The projection can be tilted if the particles are falling at an angle. It wouldn't work too good when it's at flatter angles (particles would get blocked by things on the ground near the edges of the loaded cells, and very rarely appear around the player), but it should be fine as long as it's mostly downward-facing.CMAugust wrote:A top-down projection doesn't sound like it can account for particles blowing sideways, for one thing.
In the case of Skyrim SE, it's also possible they're using DX11 ComputeShaders (OpenCL for us) to have GPU-accelerated particles with collision detection.