Yay! It took quite a bit of wrestling with formats, but it seems to work now. At least on one PC.

I greatly reduced the ambient lighting and removed all other light sources for testing, so the shadows are rather stark and flat. Also there are a metric fuckload of subtle problems - light bleeding where the depth complexity is even moderately high (the downside of using VSM), speckles due to depth precision problems, streaks and blobs caused by the low resolution shadow map... I'm sure there are plenty more, too.

But the important thing is it's a start, and it's quite essential for that claustrophobic RL feeling.

Next big problem will be making sure this is consistent with logical line-of-sight. LoS calculations need to be somewhat permissive to account for the character moving around within its cell, and only take into account the logical properties of grid cells (i.e. blocks vision or... not) rather than the exact nature of any occluding geometry. Shadow maps, by contrast, are far more precise. Hmm. Giving information to the player based on what their character could see may make the shadows seem wrong or incomplete (e.g. knowing that there's a goblin fishmonger a few squares to your right, despite it being entirely in shadow), whilst denying them information about, say, a monster which is clearly illuminated by the character's light source (but 'shadowed' according to logical LoS) is also broken.