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CS361

Week 2 - Friday. CS361. Last time. What did we talk about last time? Graphics rendering pipeline Geometry Stage. Questions?. Project 1. Assignment 1. Backface culling. I did not properly describe an important optimization done in the Geometry Stage: backface culling

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CS361

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  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. 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 XNA and OpenGL • You just have to turn it on • Beware: If you screw up your normals, polygons could vanish

  7. 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:

  8. Student Lecture: Rasterizer Stage

  9. Rasterizer Stage

  10. 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 short for "picture element"

  11. More pipelines • As you should expect, the Rasterization Stage is also divided into a pipeline of several functional stages:

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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)

  17. 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

  18. Pros and cons of the Z-buffer • Pros • 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

  19. 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

  20. 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

  21. Upcoming

  22. Next time… • GPU architecture • Programmable shading

  23. Reminders • Read Chapter 3 • Start on Assignment 1, due next Friday, February 1 by 11:59 • Keep working on Project 1, due Friday, February 8 by 11:59

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