Interesting, that looks like it basically turns the brightness into an alpha channel, so the lighter the colour, the more transparent it is. So it works for white, but not if it's a black background. It also seems to grey out a lot of the image, as (unless the image is 100% black) the quickmask masks some of the coloured regions.
Another way to do something similar is:
1. duplicate layer
3. Use a levels adjustment and pull the white and the black sliders in to increase the contrast. You can use threshold, but that will give jagged edges.
4. Copy the layer (and then turn it off).
5. Add a layer mask to your original layer.
6. Option click the layer mask (your screen should now be white) and paste.
You should now have a mask that corresponds to the layer you created. You probably need to invert this to get what you want.
You can start with the red, green or blue channels instead of your full image to get some better results.
Fantasy Map Blog
| My food illustration
Everything I post is free for use and redistribution under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 licence, except where noted otherwise in the thread.
Couldn't "Inverse" (ctrl+I) solve this?
Originally Posted by torstan