I seem to remember reading somewhere this might have something to do with faces, but I can’t find any specific reference to it any more.
Windows especially are giving me problems. For example, I made this set of french doors - really doesn’t make sense, because I never touched the glass material, I just modified the doors that I downloaded from a manufacturer, and applied some Sketchup material, then applied the Twilight material, it’s consistent on each left and right side door, but two weird things:
When I turn on shadows, only one side casts a shadow of the door handle, and when I render, only one side looks like a mirror instead of transparent glass.
This is so frustrating! When I use the paint dropper, the SU materials are identical, and the Twilight material applied is identical, why are they behaving differently?
interior-french-doors.skp (524.5 KB)
if you go into monochrome view you can see the issues with the textures not matching, likely reversed faces that need to be flipped and the glass material applied.
Also, to add, as this is a DC you might have to make the edge glass sections “unique” otherwise when you change them they will flip in the other door too, and you’d have the same issue.
Also, not sure how twilight handles glass, you may need to apply their glass material?
Some renderers will handle two sided materials, if one needs a different texture on each side, but glass in reality isn’t a thin plane but has thickness, and so it has two independent sides with a solid interior.
Unless you specifically need to see the reflections in the glass, you could eliminate it altogether? render times can go up significantly trying to resolve light through glass that has refraction properties etc.
Thank you so much! Monochrome view is what I was trying to remember, I think it was mentioned for a split second on a video and I missed it…
And yes, I am aware of possible problems I need to avoid by making components unique, I have learned that lesson many times over, some really unexpected and kinda hilarious surprises happen when you make edits to components outside your structure and stuff is happening inside!
Yes, it’s much easier to make glass two dimensional whenever possible. Their material does still have to be applied, however, but just once, as long as you don’t do anything silly like put different SU material on each side.
As I’m a Twilight user thought I’d take a quick look at your door model. I agree with whiterabbit’s comments and would like to add a few more. It appears that the glass in the door is just a single face. Make sure in Twilights Material Templates you use one of the “Architectural Glass” templates, they are meant to be used when your glass is a single face. Also Twilight has a known issue if an object has multiple nested groups/components ( more than 5 or 6 ) . I lost count how many groups I had to bore down into to get to the glass. You could possibly have some unexpected results with rendering this door due to that.
Thank you for the feedback!
I am using the Architectural Glass template for rendering.
I wasn’t finished with the door yet; I exploded the heck out of it, lol! 
If you happened to view hidden geometry, I’m not sure if the version I uploaded here was before or after I took care of that mess, but YIKES! 
Here’s the (more or less) finished product if you want to take a look. I’m always open to suggestions, if you see anything else I can improve, however!
interior-french-doors.skp (399.1 KB)
