SphereAO is different from spheremask. It is meant to return the fraction of a hemisphere that would be occluded by a sphere of a given size and location. It returns this value for a given position in the world. It can be used in lighting functions or used directly but it probably will not look that interesting by itself.
Sphere AO shown as an unlit material. Note that sphere AO goes black when the geometry is inside of the sphere radius as shown when the sphere is hidden:
Here is the whole material. A blueprint sets the S parameter based on an editable vector variable that I marked “show 3d widget” which lets it be dragged around. Great feature for debugging:
Mesh decals are more intended for things like destruction of edges where you can model the decal geometry to fit exactly. For foot shadow type stuff youd probably want to stick to either regular decals or box mesh with world position behind translucency.
The more uneven gaps you want to support, the more conservative you need to be with the oversizing of your geometry. Ie in the bullet train case they could be relatively flat since we knew the floor was flat in the gameplay area and the effect was disabled until enemies got there.
