Week 2 - Friday CS361

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

Questions?

Project 1

Assignment 1

Let's see those matrices in MonoGame again

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

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

Student Lecture: Rasterizer Stage

Rasterizer Stage

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"

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

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

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

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

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

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)

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

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

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

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

Upcoming

Next time… GPU architecture Programmable shading

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