Linux – Batch Convert PNG to JPG

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

ImageMagick will always compress PNG files. The worst compression you can get is using:

convert -verbose -quality 01 input.png output.png

but it depends on the image content (the 0 will use Huffman compression which sometimes compress better than zlib).

You can try other tools like pngcrush (http://pmt.sourceforge.net/pngcrush/) to disable the compression:

pngcrush -force -m 1 -l 0 input.png output.png

which create a file the same size GIMP create when using Compression Level 0 (few bytes more or less).

Some example sizes (for a photographic PNG, 1600x1200):

• original: 1,693,848 bytes.
• after IM: 2,435,983 bytes.
• after GIMP: 5,770,587 bytes.
• after pngcrush: 5,802,254 bytes.