AMCS / CS 247 – Scientific VisualizationLecture 10: GPU Basics

Markus Hadwiger, KAUST

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Reading Assignment #5 (until Sep 22)

Read (required):• Real-Time Volume Graphics, Chapter 2 (GPU Programming)

• Real-Time Volume Graphics, Chapter 5.3 (Gradient-Based Illumination)

• Real-Time Volume Graphics, Chapter 5.4.1 (Blinn-Phong Illumination)

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What‘s in a GPU?

Lots of floating point processing power• Stream processing cores

different names:stream processors,CUDA cores, ...

• Was vector processing, now scalar cores!

Still lots of fixed graphics functionality• Attribute interpolation (per-vertex -> per-fragment)

• Rasterization (turning triangles into fragments/pixels)

• Texture sampling and filtering

• Depth buffering (per-pixel visibility)

• Blending/compositing (semi-transparent geometry, ...)

• Frame buffers

What can the hardware do?

RasterizationDecomposition into fragmentsInterpolation of colorTexturing

Interpolation/Filtering Fragment Shading

Fragment OperationsDepth Test (Z-Test)Alpha Blending (Compositing)

Pixels

Graphics Pipeline

Vertices Primitives Fragments

GeometryProcessing

FragmentOperations

Scene Description Raster Image

Rasterization

Geometry Processing

Per-VertexLightingClipping,

Perspect.DividePrimitiveAssemblyTransformation

Multiplication withModelview and

Projection Matrix

Per-VertexLocal Illumination

(Blinn/Phong)

GeometricPrimitives

(Points, LinesTriangles)

Primitives

Clip SpaceTo

Screen Space

Vertices

GeometryProcessing Rasterization Fragment

Operations

GeometryProcessing Rasterization Fragment

OperationsFragment

Operations

TextureFetchTexture

ApplicationPolygon

Rasterization

PrimitiveVertices

Decompositionof primitives

into fragments

Interpolation oftexture coordinates

Filtering oftexture color

Primitives Fragments

Rasterization

Combination ofprimary color with

texture color

Combination ofprimary color with

texture color

Fragment Operations

StencilTestAlpha

BlendingDepthTest

AlphaTest

Discard allfragments within

a certain alpha range

Discard afragment ifthe stencil buffer is set

Discard alloccludedfragments

GeometryProcessing Rasterization Fragment

Operations

Pixels

Graphics Hardware

Vertices Primitives Fragments

GeometryProcessing

FragmentOperations

Scene Description Raster Image

RasterizationVertexShader

FragmentShader

Programmable Pipeline

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Direct3D 10 Pipeline

New geometry shader stage:• Vertex -> geometry -> pixel shaders

• Stream output after geometry shader

Courtesy David Blythe, Microsoft

Programmable Processing Stages

GLSL 1.50 (OpenGL 3.2)• Vertex shaders (run on vertex processors)

• Geometry shaders (run on geometry processors)

• Fragment shaders (run on fragment processors)

From the language / API perspective it is useful to consider separate types of processors, even when all of these shaders are in reality executed on identical processing cores on current GPUs!

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Vertex Shader (1)

Process vertices and their attributes• Position

• Color, texture coordinate(s), ...

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GLSL 1.20

Vertex Shader (2)

“Pass-through“ example• Pass through per-vertex color

• Transform vertex position with OpenGL Model-View-Projection matrix

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Geometry Shader (1)

Process whole primitives• Emit vertices

• Emit primitive(s)

14GL_EXT_geometry_shader4

Geometry Shader (2)

“Pass-through“ example• Pass through (emit) all vertices

• Pass through (emit) one primitive

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Fragment Shader (1)

Process fragments• Write one or more output fragments

• Use input color, texture coordinates, ...

• Compute shading, sample textures, ...

• Optionally discard fragment

• MRT: multiple render targets

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GLSL 1.20

Fragment Shader (2)

“Pass-through“ example• Pass through interpolated color as fragment color

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Thank you.

Thanks for material• Helwig Hauser

• Eduard Gröller

• Daniel Weiskopf

• Torsten Möller

• Ronny Peikert

• Philipp Muigg

• Christof Rezk-Salama