Programmable Graphics Hardware and Shaders

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Fac. of Comp., Eng. Tech. Staffordshire
University
3D Computer Graphics
Programmable Graphics Hardware and Shaders
Dr. Claude C. Chibelushi
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Outline

• Introduction
• Programmable graphics hardware
• Graphics processing unit (GPU)
• GPU programming model
• Shaders
• Vertex shader
• Pixel shader
• Shader languages
• Summary

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Introduction

• Shader custom program segment inserted into
  graphics pipeline and executed on GPU
• replaces specific parts of fixed-function
  pipeline
• comprises instructions that operate on
• vertex data (vertex shader) vertex
  transformation and lighting calculations
• or pixel data (pixel shader) e.g. pixel lighting
  calculations

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Introduction

• Shaders offer code development flexibility and
  processing speed
• flexibility shader implements customised vertex
  or pixel processing
• speed shader runs on fast specialised chips
• many current graphics cards support shaders
• Shaders often used for better visual quality in
  real time

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Introduction

• Recent feature of DirectX Graphics and OpenGL
• addition of programmable pipeline
• API provides high-level or assembly language
  interface to processing hardware
• specialised language for vertex and pixel
  processing
• developer can program special code for real-time
  graphics applications
• e.g. rich graphical effects such as simulation of
  cloth, hair or fur

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Programmable Graphics Hardware

• Graphics processing unit (GPU)
• Graphics cards used to be fixed-function
• graphics processors hard-wired for specific
  functions
• graphics limited by implementation of device
  driver and underlying hardware
• fixed-function constraints overcome by running
  code on
• general-purpose CPU (has limited support for
  graphics, hence slow)
• programmable graphics processor(s) known as GPU
  (fast)
• code downloaded onto GPU during rendering

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Programmable Graphics Hardware

• Graphics processing unit (GPU)
• Special purpose microprocessor(s)
• multiple programmable processing units for
  concurrent processing on single chip
• optimised for graphics processing, e.g. vector /
  matrix manipulations
• used in computer graphics (accelerator) card
• for manipulation and display of graphics data
• can perform some tasks of pipeline
• evolving hardware capability improving with time

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Programmable Graphics Hardware

• DirectX 9 graphics pipeline

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Programmable Graphics Hardware
CPU
GPU
VertexProcessor
FragmentProcessor
Frame bufferOperations
Assembly Rasterization
Application
Frame buffer
Textures
GPU programming model (Source Bill Mark,
NVIDIA Programmable Graphics Technology,
SIGGRAPH 2002 course)
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Shaders

• Definition fragment corresponds to pixel
• has attributes such as colour, depth, texture
  coordinates
• fragments are generated by rasterisation

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Shaders

• Vertex shader
• Vertex shader bypasses fixed-function
  transformation and lighting pipeline segment
• processes vertex
• runs on programmable vertex processor
• example functionality vertex displacements for
  mesh deformation (e.g. for fish-eye lens effect)

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Shaders

• Vertex shader
• Vertex shader outputs attributes that can be
  interpolated during fragment processing
• fragment processing done in subsequent stages of
  graphics pipeline
• e.g. light reflected at vertex can be calculated
  in vertex shader for use in pixel shader

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Shaders

• Pixel shader
• Also known as fragment shader
• processes pixel / fragment
• runs on pixel / fragment processor during
  rasterisation
• example functionality shading / texturing
  effects, per-pixel lighting
• e.g. accurate lighting models, simulation of
  complex surface properties or natural phenomena

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Shader Languages

• Shader programming requires language specialised
  for graphics
• needs graphics-specific data types (e.g. vector,
  matrix) and operations (e.g. dot product, matrix
  multiply)
• language often tied to specific graphics API
• Actual shader code is assembly code
• but high-level language can be compiled to
  assembly
• compilers available

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Shader Languages

• High-level languages
• historically preceded by various low-level
  languages such as Microsofts ASM
• low-level programming has low productivity,
  limited portability and low code readability /
  maintainability
• variety of languages, e.g.
• OpenGL shading language (GLSL or glslang)
• for use with OpenGL.
• C-like languages jointly developed by Microsoft
  and NVidia (essentially same language but
  different brand names)
• High-Level Shader Language (HLSL) (Microsoft)
• for use with DirectX HLSL compiler is part of
  D3DX library
• Cg (NVIDIA)
• support for DirectX Graphics and OpenGL

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Shader Languages

• HLSL
• Sample intrinsic data types
• vector types contain from 1 to 4 components,
  e.g.
• int1 vector containing 1 int float3 vector
  containing 3 floats double4 vector containing 4
  doubles
• matrix types contain rows and columns of
  same-type data, e.g.
• int2x1 integer matrix with 2 rows and 1 column
  float4x4 float matrix with 4 rows and 4 columns

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Shader Languages

• HLSL
• Sample intrinsic functions
• dot(a, b) returns dot product of vectors a and b
• mul(A, B) returns product of matrices A and B
• reflect(i, n) returns reflection vector given
  incident ray direction i and surface normal n
• round(x) returns x rounded to nearest integer

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Shader Languages

• Sample code simple HLSL shaders
• Example 1 (from DirectX Graphics documentation
  (http//msdn.microsoft.com/library/default.asp?url
  /library/en-us/directx9_c/HLSL_Shaders.asp))
• / Vertex shader
• Functionality
• transforms vertex position from object space to
  projection space
• assigns per-vertex colour from vertex input /
• // Note POSITION and COLOR0 are
  semantics that tell compiler usage of variable
• float4x4 mWorldViewProj // World View
  Projection transformation
• // (global variable set by application)
• // shader output structure
• struct VS_OUTPUT
• float4 Position POSITION // vertex
  position
• float4 Diffuse COLOR0 // vertex
  diffuse color

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Shader Languages

• Sample code simple HLSL shaders
• Example 1 (ctd.)
• VS_OUTPUT Vertex_Shader_Transform( in float4
  vPosition POSITION, in float4 vColor COLOR0)
• VS_OUTPUT Output
• // Transform the vertex into projection
  space.
• Output.Position mul( vPosition,
  mWorldViewProj )
• // Set the diffuse color for output
• Output.Diffuse vColor
• return Output

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Shader Languages

• Sample code simple HLSL shaders
• Example 1 (ctd.)
• / Pixel shader /
• Functionality copies interpolated vertex colour
  data, and outputs pixel colour
• float4 Pixel_Shader( VS_OUTPUT in ) COLOR0
• float4 color in. Diffuse // copy input
  colour
• / float4 color in.Color / // note error in
  original example code
• return color

Output semantic
Input parameter
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Shader Languages

• Sample code simple HLSL shaders
• Example 2 Ambient and diffuse reflection models
  to simulate matte surface (adapted from
  http//www.toymaker.info/Games/html/pixel_shaders.
  html6)
• / Vertex Shader
• Functionality
• transform vertex position, normalise light vector
  and vertex normal,
• transform vertex normal /
• // shader output structure
• struct VS_OUTPUT
• float4 Pos POSITION // vertex position
• float3 Light TEXCOORD0 // normalised light
  vector
• float3 Norm TEXCOORD1 // normalised vertex
  normal

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Shader Languages

• Sample code simple HLSL shaders
• Example 2 (ctd.)
• // Vertex shader
• // Note matWorldViewProj to be passed in from
  application, given semantic WORLDVIEWPROJECTION
• VS_OUTPUT vertShader(float4 vertPos POSITION,
  float3 vertNorm NORMAL)
• VS_OUTPUT Out (VS_OUTPUT)0
• Out.Pos mul(vertPos, matWorldViewProj) //
  transform position from model to screen space
• Out.Light normalize(vecLightDir) //
  normalise light vector
• Out.Norm -normalize(mul(vertNorm,
  matWorld)) // transform normal from model to
  world space, and normalize
• return Out

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Shader Languages

• Sample code simple HLSL shaders
• Example 2 (ctd.)
• / Pixel Shader
• Functionality
• output pixel colour calculated using ambient and
  diffuse reflection models
• (from light and normal vector input from
  pipeline) /
• // Note some values set by application, e.g.
  global variables Dintensity, Dcolour, Aintensity,
  Acolour.
• float4 pixShader(float3 Light TEXCOORD0, float3
  Norm TEXCOORD1) COLOR
• // calculate diffuse reflection
• float4 difsRefDintensityDcolour(dot(Norm,Light
  ))
• // combine diffuse reflection ambient light
  reflection
• return AintensityAcolourdifsRef

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Suggested Reading

• http//www.microsoft.com/directx/default.asp
  (Microsoft DirectX homepage)
• http//developer.nvidia.com/page/cg_main.html (Cg
  homepage)

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Summary

• Shaders offer code development flexibility and
  processing speed
• Programmable hardware pipeline on GPU
• concurrent processing on single chip
• Shader program segment inserted into graphics
  pipeline and executed on GPU
• replaces section of fixed-function pipeline
• vertex shader or pixel shader
• Variety of shader languages