Bug #61221 [Ana]: 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 Type: Bug Package: GD related Operating System: * PHP Version: 5.6.9 Block user comment: N Private report: N New Comment: 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. Previous Comments: ------------------------------------------------------------------------ [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); ------------------------------------------------------------------------ [2012-03-01 14:56:41] efbiaiinzinz at hotmail dot com Description: ------------ When issuing imagegammacorrect function on an image resource that has alpha channel, the alpha channel gets lost. I looked at the source of 5.4.0RC8 and have also made a minor patch that adds alpha channel support by modifying the ext/gd/gd.c file. Test script: --------------- $img = imagecreatefrompng('input.png'); imagegammacorrect($img, 1.0, 1.0); imagepng($img, 'result.png'); Expected result: ---------------- I would expect to see output.png retain its alpha channel. Actual result: -------------- The result file does not have any transparency, since the current source code indeed does not do gamma correction to alpha channel. ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=61221&edit=1