Chaos Corona for 3ds Max > [Max] General Discussion

How to handle backplates?

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aaouviz:
So, please bare with me here while I explain the conundrum Corona render users find ourselves in...

The situation: I want a image (let's say a city skyline) in the back of my render.

Option a) I can either set my 'direct visibility' override to use this bitmap texture.
or
Option b) I can put the bitmap texture on a plane (corona light mtl) and place the plane in my scene behind my geometry.

Problems with option A:
If I want this city skyline visible through glass I must also set the 'refraction' (and probably 'reflection') overrides. This is problematic as it creates the noisy bright spots all over the image. Not ideal. I know corona devs are aware of this problem, but I'm pretty sure it's not fixed yet, correct?

Problems with option B:
I must set 'visible in reflection/refraction' if I want to see this backplate skyline through glass. This causes the other error of weird sun shadow artifacts on walls. I can turn off 'visible in reflection' (leaving only 'refracton' ticked) and the error disappears, but I now have incorrect reflections (ie; the skyline isn't reflected).

So... basically we have no bug-free solution to put a backplate in the scene.

What am I missing? What am I to do?

Hope this is clear. Thanks!

CJRenders:
I've run into this issue a lot doing product scenes. I typically blur the skyline similar to a DOF and then adjust the color values through corona color correct to get some vivid details to pop through so you can get some detail still through the blur. OR you could remove the glass in the windows if they aren't actually reflecting anything around in the scene. but that'll only work for still shots.

marchik:
I also wondered for a long time why such  engine as Сorona Renderer, which, frankly, is strongly focused on architectural visualization, still does not have a separate feature for correctly displaying backplates with variable bells and whistles.
I suggest to rename this topic to CoronaBackplateMtl and move it to the "feature request" section:D

theredmage333:
OP you solved for me thank you!! Racking my brain on this for days now, always had a few scenes with this issue but never was enough to fully investigate. Finally had a scene with a lot of weird shadow effects. Went back and forth on all the settings of the lights, the sun, materials, etc... even enabled the Debugger and played with Terminator and no luck. Switched off reflections in the light material that I was using as a backplate and boom no more weird shadow effect... Changed the lighting of the whole scene but probably is actually more accurate now as it is less dark.

matthew07:
A CoronaBackplateMtl would be indeed a dream.
@aaouviz, the workaround for option B is to use 2 corona lights, one plugged to direct visibility as it is, the other plugged to the reflection slot of your RaySwitch, but this with an opacity map applied in order to cut out the sky, which is the one usually interfering with the sun, causing the artifacts. In this way you will have the skyline reflected but not the sky (I know...).

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