I’ve already tried with the “blend mode” “Translucent” and it just creates artifacts, since it cannot display the material through all the layers of the curtain. I know there is a “lightmass replace” node, but I have no place to plug it in, since it is “opaque” “blend mode”.
You need a second copy of your mesh with the translucent blend mode. You need to make sure its set to hidden and also cast hidden shadow (altho that may only be for dynamic lights I’d flip it on just in case).
Then you need to set the opaque, visible copy to not cast any shadows.
I’ve run into a problem.
I can’t use this method, since the translucent shadow requires the directional light to be static, and the sub-surface shading model requires the directional light to be stationary.
So I’ve decided to work with translucent blend mode (without using two different curtians). But when I bake the lightmap, the curtain becomes black. I think this might be an issue with ver. 4.11, since I’ve followed this guide Using Colored Translucent Shadows | Unreal Engine Documentation and in this tutorial the object that casts the translucent shadow remains the same, after lightmap baking.
This is what the material looks like:
This is what the curtain looks like before I bake it:
This is what happens after I bake the lightmaps (The directional light is set to static)
In answer to your other question, it should not affect performance in any way to have a hidden, static shadow casting mesh. The only impact will be a very minor affect on static lighting rebuild times.