Gimp Color To Alpha removes too much opacity

Question

I've used Gimp's "Color To Alpha" (C2A) in the past, and it was always the case that, if you placed the resulting layer over a background of the "removed color", that you'd get back the original image. However, having just installed v2.10.34, there seems to be a new dialog for C2A and the filter seems to now produce too much transparency.

Here's a minimal example of what I mean. I've set up an image with two layers, bottom layer pure white, top layer white background with a black loop. Running C2A on the top layer removes all white, but the alpha values for the resulting image are too low for grey pixels, so you can not get back to the original value with the white background from the bottom layer. On the right is the original loop, left the filter's result over the white background.

Manipulating the "Opacity threshold" slider reduces the problem, but then there will be white remaining in the output. Is there another setting I am missing? Was the definition of C2A changed? Is this just a bug?

Answer 1

Try this.

Set the layer blending modes to the Legacy setting - that will set the layer blending mode to Normal (I), which is the Normal (legacy) blending mode used in older versions of GIMP prior to 2.10. Then it should work as expected.

An example

Answer 2

Using the "standard" layer modes, the exact equivalent of C2A is to paint in Color erase mode with the color you want to make transparent.

You can also get the same result in a non-destructive way by using a layer in Color erase blend mode (and putting things in a group to limit the scope of the color erasure):