1. CS5500 Computer Graphics April 17, 2006 CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

2. Today’s Topic Overview of 3D pipelines. Scope of the course project.
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

3. Many Views of Graphics Pipeline
Simple “Front-End/Back-End” view.
Textbook version in [Foley/van Dam].
David Kirk’s (nVidia CTO) version presented in EG Hardware Workshop 1998: (slide 05)
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

4. Simplified View The Data Flow:
3D Polygons (+Colors, Lights, Normals, Texture Coordinates…etc.)
2D Polygons
2D Pixels (I.e., Output Images)
Transform
(& Lighting)
Rasterization
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

5. A Quick Review By default, graphic pipeline will do the following:
Take as input various per-vertex quantities (color, light source, eye point, texture coordinates, etc.)
Calculate a final color for each vertex using a basic lighting model (OpenGL uses Phong lighting)
For each pixel, linearly interpolate the three surrounding vertex colors to shade the pixel (OpenGL uses Gouraud shading)
Write the pixel color value to the frame buffer
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

6. The View of DirectX 8
(Note: This figure is overly crowded, so don’t worry about it if you can’t understand it at the first look. The next slide might give you a better idea of the pipeline.)
CS5500 Computer Graphics
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7. CS5500 Computer Graphics
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8. And a really scary one… CS5500 Computer Graphics
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9. CS5500 Computer Graphics
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10. The next 6 slides are borrowed from UNC-CH COMP236 Course Slides (Spring 2003)
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

11. CS5500 Computer Graphics
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12. CS5500 Computer Graphics
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13. CS5500 Computer Graphics
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14. CS5500 Computer Graphics
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15. CS5500 Computer Graphics
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16. CS5500 Computer Graphics
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17. CS5500 Computer Graphics
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18. Implementing a 3D Pipeline
A case study -- MESA.
Mesa 3D Graphics Library
A famous open source effort to implement OpenGL.
Pure software implementation, meaning all computation is done on CPU, not on GPU.
Used to call MesaGL, but SGI complained about it due to customer support issues.
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

19. Two Different Approaches
Consider how you expect the users to create the contents to your 3D pipeline.
Method 1: by providing 3D model files (e.g., in the OBJ or VRML format)
Our approach since it’s easier to implement.
Method 2: by writing an OpenGL program.
MESA’s approach.
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

20. 3D Models 3D Renderer MESA OpenGL commands OpenGLDrivers CPU & GPU
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

21. Input File Format
Very similar to an OpenGL “command stream,” for example:
Rotate angle, x, y, z
Translate x, y, z
Color R, G, B, A
Begin Triangle
Vertex x, y, z
...
End
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

22. Major Components Data structures for: Transformation Lighting
Points, vectors, matrices
Lines and polygons (or just triangles)
Frame buffer and textures
Transformation
Lighting
Clipping & Projection
Rasterization & texture mapping
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

23. Don’t Reinvent the Wheel
A few useful 3D vector and matrix code:
Or borrow one from your friends, or find a good one from the Internet.
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

24. Graphics Hardware
Why do we study the graphics pipeline in such depth? Why not teaching more Cg or shader programming?
You’ve got to know some hardware!
They are all built upon the traditional 3D pipeline.
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

25. Why Need Hardware
All parts of graphics pipeline can be done in software.
But very slowly.
Example: mesaGL
For some applications, speed is beauty
Games
Walkthrough
Visualization
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

26. Evolutions of Graphics Hardware
Gouraud-shaded polygons.
Then came antialiasing.
Then came texture mapping.
Now comes programmable shading.
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

27. John Poulton’s Chart CS5500 Computer Graphics
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28. Fixed vs. Programmable
Starting in 1999 some graphics cards used the standard lighting model and Gouraud shading to draw polygon fragments entirely in hardware
Implementing the pipeline in hardware made processing polygons much faster, but the developer could not modify the pipeline (hence “fixed function pipeline”)
New programmable hardware allows programmers to write vertex and pixel programs to change the pipeline
1. Fixed function pipeline: some control through use of special flags, extensions,
2. Use pixel shader do Phong shading
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

29. OpenGL Fixed Function Vertex
Vertex (object)
Vertex (clip)
Transform [MVP],[MV],[MV]-T
Normal
Vertex (eye)
Color SecondaryColor
Front&Back Color
[0,1]
Lighting
Front&Back SecondaryColor
[0,1]
Texgen
Texture Matrixn
TexCoordn
TexCoordn
EdgeFlag
EdgeFlag
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

30. GL2 Vertex Processor Uniform Vertex Shader Temporaries Vertex (object)
Vertex (clip)
Uniform
Normal
Vertex (eye)
Color SecondaryColor
Vertex Shader
Front&Back Color
Front&Back SecondaryColor
TexCoordn
Temporaries
TexCoordn
EdgeFlag
EdgeFlag
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

31. Programmable Shaders A concept made popular by Pixar’s RenderMan.
First appeared in hardware: UNC PixelFlow
See SIGGRAPH papers by Molnar 1995 and Olano 1997.
Made affordable by nVidia GeForce3 and XBox.
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

32. Faked Global Illumination
Shadow, Reflection, BRDF…etc.
In theory, real global illumination is not possible in current graphics pipeline:
Conceptually a loop of individual polygons.
No interaction between polygons.
Can this be changed by multi-pass rendering?
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

33. Future Trends Vertex/Pixel shaders will become more and more flexible
Less limits on program size
Able to execute branch instructions
Capable of moving complicated effects (like those in Renderman) onto the GPU
More and more operations executed per-pixel rather than per-vertex
As people get more creative with the hardware we will see more techniques for non-photorealistic rendering
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006

34. Future Trends Real-time fur CS5500 Computer Graphics
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35. Future Trends More realistic skin Subsurface scattering approximation
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36. References Brown U. CS123 programmable hardware lecture
developer.nvidia.com
CS5500 Computer Graphics
© Chun-Fa Chang, Spring 2006