02-02-2012, 10:58 AM
Ooh! There really are a ton of ways to do this, but it depends slightly on your render setup. I'm assuming you're using Max, but these should all be relevant to any software really.
Remember one thing, though: You need mattes to "capture" shadows because shadows are an absence of light, and it's impossible to capture an absence of something normally. A matte works almost opposite to reality, where the non-black bit is actually the area with the least light. This isn't the case for light! You don't need any fancy material or shader, really, as the light is what's primarily rendered! As such, the most tried and tested way to do this is simply to do a render pass with all other lights turned off. If you specifically want this without the neon itself visible (as a source) then I'm sure you can make it invisible - if it's a VRayLight there's an "invisibility" parameter, and if it's a vraylight material you can make the geometry that it's assigned to not visible to camera in the object properties.
Another way, that might save you time, is to have a VRayLight pass, though this will only work if it's a VRayLight (if it's neon, then probably set to a mesh that you select. This way you get the advanced features of the VRaylights, as well as the cool shapes!). Here, in the render elements list, you can choose specific lights for it to spit out a pass for. If you just select that specific light, then you'll get a pass such as that above, but without having to render twice! Magic!
thanks very much; is that the "light select" render element you're referring to at the end? or another element?
one problem I've read about with light select is it cannot capture GI, only direct light... but maybe that's all I really need in the first place...
02-02-2012, 03:21 PM
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