gecko-dev/third_party/rust/wpf-gpu-raster

This is a port of the WPF hardware rasterizer code to Rust. That rasterizer is predecessor to the Direct2D rasterizer. Direct2D still uses a similar technique when run on hardware that does not support Target Independent Rasterization.

Design

The general algorithm used for rasterization is a vertical sweep of the shape that maintains an active edge list. The sweep is done at a sub-scanline resolution and results in either:

1. Sub-scanlines being combined in the coverage buffer and output as "complex scans". These are emitted as lines constructed out of triangle strips.
2. Simple trapezoids being recognized in the active edge list and output using a faster simple trapezoid path.

Bezier flattening is done using an approach that uses forward differencing of the error metric to compute a flattened version that would match a traditional adaptive recursive flattening.