Reflections and lights drastically changes when using shadowcatcher

[ilmelo]

Hi all,
sorry if this has been already written somewhere but I searched the internet, followed various tutorials, etc. but it seems I cant understand and fix it by myself.
I usually work in vray but I'm now testing Corona for a product rendering job. The issue as you can see from the attached image is that when using shadowcatcher for post compositing the reflections and lights on the objects drastically change and I can't obtain the same exact result when using shadowcatcher.
In the attached image the top left part is the correct render with a grey plane directly from 3ds max render, in the top right part in the red boxes are the visible differences and problems, and in the bottom part are the actual settings (i tried a lot of mix and combinations, but this is the best I can achieve).
What am I doing wrong? Can someone please help and give me some advice?
Thank you very much in advance

[TomG]

Is the Corona Bitmap itself set to Screen mode for environment, or spherical?

[ilmelo]

The bitmap is an hdr used for reflections and for lighting the scene, and is set as spherical in the coordinates section of the map. I use one for everything, do I have to split it and use a specific one with different settings for the shadowcatcher? I tried also enviro setting in corona shadowcather but nothing changes...

[romullus]

It would be best if you could upload the scene, because it's hard to know how you actuall build it,

[PROH]

Hi. Hope I understand you correct. If you want to replicate the top left render with a grey plane, then the shadowcather mtl should only be filled with the grey color used on the original plane - NOT the HDRI used for lighting.

[ilmelo]

@PROH
tried with filling only the grey color and disconnecting the hdri. I think it's even worse (see attached image)... :(

@Romullus
I cant share the scene due to copyright, I'll try to make a simpler scene to be able to post it with simple test geometry...

Thank you guys

[PROH]

Hmm... I've done this many times, and never seen anything like this. Looks like there's something wrong with your setup. Why does the light change because the shadowcatcher color is changed? Are you using the shadowcatcher as lighting?

[ilmelo]

@PROH:
sorry for the late reply but I've been busy with another work. I built a test scene starting from the scene I cannot share, but environment, maps and material are the same.
As you can see in the attached image, the correct rendering (with no shadowcatcher) is totally different when using the shadowcatcher (no hdri plugged).
I didnt upload the hdri map, but you can plug what you want (I dont want to break any copyright sharing it).
I use max2020 but I saved the scene also in 2017 version.

[TomG]

It's the reflection and not the lighting, and because it has no HDRI it is reflecting the Environ/Backplate color, which is bright (see later about that).

Best to give it the HDRI that is being used in the environment - since it is not a spherical image, would have to use Screen projection in the bitmap, and I used Do not alter projection in the Shadowcatcher. Final image rendering dimensions will need to match the backplate to avoid squashing or stretching it. I used Always Solid mode so that we could grab the background in the image rather than composite in post for these IR tests. The reflection changes of course, as one has the solid material it is reflecting, the other has the backplate through the shadowcatcher.

Or you could use a much darker Environ/Backplate color in the shadowcatcher. Also note that the offscreen rays play a part - since this is a screen mapped image and not a spherical HDRI, when rays go off the screen then I am not sure what color they get :) I got a close match by grabbing the color from an IR of the solid material into a CoronaColor, passing it through a tonemap so it is not affected by post processing (as the color you have grabbed from the VFB for the solid material has already gone through that post processing, with an amplified Exposure, so using that same color as the background color just gets brighter as the Exposure is applied to it again, in effect exposing it twice), and using that result in the Backplate and the Off-screen color map (disabling Use enviro for offscreen rays).

As a note, there was a mapping offset of 0.5 in the U of the bitmap used as an environment, not sure what effect that might have too.

[PROH]

@ilmelo - I couldn't test your scene, but I just tried building a similar scene, and despite what I wrote before, you're absolutely right.

There's definately something unexpected happening with the shadowcatcher, and the further away the EV value is from 0, the bigger "trouble". You can minimize the problem a bit by putting the color-map inside a CoronaTonemapContro with "exposure" unchecked, but there are still some difference.

Don't know why I haven't noticed it before. Is this something newly introduced?

Hope to see a solution soon.

Regards

[TomG]

I installed Corona 2, and get the same result - which comes from the exposure being applied to the color of the shadowcatcher (the color there is taken from the background color after exposure was already applied, in effect applying exposure twice to that color and making the environment seen in the reflections of the shadowcatcher be twice as bright as expected). Either adjusting the color, or adding the ToneMap control, removes that "doubling up" of the exposure on the color.

[ilmelo]

Thank you very much for your efforts. I will put all your suggestions to test as soon as I can.

@TomG: interesting, I will try what you say, but there are still big differences in the reflections of your images (see near the borders of object, between "shadowcatcher" and "solid") and I need to make them the exact same.

I will do other tests and I will post the results here asap

Thank you again guys for all your help, I really appreciate that