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Shaders

Shaders. Chris Kerkhoff Matthew Sullivan 10/16/2009. What is a Shader ?. Shaders are simple programs that describe the traits of either a vertex or a pixel. Shaders replace a section of video hardware that's typically called the Fixed Function Pipeline (FFP).

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Shaders

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  1. Shaders Chris Kerkhoff Matthew Sullivan 10/16/2009

  2. What is a Shader? • Shaders are simple programs that describe the traits of either a vertex or a pixel. • Shaders replace a section of video hardware that's typically called the Fixed Function Pipeline (FFP). • The FFP performs lighting and texture mapping in a hard-coded manner, while shaders let you replace this hard-coded approach with a programmable one.

  3. Three types • Pixel • Calculates actual color for a pixel • Vertex • Projects 3D points onto 2D plane (the viewport) and calculates Z depth • Geometry • Modifies existing 3D meshes or creates new ones from formulas

  4. Simplified GPU pipeline • The CPU sends instructions and geometry data to the GPU. • Within the vertex shader, the geometry is transformed and lighting calculations are performed. • If a geometry shader is in the graphic processing unit, some changes of the geometries in the scene are performed. • The calculated geometry is triangulated (subdivided into triangles). • Triangles are transformed into pixel quads (one pixel quad is a 2 × 2 pixel primitive).

  5. The Need • Real world optics and lighting are far too complex for even the fastest computers • Ray tracing which is state of the art for 3D graphics is still too complex to run in realtime for scenes of a high enough complexity to look real • Shaders are written to apply transformations to a large set of elements at a time • Modern GPUs have multiple shader pipelines

  6. Pixel Shader • The only type of Shader that actually shades anything • Takes a color from a texture map and modifies by the effect of various light sources on the surface • A simplified model is: Ambient+Lambertian+Specular • Ambient is fake light to avoid the need for environmental lighting to fill in shadow • Lambertian represents the coloring of a perfectly diffuse (or flat) surface • Specular highlighting is the result of a perfectly glossy surface; the real world is combination of the two.

  7. The GPU Today • Large Frame Buffer • Complicated Pipeline • It’s fixed-function • But we can specify shader programs that execute in certain pipeline stages

  8. Shader Program Limitations • No random-access memory writes • Can write to current pixel in frame buffer • Can’t create data structures • Can’t traverse data structures • Can hack it using texture accesses • Hard to share data between main program and shader programs • Weird programming language • HLSL (High Level Shader Language, similar to Cg)

  9. Programmable Shaders • Manipulate vertices and textures • Implement oversampling and interpolation techniques • Most of these computations involve matrix and vector operations • Good for Scientists and Engineers

  10. Programming Shaders • GLSL (OpenGL Shading Language)in OpenGL (1.5+) • HLSL (High Level Shader Language) in Microsoft Direct3D API (Direct3D 9+) • Cg (C for Graphics) developed by Nvidia for programming vertex and pixel shaders

  11. Universal Shaders • Early video cards had dedicated processors for different shaders (Vertex, Geometry, Pixel) • This simplified the hardware requirements for each type but was inflexible • Modern video cards (DirectX 10 and newer; GeForce 8xxx+, Radeon HD) have universal shaders which can be assigned to the different functions at the discretion of the programmer.

  12. Universal Shaders and GPGPU • There are a few early programs that attempted to use dedicated shaders for GPGPU, but the limitations make this very challenging and inefficient • Universal shaders with their increased flexibility allowed for GPGPU to really take off

  13. DirectX 11-3 new Shader types

  14. Links • http://graphics.cs.williams.edu/archive/SweeneyHPG2009/TimHPG2009.pdf • http://wapedia.mobi/en/Shader_Model • http://www.absoluteastronomy.com/topics/Pixel_shader • http://en.wikipedia.org/wiki/Shaders • http://en.wikipedia.org/wiki/Shader_%28realtime,_logical%29 • http://my.opera.com/Vorlath/blog/2008/01/22/opengl-pixel-shaders-and-why-the-future-of-software-depends-on-it

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