Soft particles
Fading a particle's transparency where it cuts into solid scene geometry, so smoke and fog melt into the floor instead of being sliced off by a hard line.
A particle system usually draws smoke as flat billboards that always turn to face the camera. Where such a plane cuts through a wall or floor, a knife-sharp intersection line appears and immediately gives away that the smoke is flat — one of the clearest tells of faked volume.
The fix, popularised by NVIDIA around 2007, is to let the shader read the z-buffer and compare the particle's depth with the already-drawn geometry's. When they are close, the particle's alpha fades toward zero and the intersection line dissolves into a soft transition. The price is one extra depth read per pixel and a dependency on the scene depth being available.