I have an indoor room scene with a world environment HDR and sun light.
The sun and HDR are lighting the room through windows (no glass layers - the windows are simple openings).
The windows are partly covered by curtains which use a principled Shader with SSS enabled.
What I am missing in the render is light that travels through the curtains and contributes to the direct and indirect light in the room.
Appreciate any hints I can get (new to Blender; switching from Lightwave).
EDIT: I moved this from Technical to Lighting because I understood that technical is more about hardware and drivers.
EDIT2:
File was to big to post here. But I made stripped down version of the scene and uploaded it to dropbox:
Not in a structured way. Actually working on understandig how materials, lights and rendering works in Blender, especially with regard to SSS. I was not able to get the desired result with Lightwave even in its latest 2020 version. So, I decided it is time to have a real look into Blender.
Cycles mixes the shadows and everything on different layers. As such the caustics and everything is kind of faked even though it’s all calculated. This is the way it is with all render engines, even Maxwell Render, as it’s all a computer simulation. The freedom this bring in Blender is you can change it to whatever you like independently. The other thing I found is the shadow from your curtains are falling on the inset door which is making it look like it’s almost not there even when it is there. Long story short I think this is the effect you are looking for. Just change the transparent to whatever color and darkness you like. Without doing it this way the SSS scatters the light so much the effect is probably not given that is wanted.
I made some more tests and my conclusion is, that SSS indeed contributes to the GI calculations. The image shows the diffuse indirect pass (no specular / glossy in the materials). The orange tint comes from the SSS.
Do you need a solid curtain though? I would simplify and use a thin geometry curtain, possibly only solidify the edge for the folds using vertex group, and just go with diffuse/translucency mix instead. If you want actual semi-see-through as well, doing it with pattern masking should be ok, doing it with only angles it can get a bit messy.
If using fresnel for any glossy, remember to invert the IOR for the backface.
No, no real need for a solid curtain. It is just a test scene. I was curious about SSS on thick curtains and of course how shaders in Cycles generally work. Learning mode so to say. But it is quite similar to how Lightwave handles shading - I use nodes there as well.
