Description:
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In http://lxr.php.net/source/php-src/ext/gd/gd.c#4064, the function php_image_filter_contrast calls gdImageBrightness where it probably should call gdImageContrast. This seems to work and seems to make sense, since gdImageContrast does not seem to be called from anywhere else.

Reproduce code:
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 $img = imagecreatefromgif("http://images.slashdot.org/title.gif");
 imagefilter($img, 3, 50);
 imagejpeg($img);