TFD Holdout Objects and propper alpha channel for fire Tweet

[rustyippolito]

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.

Here is the fire appearing to multiply by the log geometry

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

Please help:)

[haze]

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.

[rustyippolito]

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

[haze]

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

[lstudios3d]

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.

[fantome]

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