GD library , drawing an ellipse filled with transparent color

[Matth_S]

Hello,

I'd like to draw an ellipse with the GD library. My problem is :
how to draw an ellipse keeping the background of the pic on which I draw this ellipse as it is, but changing the gamma correction of this background. I mean (I'm not sure it's clear) I want to apply a gamma correction on the area over which the ellipse has been drawn.

I'm not even sure it's possible with GD library ; do you know any way to do so ?

Thanks !