Karma Fire Has No Alpha Channel [SOLVED]

[GlennimusPrime]

I've included a quick build of a similar setup, as I'm unable to share the original file due to work reasons.

I've built this setup in a couple of different ways to try help explain the issue..

I started out with a redshift fire render, but unfortunately the flame volume does not have an alpha channel (which we need in this particular instance).
I can remedy this by simply adding the flame volume to the density field on the redshift volume material, which works without any issues.


So far so good, although I need to make use of a deep camera map for volume depth camera blur in post.. So time to make this work in Karma.

Karma's alpha channel is not solid in the areas where I have flames - Only where the smoke density is.

How can I achieve a more solid alpha channel in place of the flame areas?

I have tried deep diving inside the pyro material, but there's so much going on in there, I'm not sure where to start.

fire_problem.hiplc

Edited October 6, 2022 by GlennimusPrime

[davpe]

to me, it looks like Redshift and Karma are just using different pyro fields to render alpha? 

[ryew]

I'm unable to dig into the scene but I agreed with @davpe as it seems like you pointed yourself in the right direction with your post - have you tried piping the 'flame' field into what your shader in karma is using for the 'density' field (presumably density) and adjusting from there?

[GlennimusPrime]

On 8/25/2022 at 4:29 AM, davpe said:

to me, it looks like Redshift and Karma are just using different pyro fields to render alpha? 

Yes that's correct. The redshift render has the flame field piped into the density to render, whereas the karma version is only using density for the alpha.

 

On 8/25/2022 at 9:19 AM, ryew said:

I'm unable to dig into the scene but I agreed with @davpe as it seems like you pointed yourself in the right direction with your post - have you tried piping the 'flame' field into what your shader in karma is using for the 'density' field (presumably density) and adjusting from there?

That's a good idea - I can try to mimic what I've done in Redshift, although there's so much going on inside the pyro shader, I'm not sure where to start.
I would also hope to optimise the number of renders I'll need to output. It would be great to be able to output just the one render, but have the alpha of the flame included the same as it currently displays with density.

I do understand that that the normal fire compositing workflow is to simply add the fire on top, although I would prefer to have a less see-though flame, hence why I'm on a mission to create a more solid alpha for the flame.

[dleonhardt]

You can connect a Bind Export VOP anywhere in the shader and use whatever goes into it as extra AOV.

dl_fire_problem.hipnc

[GlennimusPrime]

@dleonhardt I didn't quite get the desired result with this method, but I did end up solving the issue by altering some parameters within the Pyro bake Volume SOP itself (it actually gave me a little more control doing it this way too).

It turned out to be a bit more simple than I initially thought!

I wanted to use the temperature field to control and render the flames, so I used the Bindings tab to re-route the temperature into the Smoke Volume, rather than using the default density attribute.


Then I was able to use the Density Scale control to set the desired amount of alpha, and then colour the flames using the Smoke Colour Ramp control.
The Density Scale control was super helpful, as it allowed me to dial in the desired amount of alpha on the flames.


I did however need to render separate flame and smoke layers this way, but it did solve the issue of having no alpha channel on the fire.