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Incomputer graphics,rasterisation (British English) orrasterization (American English) is the task of taking animage described in avector graphics format (shapes) and converting it into araster image (a series ofpixels, dots or lines, which, when displayed together, create the image which was represented via shapes).[1][2] The rasterized image may then be displayed on acomputer display,video display orprinter, or stored in abitmap file format. Rasterization may refer to the technique of drawing3D models, or to the conversion of 2Drendering primitives, such aspolygons andline segments, into a rasterized format.
The term "rasterisation" comes from German Raster 'grid, pattern, schema' and Latin rāstrum 'scraper, rake'.[3][4]
Bresenham's line algorithm is an example of an algorithm used to rasterize lines.
Algorithms such asMidpoint circle algorithm are used to render circle onto a pixelated canvas.
Rasterization is one of the typical techniques of rendering 3D models. Compared with other rendering techniques such asray tracing, rasterization is extremely fast and therefore used in most realtime 3D engines. However, rasterization is simply the process of computing the mapping from scene geometry to pixels and does not prescribe a particular way to compute the color of those pixels. The specific color of each pixel is assigned by apixel shader (which in modernGPUs is completelyprogrammable). Shading may take into account physical effects such as light position, their approximations or purely artistic intent.
The process of rasterizing 3D models onto a 2D plane for display on acomputer screen ("screen space") is often carried out by fixed function (non-programmable) hardware within thegraphics pipeline. This is because there is no motivation for modifying the techniques for rasterization used at render time[5] and a special-purpose system allows for high efficiency.
Polygons are a common representation of digital 3D models. Before rasterization, individual polygons are typically broken down into triangles; therefore, a typical problem to solve in 3D rasterization is rasterization of a triangle. Properties that are usually required from triangle rasterization algorithms are that rasterizing two adjacent triangles (i.e. those that share an edge)
This leads to establishingrasterization rules to guarantee the above conditions. One set of such rules is called atop-left rule, which states that a pixel is rasterized if and only if
Atop edge is an edge that is exactly horizontal and lies above other edges, and aleft edge is a non-horizontal edge that is on the left side of the triangle.
This rule is implemented e.g. byDirect3D[6] and manyOpenGL implementations (even though the specification doesn't define it and only requires a consistent rule[7]).
The quality of rasterization can be improved byantialiasing, which creates "smooth" edges.Sub-pixel precision is a method which takes into account positions on a finer scale than the pixel grid and can produce different results even if the endpoints of a primitive fall into same pixel coordinates, producing smoother movement animations. Simple or older hardware, such asPlayStation 1, lacked sub-pixel precision in 3D rasterization.[8]