In gimp how to create a silhouette/"Burn" a layer into another layer?

Question

Well I have an (set of) image(s) and I wish to create silhouettes out of those. Based on the alpha value of the image (alpha is either 0 or 1 no intermediate values).

The idea I had was to create three layers: a white background layer, a black foreground layer and a third top layer with the image.

Then for the top layer select the right image each time and would pick a blend/layer mode so that it "burns" the alpha from the top layer on the bottom layer. Then I could just load all images into the layers, set the blend modes and make them visible one by one.

Is this possible, or would I have to go through the lengthy process of creating a selection, and making a path from this selection and then using that path as object to fill in the "black" layer. (which requires many more steps and with around a 100 images to process I'm not looking forward to that).

So say I have this image (and/or with the white replaced by a transparent region):

And I wish to create the following:

Answer 1

There are probably many ways you could do this. Here's one.

1. Right click the image layer and do Alpha to Selection

2. Create a new transparent layer, and make sure you select it in the layers panel

3. Do Ctrl+, to fill with FG colour

Another possibility is to use Colors > Components > Extract component, select "Alpha" as the component. Do it on a copy of the image layer if you want to keep the original

Answer 2

You want something like this:

Your image is in Split mode in a group over a layer filled with the background color, and the group itself is over a layer filled with the silhouette color.

If you want to handle several images, you add them to the group. If you do not want to set every image to Split mode, put a layer group in Split mode where the image is, and add the images to is in Normal mode.

Edit #1: why the groups

Layer groups are not just a convenient way to keep layers together. They also act like parentheses do in algebra.

• Without a group the layer stack above is evaluated bottom to top as (white overlaid_by black) split_by image and (white overlaid_by black) is just black
• The presence of the group changes this order to white overlaid_by (black split_by image). Since black split_by image is black with a hole that silhouettes the image, the final result is the white showing through the image silhouette.

Edit #2: the Erase mode (instead of Split) works just as well, and could be more appropriate (these blend modes are new and ill-documented)