ImagePngAlpha() different outputs with imagick and gd

[SoulFliktioN]

I encountered a problem with pngs with semi transparent areas. In my case I got a glow around a logo. Local everything was ok but not online. I figured out that local Imagick is enabled and online not. As I do not have the possibility to activate it online i was looking for an solution. I think I got it but would need some help!

So what I did was to change the try block for gd extension to:

// extracting alpha channel
$img = imagecreatefrompng($file);
imagesavealpha($img, false);
imagepng($img, $tempfile_plain);
imagedestroy($img);
			
// creating the alpha image	
$img = imagecreatefrompng($file);
$imgalpha = imagecreate($wpx, $hpx);
				
for ($c = 0; $c < 256; ++$c) {
 ImageColorAllocate($imgalpha, $c, $c, $c);
}

for ($xpx = 0; $xpx < $wpx; ++$xpx) {
 for ($ypx = 0; $ypx < $hpx; ++$ypx) {
  $index = imagecolorat($img, $xpx, $ypx);
  $rgba = imagecolorsforindex($img, $index);
  $alpha = abs($rgba["alpha"] - 127) / 127 * 255;
  // not sure about the gamma correction
  //$alpha = (int) (pow(($alpha / 255), 2.2) * 255);
  imagesetpixel($imgalpha, (int) $xpx, (int) $ypx, (int) $alpha);
 }
}
							
imagepng($imgalpha, $tempfile_alpha);
imagedestroy($imgalpha);
$parsed = true;

In fact I just try to reproduce the result of the Imagick version.