Windfoil brings analytic Bézier coverage to WebGPU

A Windfoil paper by Matt DesLauriers presents a WebGPU renderer that calculates per-pixel coverage for quadratic Bézier shapes with a closed-form method. The same analytic kernel supports interactive rasterisation and gradient-based image fitting in browsers or server-side Deno, with reported performance on a consumer laptop.

Vector rasterisers need accurate coverage values to produce smooth edges, while differentiable renderers also need derivatives that connect output pixels to path parameters. Windfoil addresses both requirements with one coverage function, reducing the sampling cost and gradient noise associated with earlier differentiable approaches.

Coverage becomes an integral

Each output pixel needs a fractional coverage value describing how much of its square area lies inside a path. Inaccurate estimates create jagged edges and temporal shimmer; discontinuous or noisy estimates also make gradient descent harder to use.

Windfoil calculates coverage from the box-filtered winding number. A winding number counts how many times a contour wraps around a point, which determines whether that point belongs to the filled region. Averaging that value across the square footprint of a pixel produces its antialiased coverage.

A quadratic Bézier segment consists of two endpoints and one control point. Windfoil integrates the winding contribution of these segments over each pixel footprint in closed form. The paper describes the result as exact for well-formed quadratic contours under its stated assumptions, avoiding repeated subpixel samples and signed-distance approximations.