Re: Alphas, mattes, adds and overs

["Ponthieux, Joey" <j.ponthieux(at)nasa.gov>]

After looking at your scene what I see here makes perfect sense. A 
premultiplied image is optimized for an "over" like composite, in other 
words its expecting to be composited over something else, not joined or 
mixed. You are attempting to create a boolean of object A to object B, 
this is why ADD works and OVER doesn't. The object edge which has 
background color mixed into it will not join with the second object 
which also has background color mixed in.

Using OVER one object is being placed over the second which means that 
the other object is acting like a background, in other words it doesn't 
care what the alpha is for the second object. Since Over is optimizing 
the top layer with the background(second object) it is more or less dumb 
and not getting any premultiplication treatment that you would expect 
from the second object, so the black and transparency in the edge is 
finding its way through the top layer from the background(second object) 
layer.

Using ADD, you are technically mixing the two where there is color or 
value. Color and alpha from one is being added to Color and alpha from 
the other. Where there is no color nothing is added. (I assume you are 
using Math Composite/Math op/Add in FXTree).  Add really isn't a 
composite tool like Over as much as it is a "joining" or boolean tool. 
Adding the RGB as well as the alpha is probably not a problem as long as 
you are adding the two objects together over black and output that 
result for later use, any other circumstance will more than likely yield 
unwanted results. This result will then need to be comped OVER the rest 
of your visuals. This is easy to do with Shake, FXTree etc, since you 
can isolate a node structure separately, but probably a real pain in AE 
since it is layer based and is wanting to add everything from lower 
layers to the top layer(as you noted).  If there is a way to add the two 
at the bottom  of an AE  project then may you can OVER them later, I 
don't know,  I use Shake here and though I have AE I rarely use it.

In any case your original assumption is correct, Over will not yield the 
results you want and Add, while probably not the only method sure looks 
like the best one for this problem.

Joey Ponthieux
NCI Information Systems Inc.
NASA Langley Research Center
____________________________________________________________
Opinions stated here-in are strictly those of the author and
do not represent the opinions of NASA or any other party.



adrian wrote:
> we're rendering premultiplied, and the result is fine, as alan says, 
> if we composit using node based ADD, then  your background gets dropped
> in later down the tree
>
> however, lots of people still comp in afterfx (cos it's better damnit, 
> don't argue!)
> the issue then is when you ADD on layer over another, any layers 
> UNDERNEATH are also added to, and you get gohstly glowing 3d over
> your backgrounds..... not cool!
>
> i have put a scn file here, if anyone wants repro;
>
> http://www.fluid-pictures.com/public/matte_conundrum.scn
>
> a
>
> Adrian Wyer
> Fluid Pictures
> 33 Glasshouse Street
> London
> W1B 5DG
> T: +44 (0) 207 434 5133
> www.fluid-pictures.com
> ----- Original Message ----- From: "takita" <takita(at)earthlink.net>
> To: <XSI(at)Softimage.COM>
> Sent: Tuesday, June 26, 2007 4:49 PM
> Subject: Re: Alphas, mattes, adds and overs
>
>
> > Are you rendering unmultiplied (e.g. what AE folks call "straight") 
> > out of CG?
> >
> > What might work (dodgy sometimes) is to render premultiplied and run 
> > it through the unmult op in FXtree or AE's unumult plugin (lame, I 
> > know...).
> >
> > But sometimes this helps make the edge consistent at least.
> >
> > -T
> >
> > --
> >
> >
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