the doc says Object3d should be added to 1 world only.
i wonder if an Object3d is added to 2 worlds and render them, what will happen, - only for rendering purpose.
1 of the World is used for shadow.
because the Object3d should be isolated in shadow World by my design, so i have to use another World, or i must call setVisibility many times.
never mind this. i would use multiple Object3D with the same Mesh for my purpose.
Yes, that's how to do it. The objects can share some data though, but you have to use different instances for multiple worlds.