Replacing the white with transparent in the gimp

published Mar 30, 2013 05:10 by admin ( last modified Mar 30, 2013 05:18 )

I had to adjust the look of a navigation arrow, that had been anti aliased against a white background, but now the arrow should function as a semi transparent overlay on a slideshow. So I needed the Gimp to not only convert pure white to transparent, but somehow understand that white should in general mean "transparent" so that areas fading to white should in fact be fading to transparent. It did not matter if the whole image got a bit semi transparent due to this.

Old version, you can see the anti-aliasing to white, giving a halo to the image as it is being laid over a slide

New version, the whole arrow became semi-transparent, but that is ok for this purpose. Note that the pesky halo is completely gone

Turned out to be really simple: Select Colors->Colors to Alpha in the menus and go with the default white.

The canonical way is to use Color/Color to alpha and give the background color, but that doesn't work too well with white because it makes a lot of things semi-transparent.