Correct way to render and comp env fog in passes?

[SonyBoy]

I need some clarification about the correct way to render a scene with fog in passes for comping and post work. I have a helicopter emitting light beams into environment fog. I want to be able to render the fog completely separately for total control in post, however the obvious way to do it -- by adding a lighting pass (or in this case light selects which are then combined) as well as an atmospheric effects pass, doesn't give the expected results. The atmos pass works great, but as you can see in the attached image the lighting passes also incorporate the illuminated fog in areas that overlap geometry on the helicopter. Obviously I don't want this as then adding the atmos back in will double up the fog on those overlapping areas.

This seems unintuitive behavior in the lighting passes. Am I going about this the wrong way, or otherwise what's the correct procedure for this? I can fudge this in comp by using a copy of the atmos pass as a difference layer, but I figure there's got to be a more elegant way to get the fog and the lighting completely separated.

[hermit.crab]

I'm not getting where that atmosphere info is coming from if that light is not included in the LightSelect passes. Could you share a simplified scene? Otherwise, LPEs might help, but I need to check the scene first.

[SonyBoy]

Just realized my post may have been misleading/confusing as the center image of the light selects shows the fog clipped to just where it overlaps the geo. This is not how it's rendering -- I am getting the same fog as shown in the RGB and atmos passes, I just had an overall alpha on the heli clipping it when I output that.

But the problem remains and is basically just that I'm getting fog rendered into my light selects when I don't want it, and that would seem to be the point of the atmospheric effects AOV anyway. I am not familiar with the term LPEs. I can't share a file here since the 2Mb file size limit prevents an attachment (isn't it time this was increased -- you guys frequently ask for scene files and they are rarely that small?)

[hermit.crab]

I see - you basically wish for a Full Light Select that excludes the atmosphere. You can do that with LPEs. Do the following: Switch your existing VRayLightSelects' type to Light Path Expression. In the text field type:

C.[^V]*L

Regarding scene sharing - using the contact form is always an option for larger scenes.

[SonyBoy]

Thanks, haven't come across LPEs before. They seem bewilderingly complicated. And, unfortunately the code you provided just results in an empty light select (or perhaps not empty but the light in question cast so little direct illumination on anything else in the scene it's hard to tell). But the light source itself is not visible. Trying out some of the other presets and attempting to wrap my head around the syntax to see what other code might get me that.

[hermit.crab]

You can include the self-illumination of the light like so:

C([^V]*L)|([OL])

You can read more about LPEs here.

[SonyBoy]

Thanks, that's what I needed.

[Twist]

You can include the self-illumination of the light like so:

C([^V]*L)|([OL])

You can read more about LPEs here.

Could you please make this simpler than having to type code in a future update please. Thank you

[Buck]

I generally render any volume effects as a separate pass. Usually because I don't always want all of my lights to be calculated when doing volume.