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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_ranger 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 stone 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 > _______________________________________________ > gimp-user-list mailing list > gimp-user-list gnome org > https://mail.gnome.org/mailman/listinfo/gimp-user-list |