Fire Ignored By Alpha?

[Teriander]

Im trying to render a scene for compositing. Everything is working fine except the fire is not displaying in the Alpha channel. Smoke works, but fire is ignored. I've tried adjusting almost every setting in the "Colors and Transparent" options but nothing seems to help, any suggestions?

[Ivaylo Katev]

this happens when the checkbox "ignore alpha" is checked, see in the emissive panel.

[Teriander]

The checkbox was already unchecked, in fact most of my emissive panel options are default except for light count and multiplier, but that didn't solve it either. Any other possible causes of this?

[Teriander]

Here are some images to help explain my problem. See what I mean about the fire missing from alpha? And the "ignore alpha" is unchecked. This was rendered with Vray using vray frame buffer, brute force and light cache. Transparency/Opacity settings are default...

Rendered Image
Alpha Render
Colors and trans settings

[Ivaylo Katev]

actually there is no separate alpha for smoke and fire, but i thing the compositing should be possible. the currently used shading model is something like this: very small solid pieces in the air, having its own color (this is the diffuse color), and emitting certain light (this is the emissive color). the "surface" of these particles is the alpha. the same surface produces the emission and the reflection, so there is no separate alpha. this model is physically correct and rules the common fires in the nature, but many people find it hard to control due the shared alpha. because of this the checkbox "ignore alpha" is introduced, it changes the shading model and the alpha is used only for the diffuse color. with this option turned on the emissive component does not have an alpha. if the compositing software requires alpha for blending, you can manage this using some extremely low value (let say 0.0001) and multiplying the emissive render element by the inverse of the used value, in case of alpha 0.0001 this is 10000.

[vlado]

To add to this, since the fire is mostly just emissive (it only adds light), it is normal that it has little or no alpha, in difference from smoke which blocks light.

Best regards,
Vlado

[Paul Oblomov]

For compositing need - why don't you use ur fire channel to create luma matte ?

[Teriander]

I can't find an option to view or seperate the "fire" channel, where can I Find this?

[Ivaylo Katev]

there is no separate "fire" channel (yet), but you can use the emissive one (if i'm not wrong the name is self illumination or something like that).