Particles emit from origin when boolean intersect or difference results in no faces. Is there a solution for this?

Question

I'm not sure if this is a bug and if there's a work around. Let me know if there is.

To replicate, make two cubes: one smaller, one larger. Add a particle system to the smaller cube. Put a Boolean modifier on the smaller cube, select the larger cube as the object in the Boolean modifier, and overlap it with the smaller cube to cut a portion of it. Check "Use Modifier Stack" so that the particle will account for the Boolean modifier and make sure the Boolean modifier is above the Particle System in the modifier stack.

Everything works as expected.

However, when you move the larger cube to completely overlap the smaller cube, something weird happens. The particles emit from the origin of the smaller cube (I set the origin to outside the cube to show it's not just emitting from the volume) despite the particles source being set to emit from faces which there are none of.

Answer 1

When the particle system is set to emit from faces, it uses all faces that are available (as long as there are no specific vertex groups selected to limit emission). However, this does not work as an on/off switch for the particle system. If frames are within the range where the system is expected to emit particles but there are absolutely no faces, it is not switched off - so in absence of any faces it just emits from the origin point.

You can get the same result for example if you have a cube, go into Edit Mode, select all vertices and delete them. The object itself is still there, it just has no mesh anymore. In this case a particle system set to faces will also emit from the origin, this has nothing to do with the Boolean modifier.

By the way, the same is valid for edges as well. If there are no faces, only edges, the system will not emit particles from the edges, but the origin.