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Ok, but is it actually computing the same thing? Are the blurred images from photopea and photoshop identical? The article mentions always doing no more than N*10 steps -- is this an approximation algorithm?

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Photopea and Gimp do compute the same thing (Surface Blur by definition). However, in Photoshop, the results at the radius >= 50px are slightly different. I have a suspicion, that Photoshop uses an approximation algorithm.

N*10 algorithm has an overhead, it needs a little more time (and memory) per pixel, but it does not depend on the radius. It is an exact algorithm (no approximation).

I can't speak to the actual algorithm, but Photoshop also does various types of color space conversions unless you explicitly set it not to. I didn't see anywhere in the article where that was mentioned, or whether Photopea does those same conversions.

I hope that Photoshop does not do any conversion, when it is not necessary. Color space conversion is performed e.g. when changing saturation, vibrance etc. But I think it is not necessary in this case.

Blurs can look a bit more natural when done in a linear color space. Does your app work in linear or sRGB or something else?

It works in sRGB.

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