How to export the projection

[ittayd]

I'm so new to Gimp that I'm not even sure I'm using the right terms. I have a bunch of layers (photos) in a group and the group has a circle mask. I want to export the layers with this mask, so the result is a square image with only a circle of content. I think this is called 'projection'.
Alternatively, I want to apply a mask before the export (that is, I don't care if the mask is applied in Gimp, I need it just for the result output)

[ittayd]

I tried to apply a procedure and got an error:
Failed to apply procedure "gimp-layer-add-mask" on "2023_09_07_181805.jpg" because:

Procedure 'gimp-layer-add-mask' has been called with an invalid ID for argument 'mask'. Most likely a plug-in is trying to work on a layer that doesn't exist any longer.

I tried to use a layer outside of the group, same results.

[kamilburda]

I found a solution, although it might be somewhat inconvenient as it involves multiple steps.

I created a Python script that applies a layer mask from a layer specified by its name to the current layer.

1. Install the script (link below) the same way as Export Layers by copying it to the plug-ins directory.
2. Switch to batch editing (Settings -> Batch Editing)
3. Add the python-fu-apply-mask-from-layer procedure and specify the name of the layer with the mask
4. Click Run
5. Switch back to the export mode by unchecking Settings -> Batch Editing
6. Uncheck python-fu-apply-mask-from-layer
7. Export

If you do not wish to keep the masks, you can simply undo the change.

The masks will not be created/overridden if a layer already has one.

apply_mask_from_layer.zip

[ittayd]

Thank you. I copied the file to ~/.var/app/org.gimp.GIMP/config/GIMP/2.10/plug-ins/ (i have a flatpak installation) but after restarting gimp I can't find it in the custom procedures. Gimp is 2.10.36

[kamilburda]

If you use a Unix-based system, make sure the .py file is executable.

You should be able to see a new menu entry under Layer -> Apply mask from layer.

Also maybe check if the plug-ins folder is added in GIMP (Edit -> Preferences -> Folders -> Plug-ins).

[ittayd]

Yep, the executable bit was it. Thanks!

[ittayd]

I created a layer called Mask and added the mask to it (it's icon is a black rectangle with a white circle). Didn't apply the mask. The batch operation iterates over all layers and applies a mask, but apparently it's a 'do nothing' as it is all white (white rectangle in the icon).

[kamilburda]

The mask is not scaled and is pasted in the same offsets as the layers it is applied onto.

One thing I forgot to mention when batch-editing (before step 4) is to uncheck Use layer size as that will modify the image size every time another layer is being processed.

If that doesn't help, maybe you could send an example of layers and a mask if you're OK with it,

[ittayd]

All my layers are of the same size.

Attached is an example,
example.zip

[kamilburda]

When I cleared the selection (before calling the script), the mask was copied with the black background and the white circle (which I assume is the expected result). The script assumes that the selection is empty, which I did not anticipate. If you need to preserve the selection, you can create a new channel from selection (Select -> Save to Channel).

[ittayd]

Works like a charm! Thanks!