I've been modeling the Sun-Earth-Moon system to simulate the Aug. 21 eclipse. THREE.js is working fantastically.
The only hangup I have is with making shadows that overlap to create darker shadows in the joint area. I earlier asked about creating umbras and penumbras, but it turns out this requires making many, many directional lights, which is a performance killer. So I came up with a solution I think is rather clever: I'm creating various invisible, concentric Moons of different radii that cast shadows of different radii, centered on the same spot.
I already have an ambient light that tones down the opacity of one shadow. The trouble is that, if I have two objects individually casting their own shadows, one inside the other, they don't multiply. All I see is the biggest shadow.
Is there any way to make shadows multiply?