1. Week 2 - Friday
CS361

2. Last time What did we talk about last time?
Graphics rendering pipeline
Geometry Stage

3. Questions?

4. Project 1

5. Assignment 1

6. Let's see those matrices in MonoGame again

7. Backface culling
I did not properly describe an important optimization done in the Geometry Stage: backface culling
Backface culling removes all polygons that are not facing toward the screen
A simple dot product is all that is needed
This step is done in hardware in MonoGame and OpenGL
You just have to turn it on
Beware: If you screw up your normals, polygons could vanish

8. Graphics rendering pipeline
For API design, practical top-down problem solving, and hardware design, and efficiency, rendering is described as a pipeline
This pipeline contains three conceptual stages:
Produces material to be rendered
Application
Decides what, how, and where to render
Geometry
Renders the final image
Rasterizer

9. Student Lecture: Rasterizer Stage

10. Rasterizer Stage

11. Rasterizer Stage
The goal of the Rasterizer Stage is to take all the transformed geometric data and set colors for all the pixels in the screen space
Doing so is called:
Rasterization
Scan Conversion
Note that the word pixel is actually a portmanteau for "picture element"

12. More pipelines
As you should expect, the Rasterization Stage is also divided into a pipeline of several functional stages:
Triangle Setup
Triangle Traversal
Pixel Shading
Merging

13. Triangle Setup Data for each triangle is computed
This could include normals
This is boring anyway because fixed-operation (non-customizable) hardware does all the work

14. Triangle Traversal
Each pixel whose center is overlapped by a triangle must have a fragment generated for the part of the triangle that overlaps the pixel
The properties of this fragment are created by interpolating data from the vertices
Again, boring, fixed-operation hardware does this

15. Pixel Shading This is where the magic happens
Given the data from the other stages, per-pixel shading (coloring) happens here
This stage is programmable, allowing for many different shading effects to be applied
Perhaps the most important effect is texturing or texture mapping

16. Texturing Texturing is gluing a (usually) 2D image onto a polygon
To do so, we map texture coordinates onto polygon coordinates
Pixels in a texture are called texels
This is fully supported in hardware
Multiple textures can be applied in some cases

17. Merging
The final screen data containing the colors for each pixel is stored in the color buffer
The merging stage is responsible for merging the colors from each of the fragments from the pixel shading stage into a final color for a pixel
Deeply linked with merging is visibility: The final color of the pixel should be the one corresponding to a visible polygon (and not one behind it)

18. Z-buffer
To deal with the question of visibility, most modern systems use a Z-buffer or depth buffer
The Z-buffer keeps track of the z-values for each pixel on the screen
As a fragment is rendered, its color is put into the color buffer only if its z value is closer than the current value in the z-buffer (which is then updated)
This is called a depth test

19. Pros and cons of the Z-buffer
Polygons can usually be rendered in any order
Universal hardware support is available
Cons
Partially transparent objects must be rendered in back to front order (painter's algorithm)
Completely transparent values can mess up the z buffer unless they are checked
z-fighting can occur when two polygons have the same (or nearly the same) z values

20. More buffers A stencil buffer can be used to record a rendered polygon
This stores the part of the screen covered by the polygon and can be used for special effects
Frame buffer is a general term for the set of all buffers
Different images can be rendered to an accumulation buffer and then averaged together to achieve special effects like blurring or antialiasing
A back buffer allows us to render off screen to avoid popping and tearing

21. Finals words on the pipeline
This pipeline is focused on interactive graphics
Micropolygon pipelines are usually used for film production
Predictive rendering applications usually use ray tracing renderers
The old model was the fixed-function pipeline which gave little control over the application of shading functions
The book focuses on programmable GPUs which allow all kinds of tricks to be done in hardware

22. Upcoming

23. Next time…
GPU architecture
Programmable shading

24. Reminders Read Chapter 3
Start on Assignment 1, due next Friday, February 3 by 11:59
Keep working on Project 1, due Friday, February 10 by 11:59