rendering fire with Deep data (no holdout) for comp?

[JasonH]

Hi all,

I am seeking suggestions we would like to render fire with Deep output for comp but facing issues in productions.

scenario:
1) Karma as the production renderer. H20.5
2) DEEP output with NO matte holdout setup in 3D/Houdini is preferable. (to exchange artist time of setting up holdout with render time and disk space)

FX dept provides shaded fire as VDB, depending on the shots and type of fire, the fire density can be low or high, which yields following issues:

Issue 1 - lacking alpha or color correction mattes: varying fire density translates to less or more solid alpha. Compositors needs to pull custom mattes to color-correct the fire for desired look. Is there a recommended approach that can be done either in FX or LIGHTING that would provide proper mattes for the fire?

Issue 2 - DeepRecolor and DeepHoldout: when the fire density is low, the DeepRecolor-ed RGB is unusable (see attached pic), which makes writing out Deep for holdout a moot.

So, in productions where Deep output is allowed/preferred, I would like to learn what I am missing in terms of setting up the fire (in FX) and rendering the fire (in LIGHTING) so comp can do accurate holdout and color-correct the fire easier (with mattes provided, NOT pulling luminance matte in comp)?

PS. if the issue has to do with how the pyro shader is setup, please share thoughts like I don't know much about it.

Edited June 4 by JasonH

[sipi]

I don't know the industry standard solution to this. I'm really curious how the big guys handles it.

 

As far as I know alpha for smokeless fire is always a problem if you're working with deep.

What we do most of the time is that we use the fire field as smoke and render enough density->alpha that the edges won't be grainy. Of course that will reduce the additive nature of the fire shader to some extent. But you can find a balance/middle ground, where you have enough alpha for proper deep, but the fire still not looking (noticeably) flat.

 

Another approach to do 2 renders. 1 with enough alpha for clean deep, and 1 without alpha, so the fire can be fully additive. Then recolor with the additiv fire. Expensive solution since you're rendering 2 layers, but works. However over the years, I heard some comp guys claim that this alters the deep data in a bad way, and ruin DOF, if they're calculating DOF from deep or something...(?) I never really understood it unfortunately.

Edited Wednesday at 08:44 PM by sipi