Premultiplied Alpha

[glorybound]

Premultiplied Alpha or not? I need to comp skies into some scenes, however, I always get severe halos, so I avoid doing it. What's best, Premultiplied Alpha or not?

[Macker]

Have you tried the "remove white/black matte" in photoshop? Or Defringe? All found under "Layer/Matting".

[glorybound]

Yes, I tried everything, but the halo is awful. If I have a really light sky, it somewhat works.

Have you tried the "remove white/black matte" in photoshop? Or Defringe? All found under "Layer/Matting".

[Macker]

I'm not entirely sure you'll get an enormous difference between straight and premultiplied alpha's?

You could try selecting the sky (using the alpha) and simply expanding the selection by a pixel or two, and creating a new alpha from that selection, thus getting rid of the halo?

[Joelaff]

If you use premultiplied be sure your background color is black. You may need to "Remove Black Matte" depending on what format you save as. This is applied to a layer with layer transparency (not a layer mask, but actual layer transparency).

Note that any format that is not floating point will clip when compositing with certain transparent objects. The only proper way to avoid this is to comp in floating point space.

[Garryclarke]

If you want a clean comp then render against a black background. Save as .tga 32bits per pixel (this gives you your alpha) and set the 'Pre Multiplied' tick box to off.

When you open it in PS you'll notice that the edges are very jagged, just cut the image out using the alpha and it'll be perfect with no halos, even with semi transparent objects.

[glorybound]

If you want a clean comp then render against a black background. Save as .tga 32bits per pixel (this gives you your alpha) and set the 'Pre Multiplied' tick box to off. When you open it in PS you'll notice that the edges are very jagged, just cut the image out using the alpha and it'll be perfect with no halos, even with semi transparent objects.

I'll try it, thanks.

[Joelaff]

If you want a clean comp then render against a black background. Save as .tga 32bits per pixel (this gives you your alpha) and set the 'Pre Multiplied' tick box to off.

When you open it in PS you'll notice that the edges are very jagged, just cut the image out using the alpha and it'll be perfect with no halos, even with semi transparent objects.

It will not have halos, but it will indeed suffer from some clipping depending on the content. When you comp in integer space you get clipping because the unmultiply operation (which is done internally when compositing, or when you save a straight alpha image) results in values > 1.0 if the RGB values exceed the A value. This is harder to do in Vray due to energy conservation helping out, but it is still possible.

[Garryclarke]

Yes, Joelaff's right.

I should have mentioned that the saved render will be 8 bits per channel. Not sure what setting you'll need for higher quality file formats.

[Art48]

I render 32bit EXRs on black background and never had problems.