Add a layer on top of all the others to specifically add transparency in GIMP

Question

I have hundreds of layers on an image. I would like to add a layer especially dedicated on transparency - so that I can add transparency to the areas of the image, without modifying the other layers. How could I do that without flattening the other layers ?

Here is the beautifully drawn example :

I have multiple layers

I would like to add one layer on the top to determine what areas are transparent (the white part should be transparent)

I cannot merge the other layers, and adding the same layer mask to all the layers is not possible.

Answer 1

GIMP Layers transparency don't affect sibling layers beneath it - those would just show through. This idea, however, would be feasible by putting all layers inside a group - that would contain the whole image, and then setting up a mask to that group layer. The bad news is that masks are not implemented in group layers, so one simply can't do that.

So, the workaround is to duplicate the image (image->duplicate), flatten the resulting image (Image->Merge visible layers...) and adding a layer mask to the resulting layer. This mask could be pasted from a reference mask. Unfortunately, this workaround is "static": it allows you to render a final image with the desired masking, but not editing your image with the mask applied.

update: On the development version, being able to have a layer mask to a layer group, which would resolve this problem, is counted as main blocker for the release of GIMP 2.10, due this year (2017). Expect this to be implemented on the development version anytime soon.

Answer 2

There is a way to do this using a layer blending mode and group. It's not obvious.

Put your layers inside a group. And set the blending mode to "Hard Light".

Apply the layer mask to your background.