PHP: grayscale jpgs, gifs, pngs... also with alpha channels? - php

Question

$im = ImageCreateFromString(file_get_contents($source_file));
ImageFilter($im, IMG_FILTER_GRAYSCALE);
any idea what i could do, to properly grayscale gifs and pngs with transperancy? This snippet actually works good, it transforms jpgs and pngs to grayscale. However gifs are a little bit "buggy" - they don't always work, it depends on the image. Sometimes there are a few pale colors left in them. Moreover this snippet doesn't work with alpha-channels. If i convert a gif or a png with transparancy the transparent parts always get blackened.
Of course im querying the image-type and after "grayscaling" it, i'll set the proper type again.
Have you any ideas?

Answer 1

This code should preserve the alpha, but it's slower than imagefilter:
$im = ImageCreateFromString(file_get_contents($source_file));
$width=imagesx();
$height=imagesy();
for($x=0;$x<$width;$x++)
for($y=0;$y<$height;$y++)
{
$rgb=imagecolorsforindex($im,imagecolorat($im,$x,$y));
$average=ceil(($rgb["red"]+$rgb["green"]+$rgb["blue"])/3);
imagesetpixel($im,$x,$y,imagecolorallocatealpha($im,$average,$average,$average,$rgb['alpha']));
}
If you still have problems try to write this after the image creation (before the $width=..):
imagesavealpha($im,true);

Answer 2

For pngs a simple call to imagesavealpha() solves the problem of black pixels on the alpha channel, complete code:
$im = ImageCreateFromString(file_get_contents($source))
imagefilter($im, IMG_FILTER_GRAYSCALE);
imagesavealpha($im,true);
imagepng( $im, $output );