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CS5500 Computer Graphics

CS5500 Computer Graphics April 19, 2007 Today’s Topic Overview of 3D pipelines. Scope of the next 2 or 3 assignments. Many Views of Graphics Pipeline Simple “Front-End/Back-End” view. Textbook version in [Foley/van Dam].

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CS5500 Computer Graphics

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  1. CS5500 Computer Graphics April 19, 2007 CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  2. Today’s Topic • Overview of 3D pipelines. • Scope of the next 2 or 3 assignments. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  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) http://www.merl.com/hwws98/presentations/kirk/index.htm CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  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 2007

  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 2007

  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 © Chun-Fa Chang, Spring 2007

  7. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  8. And a really scary one… CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  9. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  10. The next 6 slides are borrowed from UNC-CH COMP236 Course Slides (Spring 2003) http://www.unc.edu/courses/2003spring/comp/236/001/handouts.html CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  11. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  12. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  13. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  14. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  15. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

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  17. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  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 2007

  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 2007

  20. 3D Models 3D Renderer MESA OpenGL commands OpenGLDrivers CPU & GPU CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  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 2007

  22. Major Components • Data structures for: • 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 2007

  23. Don’t Reinvent the Wheel • A few useful 3D vector and matrix code: http://www.cs.nthu.edu.tw/~chunfa/code/algebra3.zip • Or borrow one from your friends, or find a good one from the Internet. CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  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 2007

  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 2007

  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 2007

  27. John Poulton’s Chart CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  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 CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  29. OpenGLFixed Function Vertex Vertex (object) Vertex (clip) Transform[MVP],[MV],[MV]-T Normal Vertex (eye) ColorSecondaryColor Front&BackColor [0,1] Lighting Front&BackSecondaryColor [0,1] Texgen TextureMatrixn TexCoordn TexCoordn EdgeFlag EdgeFlag CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  30. GL2 Vertex Processor Vertex (object) Vertex (clip) Uniform Normal Vertex (eye) ColorSecondaryColor VertexShader Front&BackColor Front&BackSecondaryColor TexCoordn Temporaries TexCoordn EdgeFlag EdgeFlag CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  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 2007

  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 2007

  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 2007

  34. Future Trends • Real-time fur CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

  35. Future Trends • More realistic skin • Subsurface scattering approximation CS5500 Computer Graphics © Chun-Fa Chang, Spring 2007

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