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GLSL Shading withOpenSceneGraph Mike Weiblen July 31, 2005 Los Angeles Rev B 1

Summary of this talk Overview of GLSL pipeline & syntax OSG support for GLSL Tips, Tricks, Gotchas, and Tools Demos What we’re not covering Our focus will be the OSG API, not OpenGL API Not lots of detail on the GLSL language itselfCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 2

Our dependencies OpenGL 2.0 OpenGL Shading Language 1.10 rev 59 Specs on opengl.org and CDROM OpenSceneGraph 0.9.9 Note: GLSL support continues to evolve in CVSCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 3

GLSL / OSG timeline Fall 2001: 3Dlabs “GL2.0” whitepapers, shading language is the centerpiece July 2003: ARB approves GL1.5, GLSL as ARB extension Fall 2003: osgGL2 Nodekit Sep 2004: ARB approves GL2.0, GLSL in the core. Spring 2005: 2nd generation GLSL support integrated into OSG core Supports both OpenGL 2.0 and 1.5 extensionsCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 4

GLSL pipeline architecture GLSL exposes programmability at two points in the OpenGL 2.0 pipeline Vertex processor Fragment processor Compiled code to run on a particular processor is a glShader A linked executable unit to be activated on the pipeline is a glProgramCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 7

GLSL compilation model Similar to familiar C build model glShader = “object file” Contains shader source code Compiled to become an “.obj file” Must be recompiled when source changes glProgram = “executable file” Contains a list of shaders Linked to become an “.exe file” Must be relinked when set of shaders changes As with C, a glShader “.obj” can be shared across multiple glPrograms “.exe”Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 8

Vertex Processor Overview Standard OpenGL Generic attributes attributes gl_color 0, 1, 2, … gl_normal etc. Vertex User-defined uniforms: epsilon, myLightPos, surfColor, etc.Texture Maps Processor Built-in uniforms: gl_FogColor, gl_ModelViewMatrix, etc. Standard Special User-defined Varying Variables Varying gl_FrontColor gl_Position normal gl_BackColor gl_ClipVertex refraction etc. gl_PointSize etc. Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 9

Fragment Processor Overview Standard Special User-defined Varying Variables Varying gl_Color gl_FragCoord normal gl_SecondaryColor gl_FrontFacing refraction etc. etc. Fragment User-defined uniforms: epsilon, myLightPos, surfColor, etc.Texture Maps Processor Built-in uniforms: gl_FogColor, gl_ModelViewMatrix, etc. Special Variables gl_FragColor gl_FragDepth gl_FragData[n] Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 10

GLSL language design Based on syntax of ANSI C Includes preprocessor Additions for graphics functionality Additions from C++ Some refactoring for cleaner design Designed for parallelization on SIMD arrayCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 11

Language enhanced for graphics Added Vector and Matrix types Added Sampler type for textures Qualifiers: attribute, uniform, varying Built-in variables to access GL state Built-in functions Vector component notation (swizzling) discard keyword to cease fragment processingCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 12

Differences from C/C++ No automatic type conversion Constructor notation rather than type cast int x = int(5.0); Function parameters passed by value- return No pointers or stringsCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 14

GLSL compiler The GLSL compiler is in the GL driver and part of the core OpenGL API No external compiler tools required. So the compiler is always available at runtime Compile shaders whenever convenient Tight integration allows every vendor to exploit their architecture for best possible performanceCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 15

Comparing architectures Cg GLSL Cg Code Cg Code GLSL Code GLSL Code Cg Compiler Cg Compiler Intermediate Lang Intermediate Lang Too (e.g. ARB vp/fp) (e.g. ARB vp/fp) far apart IL Translator IL Translator Compiler Compiler Driver Tightly Driver Driver Driver coupled Graphics HW Graphics HW Graphics HW Graphics HWCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 16

GLSL datatypes Scalars: float, int, bool Vectors: float, int, bool Matrices: float Samplers Arrays Structs Note int and bool types are semantic, not expected to be supported natively int at least as 16 bits plus sign bitCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 17

GLSL datatype qualifiers uniform Relatively constant data from app or OpenGL Input to both vertex and fragment shaders attribute Per-vertex data from app or OpenGL Input to vertex shader only varying Perspective-correct interpolated value Output from vertex, input to fragment const in, out, inout (for function parameters)Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 18

OpenGL state tracking Common OpenGL state is available to GLSL via built-in variables Built-in variables do not need declaration All begin with reserved prefix “gl_” Includes uniforms, attributes, and varyings Makes it easier to interface w/ legacy app code However, recommend just defining your own variables for semantic clarity; resist temptation to overload built-ins FYI OpenGL/ES will have no or few built-insCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 19

uniform variables Input to vertex and fragment shaders Values from OpenGL or app e.g.: gl_ModelViewProjectionMatrix Changes relatively infrequently Typically constant over several primitives Queriable limit on number of floats App sets values with glUniform*() APICopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 20

attribute variables Input to vertex shader only Values from OpenGL or app e.g.: gl_Color, gl_Normal Can change per-vertex But doesn’t have to Queriable limit on the # of vec4 slots Scalars/vectors take a slot Matrices take a slot per column Apps sends with per-vertex API or vertex arraysCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 21

varying variables Output from vertex, input to fragment Name & type must match across shaders Values from vertex stage are perspective- corrected, interpolated, sent to fragment stage Queriable limit on number of floats Usually defined by GLSL code although GLSL defines some built-ins; e.g.: gl_FrontColor (necessary when combining GLSL with fixed-functionality)Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 22

Defining variables in GLSL Uniform, varying, attribute must be global to a glShader Over-declaring variables in GLSL code doesn’t cost Only those variables actually used in the code (the “active” variables) consume resources After linking, the app queries uniforms and attributes from glProgram Runtime introspection; useful e.g. for building a GUI on the flyCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 23

Texture access GLSL supports texture access in both vertex and fragment stages However, some hardware may not yet support texture access in vertex Vertex texturing is available when GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS > 0 Mipmap LOD is handled differently between Vertex and Fragment stagesCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 24

Shader Configurations May have more than 1 glShader per stage attached to a glProgram But there must be exactly one main() per stage Useful for a library of shared GLSL code to be reused across several glProgramsCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 25

Program Configurations GLSL permits mixing a fixed-functionality stage with a programmable stage Prog Vertex + Prog Fragment Prog Vertex + FF Fragment FF Vertex + Prog Fragment GLSL Built-in varyings are key when mixing programmable stages w/ FFCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 26

GLSL Versioning & Extensions #version min_version_number Default is “#version 110” Good idea to always declare expected version #extension name : behavior Default is “#extension all : disable” Extension names/capabilies defined in usual GL extensions specifications Special name “all” indicates all extensions supported by a compiler Details in GLSL 1.10 spec pp11-12Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 27

Using GLSL extensions Recommended approach #ifdef ARB_texture_rectangle #extension ARB_texture_rectangle : require #endif uniform sampler2DRect mysampler;Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 28

GLSL Future Expect GLSL to evolve Possible new language features More built-in functions, datatypes Interfaces Shader trees Possible new programmable stages in pipeline Geometry Blending Use #version and #extensionCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 29

OSG support for GLSL 30

OSG GLSL design goals Continue OSG’s straightforward mapping of classes to the underlying GL concepts Leverage OSG’s state stack to apply GLSL state with proper scoping App specifies where/what GLSL will do. OSG determines when/how to apply, restoring to previous state afterwards. Let OSG deal with the tedious GL stuff Management of contexts, constructors, indices, compile/link, etc.Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 31

OSG GLSL benefits over GL Decouples shaders from GL contexts Handles multiple instancing when multiple GL contexts osg::Uniform values correctly applied via OSG’s state update mechanism at the appropriate time Compilation and linking automatically handled when osg::Shader/osg::Program are dirtied by modificationCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 33

osg::Shader Derived from osg::Object Stores the shader’s source code text and manages its compilation Attach osg::Shader to osg::Program osg::Shader be attached to more than one osg::Program More than one osg::Shader may be attached to an osg::Program Encapsulates per-context glShadersCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 34

osg::Shader API subset Shader Type VERTEX or FRAGMENT Sourcecode text management setShaderSource() / getShaderSource() loadShaderSourceFromFile() readShaderFile() Queries getType() getGlShaderInfoLog()Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 35

osg::Program Derived from osg::StateAttribute Defines a set of osg::Shaders, manages their linkage, and activates for rendering osg::Programs may be attached anywhere in the scenegraph An “empty” osg::Program (i.e.: no attached osg::Shaders) indicates fixed- functionality Automatically performs relink if attached osg::Shaders are modified Encapsulates per-context glProgramsCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 36

osg::Program API subset Shader management addShader() removeShader() getNumShaders() getShader() Attribute binding management addBindAttribLocation() removeBindAttribLocation() getAttribBindingList() Queries getActiveUniforms() getActiveAttribs() getGlProgramInfoLog()Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 37

osg::Uniform Derived from osg::Object Attaches to osg::StateSet May be attached anywhere in the scenegraph, not just near osg::Program e.g.: set default values at root of scenegraph Their effect inherits/overrides through the scenegraph, like osg::StateAttributes OSG handles the uniform index management automaticallyCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 38

osg::Uniform API subset Uniform Types All defined GLSL types (float, vec, mat, etc) Value management Many convenient constructors Many get()/set() methods Callback support setUpdateCallback() / getUpdateCallback() setEventCallback() / getEventCallback()Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 39

Simple source example Putting it all together…osg::Program* pgm = new osg::Program;pgm->setName( "simple" );pgm->addShader(new osg::Shader( osg::Shader::VERTEX, vsrc ));pgm->addShader(new osg::Shader( osg::Shader::FRAGMENT, fsrc ));osg::StateSet* ss = getOrCreateStateSet();ss->setAttributeAndModes( pgm, osg::StateAttribute::ON );ss->addUniform( new osg::Uniform( "color", osg::Vec3(1.0f, 0.0f, 0.0f) ));ss->addUniform( new osg::Uniform( "val1", 0.0f )); Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 40

Attributes & osg::Program GL supports both explicit and automatic attribute binding GLSL will dynamically assign attribute indices if not otherwise specified However, OSG currently supports only explicit binding, so app must assign indices Automatic binding makes display lists dependent on osg::Program, and has impact on DL sharing GLSL specifies much freedom in selecting attribute indices, but some current drivers impose restrictionsCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 41

Using textures In the OSG app code Construct an osg::Texture Load image data, set filtering, wrap modes Attach Texture to StateSet on any texunit Create an int osg::Uniform with the texunit ID, attach to StateSet In GLSL code Declare a uniform sampler*D foo; Access the texture with texture*D( foo, coord );Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 42

OSG preset uniforms “Always available” values, like OpenGL built-in uniforms In osgUtil::SceneView int osg_FrameNumber; float osg_FrameTime; float osg_DeltaFrameTime; mat4 osg_ViewMatrix; mat4 osg_InverseViewMatrix; Automatically updated once per frame by SceneView Bitmask to disable updating if desiredCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 43

OSG file formats & GLSL .osg & .ive formats have full read/write support for GLSL objects OSG formats can serve as a GLSL effect file. Today’s demos consist simply of a .osg file no runtime app other than osgviewer requiredCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 44

Tips, Tricks,Gotchas, and ToolsGotchas, 45

Tips for Shader Debugging Name your osg::Shaders/osg::Programs Review the infologs displayed at notify osg::INFO level. Assign internal vecs to color Use discard like assert Verify your code for conformance Try glsl_dataflag.osg to see values inside your scene New in CVS: glValidateProgram() supportCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 46

GLSL performance tips Put algorithm in the right stage Don’t compute in fragment if could be passed from vertex stage or app Don’t interpolate more than necessary If your texture coord is a vec2, don’t pass as vec4 Try passing data as attributes rather than uniforms Changing uniforms sometimes have a setup cost Use built-in functions and types Review the infologs Driver may give hints on non-optimal codeCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 47

GLSL language gotchas Comparing float values Use an epsilon Varyings are interpolated Interpolating from A to A may not exactly == A int is semantic (usually float internally)Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 48

GLSL implementation gotchas Drivers are new, there’s room for improvement Don’t learn GLSL empirically on your driver Example portability issues “Any extended behavior must first be enabled.” (p11) “There are no implicit conversions between types.” (p16) Writes to read-only variables Additional resource constraints (attribute slots, texture units) Loops forced to constant number of iterations Review your driver’s release notes, take heed Note the driver’s GLSL version string Depend on the GL and GLSL specs Be vigilant now for compatibility laterCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 49

On the CDROM Documentation OpenGL 2.0, GLSL 1.10 specifications GLSL Overview whitepaper & Quick Reference OpenGL manpages (HTML & VS.net help) Open-source tools from 3Dlabs website GLSL Demo GLSL Parser Test GLSL Validate ShaderGen GLSL Compiler Front-endCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 50

GLSL Validate Open source, including commercial use Uses the 3Dlabs GLSL compiler front-end to check the validity of a shader Contains both command line and GUI interface Does NOT require a GLSL-capable driverCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 51

GLSL Parse Test Open source, including commercial use Suite of over 140 GLSL shader tests Includes both known-good and known- bad test cases Compiles each shader, compares to expected results Results are summarized, written to HTML Info logs can be examined It tests a driver’s GLSL compiler, so a GLSL-capable driver required (duh)Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 52

GLSL Compiler Front-End Front-End Open source, including commercial use Part of 3Dlabs’ production compiler Compiles on Windows and Linux Performs: Preprocessing Lexical analysis Syntactic analysis Semantic analysis Constructs a high-level parse tree.Copyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 53

osgToy::GlslLintosgToy::GlslLint Proof-of-concept GLSLvalidate-like functionality integrated with OSG Uses the 3Dlabs GLSL compiler front-end No GLSL driver or hardware necessary Currently part of the osgToy collection http://sourceforge.net/projects/osgtoy/ Accessible from C++ as a NodeVisitor: osgToy::GlslLintVisitor Or from cmdline as a pseudoloader: osgviewer myScene.osg.glsllintCopyright © 2005, 3Dlabs, Inc. Ltd July 31, 2005 54

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