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Problems with holdout when rendering deep.

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Problems with holdout when rendering deep.

Unread postby sysadminGhost » Mon Aug 30, 2021 3:27 pm

I don't have so much experience with rendering deep, so maybe this is normal. I have some particles flying around and some holdout geometry I'm rendering. The non deep render looks correct and the particles are being held out both in RBG and the alpha. But the deep output has correct RGB, but the holdout geo has a solid alpha in the deep output, making it not so great to comp with.

Is it normal that holdout geo gets solid alpha when using deep? Again, the non deep output does not have the issue.

Im using 5.0 Build ca6eeb66fe

Thanks

Martin
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Re: Problems with holdout when rendering deep.

Unread postby sam » Mon Aug 30, 2021 4:58 pm

Hello Martin,

I'm not sure to understand why you are using holdout with deep. The whole point of rendering with deep is to avoid using holdouts.
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Re: Problems with holdout when rendering deep.

Unread postby sysadminGhost » Tue Aug 31, 2021 8:44 am

Hi Sam
Thank you for your reply. the reason for rendering deep in this case it was mainly to be able to do better comp de focus and to block some of the particles with cards in nuke in case there is a depth cue issue. I guess I could render the hold out geo and particles on it's own in two passes, and then subtract them in nuke, but that is is more work both in rendering and comp. (And not to forget, diskspace!). (it is +100 shots)
So I was hoping that the deep render would come out the same as the non deep one.

I think we can work around it, not using the alpha in comp. But does that mean that it is not possible to use holdouts in deep?
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Re: Problems with holdout when rendering deep.

Unread postby sam » Tue Aug 31, 2021 9:38 am

The point of the deep is to render all geometry fragments and render the scene from the camera as if it was a volume. Alpha is only weighting the transparency of geometry fragments. Using holdouts doesn't make sense. Compositing of deeps is achieved using a DeepMerge node using the camera Z component of each fragment. Alpha is only used to resolve the insertion of transparent fragments between each other.
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Re: Problems with holdout when rendering deep.

Unread postby sysadminGhost » Tue Aug 31, 2021 10:49 pm

Ok, thank you Sam. We will go with the workaround then.

/Martin
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Re: Problems with holdout when rendering deep.

Unread postby sam » Wed Sep 01, 2021 6:38 am

Sorry to be the bearer of bad news but you could try to post this as a feature request to see if that could be supported in the future. We would need to explore how we could support such feature. Then again it's really a super corner case and so far we never had such request including from studios using deep regularly.

As I said that seems to go against the general idea behind deep compositing.
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Re: Problems with holdout when rendering deep.

Unread postby RayCaster » Mon Sep 13, 2021 10:52 pm

whats the difference between a holdout mask & cryptomatte, couldnt one be used in place of the other?
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Re: Problems with holdout when rendering deep.

Unread postby mdkai » Fri Sep 17, 2021 12:49 am

No
because Cryptomatte is an automated system to create mattes based on automated ID types (Asset, Material, Object) but including pixel coverage passes for perfect AA transitions.
Holdouts simply act as cutout against a regular area of the rendering... but can be used for any type of mesh.

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