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View Full Version : drive rgb as alpha ?


deadalvs
05-27-2008, 09:23 PM
hi ...

i have two series of images. each one has an alpha.

now. i'd like to get rid of both alphas and drive one of the color channels as the new alpha for the other image.

how do i do that ?

how can i premultiply or un-premultiply ? does that matter if there's no more alpha as i've written above ?

:banghead: :banghead:

i'm rather new to the application... so many hours spent !!

scrimski
05-27-2008, 09:56 PM
now. i'd like to get rid of both alphas and drive one of the color channels as the new alpha for the other image.
Check out the Color->Reorder node.

Typing rgba in the expression field will leave everything as is, rgbn will delete the alpha, rrrr will use the red channel for green, blue and alpha and so on.

I think you even use L for luminance, not sure about that though.

deadalvs
05-27-2008, 10:19 PM
yes, i have used the reorder node and this works okay.

the question is more like: which nodes have to be plugged how to get the alpha back into one of the alpha-less images.

scrimski
05-27-2008, 10:29 PM
Use a copy node.

Shankar
07-04-2008, 02:15 AM
just connect a reorder node to the input replace rgba to any one

rgbr
rgbg
rgbb

i think u got it

ok next one

You can use MDiv to unpremultipy for tweaking color correction and then use MMult [or] over node with the premultiply option turned on. No need to worry about alpha channel Still its preserved during this operation

-Sankar

RiKToR
07-11-2008, 07:30 PM
You use a reorder node to select the channel you want as the alpha and then you use a switch matte node to use that alpha with the alphaless color.

deadalvs
07-12-2008, 07:05 AM
thanks guys !

it was a little confusing, but now that i also understand the premultiplication everything falls in place !
:applause:

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07-12-2008, 07:05 AM
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