Software rasterizing hair
In the last few years, we have seen more rendering systems that lean on software rasterization. It’s an optimal choice for small triangles (~1 px wide) or thin triangles that span many pixels (especially diagonals). Two of the most known examples are UE5’s Nanite (which I’ve reimplemented in Nanite WebGPU) and Frostbite’s hair system (which I’ve reimplemented in Frostbitten hair WebGPU). Basic software rasterization for triangles has been described countless times e.g.:
In this article, we will see the rasterization algorithm for hair. Starting from a file that contains strand points in 3D space and ending on pixel attributes inside a quad. First, we will see the required 3D transformations to produce projected vertices. Then we will go over triangle rasterization basics, and see the same function applied to quads. We will also need coordinates inside each rasterized hair segment to compute attributes. For each section I will also link code from my “Frostbitten hair WebGPU” so you can see how this works in practice.
Software-rasterized hair with analytical anti-aliasing and order-independent transparency. Screen from my Frostbitten hair WebGPU.
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