Author Topic: How to handle backplates?  (Read 2587 times)

2021-08-18, 14:42:29


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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.
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!
Nicolas Pratt
Another Angle 3D

2021-08-18, 16:35:40
Reply #1


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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.

2021-08-18, 17:46:17
Reply #2


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I also wondered for a long time why such  engine as –°orona 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

2022-08-23, 15:40:49
Reply #3


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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.

2022-09-30, 22:35:59
Reply #4


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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...).

2022-10-03, 12:30:43
Reply #5


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Another issue is that backplates in environment overrides, don't play nicely with ACES OT or LUTS etc, and the tone mapping control rarely works to reverse the effects.  So if using any tone mapping techniques in the frame buffer, then its pretty pointless getting your backplate looking how you want it to in your photo editing app beforehand, as it rarely looks like the same in render.

If you process backplates as you would a normal photograph and then use it in Corona with any sort of of Lut, ACES OT or tonemapping tweaks, you'll effectively be applying two tone curves to the backplate (one in Lightroom, one in corona) and one to the render.  Giving a mismatch.

I'm currently experimenting with processing backplates using Linear Profiles and using them that way, almost akin to prepping a proper Albedo texture.  That way the only tone curve being applied is in render and theoretically the backplate should align with the rest of the render in terms of 'look'.

Quite often we capture full 360's on site to use as reflection overrides and/or direct visibility overrides - but as already stated, Corona in it's current state doesn't really cater well for any workflow involving jpeg backplates in render.

2022-10-03, 18:03:30
Reply #6


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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?

This should have been fixed by:
Fixed issues with environment overrides, such as increased amount of noise and incorrect results
in Corona 8:

If you are still getting some similar issues with using this approach, could you please share a scene with us through this link?

Other than that, I agree that rendering background plates is difficult now. Especially after implementing the Corona Physical Mtl, which always has some glossy reflections, so the background is always reflected at least a bit.
I think the "safest" solution is using environment overrides in the Scene tab. Using a Light Mtl with "emit light" off will most likely give you the "dark highlights" issue described here -
It is similar with using a self-illuminated material with rayswitchers. Generally, this is a kind of fake that Corona handles in a way that is expected from the render engine / light simulation point of view, but not for the users. We have this properly logged with example scenes and it is acknowledged, but the main issue is that resolving it without breaking other stuff will be difficult and will take a lot of time. Hopefully it is another case of something nearly impossible that will be done one day.

Other than that, *sometimes* some workarounds are possible, for example using a combination of "emit off" and "emit on" with a rayswitcher:
Marcin Miodek |
Chaos Corona Support Team Lead | contact us

2022-10-07, 16:01:50
Reply #7


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I usually create an alpha for the sky and cut it out from any backplate, and then I do a modified texture that looks like a proper albedo texture for the stuff that's left.
If I feel real fancy I will extract reflecting parts (say windows, etc) and do a separate texture of that and place it in metalness/reflection channel.
I then assemble it either on projected geo (the best) or just a simple plane.

It won't look identical, and usually requires some back and forth until it looks right, but it responds much more natural to any tone-mapping shenanigans.

Removing all lighting info is tedious at best, I have had great success with 3dLutCreator and the volume channel for quick fixes though.