Edit report at http://bugs.php.net/bug.php?id=55034&edit=1
ID: 55034
User updated by: php at mrphlip dot com
Reported by: php at mrphlip dot com
-Summary: ImageCopyResampled doesn't calculate alpha properly
+Summary: ImageCopyResampled doesn't calculate colours
properly with alpha
Status: Open
Type: Bug
Package: GD related
Operating System: Ubuntu Natty
PHP Version: 5.3.6
Block user comment: N
Private report: N
New Comment:
Changed summary to be more accurate... it calculates the alpha fine, it's the
colours it doesn't calculate correctly in the presence of an alpha channel.
Previous Comments:
------------------------------------------------------------------------
[2011-06-11 15:48:17] php at mrphlip dot com
Description:
------------
It appears that ImageCopyResampled uses a naive averaging, which isn't correct
when there is an alpha channel involved. It should instead use a weighted
average for the colour channel, weighting each input pixel according to its
opacity (so a more opaque input pixel has more weight in the average).
This comes up particularly if you have eg a solid white shape against a solid
black-but-transparent background. The original image, every pixel is either
white or transparent. But shrink it down, and the object will have a thin dark
halo around the edge, because of averaging those black pixels into the result.
The expected result would fade from full opacity white, to half opacity white,
to transparent any-colour... but the actual result fades from full opacity
white, to half opacity *grey*, to transparent black.
Test script:
---------------
# create an image with an almost-transparent white pixel and an almost-opaque
black pixel
$img1 = ImageCreateTrueColor(2, 1);
ImageAlphaBlending($img1, FALSE);
ImageSetPixel($img1, 0, 0, ImageColorAllocateAlpha($img1, 255, 255, 255, 0x70));
ImageSetPixel($img1, 1, 0, ImageColorAllocateAlpha($img1, 0, 0, 0, 0x10));
# scale the image down to a single pixel - make it mix the two together
$img2 = ImageCreateTrueColor(1, 1);
ImageAlphaBlending($img2, FALSE);
ImageCopyResampled($img2, $img1, 0, 0, 0, 0, 1, 1, 2, 1);
# find out what colour the resulting pixel is
$col = ImageColorAt($img2, 0, 0);
print "R: " . (($col >> 16) & 0xFF) . "<br>";
print "G: " . (($col >> 8) & 0xFF) . "<br>";
print "B: " . ($col & 0xFF) . "<br>";
print "A: " . (($col >> 24) & 0xFF) . "<br>";
# clean up
ImageDestroy($img1);
ImageDestroy($img2);
Expected result:
----------------
R: 30
G: 30
B: 30
A: 64
Each of the colour channels has been weighted in the average... the
almost-transparent white pixel with a weight of 0xF (0x7F - 0x70), and the
almost-opaque black pixel with a weight of 0x6F (0x7F - 0x10), giving a
weighted average of (255 * 0xF + 0 * 0x6F) / (0xF + 0x6F) == 30. The alpha
channel is averaged in the normal way.
Actual result:
--------------
R: 127
G: 127
B: 127
A: 64
Each channel has been averaged separately, with no regard for the alpha channel.
------------------------------------------------------------------------
--
Edit this bug report at http://bugs.php.net/bug.php?id=55034&edit=1
