Problems Rendering Animation!

[dfcorona]

I finally finished this project and it's ready to be rendered out as an animation. I rendered the first camera scene that was 253 frames. But certain frames all of a sudden the materials change there reflections and refractions, some objects losing reflections all together.  I even tried rendering just those frames and it still happened. I don't know why this is happening. I am rendering locally on one machine, so it's not a network issue. 


Windows 11 Latest
3ds Max 2025.3
Corona Update 1 Hotfix 1
TR 7980x
192GB Ram

[dfcorona]

Just tried "Corona 12 Update 2 RC1". Still the same problem.

Update: Tried merging into a new scene and everything, no change.  Frame 104 renders perfect, frame 105 renders with materials all messed up every time.  Makes no sense.

[maru]

Please check this article: https://support.chaos.com/hc/en-us/articles/33844546706193
Most likely, there is an open (non-capped) object with refraction and/or volumetric effects close to your camera. This has the same effect as submerging your camera in a refractive/volumetric medium. The solution would be to find the problematic object and make sure it is closed (capped, no open loops), or change its material to remove refraction or volumetrics. There is a script at the end of that article which you can use to remove volumetric effects from all scene materials. While it's not a solution, it would at least tell us whether this is really the problem.

[dfcorona]

Hi Maru,

After hours I found the problem. This definitely looks like a BUG. It came down to a lamp that had a shade that used translucency and refraction on the same material. The minute I turned off the refraction on the material the animation renders out perfect now with no changing materials. I can send you the lamp model if you wish to investigate?

[maru]

Sure, you can send it here: https://support.chaos.com/hc/en-us/requests/new
But this still sounds like "camera inside refraction/volume" issue. We would like to at least detect this situation and inform the user. We have an internal task for this.

[dfcorona]

Hi Maru, I sent the model. The scene never has the camera going inside the model. This is an interior animation, the model just happens to be in the scene.

[TomG]

As the article explains, the camera doesn't need to go into the bit that you think is the model - the problem is if it is not closed, then it "extends infinitely" as far as Corona is concerned, and if it passes into that zone then Corona believes the camera is inside the volume. So anything with any sort of volume effect needs to be closed geometry, otherwise when the camera "passes in front of a hole" no matter how far away it is from the model, the camera will be treated as being inside the volume.

EDIT, PS - think of it this way, in order to calculate how far through a surface a ray has gone to see just how much it is affected by absorption and scattering, it must both intersect the material for the first time, AND exit the material, so that there is an "actual distance traveled through the volume". Otherwise the distance it has traveled is infinite, it never left the volume. So geometrically, there must be a face on both sides, the geometry must be closed. This is true whenever rendering volumes, just that now that Corona can process cameras inside volumes, it can now affect cameras as well :)

[dfcorona]

TomG, thank you for such a long informative post. I understand what your saying, weird how it only affects certain frames. Never ran into this before. I work on very huge scenes so finding the culprit can be very time consuming. Maybe there can be a error message during rendering to detect what objects. Or maybe something in the Corona tools that can detect and find what materials are the culprit?

[TomG]

Yep, exactly what Marcin meant by "We would like to at least detect this situation and inform the user. We have an internal task for this." :) If the rendering effects aren't avoidable, a warning or some information would be nice, and it's on our list.

[maru]

It was indeed Corona assuming that the camera is inside a volume due to an object with no thickness with refraction enabled. In such cases, I can recommend the following:
1) Adjust the problematic material - either by disabling refraction or by enabling the "Thin (no refraction)" option. This way the camera won't behave as being inside a volume anymore.
or
2) Go to Render Setup > Performance > Development/Experimental Stuff rollout and enable the "Use legacy medium resolving" option. This should fix the rendering, but it globally changes how volumetric effects such as refraction and absorption are computed in Corona.

[TomG]

Or give it some thickness / make it closed geometry (ie make it like the real world, since no objects have zero thickness / are not closed)

[dfcorona]

Thank you guys for all your help. I fixed the issue and look forward to hopefully having an easier solution for finding the problematic material in the future, it's tough with thousands of materials.

[Aram Avetisyan]

It is actually not so hard.

On the frame where the issue appears, check camera position, check the objects above it from top view.
Almost always the problematic objects are above the camera. Select them, then "Get from selected" in material editor. This way you filter out just a few material and objects to check.

[dfcorona]

It is actually not so hard.

On the frame where the issue appears, check camera position, check the objects above it from top view.
Almost always the problematic objects are above the camera. Select them, then "Get from selected" in material editor. This way you filter out just a few material and objects to check.

Hi Aram, Yes you are correct, I just opened the scene which is large, over 2.7GB compressed. It has multiple floors in one shot and the camera is under the object looking at top down. Thanks for that, I'll keep it in mind. It just sucks that you will not find this issue till you render out an animation. out of 240 Frames of one shot, maybe 15 frames have this issue, and not all in a row. It wasn't util frame 105 that this started. But since fixing the issue changed the lighting slightly I had to rerender the entire shot again.

[mark_yang]

I have encountered a similar problem before. When rendering animations, some frames of the glass material may lose refraction or reflection, and when playing the sequence, it will flash. Later, the solution was to change all materials to physical materials, and mark the glass material as Thin shell (no inside)