double reflections not showing with HDRI

[NOOKTA]

Hello there,

following up on this issue I had in this thread (https://forum.corona-renderer.com/index.php?topic=20779.msg167464#msg167464) I found out that double reflections are not visible when using HDRI:

1. create blank scene
2. create box with 5cm thickness OR two planes with 5cm offset to each other
3. apply thin glass material, increase reflection IOR to 3 (better visibilty)
4. add HDRI
5. add cube and position to reflect in glass objects
7. render
6. double reflections only visible from cube, not from HDRI


See snapshot, yellow is my box and the red/green element is from the HDRI.
I also attached a scene, but you have to load in an HDRI yourself, because file size would be too big/resolution too low. The one I used is https://hdrihaven.com/hdri/?c=studio&h=studio_small_03

Corona Version: 5.0
Cinema version: R21.207 Cinema 4D (single-license)


Can somebody confirm this please?

Best,

Ararat


PS.: I was first suspicious of imported geometry (sketchup), but that is not the issue. Also it doesn't matter if its one box or two planes. Somehow that's different to what ficdogg said in the other thread?

[romullus]

I don't think this is bug at all, but rather physical phenomena. The closer the object is to the mirror/glass, the more distinct its double reflection can be seen. Objects that are very far from mirror, has almost indistinguishable double reflection and since HDRI is infinitely far away from every point in the scene, it can't show double reflection at all. See my little experiment attached below - red teapot is very close to the mirror, green is farther and blue is even farther away.

[NOOKTA]

Huh...interesting thoughts. I understand it now. Thanks a lot!

Another thing: I recreated your scene and can't get the glass to look exactly like yours. Could you share the glass material settings? There shouldn't be a difference from Cinema4D to 3dsmax? I can get double reflections, but the first reflection is not as strong as it is in your render. See attachment. Anything I'm missing?

Best,

Ararat

PS.: Maybe move this thread?

[romullus]

I bumped fresnel IOR quite a lot for reflections to be more prominent - it is not physically correct glass material. You shouldn't take it as a reference, if your aim is to have believable material.

[NOOKTA]

Yes I know. I was just trying to see whether there are differences between Cinema4D and 3ds max, because apparently I lose my second reflection when increasing reflection IOR.

Thanks for clearing up the double reflections issue I had, thanks a lot!

[romullus]

Glad to help you.

Regarding the difference between C4D and 3ds Max implementation, i really don't think that's the case - AFAIK, both versions share the same core code and they should produce nearly identical result. Mind you, that i did use some tricks to make reflections more prominent, especially secondary one, because by nature it's very weak. I made sure, that main light is shining from the mirror side, also it's preferable that background behind reflection would be as dark as possible, so it wouldn't overpower weak reflections. Increasing fresnel IOR also helps to boost reflections alot. In normal conditions double reflections are quite hard to spot - some people even think that it's fake or cheat in renderers :]