tiny_gfx: A tiny rasterizer

A rasterizer is a piece of software that converts arbitrary shapes into raster images, which is just a funny name for rectangular grids of pixels. A rasterizer, in some way or another, is at the heart of pretty much every modern display technology today, from computer displays to e-ink displays to printers both 2D and 3D (the graphics in laser shows are the most notable exception).

In this chapter, I will teach you a little about how rasterizers work, by describing tiny_gfx, a simple rasterizer in pure Python. Along the way we will pick up some techniques that show up repeatedly in computer graphics code. Having a bit of mathematical background on linear algebra will help, but I hope the presentation will be self-contained.

tiny_gfx is not practical in many ways. Besides being slow (see below), the main shortcoming in tiny_gfx is that shapes are all of a single solid color. Still, for 500 lines of code, tiny_gfx has a relatively large set of features:

• alpha blending for semi-transparent colors
• polygonal shapes (possibly concave, but not self-intersecting) (TODO: right now only convex polygons are supported)
• circles and ellipses
• transformations of shapes
• boolean operations on shapes (union, intersection, subtraction)
• antialiasing
• empty space skipping and fast rasterization of runs for general shapes

Maybe most interestingly, shapes in tiny_gfx are extensible. New shapes are easily added, and they compose well with the other parts of the code.

A description of the current version of the code is in doc/README.md.

A performance caveat

Rasterizers are so central to display technology that their performance can make or break a piece of software, and these days the fastest rasterizers are all based in hardware. Your videogame graphics card (and even the cheapest smartphones these days) is rasterizing polygons in highly parallel processors; 192 cores is a typical number. It should be no surprise, then, that the rasterizer we will see here is slow: if CPU-intensive tasks in Python run around 50 times slower than heavily-optimized, low-level code, and if a graphics driver has around 200 cores at its disposal, a slowdown of 10,000 times should not be surprising. In reality, tiny_gfx is closer to 1,000,000 times slower than the special-purpose graphics rasterizer from the laptop in which I'm developing it.