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I'm trying to create a "corona" effect (like the sun's corona) in an image using GIMP, but I'm having trouble editing only the alpha channel of the image.

I want the RGB to be pure white (255, 255, 255) and the alpha layer to have a brush effect applied to it. I know how to use the brush, but no matter what combination of masks or layer visibility / channel visibility settings I try, it will never edit ONLY the alpha channel, leaving the rest undisturbed.

How can I edit only the alpha channel of an image in GIMP?

Dimitar
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Knetic
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4 Answers4

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To edit the alpha channel, add a layer mask and apply the brush effect to the layer mask.

Under the Layers tab, right click the layer to edit and choose Add Layer Mask. A dialog box will ask you how you want the layer mask to be initialized. Presuming you'll start with a visible image and brush away the part you want transparent, the best choice is White (Full Opacity). Click Add.

Under the Layers tab, you should now see the white thumbnail of the layer mask, next to the thumbmail of the layer image. Click on the thumbnail to edit the layer mask. Choose your brush effect and the color black, and apply your brush effect.

If you switch to the Channels tab, you will see in real time how the alpha channel changes as you paint on the layer mask.

If you prefer to start with a transparent image and brush on the part you want visible, just start with a layer mask which is Black (Full Transparency) and apply your brush effect to the layer mask with the color white.

You can always switch back and forth between black and white paint to add or remove transparency on the layer mask.

When you want to return to editing the layer image instead of the layer mask, go back to the Layers tab and click on the thumbnail of the layer image.

MetaEd
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    This did it, i was initially confused because the channels tabs showed my circle as white on all channels, not a fading grey like in the alpha, with the rest of the color channel around the circle being black. I had thought this was incorrect until i realized what i was looking at. Thanks! – Knetic Nov 19 '11 at 21:46
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    Thanks, +1. You can also tick "show layer mask" and "modify layer mask" to edit the alpha channel only, and "disable layer mask" to edit the RGB only; this is helpful for game assets where RGB and A can be completely unrelated. – Calvin1602 Feb 05 '15 at 14:22
  • @MetaEd This answer requires screenshots. – tmighty Nov 02 '21 at 23:33
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Just a simple solution only if you want to change the transparency of the layer. Choose eraser tool. There is an option to change the opacity of the eraser under tool options. Just adjust it to your requirement(say 75). Now apply the eraser.

jjpp
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2

Just another method I found using a buggy appimage (eraser and masks not usable):

If you open the channels and deactivate the RGB channels, you can make hard selections on the image and use the fill tool, editing the opacity for gray-values.

Tcll
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  • just want to note I can't use the latest version because graphical glitches make it unusable (I have to repeatedly minimize and maximize the window to force a redraw that doesn't always work) – Tcll May 19 '19 at 14:25
  • The combination of this plus the eraser tool above worked great for me. Very helpful. – user117529 Oct 15 '20 at 00:00
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Just double click on the layer you want to change the transparency and change the opacity

mXaln
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  • I don't think this was answering the question, but did provide the solution I was searching for two years later! – Elton Clark Dec 14 '22 at 14:59