Distant Shadowing "brainfart"

Recently, I've been having a little bit of a think about how to handle realtime distant shadowing in games, in order to avoid shadow baking.

Clearly, it's entirely possible to use some kind of cascaded shadow map extending all the way out. But, this soon ends up getting expensive in terms of textures and rendering. Alternatively, you might consider using something like stencil shadow volumes, and using the extended compute capabilities of modern GPUs to clip down the shadow volume so it doesn't consume enormous amounts of rasterisation or clipping time.

Alternatively, I've been thinking a lot about reprojection. Let's say you interrogate your cascaded shadow map and use that to build a screen-space mask of the shadow response. You want soft(ish) shadows, so you commit a lot of PCF samples.

How about reprojecting last frame's shadow mask into this frame's shadow mask, then filling in the holes with some simple filtering operations? Distant shadows are typically pretty low-frequency things, so you could well get away with some blurring and imperfection in the shadow mask. Then, you need only render the geometry that became newly visible this frame into the shadow map. You could just subtract one frame's light frustum from the other, yielding a convex polytope, and only render the intersecting objects.

Just a brainfart...