From Cradle to Framebuffer
Before any pixels appear on the screen, our models must pass through an obstacle course of manipulations and challenges called the graphics pipeline. The pipeline roughly follows these stages:
- A geometric model is loaded or generated by the application and uploaded to the graphics card in vertex buffers.
- The application issues a draw call.
- The vertex shader transforms the vertices, passing each through a chain of coordinate systems and performing operations appropriate to each system.
- Primitives that extend outside of the viewport are clipped so only their visible portions continue on through the pipeline.
- The positions of the clipped vertices are transformed into pixel coordinates.
- The fragment shader computes the color of each fragment that falls between the clipped vertices. It is passed any interpolated vertex attributes.
- Fragments are tested to see if their colors should be written to the framebuffer. One typical test rejects any fragments that are behind geometry already rendered.
- The colors of the surviving fragments either overwrite or blend with colors already written to the framebuffer.
We explored some of these stages in earlier chapters. For example, we've seen how to issue draw calls and write shaders for simple models defined in the unit cube coordinate system. In this chapter, we open up all of space so that we can render complex 3D scenes containing many models. By the chapter's end, we'll be able to answer the following questions:
- What coordinate spaces does a model pass through on its way to being rasterized and what operations do we perform in each?
- How can we change the region of the viewport in which we render so that we may draw mini-maps and split-screens?
- Which color should be assigned to a pixel if more than fragment tries to write to it?
- How do we make the pipeline repeat so that viewers feel interactivity and life?
Once we've got a handle on this graphics pipeline, we'll be able to put the human viewer into a scene full of moving 3D models.