Bug #61221 [Ana->Csd]: imagegammacorrect function loses alpha channel

Edit report at https://bugs.php.net/bug.php?id=61221&edit=1 ID: 61221 Updated by: cmb@php.net Reported by: efbiaiinzinz at hotmail dot com Summary: imagegammacorrect function loses alpha channel -Status: Analyzed +Status: Closed Type: Bug Package: GD related Operating System: * PHP Version: 5.6.9 Block user comment: N Private report: N New Comment: Automatic comment on behalf of cmbecker69@gmx.de Revision: http://git.php.net/?p=php-src.git;a=commit;h=7469c7e7d0cee78abfaeeb087be494939bee82a9 Log: Fixed bug #61221 - imagegammacorrect function loses alpha channel Previous Comments: ------------------------------------------------------------------------ [2015-06-06 13:34:57] cmb@php.net 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. ------------------------------------------------------------------------ [2015-06-06 08:56:48] efbiaiinzinz at hotmail dot com 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. ------------------------------------------------------------------------ [2015-06-06 00:47:31] cmb@php.net 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. ------------------------------------------------------------------------ [2015-06-06 00:45:35] cmb@php.net The following patch has been added/updated: Patch Name: imagegammacorrect-alpha Revision: 1433551535 URL: https://bugs.php.net/patch-display.php?bug=61221&patch=imagegammacorrect-alpha&revision=1433551535 ------------------------------------------------------------------------ [2013-07-02 10:17:50] efbiaiinzinz at hotmail dot com 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); ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at https://bugs.php.net/bug.php?id=61221 -- Edit this bug report at https://bugs.php.net/bug.php?id=61221&edit=1