One way to create blur mask: box down sample full screen shadow mask to quarter rez by sampling 4 corners of 16 pixel block and average. That’s it. Sample that smaller texture at full-screen, write to stencil where you alpha test based on whether bi linear sampled texture has any value not 0 or 1. If it has a gradient i.e. blurred from box down sample, then it is a natural edge and naturally got dilated by the down sample.

But yeah, for blurring something could be done. IIRC, ATI had a technique on that back in the days. Not sure if they used stencil or just dynamic branching (might be latter, as it was to show off dynamic branching in Radeon X1xxx).

How do you create the mask? I’d imagine you either need to sample shadowmap once, detect edges & dilate them (into stencil… somehow); or sample shadowmap with a small amount of samples but quite widespread, and hope you won’t miss some very thin shadows?

You can take a bit further by creating another screen space Early-Stencil edge mask ( actually very quickly, I got mine to ~0.5ms on PS3 ) and running your screen space blur pass only on the edges of the shadow mask. I realized this couple of years ago, don’t need to run a blur operation on all white or all black pixels, won’t do anything, only need it along edges of the shadows that the mask creates. This should cut the cost of the blur to about 20% – 60% of total depending on shadow complexity in the screen at that time.

Summed up in one sentence, I am now shifting the shadow mask look-up coordinate away from polygon edges towards the polygon center on edge pixels, thus rather elegantly circumventing wrongly shadowed pixels.

Any experience on this issue?

Can still see artifacts on recent screenshots though :(