Animated VRayMtl Fog Color problem when rendering sequences

[hotbox]

Hi,

I've sent this to support but hoping someone may be able to assist...

I have a fluid surface in Maya, imported as a VRayProxy.

I have a standard VRayMtl assigned, with the Fog Colour and Fog Bias animated.

The Fog Colour is animated from white to a medium blue.

When rendering a single frame after the animated Fog Colour change, any frame, all is well.

If I try and render a sequence after the animated Fog Colour change, the Fog Colour is wrong, desaturated by around 20%.

The entire sequence is just over 8,000 frames, the colour change happens across 100 frames around frame 3,800.

No matter when after the colour change I test, if I render a single frame, say frame 4750, the Fog Colour is correct, if I render the sequence 4749-4751, the Fog Colour is wrong, desaturated by around 20%.

I've tried rendering a sequence in various ways:

1. In Maya using the V-Ray Animation option.

2. A network render managed by our render manager and running across different nodes.

Same result.

Strangely, if I submit a single frame render job to the farm, the colour is correct.

But, If I submit multiple single frame render jobs (not sequences but 1 frame per job) to the farm, the colour is incorrect.

Which makes me think it's possibly that the value is being cached somewhere? Not that this makes much sense as the multiple single frame jobs were being sent to different nodes and the first frame each rendered was already past the point of the animated colour change having compleated.

I'm out of ideas so hopefully someone can suggest something!

[yolov]

What V-Ray version is that?
Can you try if instead of animating the fog color directly, you plug in it a colorConstant or a VRayTemperature node on which the color is animated?

[hotbox]

I've had the same problem across various versions of V-Ray and Maya, but currently:

Maya 2018.4
V-Ray 3.60.04
Windows 10

The fog colour isn't animated directly but via a Ramp (2D Projection).

I experienced the same problem when the fog colour was animated directly.

[yolov]

Can you get me a scene?

[hotbox]

I've tried setting up a basic scene to replicate the problem, I can't.

The only thing I can think of is its due to the size of the alembic sequence / files the VRayProxy is referencing.

The test scene I setup is referencing a 240 frame alembic sequence with each .abc file being around 200KB.

The production scene references an 8,000 frame alembic sequence with .abc files ranging in size from 100MB to 2GB.

It could of course be something completely different, but that's the main difference between the 2 scenes.

Due to the size of the reference sequence, tricky to get the production scene to you?

[yolov]

Hmm, the way you described it in your first post made me think that it's something that's just not supported as animated in our export. Hence the workaround with an animated texture.
It shouldn't matter if there's a larger or smaller alembic sequence, as (if I'm right, of course) this is not related in any way to the vrayMtl's refraction fog color.

[hotbox]

Yes quite odd, wasn't expecting the test scene to work.

Although it probably isn't the issue, I'm planning to plug the larger alembic into the test scene this afternoon to see what happens.

As far as getting the production scene to you, as the problem presents when just rendering a small sequence I could strip out all unrelated and bundle up the production scene with just a few frames of the alembic sequence so you can see what I'm seeing?

E.g. render the single frame 4758, the fog colour is correct, render a short 3 frame sequence 4757-4759, it's different/wrong.

The problem presents throughout the second half of the animation, looking at the point I've been testing the per frame size for the alembic showing the problem is around 200MB.

If I bundle up the project file and a few frames of the .abc how is best I get this to you?

[yolov]

You can upload it anywhere (like dropbox, google drive, wetransfer) etc and send the link to alexander.yolov@chaosgroup.com
Or if you prefer - we can send you instructions on how to upload it to our ftp directly.

[hotbox]

The test scene with the larger alembic was fine, so I'll strip back and package up the production scene and send over in the next couple of hours.

Many thanks for your help with this.

[yolov]

By 'fine' you mean 'not fine', I hope - as in it reproduces the issue?

[hotbox]

Unfortunately not, a consistent error would be far too simple!

3 frame sequences from the test scene rendered fine, without any problems - I applied the material to a static object, or a small file size alembic imported as a VRayProxy, and the production alembic imported as a VRayProxy - no error with the colour.

So I went back to the production scene, removed all but the elements required to show the fog colour error on the alembic, ran a sequence test render, it rendered fine - no error with the colour.

So I went back to the production scene, didn't change a thing, ran a sequence test render (identical to the test I ran a few days ago), it rendered fine - no error with the colour.

Which is very strange.

I have 2 short sequences (3 frames per sequence) of rendered stills on the screen in front of me, all from the same Maya file, the same frames, rendered the same way using the same machines and software versions, the only difference being they were rendered a few days apart, on the first sequence the fog colour is wrong, on the second sequence it's correct.

It's been the way with this problem, intermittent thus tricky to pin down.

Almost as if...the colour is being cached somewhere. E.g. if I open the file and render a sequence after the colour change without previously rendering before or during the colour change, the output is fine, if I try and render sequences before and after in the same session, or the entire animation, the colour seems to lock around 80-90% of the way through the fog white to blue transition.

I realise this doesn't make any sense, but its as much logic as I can pull from the problem.

I'm running some tests now, rendering a few sequences locally / distributed, before, then during, and after the fog colour change.

If they look okay, I'll set the 18 x 3 frame sequences I use to spot check the different sections of the animation rendering on the network overnight.

The problem always presents when I try and render multiple sequences from the start to end of the animation, but I'll let you know the result...

The error can't just have vanished, and without pinning down and resolving what is going on I'm not confident to start a render of the entire animation (due to it's length, the water, and our limited rendering resources it takes about 6 weeks to render the full animation).

So as much as I'd like the error to have magically fixed itself, hopefully there is a way I can reliably recreate it so you can see what I am.

Of course any ideas / suggestions much appreciated?

[hotbox]

So the test renders came out without any colour problems.

Which is good.

But odd.

I'm not sure there is anything I can do but proceed as if the error has fixed itself.

I'll come back to you should it happen again.

[yolov]

Uhm, OK, that's weird. But good. But still weird.
Do let me know if you come across this again and we'll have a look.