1. The Graphics Rendering Pipeline
CSC4820/6820 Computer Graphics Algorithms Ying Zhu Georgia State University
The Graphics Rendering Pipeline

2. What is graphics rendering pipeline?
A process of generating a 2D image, given a virtual camera (eye), 3D objects, light sources, textures, etc.
The 2D image is a frame of animation.
A process for converting coordinates from its local 3D coordinate system to the final 2D window coordinate system.
Both OpenGL and Direct3D implement part of the rendering pipeline.

3. Graphics Rendering Pipeline Stages
The rendering pipeline is normally divided into 3 conceptual stages
Application
Geometry
Rasterizer
The output of one stage is the input of next stage, thus the name “pipeline”
Each stage also contains sub- stages
Application
Geometry
Rasterizer

4. OpenGL pipeline (overview)

5. OpenGL pipeline (overview)
Application
Rasterizer
Geometry

6. OpenGL pipeline (detailed view)

7. Why use pipeline architecture?
It’s more efficient
Think automobile assembly line
Explore data parallelism
Data parallelism vs. instruction parallelism
The pipeline architecture is the fundamental architecture for both 3D graphics software and hardware
Think GPU as an image assembly line

8. Application Stage Overview
3D objects are created using modeling tools
3D Studio Max, Maya, etc.
3D objects are positioned in 3D world and organized in a scene graph
3D objects can be moved by transformation
Transformation are performed on vertices using different types of transformation matrices
3D transformations are specified in this stage but carried out in the Geometry stage

9. Application Stage Overview
Other tasks in the application stage:
Handling user input
level of details
occlusion culling
The result: the 3D geometry objects to be fed to the next stage
Points, lines, polygons, etc.
3D geometry objects are eventually broken down to vertices, represented by (x, y, z) coordinates

10. Geometry Stage Overview
Responsible for the majority of per-polygon or per vertex operations
Contains several sub-stages
Model Transformation
View Transformation (often combined with model transformation)
Lighting
Projection
Clipping
Screen Mapping

11. Coordinate Systems
Multiple Cartesian coordinate systems are used at different stages of the pipeline
3D Model Coordinates (model space)
Each model is in its own coordinate system with origin in some point on the model
3D World Coordinates (world space)
Unified world coordinate system, with only one origin
3D Eye Coordinates (view space)
Camera (eye) is the origin and look straight down Z-axis
2D Screen Coordinates (screen space)
3D coordinates are converted to 2D screen coordinates for display

12. 3D Graphics Pipeline Data Flow
Objects in the 3D scene are sequentially transformed through different coordinate systems when proceeding the 3D pipeline.

13. Model Transform
Transform the vertices and normals of 3D objects from model space to world space
After the transform, all objects exit in the same coordinate system
The camera (eye) has a location in world coordinate system and a direction

14. View Transform
The camera and all the 3D objects are transformed with the view transform
The purpose is to place the camera at the origin and make it look in the direction of Z-axis.
Often combined with model transform – model-view transform

15. Projection
Transform the view volume into a unit cube with its extreme points at (-1, -1, -1) and (1, 1, 1).
The purpose is to simplify clipping.
Two projection methods:
Orthographic projection
When realism is not a concern.
Perspective projection
When realism counts.

16. Lighting
A lighting equation is used to calculate a color at each vertex of the 3D object that is to be affected by lighting.
The lighting equations often have little to do with how lights behave in the real world.
Per-vertex lighting vs. per-pixel lighting

17. Shading Each 3D object surface is divided into many triangles.
The colors at the vertices of a triangle are interpolated over the triangle.
Three shading methods:
Flat shading (operate per triangle)
Gouraud Shading (operate per vertex)
Phong Shading (operate per pixel)
Programmable Shaders

18. Clipping
Only the primitives wholly or partially inside the view volume need to be passed on to the rasterizer stage.
Primitives (line or polygon) that are partially inside the view volume require clipping.

19. Screen Mapping
Primitives that survive the clipping are passed on to screen mapping stage.
X and Y coordinates of each primitive are transformed to screen coordinates.
Z coordinates are not affected and are kept for depth buffer checking.

20. Rasterizer Stage Overview
In this stage, all primitives are converted into pixels in the window.
The goal of this stage is to assign correct colors to the pixels.
It contains one or all of the following sub-stages:
Texture mapping
Fog
Translucency Test
Depth buffering
Antialiasing

21. Rasterization

22. Texturing Texture mapping means “attaching” an image onto a 3D object.
Textures are important to bringing realism to a 3D scene.
Provide surface details
Add scene depth
Advance texture mapping
Multi-texturing
Bump mapping
Environment mapping
Pixel shader

23. Fog An optional stage but help set the mood
Also helps give a scene an addition sense of depth of field
Added realism
E.g. allow distant objects to gracefully "fade away" rather than just pop out of the scene

24. Translucency Tests Also called Alpha Test
Used to create effects such as glass, water, see-through views in CAD modeling, and translucent or transparent materials.

25. Depth Buffering Also called Z-buffer algorithm
Used for visibility check
When multiple primitives are rendered to a certain pixel, the color of the pixel is the color of the primitive that is closest to the point of view of camera.
The only exception is when primitives are transparent.
This algorithm allows the primitives to be rendered in any order.

26. Anti-aliasing
Alias effect: the jagged or stair-stepped look of diagonal or curved lines in computer graphics.
Anti-aliasing algorithms sample, or examine and evaluate, the colors and shades of pixels adjoining curved or diagonal lines to present smoother looking line.

27. Display
Finally, the final 2D image is generated and rendered to frame buffer and displayed on screen.
Double-buffer technique may be used to reduce flashing
Front buffer for display, back buffer for rendering next frame.
A long trip down the graphics rendering pipeline
If application runs at 60 frame/second, this whole process will repeat every 17 ms.

28. Summary
The 3D graphics pipeline is the underlying tool for graphics rendering.
Three major stages:
Application
Geometry
Rasterizer