Corona Rendering Black in Animation

[Otuama]

Hi all.

We have an issue that I'm finding baffling.

We're rendering an animation and frame 575 is fine but 576 onward is rendering the diffuse, reflect & refract as black.  AO etc are fine.

Frame 576 is rendering fine when in perspective mode, it just seems to be when in the camera.

The camera is not going through an object or anything like that.

Isolating an object when looking through the camera works fine but when everything is turned on it renders black.

Does anyone know why this would be? 

Thanks

Here's a recording:

[Aram Avetisyan]

This seems strange.

Does the camera have any physical size?

Can you submit the files so we have a detailed look?
Does it happen only for the specified frames, or randomly until 576 too, say at 130 or 50?

[Otuama]

Thanks for the reply.

Do you have a link for me to send this?  Can't put it on the forum

Or shall I weTransfer it and open a ticket?

[maru]

You can attach many gigabytes to our support tickets :)
https://support.chaos.com/hc/en-us/requests/new

[Otuama]

Does it happen only for the specified frames, or randomly until 576 too, say at 130 or 50?

Hi, in answer to your question, this seems to be the only shot affected. All other shots have rendered fine.

[Otuama]

You can attach many gigabytes to our support tickets :)
https://support.chaos.com/hc/en-us/requests/new

Sent, thanks

[Aram Avetisyan]

Believe it or not, the issue is in "Roof_Canopy" object (profiled trim over the window) - probably an incorrect boolean operation or Corona not being able to handle thin boolean/geometry (non-enclosed mesh).

At frame 576, camera is directly under this object (looking from top), but in frame 575 it is not, and it may make Corona misinterpret the medium of rendering, turning it to all black.

If you apply tiny shell modifier or just enclose it, the render will return back to normal.
This will surely be reported and used to even further solidify the handling of medium resolving in case of non-perfect geometry.

[Aram Avetisyan]

The issue has been resolved in the ticket.

Little specific update here:

The issue is most probably not because of the boolean/bad geometry, but that the mentioned object had volumetric scattering (CoronLegacyMtl with volume absorption).
As the object was not an enclosed mesh, Corona interpreted it as an infinite medium and rendered black.

The solution is either to enclose the mesh, either disable the volumetric absorption (it was a solid non refractive object IIRC). I would suggest doing both :)

[Jens]

The issue has been resolved in the ticket.

Little specific update here:

The issue is most probably not because of the boolean/bad geometry, but that the mentioned object had volumetric scattering (CoronLegacyMtl with volume absorption).
As the object was not an enclosed mesh, Corona interpreted it as an infinite medium and rendered black.

The solution is either to enclose the mesh, either disable the volumetric absorption (it was a solid non refractive object IIRC). I would suggest doing both :)

I'm having this issue too at 1 specific frame out of 2500. Camera doesn't pass through anything, but renders black in everything except normals render element. If I do a material override, it renders fine. How can I check my scene for that troubled object/material? Any script? I don't dare convert the whole scene in case I mess something else up.

[Jens]

I tried the script available here: https://support.chaos.com/hc/en-us/articles/4528592571537-Corona-renders-black-screen-3ds-Max

While it fixes the black frame issue, sadly it also changes the look of a lot of other materials and reflections and to a degree that you can clearly see the difference. I made a ticket as I have an urgent deadline on this animation in a few hours. Hope for quick help there.

**i was lucky to find the culprit, so all good. It was a tram model located directly above the camera (moving in a metro under ground). If the tram model is put on a hidden layer, it renders that frame fine once again. I do think this must be reported as a bug though and a fix be offered other than the script that changes the look of more than just the problematic material/model. Hope to have time to provide you with scene and a bug report at some point.**

[Aram Avetisyan]

Writing again for better visibility:

This issue mostly happens when there is a object with volumetrics enabled material above the camera, which is not enclosed (has borders) - in theory this is not supported, as volumetric materials should have an enclosed volume, to be calculated based on.
Corona uses material based medium resolving, and this causes to misinterpret the medium. Possible workaround may be to turn off material based medium resolving in development/experimental settings, correct the mesh or disable volumetric settings in the material.

Please check if this was the case, again, for you and confirm. Feel free to share if the workarounds work as well.

[Jens]

Writing again for better visibility:

This issue mostly happens when there is a object with volumetrics enabled material above the camera, which is not enclosed (has borders) - in theory this is not supported, as volumetric materials should have an enclosed volume, to be calculated based on.
Corona uses material based medium resolving, and this causes to misinterpret the medium. Possible workaround may be to turn off material based medium resolving in development/experimental settings, correct the mesh or disable volumetric settings in the material.

Please check if this was the case, again, for you and confirm. Feel free to share if the workarounds work as well.

I'm pretty sure the issue was with the glass material on the windows of the tram model. Windows were just 1 plane (no thickness or maybe some xform/scale issue). While I appreciate the explanation of the technicalities, I think for a user perspective, it would be amazing if some warning of this was implemented, in Corona, to notify the user if a material or object had this issue when rendering started.

Kind of like the warning we get if a normal map is using wrong settings. Maybe it was possible for Corona to point to object/material name of the object that has the issue, so we can much faster troubleshoot and edit it ourselves. Unless that means it has to run a check on all objects in the scene to check for if it has a thickness, if not, check if a material with any Media Options is enabled. That might take a while on big scenes no? Then maybe it could be a script in the corona converter. OR maybe I'm overreacting as not that many people have had this issue? :)

[Aram Avetisyan]

Writing again for better visibility:

This issue mostly happens when there is a object with volumetrics enabled material above the camera, which is not enclosed (has borders) - in theory this is not supported, as volumetric materials should have an enclosed volume, to be calculated based on.
Corona uses material based medium resolving, and this causes to misinterpret the medium. Possible workaround may be to turn off material based medium resolving in development/experimental settings, correct the mesh or disable volumetric settings in the material.

Please check if this was the case, again, for you and confirm. Feel free to share if the workarounds work as well.

I'm pretty sure the issue was with the glass material on the windows of the tram model. Windows were just 1 plane (no thickness or maybe some xform/scale issue). While I appreciate the explanation of the technicalities, I think for a user perspective, it would be amazing if some warning of this was implemented, in Corona, to notify the user if a material or object had this issue when rendering started.

Kind of like the warning we get if a normal map is using wrong settings. Maybe it was possible for Corona to point to object/material name of the object that has the issue, so we can much faster troubleshoot and edit it ourselves. Unless that means it has to run a check on all objects in the scene to check for if it has a thickness, if not, check if a material with any Media Options is enabled. That might take a while on big scenes no? Then maybe it could be a script in the corona converter. OR maybe I'm overreacting as not that many people have had this issue? :)

So most probably the thin glass model had volumetrics enabled, which caused it.

We have thought about warning about this but it is not as straightforward as e.g. normal map warning. With normal map gamma, it can be pinpointed easily, while it is not easy to check for all scene objects, constantly, if there is a non-enclosed volume object which has volumetrics enabled material. I believe we have added this as a case for black rendering/no rendering helpdesk articles we have. On the other hand, in some cases this can be a desired faking of an effect (when camera is not under such object, but facing towards it).

For overreacting - I think there is possibility of it :)

On simpler terms, this is closer to being a user mistake, as when applying/enabling volumetrics for an object, you need to make sure that it is modeled accordingly. Imagine applying CoronaVolumeMtl to a single plane.

I can write a script for you if this happens often in your case, the script will go through all objects (may be slow), check if they have borders (are not enclosed) and have volumetrics enabled material, then select them, so you can quickly troubleshoot/fix them.

[Jens]

thanks Aram, I think a script like that would be great and also a good supplement to the helpdesk article you have on the issue. The current script offered on helpdesk does "fix" the problem, but in my case it changed a lot of other materials that had no issue, so the render was far different than original render and couldn't be used.

Agree on it to mostly being user error, the problem is when we import a model from a 3rd party (like the tram model that had that material in this case). I don't manually go through all materials on imported models (unless they render wrongly), and in this case the issue only happened when camera was below the model in Z axis. So probably not something people have issues with every day - but very likely when doing metro/subway animations ;)

[Aram Avetisyan]

The fix in article is, if I am not mistaken, just disabling all volumetrics in materials. I think it can be improved (or another one maybe) to check for non-enclosed objects only, and disable volumetrics for those only.