I thought of how to implement a really cool rain/shower effect that reacts well to dynamic geometry

1. Render the scene depth from above using an orthographic projection. This will produce a nice heightmap for part of the scene.
2. Water falling is simulated using newtonian physics in a compute or possibly geometry shader, with one buffer storing position and another storing velocity.
3. the heightmap can be used to calculate the height and tangent space at a point, so if the raindrop goes beneath the heightmap, ie it intersects scene geometry, then use the tangent space to bounce it

This could produce a very realistic effect if a character walked under a heavily dripping pipe or a shower room or something. Furthermore, if any drops fall through the heightmap because of inaccuracy or undersampling it will just look like they were absorbed

It's been done. The STALKER games, ever since Stalker... 2? Clear Skies I think? Had a "Rain Map". Heightmap/Ovehread static shadow map like used for collision of raindrops, further used for applying a dynamic "wet" shader onto anything the rain could "see". Halo Reach actually used scene depth and normals for screen space particle collisions.

So, it's a nice idea that would work, because it already has