Generally speaking, you can't directly modify the values of a layer's alpha 
channel:  Paint tools (paintbrush, etc.) only ever increase a layer's alpha 
channel (by the tool's opacity factor), Eraser tool only ever decreases it.

A layer mask gives you direct and more precise control over a layer's effective 
alpha by separating the RGB and alpha values into separate surfaces.  When you 
paint on the mask, GIMP will internally convert the current color (even a 
pattern or gradient!) into a 0-1.0 opacity value before applying it to the 
channel.

Because they are separate mechanisms, this also means you can combine their 
effects - there may be times when you actually need a layer mask to be discrete 
from the layer's actual alpha channel.  Also, you don't have to explicitly lock 
a layer's alpha channel when using a mask - separate surfaces mean you can only 
draw on one at a time.  (Which is also the downside - you can't paint a 
transparent area of the source layer and expect it to become automatically 
opaque like you'd get with a regular alpha channel.)

Finally ... well, it seems that you can't actually add a layer mask to a layer 
group as a whole.  I don't see why not though - that would give the ability to 
mask off a group of layers all at once.

-- Stratadrake
strata_ran...@hotmail.com
--------------------
Numbers may not lie, but neither do they tell the whole truth.


> Date: Thu, 27 Sep 2012 12:09:29 -0400
> From: l.elle.st...@gmail.com
> To: gimp-user-list@gnome.org
> Subject: [Gimp-user] alpha channels vs masks
> 
> 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
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