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alpha channels vs masks

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alpha channels vs masks Elle Stone 27 Sep 16:09
  alpha channels vs masks Richard Gitschlag 28 Sep 12:07
   alpha channels vs masks Simon Budig 28 Sep 12:46
    alpha channels vs masks Richard Gitschlag 29 Sep 02:00
     alpha channels vs masks Simon Budig 29 Sep 06:43
  alpha channels vs masks Burnie West 28 Sep 19:07
   alpha channels vs masks Elle Stone 28 Sep 19:23
Elle Stone
2012-09-27 16:09:29 UTC (about 12 years ago)

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

Richard Gitschlag
2012-09-28 12:07:31 UTC (about 12 years ago)

alpha channels vs masks

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

--
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Simon Budig
2012-09-28 12:46:45 UTC (about 12 years ago)

alpha channels vs masks

Richard Gitschlag (strata_ranger@hotmail.com) wrote:

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.

...because mitch discovered a nasty bug with this and this has not been fixed yet...

Bye,
Simon

Burnie West
2012-09-28 19:07:08 UTC (about 12 years ago)

alpha channels vs masks

On 09/27/2012 09:09 AM, Elle Stone wrote:

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?

Normally, when I want a transparent background in a WEB image I will use the alpha channel. Never tried to do that with layer masks.

How do you export an irregular foreground image with only alpha background to png and retain the transparency in the bounding rectangle?

-- Burnie

Elle Stone
2012-09-28 19:23:49 UTC (about 12 years ago)

alpha channels vs masks

Hi Bernie,

You are right. Open an image, create an empty transparent layer and put the empty layer below the image layer. Create a mask on the image layer and paint black where you want a transparent background. The transparent layer automatically has an alpha channel and so does the exported png with the transparent background.

So if you want transparent pixels, I guess you need an alpha channel. However, if you are blending different layers together and you don't need any transparent or partially transparent pixels in the final image, then masks would seem to be easier to use because you can paint on them directly.

On 9/28/12, Burnie West wrote:

On 09/27/2012 09:09 AM, Elle Stone wrote:

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?

Normally, when I want a transparent background in a WEB image I will use the alpha channel. Never tried to do that with layer masks.

How do you export an irregular foreground image with only alpha background to png and retain the transparency in the bounding rectangle?

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Richard Gitschlag
2012-09-29 02:00:21 UTC (about 12 years ago)

alpha channels vs masks

...like how layer groups don't have a fixed size and their width/height are automatically derived from the size/placement of their constituent layers?

There would be an easy workaround if only we had a "Multiply with alpha" layer blending mode so I could put a white masking layer at the top of the group's layer stack ... but we don't have that, either.

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

Date: Fri, 28 Sep 2012 14:46:45 +0200 From: simon@budig.de
To: gimp-user-list@gnome.org
Subject: Re: [Gimp-user] alpha channels vs masks

Richard Gitschlag (strata_ranger@hotmail.com) wrote:

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.

...because mitch discovered a nasty bug with this and this has not been fixed yet...

Bye,
Simon

--
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Simon Budig
2012-09-29 06:43:08 UTC (about 12 years ago)

alpha channels vs masks

Richard Gitschlag (strata_ranger@hotmail.com) wrote:

From: simon@budig.de
...because mitch discovered a nasty bug with this and this has not been fixed yet...

...like how layer groups don't have a fixed size and their width/height are automatically derived from the size/placement of their constituent layers?

No, it was a more subtle bug. Masking a layer group appeared to work fine and I don't recall under which circumstances it failed.

Bye, Simon