How do I make a bitmap-based alpha mask?

[_takechiya]

Alright I've checked some older threads on what may be a similar question but so far I've been unable to correctly make this work. And tbh I'm not really sure if the past threads are talking about the same thing.

 

Essentially, I have two images in layer 1 and 2 (layer 1 being above layer 2), and a third image which is bitmap-like, in the sense that there are pixels with alpha=0 and alpha=255. What I am trying to do is to make it so that, in layer 1, the corresponding pixels in the third image have inverted alpha, that is, where alpha=255 in the third image, alpha=0 in layer 1 and vice versa. This is basically a cutout in layer 1 in the shape of image three.

 

An example of what I'm trying to do is here:

https://imgur.com/a/no0CPy2

[MJW]

There are two plugins that do that in BoltBait's plugin pack, Apply Mask and Paste Alpha.  Apply Alpha Mask uses a map from another layer; Paste Alpha uses a map from the clipboard.

 

UPDATE: I corrected the name of the first plugin, which is Apply Mask, not Apply Alpha Mask, as I originally typed.

[_takechiya]

Update: I had been trying that and got just the third image being pasted along with its transparency. I didn't realize I had to tick the alpha inversion box upon pasting. That solves my issue, hehe.

 

[MJW]

9 hours ago, _takechiya said:

I didn't realize I had to tick the alpha inversion box upon pasting.

 

I probably should have mentioned the various options in my comment. Other useful options besides Invert are the choice between using the map's alpha or gray, and the blending method. The Multiply and Minimum methods allow the alpha map to be applied to a layer that already has areas of transparency which shouldn't be replaced by the map's values.