Shadow catcher alpha path tracing

Is there a way to extract matte object alpha pass with shadow only in Path Tracer mode? Currently there is no way to render the sequence for later compositing with footage and getting the shadows in path trace mode.

I know it works in iRay, but the render times with motion blur are extreme and I could not get flow sims to work in iRay.

Maybe with some sort of custom MDL graph for path trace that could be achieved? Please…

Hello @kgr! I’ve shared your post with the dev team for further assistance.

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Hello @kgr! Just wanted to let you know that the dev team is looking into this issue.

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@kgr I confirm this feature is coming in the next Kit 104.1 release. Stay tuned!

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Thank you for addressing this issue in 104.1! I have been testing it out and now alpha is indeed produced in Path Trace mode. However, It does not seem accurate and is smeared and distorted when objects pass by the shadowcatcher. The material of the “shadowcatcher” has Reflection, Specular, Albedo brightness -all off, but tried diffrent combinations and almost the same result in terms of distortion.

Also if check “invisible to secondary rays” is set on the Matte object it no longer works - just appears as a solid object, but with the check off it appears in reflections as a solid object.

Not sure if it’s a limitation of the current implementation of this feature in PathTrace or if I’m setting it up incorrectly as I have tried many diffrent combinations of settings. Would be great if documentation on Matte object alpha compositing could be updated in regards to Path Trace mode and motion blur.

@kgr Thank you for reporting this. I will look into it and get back to you soon.

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Thank you!

To add to this I just found that if you want to render an image composited in editor in front of the backplate (without alpha) motion blur object that moves in front of the backplate added through post processing leaves a black trail during substeps rendering - the more substeps the bigger the trail. To fix this I’ve created a large cube behind the object and set it as holdout object and turned off cast shadows and secondary rays on the cube.

Maybe this is related to the issue above as it looks like something related to alpha.

@kgr FYI we have logged a bug for the Invisible to Secondary Rays although the fix will likely be in Kit 105 rather than in the upcoming minor Kit 104.2.

For the Motion Blur, I’ll get back to you. Sorry for the delay.

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