Hello,
I’ve had this problem a couple of times now.
Is this a bug or are some settings responsible for this?
max 2021, corona 6 hf2, standard settings. (I even did a render engine reset just to check, and this still happens)
please see attached video.
corona_ev_i.mkv (11.7 MB)
Looks like you have piped your environment map through CoronaTonemapControl with unticked “Exposure”?
Good Luck
no, nothing of that sort.
I did reset once again and only use a newly generated sun and sky with the “add coronaSky env” button.
The problem seems to be tied to the scene surroundings. It is quite large in scale (although not very high in polycount).
When I hide the surroundings the problem seems to be gone.
This is weird.
Probably would need the problematic scene sent in to us.
(also though - Corona version, host software version, all scenes or just this one in particular, etc. 
Just to have it written in text rather than “discoverable by watching the video”)
Maybe there is any map used in that surroundings together with ToneMapControl? A site plan/master plan mapped to the ground for example? I still think it is caused by any ToneMapControl map. Just to test, try merging all xrefs (if present) and paste
for map in getclassinstances CoronaTonemapControl do format "%: ExposureAffected: %\n" map.name map.ExposureAffected
into the maxscript listener. If it reports nothing but “OK”, I was wrong
Good Luck
Yep it really does look like the ToneMap override - the only other thing I can think of is the camera is overriding it in some way.
it only reports “OK”. So no maps…
@TomG I did write which max version and corona version in my initial post. Also that it has happened before. Not very often thought…
Ok, then this option But was worth a try.
Good Luck
it is the strangest thing.
I now merged the whole scene into a new one and used an override material for everything.
corona_ir_2.mkv (4 MB)
Agree. Sending or attaching the scene would be the best now. I’m really curious what is causing this one. You could also test Tom’s guess by switching to perspective view or using a newly created camera maybe.
Good Luck
perspective view or new camera makes no difference.
I uploaded the file.
filename = 1624894136_corona-ev-issue.zip
My guess would’ve been that you are using advanced bokeh vignetting in a CoronaCam with high values (which darkens the first render iterations quite a bit) but if it happens in perspective view as well then it’s something else.
It looks like this: https://support.corona-renderer.com/hc/en-us/articles/4402317843345
Does the issue go away if you go to Render Setup > Performance and set MSI to 0?
I just did a quick test and it looks like the issue is gone when setting MSI to 0.
What does this mean now? It seems like that is not really a solution since:
Value 0 is a special case that enables an unbiased mode (this mode is not production-ready as it will never clear noise in some scenes).
Please note that your environment plane (ground mesh) significantly affects your overall global illumination, in your scene. You can either hide it and try with default MSI values, or simply block the back wall windows (behind your cam.) and introduce some interior lighting. Lastly, due to the override all scene materials are essentially reflecting light much more so avoiding the override in that case as well as properly texturing most assets will again balance this issue out.
