Thanks for the response! It’s always nice to know when some incidental thing you remark on has helped someone. Thinking back, I suppose I struggled with understanding the SVG <feGaussianBlur stdDev> parameter when messing around with complicated filters in Inkscape a few years back, for much the same reasons. And I didn’t actually know the ¾√2π until today, I just knew that a factor of 2 was a little more than enough. I’m glad for the note being attached to the spec, it’s a great note.
Although people often talk of blur radii, it might help to remember it’s a standard deviation. Take the curve in https://en.wikipedia.org/wiki/Gaussian_filter#/media/File:Ga..., and a Gaussian blur is taking so much of each pixel value, most from the closest ones, less from further away, negligible by two standard deviations away, but only asymptotic to zero; hence the “theoretically infinite extent” the spec note mentioned. But the ¾√2π thing will probably be good enough forever.
Although people often talk of blur radii, it might help to remember it’s a standard deviation. Take the curve in https://en.wikipedia.org/wiki/Gaussian_filter#/media/File:Ga..., and a Gaussian blur is taking so much of each pixel value, most from the closest ones, less from further away, negligible by two standard deviations away, but only asymptotic to zero; hence the “theoretically infinite extent” the spec note mentioned. But the ¾√2π thing will probably be good enough forever.