Emission element alpha?

[JoshJ]

I have a pyro element with smoke and fire, rendering with the billowy smoke shader.

I'm looking to render a matte for the fire emission while still maintaining the look of the fire.

I tried using the luminance of the emission to pull a matte, but it isn't very clean, as the smoke is very dark.

Also, part of the look of the fire is defined by the darkness and density of the smoke, so if I completely don't render the smoke, it's really blown out and won't work for me.

Something like applying a shadow matte on the density might work, but I can't quite figure out how I'd do that in the shader to output the pass correctly.

Fumefx has a fire render element that does a pretty good job with providing a good alpha for the fire in this kind of situation. Can anyone share any approaches on how you'd do this in houdini?

Thanks.

[LaidlawFX]

I have a pyro element with smoke and fire, rendering with the billowy smoke shader.

I'm looking to render a matte for the fire emission while still maintaining the look of the fire.

I tried using the luminance of the emission to pull a matte, but it isn't very clean, as the smoke is very dark.

Also, part of the look of the fire is defined by the darkness and density of the smoke, so if I completely don't render the smoke, it's really blown out and won't work for me.

Something like applying a shadow matte on the density might work, but I can't quite figure out how I'd do that in the shader to output the pass correctly.

Fumefx has a fire render element that does a pretty good job with providing a good alpha for the fire in this kind of situation. Can anyone share any approaches on how you'd do this in houdini?

Thanks.

Correct me if I'm wrong, but along with your beauty pass you want a matte of the fire? Can you put up a simple test scene with the fire in it, and an image, it would be easier to tell you exactly what you want.

From the hip, I would say try pulling Ce as an extra image layer, but don't know it the is what you're defining as smoke and fire. The ifDensityNotZero is the part that calculates the smoke from within the shader, so you may just want the components from the purple temperature parameter node that should be easy to pull from the multiply2 node with Ce, but if the fire part you want is some combo of the multiply3 you might need to disconnect to find what parts create what you want.

[JoshJ]

Hi, here is a prototype file demonstrating the question. Have a look at the cop network, there are some notes describing the situation. you can remove the txt extension and unzip the attached file for the images referenced in the cop network. Thanks!

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Here is a reference image of fire in front of a blue sky:

http://www.gettyimages.com/detail/ST001146/Photodisc

In this reference image, the fire doesn't carry any dark edges where it meets the blue sky, yet it also won't work to use "screen" blending mode either, as the fire has its own opacity.

Of course, in my file, there is thick dark smoke blocking the fire. I needed to do this to create the shape and look of the fire. However, I'd like to be able to composite the fire over another image without including the darkness of the smoke.

So, I'm interested to know if there are any better ways to render out the fire part without resorting to compositing techniques like using curves to adjust the matte.

firealpha_test.hipnc

images.zip.txt

Edited September 11, 2011 by joshjordan

[LaidlawFX]

Of course, in my file, there is thick dark smoke blocking the fire. I needed to do this to create the shape and look of the fire. However, I'd like to be able to composite the fire over another image without including the darkness of the smoke.

Ok so you definitely don't need the black smoke that is blocking the fire, you need to incorporate that as part of the color range. The default billowy smoke has a black edge to where it goes towards the zero side of the 0 to 1 ramp. You remove this and your problem goes away. I would certainly remove this inorder to make the ref image, and shape the fire better to compensate for that black edge, probably just clamp/fit range the volume area that makes it black with a vop sop.

So, I'm interested to know if there are any better ways to render out the fire part without resorting to compositing techniques like using curves to adjust the matte.

Using compositing with curves to adjust the matte ain't a bad solution, i think it will come down to that at the end if you keep the black in there with the temperature ramp.

To make it easier for the compositor you can just make a duplicate copy of the shader, and set the temperature setting 1 across the whole ramp and output the default emission like that. If you keep the black in the temperature ramp on the duplicate node you can make a constant change from where it goes from black to white. Kind of the same game you'll play in comp, a little cleaner for them.

As a heads up you're example has the billowy smoke rewired from it's default.

[JoshJ]

Oh yeah, I didn't realize that the black color of the ramp was contributing to the emission color. I think I know where to go from here, thanks for your help!