or any combination thereof. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims and that the invention is intended to cover all modifications and equivalents within the scope of the following claims.
• All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

Claims (28)

1. A motion simulation system for simulating interactions between two different materials, the system comprising:

at least one processor;

a computer-readable medium storing instructions, which when executed by the at least one processor, cause the system to:

represent a primary material as a plurality of first particles;

represent a layer portion of a secondary material as a grid-based fluid volume, the secondary material comprising the layer portion and an outer portion, the layer portion being positioned between the plurality of particles and the outer portion;

assign at least one boundary condition to a boundary positioned between the layer portion and the outer portion based at least in part on pressures of the outer portion and configured to affect the layer portion, the at least one boundary condition comprising at least one pressure value;

calculate motion parameters for the primary material based on the plurality of particles and the layer portion; and

calculate motion parameters for the secondary material based on the plurality of particles, the layer portion, and the at least one boundary condition, independent of the outer portion.

2. (canceled)

3. (canceled)

4. The motion simulation system ofclaim 1, wherein the layer portion has a thickness that ranges between 0.1 and 10000 times an average particle size of the plurality of particles.

5. The motion simulation system ofclaim 1, wherein the layer portion has a thickness that ranges between 10 and 1000 times an average particle size of the plurality of particles.

6. The motion simulation system ofclaim 1, wherein the layer portion has a thickness that depends on a difference between a first density of the primary material and a second density of the secondary material.

7. The motion simulation system ofclaim 1, wherein the outer portion of the secondary material is does not move as a result of interaction with the primary material.

8. (canceled)

9. (canceled)

10. The motion simulation system ofclaim 1, wherein the at least one boundary condition includes a hydrostatic pressure at the boundary configured to exert force on the primary material and the layer portion.

11. The motion simulation system ofclaim 10, wherein the exerted force affects a shape of the boundary.

12. The motion simulation system ofclaim 1, wherein the boundary is a first boundary, the at least one boundary condition is at least one primary boundary condition, and the instructions, when executed by at least one processor, further cause the system to:

assign at least one secondary boundary condition to a second boundary positioned between the primary material and the layer portion, the at least one second boundary condition comprising a free surface boundary condition.

13. The motion simulation system ofclaim 12, wherein the at least one primary boundary condition comprises a draft force in a vicinity of the first boundary.

14. (canceled)

15. The motion simulation system ofclaim 1, wherein the instructions, when executed by at least one processor, further cause the system to:

calculate a primary velocity field for the primary material, wherein the primary velocity field is configured to store a primary velocity value that indicates how an environment affects motion of the plurality of particles; and

calculate a secondary velocity field for the secondary material, wherein the secondary velocity field is configured to store a secondary velocity value that indicates how the environment affects motion of at least a portion of the secondary material.

16. (canceled)

17. (canceled)

18. (canceled)

19. The motion simulation system ofclaim 1, wherein the primary material is a liquid and the secondary material is a gas.

20. The motion system ofclaim 1, wherein the primary material is falling through the secondary material under the influence of gravity.

21. The motion system ofclaim 1, wherein the primary material is weightless with respect to the secondary material.

22. The motion simulation system ofclaim 1, wherein the primary material is a gas and the secondary material is a liquid.

23. The motion system ofclaim 1, wherein the primary material is rising buoyantly through or suspended buoyantly within the secondary material.

24. The motion simulation system ofclaim 1, wherein the primary material is a solid and the secondary material is a liquid.

25. The motion simulation system ofclaim 24, wherein the solid comprises hair, cloth, or sponge.

26. The motion simulation system ofclaim 1, wherein the primary material is a solid and the secondary material is a gas.

27. The motion simulation system ofclaim 1, wherein the instructions, when executed by the at least one processor, further cause the system to generate one or more visual representations of the primary material interacting with the secondary material based on the values of the motion parameters.

28. The motion simulation system ofclaim 1, wherein either the primary material or the secondary material is invisible in the one or more visual representations.

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