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

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

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 2006

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

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

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 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

And a really scary one… CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

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 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

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

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

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

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

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

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 2006

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

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

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

John Poulton’s Chart CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

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

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

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

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

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

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

Future Trends Real-time fur CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

Future Trends More realistic skin Subsurface scattering approximation CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006

References Brown U. CS123 programmable hardware lecture developer.nvidia.com www.cgshaders.org CS5500 Computer Graphics © Chun-Fa Chang, Spring 2006