ABoxBlur

[GameMaker] Arbitrarily sized box-blur for any pixel.

This library provides following function, which can be used to apply box-blur;

ABoxBlur(_dst, _src, _blur, _strength, _fallback);

• dst is destination surface
• src is source surface
• blur-mask is surface, which tells how strongly blur should be applied.
• strength is multiplier how strongly blur-mask is applied.
• fallback is whether use compatibility rgba8unorm-version (no float-textures). Default is false.

GitHub Link

This asset works by applying parallel prefix sum in two axis to generate summed-area table. This table is sampled at the corners of desired box-blur area to calculate the sum of the area. As area is known, it can be use to divide the sum, calculating the average, which is the box-blur.

The parallel prefix scan is implemented in Hillis-Steele style, which is easy but work-inefficient approach. Blelloch would be better option, but it's harder to implement with fragment shaders (but not impossible).

The asset currently always recreates the sum-table. This in some cases is not necessary (when source image doesn't change), and as most of the work is done for creating the sum-table, it would be much more performant if it didn't do at those cases. As blur mask and sum-table are separate, you could change blur-mask without recreating sum-table. 

Although asset only does box-blur, you could technically emulate different shapes by taking more boxes. Getting sum of area requires 4 sum-table samples, so with three overlapping boxes (12 samples) you could already do rough approximates of other blurs.

Note! Not mobile browser friendly (most likely glitches).