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View Full Version : Alpha color space, etc… to get clean edges ?



AbstractTech3D
08-11-2011, 06:24 PM
Hi There

I'm trying to render out a collection of text animations with alphas, as linear color space open EXR - pass them through After Effects to lower the gamma, then into Avid for final composite in the edit. I'm using this work flow to get a 10 bit image into the Avid (which can't import open exr's - crying shame!)

However, am having problems with the edge matting of the LW rendered text.

So am trying to work my way through best choices as far as whether to premultiply or un-premultiply the alpha coming out of LW, the color space for the alpha (maybe linear is not the best choice?), then the choices in After Effects in how to interpret...

This seems more complicated than it should be! (Probably due to my ignorance).

Any help greatly appreciated!

Thanks!

toby
08-11-2011, 07:26 PM
You have to unpremultiply the image before you do any color correction, then premult it back again, otherwise you will color correct the edge that's supposed to fade to black; and that gives you the matte lines.

I don't think AE has unpremult, so I guess you'd bring it in unpremulted, and with a separate alpha, use the track matte option?

Or can't you just gamma it down in Avid (I know nothing about avid)?

If I understand you correctly -

AbstractTech3D
08-12-2011, 02:24 AM
Thanks for the advice.

The issue with the Avid relates to the fact that it can't import OpenEXR's which if I'm not mistaken, seem to be the most data efficient way of getting more than 8 bit images out of LW (Floating point tiffs can be comparatively very large). In After Effects I'm exporting to PNG (which is reasonably data efficient), with options for Trillions of Colors+. PNG export in LW doesn't have such settings (that I know of).

toby
08-12-2011, 03:20 AM
Right on all counts, exr is best for float, and lw doesn't do 16bit png -

Did you try to do the unpremult - gamma - premult steps in ae? You could also try a post-process gamma in lw. Or could outputting 10bit cineon do the trick?

AbstractTech3D
08-12-2011, 05:12 PM
Yes, unpremult-tamma-premult in ae seems to work well. Thanks. I prefer to keep the gamma reduction in post (after lightwave) for more colour correction flexibility, closer to the final edit. Haven't yet tried 10 bit cineon. That's a new one to me...

AbstractTech3D
08-13-2011, 01:51 AM
…It turns out that Cineon does not feature an alpha. Could still export a separate alpha of course… but then it would effectively be premultiplied, wouldn't it?

toby
08-13-2011, 02:47 PM
Whoops - I don't know much about cineon except that it's 10bit and commonly used for TV, so I thought it might suit you.

dwburman
08-13-2011, 03:07 PM
You can can use unpremultiplied alphas in AE, you just need to tell it to interpret the alpha as 'Straight' when you import the file (or "interpret file" in the project panel).

toby
08-13-2011, 03:28 PM
…It turns out that Cineon does not feature an alpha. Could still export a separate alpha of course… but then it would effectively be premultiplied, wouldn't it?

Whoops - I don't know much about cineon except that it's 10bit and commonly used for TV, so I thought it might suit you.

Effectively premulted? I don't think so - it should be the same as other formats.

Lightwolf
08-13-2011, 04:43 PM
Whoops - I don't know much about cineon except that it's 10bit and commonly used for TV, so I thought it might suit you.

Which is odd, considering that the roots are film.
The successor file format (DPX) is a lot more flexible and afaik even supports alpha channels.

Cheers,
Mike

AbstractTech3D
08-14-2011, 01:48 PM
Thanks guys.

DPX could be a solution, it sounds like. Although it would seem the import process into Avid MC (version 5) is a bit of a process, needing to go through the MetaFuze tool first:
http://community.avid.com/forums/p/85322/482383.aspx

(For anybody else's future reference)

Cheers