想象一下PHP中的PNG压缩

Question

I have been scouring stackoverflow as well as PHP / Imagick documentation in order to get this to work. I have PNG images stored on my server, which are then returned to my iOS app. I need to return them as resized and cropped images, so I am using Imagick. Thus, so far I have:

$image_name = $_POST['filepath'];
if(!file_exists($image_name)){ return ""; }
header("Content-Type: image/png");

$image = new Imagick($image_name);
$image->resizeImage($_POST['width'], $_POST['height'], Imagick::FILTER_LANCZOS, 1);
$image->roundCorners($_POST['width'], $_POST['height']);
$image->setImageFormat("png");
$image->setImageCompression(\Imagick::COMPRESSION_UNDEFINED);
$image->setImageCompressionQuality(0);
$image->stripImage();
echo $image;

For setImageCompression, I have tried every single compression constant listed here: http://php.net/manual/en/imagick.constants.php , as there is no specific one for PNGs, but none of them seem to change the file size. (Is there a particular one supposed to be used for PNGs?) I have also changed the value in setImagesCompressionQuality around, but none of these changes seem to make any difference either. I've seen that some people were never able to get Imagick to work, while others were, and was hoping for a more updated answer. Any help appreciated!

Answer 1

It is the function setImageCompressionQuality to set the compression level that is done for PNG images. As PNG is a lossless image format, this does not affect the actual image quality (like it does for JPEG images) instead it just tells the library how much effort, and which strategy to use when compressing the images. The code below runs through all of the possible options.

I also tried pngcrush with the command line: pngcrush -brute Original.png pngcrushBrute.png to see how much more space could be saved.

• Original.png: 192,382 bytes - Without setting any specific compression level
• FullColor92.png: 181,832 bytes - the best output from all the possible options.

• pngcrushBrute.png: 178,129 bytes - which is probably more due to removing extra headers than that much better compression.

  $imagick->setImageFormat('jpg'); $imagick->setImageCompressionQuality(80);

Original.jpg: 27,626 bytes \\o/

To summarise:

• Imagick/ImageMagick already gets close to the best compression with the default settings.
• If you need to get the best compression, then using PngCrush will give a slightly better result than Imagick can.
• If the image is a photo, serve it as a jpg. It it's a diagram/must be served with a png, you could try color reduction, which would give good 'compression' but isn't suitable for photos, as it looks crap.

The code used:

//10's digit:
//
//        0 or omitted: Use Z_HUFFMAN_ONLY strategy with the
//           zlib default compression level
//
//        1-9: the zlib compression level
//
//     1's digit:
//
//        0-4: the PNG filter method
//
//        5:   libpng adaptive filtering if compression level > 5
//             libpng filter type "none" if compression level <= 5
//or if image is grayscale or palette
//
//        6:   libpng adaptive filtering
//
//        7:   "LOCO" filtering (intrapixel differing) if writing
//a MNG, otherwise "none".  Did not work in IM-6.7.0-9
//and earlier because of a missing "else".
//
//8:   Z_RLE strategy (or Z_HUFFMAN_ONLY if quality < 10), adaptive
//             filtering. Unused prior to IM-6.7.0-10, was same as 6
//
//        9:   Z_RLE strategy (or Z_HUFFMAN_ONLY if quality < 10), no PNG filters
//             Unused prior to IM-6.7.0-10, was same as 6

$imagick = new Imagick("./Biter_500.jpg");

$imagick->setImageFormat('png');

$imagick->writeimage("./output/original.png");
compressAllTypes($imagick, "./output/FullColor");


function compressAllTypes(Imagick $imagick, $filename) {
    for ($compression = 0; $compression <= 9; $compression++) {
        echo "Compression $compression \n";
        for ($filter = 0; $filter <= 9; $filter++) {
            echo "Filter $filter";
            $output = clone $imagick;
            $output->setImageFormat('png');
            //$output->setOption('png:format', 'png8');
            $compressionType = intval($compression . $filter);
            $output->setImageCompressionQuality($compressionType);
            $outputName = $filename."$compression$filter.png";
            $output->writeImage($outputName);
        }
        echo "\n";
    }
}