recover the alpha transparence of pixels

[COUTIER Eric]

Hello,

Here's the problem:

An image in classic format (gif, bmp, ...). In this image, a red curve on
white background. This curve has been drawned with antialiasing option. Now,
added pixels for antialiasing are in progressive light red (and not
transparent). Is there a tip to perfectly transform these pixels in
semi-transparent pixels.

I've tested this solution but it doesn't give not good results:
.Select->by color:select white area
.select->feather: about 2 pixels
.edit/cut

Regards. Eric.