That is cool! I think I may be able to explain the gist (though not the full details). It uses an IIR filter, or Infinite Impulse Response filter. This is a d

That is cool! I think I may be able to explain the gist (though not the full details). It uses an IIR filter, or Infinite Impulse Response filter. This is a digital filter (could be either software or hardware) where there’s a feedback loop from the output to the input. The advantage of an IIR is that it can produce a long output (potentially infinite, hence the name) from a single input. The disadvantage is that IIRs can easily get unstable and hard to work with; hence many (most?) applications use FIRs, Finite Impulse Response, where there’s no feedback and the output is strictly bounded. It looks like they’re using an IIR tuned to produce a Gaussian bell curve as the impulse response. By feeding each pixel through the filter in sequence, each pixel produces a bell curve and those get blended together, exactly what you what. So, that tight inner loop consists of reading in the next pixel and blending it with the intermediate results of earlier pixels. Very efficient on a CPU where you can stream the data through, but it wouldn’t work in an OpenGL shader as that expects to work on each pixel in parallel, not sequentially.