The University of North Carolina - Chapel Hill

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GPGP - Background Lecture

• Graphics Pipelines, OpenGL, Fragment Programs,
  First GPGP Programs

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Purpose

• Give sufficient background to non-graphics
  students to program a simple GPGP program
• Attempted to minimize any assumptions prior
  knowledge of Graphics

3
My Background in GPGP

• 06/2003 - Worked with Bill Baxter on paint work
  using GPU to transform volume representation of
  paint to RGB
• 01/2004 - Worked with Brandon Lloyd on shadow
  work
• Didnt touch GPU work, but understood what it did
• 09/2005 - Taught COMP136 and we talked about GPU
• 10/2006 - Wrote a simple program to remove radial
  distortion from images in real time
• 01/2007 - Prepared for this lecture

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Overview

• Raster Graphics
• Where did this start?
• Graphics Pipeline
• Whats the hardware like?
• OpenGL
• How to talk to the hardware?
• GPU Programming
• How to make it do something non-standard?
• A First GPGP Program
• Hello World!

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Raster Graphics --Utah Rendering

• 1960s-1970s The University of Utah led raster
  graphics research
• Ivan Sutherland
• How do we convert mathematical representations of
  objects into images on the screen?
• Conversion of continuous to discrete
• Solution of light-surface-eye interactions

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Object Representations

• MANY continuous representations exist for objects
• Planar polygons, quadrics, splines, M-Reps,
  general equations
• Discrete representations are sparser
• Generally some interpretation of an array of
  numbers

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The Winner!

• In modern raster graphics, the triangle is king
• It is the simplest continuous representation that
  can approximate all other surface-types
• All other continuous representations must be
  triangulated before being rasterized
• Unless GPGP is used!

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Rasterization Terms

• Tessellate - convert some other continuous
  representation to planar polygons
• Triangulate - converting some other continuous
  representation to triangles
• Vertex - a point in some nD space
• Rasterize - convert a triangle to fragments
• Fragment - a potential pixel

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Utah Graphics Pipeline

• Obtain triangulation of model
• Affine Transforms
• Projective Transforms
• Clip to viewable region
• Rasterize

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Overview

• Raster Graphics
• Where did this start?
• Graphics Pipeline
• Whats the hardware like?
• OpenGL
• How to talk to the hardware?
• GPU Programming
• How to make it do something non-standard?
• A First GPGP Program
• Hello World!

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Why Specialty Hardware?

• CPU can do all Turing complete operations

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Earliest Commodity Hardware
Vertex Transformation

• State of pipeline initialized
• Vertices come down the pipe
• Framebuffer and depth buffer set by the end

Clipping
Rasterization
Fragment Operations
Visibility
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Cutting-edge Commodity Hardware
Vertex Transformation

• Okay, not much changed
• Orange denotes programmability
• Power of standard set of settings increased
• Output can go to any/many buffers

Clipping
Rasterization
Fragment Operations
Visibility
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Overview

• Raster Graphics
• Where did this start?
• Graphics Pipeline
• Whats the hardware like?
• OpenGL
• How to talk to the hardware?
• GPU Programming
• How to make it do something non-standard?
• A First GPGP Program
• Hello World!

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Talking to the GPU

• Two parts
• STATE The majority of OpenGL calls modify the
  state machine
• INPUT Vertices
• Three vertices make a triangle
• Once a triangle is complete, the GPU runs with it

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OpenGL Overview

• Most of the features in OpenGL will probably
  never be used in this class
• For the majority of GPGP work, you render a quad
  (two triangles) that fills the screen on a
  one-input-texture-to-one-output-pixel basis

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Note on OpenGL

• Although OpenGL calls are supported by nVIDIA or
  ATi drivers, some windowing system must be used
• Native to OS - a pain
• GLUT - quick, easy, small, has some issues with
  the nicities of coding
• Almost all windowing toolkits support OpenGL
• FLTK, Qt, WxWindows, etc.

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Lets go to the code

• All code is available at http//www.cs.unc.edu/jw
  endt/classes/GPGPLecture

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OpenGL Gnitty-Gritties

• Three more important OpenGL features
• Multi-pass rendering
• Read-backs
• Extensions

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Overview

• Raster Graphics
• Where did this start?
• Graphics Pipeline
• Whats the hardware like?
• OpenGL
• How to talk to the hardware?
• GPU Programming
• How to make it do something non-standard?
• A First GPGP Program
• Hello World!

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History of Commodity GPU Programming

• Pre-1999 - Basic rasterizers
• Some texture combination ability
• Vertex transformation occurs on CPU
• 1999-2000 - Slightly configurable
• Cube maps, signed math ops
• Vertex transforms added to GPU
• 2001 - Vertex programmability
• 2002-present - Vertex/fragment programmability

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History of GPU non-Commodity Programmability

• mid-1990s - UNC PixelFlow
• later-1990s - Stanford RTSL

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GPU Programming Languages

• Assembly language
• Cg and HLSL
• GLSL

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Types of GPU Programs

• Vertex Programs
• Required Outputs Vertex position and Vertex
  color
• Optional Outputs Hardware/language dependant
  maximum number of output values
• Fragment Programs
• Required Outputs RGBA color
• Optional Outputs Writing to multiple sources

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Communicating with GPU Programs

• There are two ways of sending information to GPU
  Programs
• Explicitly setting parameters using specified
  function calls
• Sending down standard values by setting OpenGL
  state

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For More Info

• Tutorials, sample code, etc.
• Go to www.gpgpu.org/developer
• Cg Tutorial
• Amazon http//www.amazon.com/Cg-Tutorial-Definiti
  ve-Programmable-Real-Time/dp/0321194969
• GLSL (Orange Book)
• Amazon http//www.amazon.com/OpenGL-Shading-Langu
  age-2nd/dp/0321334892/sr1-1/qid1169220867/refpd
  _bbs_1/102-4102099-2237769?ieUTF8sbooks

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Overview

• Raster Graphics
• Where did this start?
• Graphics Pipeline
• Whats the hardware like?
• OpenGL
• How to talk to the hardware?
• GPU Programming
• How to make it do something non-standard?
• A First GPGP Program
• Hello World!

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Lets go to the code

• Borrowed heavily from gpgpu.org/developer
• All code is available at http//www.cs.unc.edu/jw
  endt/classes/GPGPLecture

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Notes

• No Vertex program! no use for one.
• The framebuffer-to-texture transfers we were
  doing are slow
• Use the framebuffer object class available from
  GPGPU.org/developer
• GLEW is downloadable from glew.sourceforge.net/
• We only passed one parameter down in this example

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Reading the Data Back to the CPU

• See function SnapShot at the bottom of the last
  file

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Debugging

• IMDebug by Bill Baxter
• http//www.billbaxter.com/projects/imdebug/index.h
  tml

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Questions?