From: gardner@netcom.com (Gardner Grout)
Newsgroups: sci.image.processing
Subject: Re: Alpha channels
Date: 27 Jul 92 15:37:40 GMT
Organization: Netcom - Online Communication Services (408 241-9760 guest)

In article <3199@blue.cis.pitt.edu.UUCP> cjk@pitt.edu (Carl J Kuzmich) writes:
>Hi all and good morning.
>
>I would like to know if someone could take the time to explain "in
>simple terms" just exactly what is an alpha channel, how they are used
>and why they are used?   It seems to me that it is an additional channel
>that can be incorporated into an image.
>
>I would also like to know why I would want to use or not use an alpha
>channel?  I had an individual that wanted to do an overlay of his face
>onto an RGB image.  I was playing with PhotoShop and could not succeed
>in doing this for him.  It would seem that there should be a way to
>"transparent" the channel that I want to overlay, so that the two images
>sort of blend into one image.
>
>Am I off-base here or am I on the right track?  Any and all help would
>be appreciated......
>
>Thanks in advance.  If I am mailed I will summarize and post.
>
>-- 
>Carl Joel Kuzmich  (412) 624-1885     | Love is a better teacher than duty |
>University of Pittsburgh              |                - Albert Einstein   |
>110 Old Engineering Hall 
>Pittsburgh, PA   15260	


I also had a lot of trouble understanding alpha channels, and now use them 
reegularly thanks to David Biedny. He wrote "TheOfficial PhotoShop Handook". 
I recommend this to anyone using PhotoShop regularly.

Basically, an alpha channel is a mask. If you are in your RGB channel in PS, 
and you make a selection, say a cirlcular one, you will have the opportunity to 
save this as an alpha channel (Selection menu/save as new channel). If you do 
this,and then move to that channel you will see a white circle with black all 
around.

Think of the black area as solid and the white as a hole. When you go back into 
the RGB channel and choose "Load selection", whatever is white on your alpha 
will appear selected on your RGB. So now if you use the airbrush tool and drad 
it across the image, only the area selected will be affected, just like a 
cardborad stencil and a can of paint.

If you bo back to your alpha channel and fill the white area with a black to 
white radial blend, then return to RGB, choose "Load Selection" and paint, 
manipulate, or change the image, the change will effect everything that is 
white 100%, and as the grey fades to black, the effect will be diminished until 
it cannot be noticed.

Once you understand alphas a new world of freedom opens up, you can do amazing 
things.


As far as composing your friends face onto an image, I would open both images, 
select you friends face in one, save the selection as an alpha, then go into 
that alpha and blur or feather the black edge, so it is not as stark. You can 
do a few things now, the simplest would be to go into RGB, and "Load 
selection", then copy the image, go to your background file and paste it. If it 
does not blend well, undo the paste, go back to your faces alpha and adjust and 
manipulate it and try again.

The other option is to Duplicate the alpa to the background file (see below). 
This way you can select the whole face file, and paste it into, behind, etc. on 
the background file. Both have advantages. 

To copy alphas to other RGB files: go to  the terrifying "Calculate" commands 
and choose 'Duplicate'. Set it to create a new channel on the file that 
contains your background from the face file's channel #4 (or whatever  the 
alpha is).

REMEMBER: Save your file as a PhotoShop document if you have multiple alpha 
files or you will LOSE them. (I THINK PICT will  keep 1 channel, but don't 
quote me).

Sorry if this is long winded, but I hope it helps, feel free to email if you 
have more questions. Anyone who disagrees with my solution, please suggest 
others, there are countless more.

=Gardner=
gardner@netcom.com