Remove black background of a shape with blurred edges

Question

I am trying to change the background colour of the image https://www.eso.org/public/images/eso0728c/ from black to white with gimp.

To do this I'm following the suggestion from this answer: https://graphicdesign.stackexchange.com/a/11080/130689

The problem now is that all colours get lighter, see the second image below. I'd like the colours to stay the same, just the background should be white, as I tried to draw manually in the 3rd image below.

Any suggestions which technique I could try?

Answer 1

If a color isn't 100% white, C2A will find some black to subtract from it which makes it a bit transparent and so appear lighter if there is white under it.

In fact you want to exclude the center of the area from C2A's reach, so that only the borders are converted to partial transparency. The fuzziness of the border makes that difficult but you can try this:

• fuzzy select the background
• Select>Grow by N pixels (I used 10 pixels on your picture above)
• Select>Feather by N/2 pixels
• Apply C2A to remove the black.

There may be ways to correct the feathering (Curves on the Quickmask, for instance) to reduce the dark border if necessary.