04 April 2010, 10:57 AM
Always render on a black background. Photoshop assumes your alpha is straight, i.e it will usually multiply it, resulting in a dark halo if you saved it as premultiplied. So when using photoshop, you should save your image from 3d-package with a straight alpha. I don't know how it's saved with PSD from 3d-package.
04 April 2010, 02:21 AM
A little bit of explanation about "alpha" and the notion of "premultiplication" might be very helpful here. (Pardon me if this is "but of course" to you...)
Everybody wants to avoid jaggies: those horrid jagged lines that naturally occur at the edges of things. The usual practice is called anti-aliasing, and it consists of defining a "soft edge" ... specifically, a semi-transparent edge ... to things (such as your leading lady).
In CG parlance, "transparency" means alpha. "Alpha = 1.0" means "solid." "Alpha = 0.5" is "a ghost." "Alpha = 0.0" is "completely transparent." This so-called alpha channel is stored alongside the color-information channels (R, G, and B).
Here's the thought-question: "What's the most convenient way to represent a solid-red pixel with 50% opacity?"
One way to do it is with (R, G, B, A) = (1.0, 0, 0, 0.5): the original RGB color-value, accompanied by an Alpha of 0.5. But another way to do it is to premultiply the RGB by the Alpha: (R, G, B, A) = (0.5, 0, 0, 0.5). (Under the right conditions, this "trick" can save a lot of messy and redundant math.) Notice that the "Alpha" figure in both cases did not change. In both cases, it's "0.5." Notice also that there's absolutely no way to tell by inspection whether premultiplication has happened or not! :buttrock:
Which is "better?" Well, that's really not the point. What does matter is... that you must always know whether "premultiplication" has been performed or not. If it has, but you act as though it hasn't, then the effects will accumulate ... to create an ugly dark shadow or muddy-spot like the one you see here.
Once you understand what is happening (and I dimly "HTH" ...), the problem is quite easy to deal with. You simply need to tell Photoshop (and all the other tools) what to do. They will all be (so to speak...) "familiar with the problem," but they have no way to know (so to speak...) how to interpret the data they have been given, by inspection alone.
There is one more "little caveat." Be sure that you have chosen an image-format that actually includes this fourth, "Alpha," channel! The .JPG file format for example does not. But other formats, such as .PNG or .EXR, do. Photoshop understands all of the image-file formats that are out there.
04 April 2010, 11:22 AM
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