Meshlights dim/flicker between frames when rendering

[julzrwood]

Hi all,

I can't provide an example scene as I am working on a project but we have an issue where our meshlights are dimming on random frames at semmingly random intervals. We've gotten away with this in the past by making sure that the shadow rays are on for the meshlight geo and by restarting frames that have dimmed/broken meshlights. We also see this happening when rendering with checkpointing turned on. For example, during the render a meshlight might look fine, the render then checkpoints, stops and when it resumes the meshlight appears much dimmer/broken than it was previously. I have tried to replicate this in a similar simple scene to pass of however this scene doesn't produce the issue. 

I've read through this post https://forums.autodesk.com/t5/arnold-general-rendering-forum/are-there-known-issues-regarding-frame... and found the solution not very helpful to what we are doing.

What are the restrictions of meshlight geometry? Like the size, normals, vert count, is it betterto breka meshlight geo into smaller components? Do meshlights need to be polymesh instead of Subdmesh.
Is there an intensity issue with the lights? Are there any known issues with meshlights in general that are ticketed?

I know it's hard without a simple scene and I am working hard to break Arnold so that I can get this to you I promise.

We are using Katana 4.5 and Arnold 7.2.4

Thanks,

Julian

Julian

[lee_griggs]

It's hard to say without seeing what the mesh_light geo and the renders look like.

How many mesh_light samples are you using? The mesh_light requires more samples than a regular light.

Have you noticed any difference in increasing the subdivision_iterations for the mesh_light geo?

Is there a texture map connected to the mesh_light.color?

[julzrwood]

Hey @lee_griggs , 

Thanks for getting back to me, I will try to get some screenshots of the geo cleared to post here tomorrow at work however, the best way I can describe them is that they are smallish, tetris shaped, individual pieces of geometry that are watertight. The tertris shapes are scattered in a ring shape and are not connected, they are however one piece of geometry in Katanas scenegraph.

When you mention `meshLight_samples` are these just regular light samples on the meshlight or is there a seperate attribute for this? I'll check this tomorrow but I think we're using the default samples of 2. Would this explain how sometimes the lights are dimming while on others they are not? If so how does this work technically? Is there an element of randomness in the sampling that says if a geo is too small to ignore or lower it's contribution?

For the shader assigned there are no textures, only a shader, It is a ramp being driven by a facing ratio. Essentially acting as an LED type light with a hotspot in the middle that gets refracted through glass and a subsurface liquid.

For the subdivisions we are using 2 iterations as I saw this in the docs and thought it would be worth a try.

I'll keep working on trying to get something uploadable tomorrow.

Thanks for your reply on this one mate, we're quite stuck at what to do here.

Julian

[lee_griggs]

Great, if you could.

>The tertris shapes are scattered in a ring shape and are not connected, they are however one piece of geometry in Katanas scenegraph.

I imagine this with just 2 mesh_light samples would be very noisy. Try more to see if it helps.

> that gets refracted through glass and a subsurface liquid.

Are the dimmed frames when the glass passes through or intersects with the mesh_light geo?