GIMP: How to apply a mask to multiple layers?

Question

I'm new to GIMP, so I'm having trouble accomplishing the following:

• I want to apply the same layer mask to all of my layers without having to merge them.

If this is not possible, the end result I'm trying to accomplish is to have an alpha-gradient effect on the whole image, not just one layer; but again, I don't want to merge the layers.

Answer 1

Since Gimp 2.10, you can have masks on layer groups, so one solution is to put the layers in a group, and add a mask to the group:

However, in the general case, this may not give exactly the same result as putting the same mask on all layers:

• with a group mask, partial opacity of pixels is applied only once
• with individual masks, partially opaque pixels in each layer are composited, and may result in a more opaque pixel in the composite image.

This said, in most cases you will want the result from the group mask...

Answer 2

Unfortunately GIMP does not support adding a layer mask to a layer group. Layer Masks can only apply to a single layer.

The closest work around is to create the layer mask you want on a single layer, then copy it to the other layers.

1. Add the layer mask to the layer (Right click -> Add Layer Mask)
2. Configure the layer mask as desired, such as adding a gradient
3. Right click the layer with the mask and select "Mask to Selection"
4. On another layer, add a layer mask. For "Initialize Layer Mask to" click "Selection"
5. Repeat for other layers as needed.

If you need to change the layer mask later, you'll need to delete all except one, change it, and then re-copy it to the other layers.

Answer 3

I had the same problem and like to share my Workaround:

1. Add the mask as a final layer inside the group
2. Set its mode to Multiply
3. Set the groups mode to Addition
4. Duplicate the mask layer
5. Invert its color
6. Move it right beneath the group

Should look as follows:

Answer 4

I don't specifically know GIMP, but i'm pretty sure the method would be the same, so in Photoshop, I would either

Group the layers under a folder, then apply a mask to the folder (this is the better method as it keeps everything editable and separate)

or

Create an empty mask on each of the layers, then create the actual mask in one of them, then copy that mask to each of the subsequent layers.

Answer 5

You'll want this method if you're going to change the foreground but not the background. Changes to the background still require re-copying the background the masked image at the top. If the opposite is true (you're going to be changing the background a lot, and the content not much), then you are better off simply merging the layer group into a single layer.

Make a clone of your background (or whatever you want to show behind the thing you're masking)¹. Then add a layer mask to this new layer that is the opposite of what you want. That is, if you want something in the other layers to show(opaque), make that area of the mask black (for transparency of the background) and if you want them to be hidden(transparent), make that area of the mask white(opaque background above them)

If you have a pre-made mask, simply apply it as normal, then with the mask selected go to Colors > Invert.