Why Arnold takes time to render matte or unlit objects ?

[idizdar]

Hi,

I was wondering since a long time why is Arnold taking time to render matte or unlit object in the scene ?

For example if I load an environment in my scene, even if I ignore all in the diagnostic panel my image (960x540) takes 3 minutes to render form a completely black frame.

It seems that if there is a geometry in the frame Arnold will take time to raytrace it even if there is no lights or any reflection, bounce ...

Any idea or explanation ?

Thanks

[lee_griggs]

What does the log file say? Are there any textures in the scene? Maybe it is converting textures to .tx in the scene?

[idizdar]

Hi Lee !

I'll check the log but Auto convert to TX is disabled and ignore texture is enabled.

I had the same issue years ago, Arnold is always faster to render on empty space than space with geometry. Since then I wonder why the buckets stuck on geometry even if there is no tracing on it.

As much as I increase the AA the buckets takes longer time to render even if there is no light in the scene.

[lee_griggs]

What if you disable textures & shaders in Feature Overrides (Diagnostics)?

Which shaders do you have? Do you get the same issue if you assign a flat shader to it? Maybe you could upload a simplified scene somewhere?

https://docs.arnoldrenderer.com/display/A5AFMUG/Flat

[thiago.ize]

Arnold still traces rays in black scenes because it doesn't know that a pixel might be black (in shadow) or might have light. It's the work to find out if the surface is lit that takes a while to perform. Even if there are no lights at all arnold still needs to do this work because some AOVs can be generated even if there are no lights, such as a Z AOV.

[idizdar]

Thanks of the explanation @Thiago.

I can understand for unlit object then, but why it stuck for matte object ? The render time off matte objects should be pretty fast but Arnold still stuck on it (with aovs or not). To optimize the render time I always set primary off rather than matte object. Is there a way to optimise the render time over matte object ?