Sorry for the delay.
Right now, that option defaults its behavior to casting shadows wherever there’s opacity on the mesh. But what if I want the shadows cast to be the inverse of that, or shadows to appear within a certain opacity range? There’s no real way to fine tune that from what I’ve seen so far.
Curious, what use case would this be useful for? Opacity defines how much the material blocks light, which is going to work the same regardless of whether that light is causing shadows or arriving at your eye. So using opacity for both is physically based. Of course, if there’s something really cool that can be done with having more control over opacity for shadowing, then it can be justified.
I’d like to create “prism masks” (for want of a better term), where I can specifically direct lightmass to influence the light based on a color map when light passes through geometry faces.
What you are describing sounds like colored translucent shadows. Lightmass supports those, but unfortunately they only work on static lights. Stationary lights don’t store their direct lighting in the lightmap and therefore LIghtmass can’t compute colored translucent shadows for it.
Outside of lightmass, a movable object with a translucent material can cast dynamic shadows on the world if you check bCastVolumetricShadow on the object and the light. Unfortunately they can only be monochrome due to some render target limitations.
Thanks for your suggestions.