Environment fog without adding to alpha

[bobvelocity]

This may be a silly question but is there a way to stop the environment fog adding to the overall alpha?

[Svetlozar Draganov]

In general - it is not possible.
The only way to disable it from alpha - is to disable "affect background" from VrayEnvironmentFog options - unfortunately this only works if the Fog is applied to Environment Shader in Render Settings window.
I don't know how useful could be this - cause the Fog won't affect background.
The other options is to try to create alpha from some other Render Element - depending on your scene this could be possible - or to render your scene on two passes.

[bobvelocity]

Thanks for the reply. I faked it with zDepth in the end. I just always want to do things in render. Bit lazy really. Thanks though it was useful to know.

Cheers

Keith

[Morbid Angel]

What I did on a recent project (still doing actually) is render the fog and geometry in two layers one matted out by the other, this way you get the fog which you simply drop ontop of your geo and there you have it.

One think you need to do is in the geo render to make fog invisible to camera rays.