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RichH
02-24-2006, 12:04 PM
Hi all - I'm new to this so forgive if i don't get this thread thing correct!

I've just started to learn about compositing after using maya for a few years i thought it would be good to learn.

I am a bit confused about the multiply / pre-multiply aspects in after-effects. As i understand it some cgi can be premultiplied which can give you a fringe when you do any sort of color correction.

I really want to know how after-effects handles the whole alpha thing as regards any color changes you make.

Any tips or help would be greatly appreciated.

Many thanks in advance

p.s if anyone wants to correct me with regards to the etiquette of posting please feel free!
Rich

Mylenium
02-24-2006, 03:20 PM
Hi all - I'm new to this so forgive if i don't get this thread thing correct!

I've just started to learn about compositing after using maya for a few years i thought it would be good to learn.

I am a bit confused about the multiply / pre-multiply aspects in after-effects. As i understand it some cgi can be premultiplied which can give you a fringe when you do any sort of color correction.

I really want to know how after-effects handles the whole alpha thing as regards any color changes you make.

Any tips or help would be greatly appreciated.

Many thanks in advance

p.s if anyone wants to correct me with regards to the etiquette of posting please feel free!
Rich

There's not much to learn. Internally AE uses unmultiplied Alphas all the time (it makes calculations simpler and faster) so once you tell it to interpret your footage one or the other way, that should not be a problem at all. The only rule you have to obey when working with premultiplied images is to have them against a totally black or white background. All other colors will give you problems since from a mathematical standpoint it's impossible to remove e.g. a lilac backround fringe without affecting the foreground colors. Of course you have to know how your 3D program spits out its Alpha channels, but in case of Maya that should not be a problem since it is an explicit setting on the render panel and can be controlled as needed.

Another matter to consider is that AE will not blend Alpha channels linearly with the default settings, only Alpha Add blend mode will do this. This is especially important when working with semi-transparent stuff.

Mylenium

RichH
03-06-2006, 07:07 AM
Thanks Mylenium

jussing
03-07-2006, 06:29 AM
Color correction is exactly the problem...

A simple import of premultiplied footage will look just fine if you tell After Effects to interpret it as premultiplied, but the moment you start doing heavy color correction (like multiplying a color pass with ambient occlusion or hdri), you will get nasty alpha edges again.

So, if you want to composite several passes of the same element, my advice is, render all clips UN-premultiplied, ignore alpha, composite them as you like, then import the alpha seperately, and apply that.


As the Swedish "Master Zap" here at CGTalk can tell you, After Effects is not doing the alpha channel math entirely correct, which is why you have to take these special measures. ;)

Cheers,
- Jonas

Vympel
03-07-2006, 02:41 PM
In AE is possible to create a mult/divide effect? More pratical

beenyweenies
03-07-2006, 03:29 PM
Hi all - I'm new to this so forgive if i don't get this thread thing correct!

I've just started to learn about compositing after using maya for a few years i thought it would be good to learn.

I am a bit confused about the multiply / pre-multiply aspects in after-effects. As i understand it some cgi can be premultiplied which can give you a fringe when you do any sort of color correction.

I really want to know how after-effects handles the whole alpha thing as regards any color changes you make.

Any tips or help would be greatly appreciated.

Many thanks in advance

p.s if anyone wants to correct me with regards to the etiquette of posting please feel free!
Rich

The whole idea is that AE can calculate your matte using the background color of your render (usually black, and referred to as premultiplied) or using nothing at all (no color included in the matte, or a "straight" matte). I exclusively use straight mattes when I can, because of the problems mentioned above - and ultimately, why would you want to include black in your matte if you can help it? It only leads to color problems and fringing down the road.

jussing
03-07-2006, 04:19 PM
I do exactly as you, beenie, but Master Zap has some good points that speaks for premultiplied, such as being able to specify additive colors in the image. (except After Effects doesn't support this)

Cheers,
- Jonas

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03-07-2006, 04:19 PM
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