TFD Holdout Objects and propper alpha channel for fire

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TFD Holdout Objects and propper alpha channel for fire

Postby rustyippolito » Fri Aug 04, 2017 12:03 am

rustyippolito Fri Aug 04, 2017 12:03 am
We are having problems getting a proper alpha channel out of Octane for our fire Sims. We have 3 logs and a campfire simulation with flickering flames and smoke. We can not figure out how to get the following:

-Logs to properly holdout the flames by being black in the alpha
-Fire to properly render... it appears to be multiplied by the logs, so it is super bright when in front of the log geometry, but falls of to barely there when on it's own.

campfire_rgb.jpg
Here is the fire appearing to multiply by the log geometry


campfire_alpha.jpg
Here is the alpha with the logs not holding out and the flames not being represented


Please help:)
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Re: TFD Holdout Objects and propper alpha channel for fire

Postby haze » Fri Aug 04, 2017 3:21 am

haze Fri Aug 04, 2017 3:21 am
Hi, a couple of things may help us figure it out -

What is the volume step length you are using? and do you have any absorption set?

Usually i would recommend adding a significant amount of absorption.
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Re: TFD Holdout Objects and propper alpha channel for fire

Postby rustyippolito » Fri Aug 04, 2017 9:27 am

rustyippolito Fri Aug 04, 2017 9:27 am
Yes, the absorption value increase definitely helped. I feel like i have to tweak the TFD simulation values because Octane is extremely sensitive to tweaks in the absorption settings. Will keep trying stuff:)

cheers
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Re: TFD Holdout Objects and propper alpha channel for fire

Postby haze » Sun Aug 06, 2017 9:02 pm

haze Sun Aug 06, 2017 9:02 pm
I am planning to make some improvements in the way we obtain particle density information for scaling the emissive power output from volumes. At the moment we use whatever the absorption grid is set to, as particle density, meaning that, as absorption increases/decreases, so does temperature. We are planning to split these into two grids, so that will give more flexibility when dealing with volumes. There is also a current issue with emissive volumes, where the step length influences the brightness. We will get to this as soon as we can
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Re: TFD Holdout Objects and propper alpha channel for fire

Postby lstudios3d » Thu Aug 10, 2017 4:07 pm

lstudios3d Thu Aug 10, 2017 4:07 pm
Sounds like you are having a similar issue to what I was trying to solve with getting your logs to be cut out of the alpha. From what I've been told octane can't do that at the moment and the way around that is using layers and a multi layer exr. I've been able to get something working with that but still a few issues I'm trying to figure out.
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Re: TFD Holdout Objects and propper alpha channel for fire

Postby fantome » Thu Jun 06, 2019 8:08 am

fantome Thu Jun 06, 2019 8:08 am
Hi guys ,

I would have the same question, is there any new official method to generate proper holdout ( matte ) object with volume object.
Cheers
E
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