I've searched for this in a bunch of places, and I've found a technique that works for me - I think my requirements are a little different from maybe what other people are looking for, so I'm writing this up for my own notes, and maybe somebody else can benefit.
Problem Statement:
I've got some black-on-white grayscale art (possibly pen-and-ink line art) that I'd like to turn into a layer of a single color, but with the grayscale value in the alpha channel.
If I was writing a Python script using PIL, I'd expect the incoming image to be a grayscale format, with 8 bits of lightness information (0 = black, 255 = white), no alpha channel, and the output image is RGBA, where every pixel is (0, 0, 0, A) where A is 255-L, L being the lightness value of the input image.
Why not make a Python script to do this? It sounds like 5 lines of Python or less, right?
Yeah, maybe.
Still, this is how I do it in GIMP. Maybe I want to do additional processing in GIMP, and having it already open might be handy. Or something.
Step 1
Ok, I'm starting with my image loaded in GIMP. I'm not going to help you get to this point.
Optionally, crop the image - for this image, I tried "Image > Crop To Content", which didn't get as tight on the left hand side as I expected, so I followed up by using the rectangular selection box from the toolbar, selected a generous box, and manually pulled in the left side to where I wanted it. And then "Image > Crop To Selection".
I suppose you could do a bunch of other stuff here, too, like resizing the image. Live your best life.
Step 2
Duplicate the layer
This can be done by right clicking on the layer in the Layers panel.
Step 3
Invert the layer. I had best results with "Colors > Linear Invert". You could try the normal invert, I suppose.
Step 4
Right click on the new layer, select "Add Layer Mask"
Select the Grayscale copy of layer. (Why is it grayed out AND selected?)
Step 5
With the paint bucket tool, fill the layer (not the mask) with black, using "Fill whole selection", making sure your selection is all or none of the image.
Step 6
No, there's no step 6. You're done. Make a sweatshirt or whatever it is that you were going to do with your grayscale Half-Elk Fighter-Master. Or whatever.
