I’m going through BlenderGuru’s tutorial and he’s using Principled BSDF as a glass material and Cycles as the render engine. The cup by itself looks fine, so does the plate, but if you look through the cup at the plate, it’s all weird and kinda dark. Same if you look through one cup at another cup.
I’ve set the Base colour of both the cup and plate’s material to pure white (it’s the same material anyway), same with Surface Colour. Roughness is set to 0, IOR is 1.5 and in Render Properties Light Paths>Max Bounces Total, Transparency and Transmissions are all set to 8 (I increased these to 12 to no avail). I also turned on Reflective Caustics and Refractive Caustics.
The last image is what BlenderGuru’s realtime render looks like. No weird transparency issues there.
Anything else I can do?
Add a lip to the cup. This will help guide the direction of refraction rays. Most cups have a rim, and the glass shader is physically-based, so you're probably going to get weird results if you deviate from the real world.
Increase glossy bounces to something dramatically higher, like 64-128. This will help ensure enough glossy (reflection) bounces hit the rim of your cup. Also up transmission bounces to something like 64. This will help stop refraction rays from terminating too early.
Add an HDRI, more lights, and/or simply add a box or some other objects surrounding your cup. The environment is mostly causing this. At the moment, your world color is just a flat color, and you only have a small pink plane that the cup rests on. There's very litle for light to reflect off of, so you get too much refraction where there should be reflection.
This is how it looks when I upped glossy bounces to 128 and transmission bounces to 64. I think it's significantly improved already. It's not perfect though, and you can further improve the realism following my other steps. Most important is an HDRI or some other more realistic environment instead of endless empty space.
Correct answer by Joe on August 14, 2020
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