Converting PNG32 file to PNG8 without losing alpha channel

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I'm using Linux and have no access to any of Adobe's 'fancy' programs.

I'm trying to convert an existing PNG32 image with alpha channel to PNG8. I have tried the following methods:

  • convert original.png PNG8:new.png – Horribly distorts the image and preserving only binary alpha (Not Indexed alpha)
  • GIMP – Fails as well, but produces better quality (good color quantizer) than ImageMagick.
  • pngcrush -rem alla -reduce -brute original.png new.png – Made the image smaller but didn't take quantization into account (Image has less than 256 colors), so output was still PNG32.

What else can I try?

Best Answer

This PHP script does the trick with libgd:

<?PHP

if(!isset($argv[1]) || !is_readable($argv[1])) {
    echo "Creates an 8-bit PNG from a 32-bit PNG\n\n";
    echo "Usage:\n";
    echo "\t" . $argv[0] . " input.png > output.png\n";
    echo "\t" . $argv[0] . " input.png output.png\n";
    die();
}

$inFile = $argv[1];
$outFile = $argv[2] or STDOUT;

$inImage = imagecreatefrompng($inFile);
$outImage = imagecreate(imagesx($inImage), imagesy($inImage));

imagecopy($outImage, $inImage, 0, 0, 0, 0, imagesx($inImage), imagesy($inImage));

imagepng($outImage, $outFile);

Dump that into a file and run it as:

php convert.php input.png output.png
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