[Gimp-user] alpha channels vs masks

[Elle Stone <l elle stone gmail com>]

Hello Gimp Users,

Is there anything that can be done using an alpha channel, that can't
be done using masks and layers, or vice versa?

Context of question: I've worked extensively with masks and layers.
I've never worked with alpha channels.

For example:

Using an alpha channel:

1. Open two single layer images, A and B, neither one of which has an
alpha channel.
2. Drag the image B over to A and close B.
3. Now image A has two layers, A and B. Put layer A on top, select
layer A, set the blend mode to Normal,  and add an alpha channel.
4. Erase part of layer A, where-ever you want layer B to show through.
At this point, layer A has an alpha channel with white where you want
layer A to show, and black where you want layer B to show, but it does
not have a mask.

OR

Using a layer mask:

1. Open two single layer images, A and B, neither one of which has an
alpha channel.
2. Drag image B over to A and close B.
3. Now image A has two layers, A and B. Put layer A on top, select
layer A, and set the blend mode to Normal (don't add an alpha
channel).
4. Create a solid white mask for layer A. Paint black on the mask
where you want layer B to show through.
At this point, layer A has a mask with white where you want layer A to
show, and black where you want layer B to show., but it does not have
an alpha channel.

In both cases, using a mask or using an alpha channel, the net result
is the same.

So again, is there anything that can be done using an alpha channel,
that can't be done using masks and layers, or vice versa?

A puzzled and hoping to be enlightened,
Elle Stone

-- 
http://ninedegreesbelow.com
Articles and tutorials on open source digital imaging and photography