Black Outline in Alpha Channel?

[djmiroku]

Hi, does anyone know why Maya renders have a black outline in the alpha channel? Can I get rid of it? Note: this is happening when I just render a still image with a black (transparent), no motion blur or anything.

-Pete

[gmask]

It would help to beable to see the image you refer to. My first response is that maybe you're looking at this image in a compositing program as if it had straight alpha rather than premultiplied. What program are you using to view this image?

[djmiroku]

I'm looking at it in Photoshop. The black outline is most apparent when you select the rendered objects (via CTRL+CLick) on the alpha channel and then make a new layer which is put onto a white background. It looks to me as though the outline is actually the anti-aliasing from the black background of the original rendering, but I would think that should'nt be visible at all.
If you duplicate the new layer 5 or 6 times in Photoshop you'll see the black outline building up with each new layer.

I hope I make sense I'm gonna post an example of this on my website soon.

[ivorg]

hi,

The "problem" is indeed with how you use your
alpha channel in Photoshop, not with the Maya
render. As mentioned in the above post, you
need the alpha pre-multiplied, otherwise you
will see a "rim" of colour.

Cheers,

IvorG

[djmiroku]

OK. Do you have any advice as to how to properly use my alpha channel so that I dont get the rim of black? I've never seen any pre-multiply options in Photoshop...

Anyways, here is what this rim looks like in an image that I made in Photoshop.

http://www.peterellis.com/NURBS/bla05 copy.jpg