AWS Thinkbox Discussion Forums

Using one PRT object as a holdout for another PRT

Is it possible to use the particles in one PRT Loader object as a holdout for another PRT Loader with a different particle sim? Currently I have 2 different effects with the second one being “inside” the first effect, hoping to simplify the compositing by using the 1st effect as a holdout to the 2nd one and simply merging them over in nuke. Is this possible? Currently I can get magma to render the 1st PRT as black but it still shows in the alpha, which will present a problem when I merge over. I can’t seem to find any control over the output alpha, only color, density, etc.

Compositing volumetric rendering is a tricky business, and even deep compositing (once we support it) would not solve all problems, esp. when compositing two semi-transparent volumetric clouds - the safest way is always to render them at once. This is especially true for the shadow casting from one particle system to another.

A possible solution would be to render the one particle system out with a Z-Depth pass, then load the Z-Depth pass as Initial Depth Map. Unfortunately, this feature of Krakatoa is not in the Maya version yet, but it is already in the works internally and should be part of the next KMY update.
Currently, only the KMX version supports loading of Z-Depth maps, the Krakatoa MY version can only load DTEX sequences. I wonder if there is some tool out there that can save DTEX from Z-Depth files? I am under the impression that only PRman saved DTEX, and 3rd parties can only load the data via the public loader library, but I could be wrong.

So in the current version, you would theoretically
*Render Particle System B to RGBA sequence with Z-Depth pass enabled.
*Convert the Z-Depth to DTEX (not sure if there is such a tool - Nuke can convert a Z-Depth to Deep data, but I am not sure if it can save DTEX files)
*Load the DTEX sequence in Krakatoa
*Render Particle System A to RGBA - the particles behind the Z-Depth of the Particle System B will be matted out.

Once we have support for Initial Depth Map loading in KMY, you would skip the second step above.

We had some internal discussion about what we could do to automate this in the future, and we have some ideas, but we will have to test them out first…

Sorry for the long reply, I can try these methods to get them to work. The only thing stopping me from rendering them together is one system is rendered with Force Additive and the other is emissive. It’s not adding to white but it also does not need lighting information. Normally I would follow the procedure to just add the Forced Additive system on top in comp, but the emissive system is supposed to cover part of it, but also be covered by another area of the additive system.

I’ve been resorting to using magma to change the additive system to black, turning off additive mode, and using the RGB information as a sort of imperfect alpha.

Hope to see the update for KMY Z-Depth maps soon! :slight_smile:

Now that you explain the setup, you might be able to do it in one go!

All the Force Additive does is:
*Copy the Color channel into Emission
*Set Color and Absorption to black 0,0,0

So if you add a Magma to the “force additive” system and make sure the Emission channel is set to the Color channel, the Color channel is set to Black, the Absorption channel is also set to Black and the Use Emission and Use Absorption options are both checked in the render setup, your system should render exactly the same as with Force Additive on! At the SAME time, you should be able to set up the Color, Emission and Absorption channels of your other system to produce solid volumetric rendering (with or without lighting) that integrate with the rest. Basically the Color should be non-black, and the Absorption can be left untouched.

See if this helps!

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