Edit report at https://bugs.php.net/bug.php?id=61221&edit=1
ID: 61221
Comment by: efbiaiinzinz at hotmail dot com
Reported by: efbiaiinzinz at hotmail dot com
Summary: imagegammacorrect function loses alpha channel
Status: Open
Type: Bug
Package: GD related
PHP Version: 5.4.0RC8
Block user comment: N
Private report: N
New Comment:
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);
Previous Comments:
------------------------------------------------------------------------
[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.
------------------------------------------------------------------------
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Edit this bug report at https://bugs.php.net/bug.php?id=61221&edit=1
