Volumetric lighting and alpha channel

[RoyBU]

I have some live-action footage I am using in an animation. The footage used a green screen and in After Effects I eliminated the green to leave only the actors and an alpha channel. I imported the footage into AM as a TGA sequence. So far, so good.

 

I want the footage to be lit by a klieg light that has volumetric turned on. The problem is that the volumetric effect is lighting up the transparent part of the footage somewhat brighter than background objects, so the actors appear to be in a rectangle that is unnaturally brighter than the surrounding scene. I don't understand what is causing the klieg light to (apparently) reflect off the transparent part of the footage, so I can't figure out how to stop it. I tried turning off shadows and reflections for my footage, but that made no difference. I welcome your suggestions.

 

Thanks,

Roy

[KenH]

A picture paints a thousand words. So we'd be 10 times more clued in.....

[RoyBU]

OK, in getting a still ready to post I discovered that this problem does not appear in a final render, only in a shift-Q quick render. Below you can see the difference between the two (the one on the left is the quick render). I turned the intensity of the light way up to make the "problem" obvious. Too bad that quick render isn't a more accurate representation; 3-4 minutes is a long time to wait every time I want to see the result of a lighting tweak.

[attachmentid=17043] [attachmentid=17044]

[spikerthree]

I suggest you do the compositing completely in After Effects. Render out your AM scene in chunks. For example, render the background as a single layer, then render that left hand column as another. Sandwich your actors between them and Presto! for volumetrics, you could render your light seperatly and layer that on top of your actors.

 

Grayson

[RoyBU]

Yes, I've thought of doing that, and for a scene with a static camera I think that's the way to go. For this particular scene, however, the camera is going to be moving and the nice thing about staying in AM is I can do that and everything stays in proper perspective.

 

Thanks for the suggestion.

[Pengy]

what about using composite and the exr. then theres no need to wait for lighting tweaks.

[RoyBU]

Thanks for the tip. I looked up "composite" and "exr" in the help index and found the relevant page, but confess I don't understand it very well. Is there a tutorial somewhere that goes into more depth on this?