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?
Answer
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.