Blender's Fast Blur Isn't Fast: A New Gaussian Blur That Handles Animated Radii
Fast Blur with Animated Radius
Aras Pranckevičius noticed Blender's Fast Blur mode is slow for large radii and built Smol Gaussian: downsample, apply a small separable Gaussian, then reconstruct. It matches Skia's GPU blur but adds pixel-area integration for odd sizes, variance subtraction from downsampling, and a cubic B-spline upsample to avoid bilinear slope steps on HDR highlights. An interactive WebGPU playground compares it with Dual Kawase, Skia Gaussian, Blender's Fast Gaussian, and Ryg Blur.
The algorithms are more or less constant work independent of the blur radius, which is very nice. However, they are also from 25+ years ago, and are not 'embarrassingly parallel' that would fit a GPU (or even a many-core CPU) very well. So despite the name, they may or might not be very fast :)