In the original proposed patch, alpha shannel should not actually be modified by 
imagegammacorrect, only rgb should get multiplied.

Sample test case to show alpha channel loss:

<?php
$imagew = 50;
$imageh = 50;
//truecolor means color allocations just combine ARGB into one integer value
$img = imagecreatetruecolor($imagew, $imageh);
//make sure alpha gets stored
imagesavealpha($img, true);
$blacktransparent = imagecolorallocatealpha($img, 0, 0, 0, 127);
$redsolid = imagecolorallocate($img, 255, 0, 0);
//fill with transparent black background
imagefill($img, 0, 0, $blacktransparent);
//draw solid red cross over the image for contrast
imageline($img, $imagew / 2, 0, $imagew / 2, $imageh - 1, $redsolid);
imageline($img, 0, $imageh / 2, $imagew - 1, $imageh / 2, $redsolid);
//execute imagecammacorrect with same input-/output gamma values so that colors 
should actually remain same
imagegammacorrect($img, 1, 1);
//ask top-left corner color, for truecolor it is ARGB combined into one integer 
value
$color = imagecolorat($img, 0, 0);
//correct result should be same black-based transparent color that we used for 
filling
echo $color === $blacktransparent ? 'ok' : 'failure';
//comment the echo line above and uncomment the two lines below to see visually 
how transparent color gets changed to solid black
//header('Content-Type: image/png');
//imagepng($img);
imagedestroy($img);

I can still confirm the issue with current master (1123032) – even
when `imagesavealpha($img, true)` is added to the supplied test
script, the alpha channel is lost when `imagegammacorrect` is
applied. I agree that this is a bug.

I'm in no way an expert on image manipulation, but I agree that
the alpha channel should not be modified in *any* *way* when
`imagegammacorrect` is called, i.e. the given patch should not be
applied, but rather imagegammacorrect-alpha.patch.

Glad that someone finally discovered this issue after 3 years :)

Yes indeed, the original patch I did was made in a hurry and then forgotten.
Keeping the alpha value the same is correct behaviour and imagegammacorrect-alpha thus should work correctly.

I wrote a PHP function to work around the issue at the time of the report, discovered the alpha problem some time later in my code but didn't bother to add info here after a year or so total silence from PHP team.

Thanks for the confirmation regarding the patch, and sorry for
having had to wait so long. And, of course, thanks for your report
and the supplied test case.

I have submitted a respective PR now.