View Full Version : Rendering fluid flame - white background

01 January 2011, 07:08 AM
Hi guys. Has anyone successfully rendered the presets 'flame' or 'turbulent flame' with a white bg?

Obviously I would render with no background (or background shader on objects in the background) and comp it with a white layer underneath. However, when composited, the flames still come out opaque, and with terrible edge sharpness.

Any tips to make it look as good as when it's rendered over black, yet still keep some transparency like a true flame?


01 January 2011, 09:56 PM
To get fire to look correct on bright backgrounds you are going to need to help it a bit in comp. Try using screen or plus methods to merge in onto your background and you may need to create your own alpha channel using the flames luminance values.

01 January 2011, 10:44 PM
all those suggestions and it is also a good idea to tweak your fire shader in maya with the BG plate visible.

01 January 2011, 06:27 AM
Screen and add just make it even more opaque.

Afaik there is no fire shader - just the shading settings in the fluidshape node.

If I render it in Maya with a white background, nothing shows up, just white.

01 January 2011, 06:33 AM
Would it be possible to render it out with a black background, but a high matte opacity, then render it out again with a regular matte opacity, and use the alpha from the second one as the alpha for the first?

01 January 2011, 09:55 PM
You shouldn't be rendering in maya with a white background. You should be rending the fire as it's own layer and then comp it using AE/nuke/fusion. You can render to versions if you want. But it's usually adjust alpha and apply modes in comp.

01 January 2011, 10:39 PM
I don't actually attempt to render it with a white background, was just fiddling with techniques suggested above. What sort of tweaks should I do to my alpha channel? And is the standard Matte Opacity setting of "Opacity Gain" at 1 the best setting to use?

01 January 2011, 01:39 AM
In my experience a matte opacity of 1 is fine. There are probably many ways to comp fire but here's two of them that have worked for me:

1) Merge the fire twice. Merge the fire once using an over method. Then merge the fire on top of itself again using a screen or plus method. Adjust the mix of you screen merge and your glows to get the look you want.

2) Merge using a screen mode but get your alpha from the lumance of the fire. IE Hot spots will be opaque and cool spots transparent.

You can also combine these techniques.

PS I composite in nuke so I'm using nuke terminology. So if you're using something else, you're going to have translate these ideas into that program.

01 January 2011, 02:52 AM
What do you mean by 'merge'? Merge with what?

Do you mean combine the 2 layers into one layer, with the top layer of the two layers using an over(lay) mode, and the bottom one using normal mode?

Then take that combined layer and combine it on itself with the top layer being screen?

01 January 2011, 08:24 PM
By merge I mean combining your fire image sequence with the rest of your comp. In nuke that's done using a merge node.

CGTalk Moderation
01 January 2011, 08:24 PM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.