Compositing Clouds - What not to do.
The scene pictured is from a fully 3d shot at HD res I created using Maya. Yep, it’s an aircraft p.o.v flying through clouds. The clouds are multiple layers of maya fluid effects all composited in SHAKE.
One long term problem I have had using iadd or screen nodes with such scenes in SHAKE is that the cloud or dust elements end up lightening the background plate. This problem was driving me crazy and I even asked a few SHAKE people for their solutions all to no avail. Sometimes it’s the simple things I find, that are… well… they just aren’t! Not until you find the real solution that is.
To cope with this luminance lift on the b.g plate I came up with an elaborate counter measure by multiplying an inverted version of the clouds. Though misguided it kind of worked as you can see from the image.
Recently I have been corresponding with the mighty Frank Rueter (see his posts at the Nuke section of fxshare). I asked him for his advice on the matter and he pointed me to the solution …luma keying.
Frank says, “Just keep in mind that if you’re comping stuff that is light (i.e. reflections, lens flares, glints, volume light, etc) use additive methods as light behaves in an additive manner. if you need control over volume (smoke, dust, water, …) you will want to use alpha comping to control that.”
Armed with this new knowledge I set up some tests and was immediately thrilled with the results. I have found I can use two luma keys of the same cloud element with different chews on the key so you can have fleecy transparent edges transitioning to a voluminous opaque center.
Hurray!