Hard Edges Where Smoke Meets Geometry
Visible cut lines where a sprite intersects a surface
Also known as: Soft particles · Depth fade · Particle intersects floor · Smoke hard edge · Unity soft particles · DepthFade Unreal · Particle clipping geometry
Quick answer
Hard lines appear where a particle sprite intersects the floor or a wall because the flat sprite is cut by the surface. Fix it with soft particles, also called depth fade, which fade the particle's opacity out as it gets close to the geometry behind it. In Unreal Engine, use the DepthFade node in the material's opacity, and in Unity enable Soft Particles in the particle shader, which also needs the depth texture enabled in the render pipeline asset.
How to fix it
Set the fade distance to match the size of the effect: too small and the edge remains, too large and the particle becomes too transparent. Soft particles need the depth buffer, so they cost a little more and do not work in every pipeline setting. For effects that must touch the ground, a mesh, a decal or spawning the effect slightly above the surface also avoids the problem.
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See also
Depth Fade (Soft Particles)
Fades a surface where it meets other geometry
Depth Buffer
Per-pixel distance used to sort what is visible
Blend Mode
How a material combines with what is behind it
Particle System
Spawns and animates many small elements
Overdraw
Redrawing same pixels
Decal
Projected texture detail on existing surfaces