PHP Imagick：如何将自定义图像属性保存到文件

Question

I have a PHP app that is processing a large collection of images using Image Magick ( Imagick() ) and am attempting to save a piece of calculated data to the images to be read if available to save processing time or to be calculated and saved for use next time using Imagick::setImageProperty() and Imagick::getImageProperty() . I have a test controller in CodeIgniter 2 as follows:

class Example extends CI_Controller
{
    public function __construct()
    {
         parent::__construct();
    }

    public function property()
    {
        $im = new Imagick();
        $im->newimage(50, 50, 'blue');
        $im->setimageformat('jpg');
        $im->setimageproperty('My-App:My-Prop', 'rawr');
        var_dump($im->getimageproperty('My-App:My-Prop'));
        $im->writeimage(getcwd().'/images/test/output.jpg');
        echo '<br />'.var_dump($im->getimageproperty('My-App:My-Prop'));
        echo '<img src="/images/test/output.jpg" /><br />';

        $this->read();
    }

    public function read()
    {
        $im = new Imagick(getcwd().'/images/test/output.jpg');
        var_dump($im->getimageproperty('My-App:My-Prop'));
    }
}

The property is successfully read the first 2 times, but once a new Imagick object is instantiated based off the saved image, the property cannot be read and returns false :

string(4) "rawr" string(4) "rawr"

bool(false)

Is saving a custom image property to a file not possible, or am I misusing the class?

PS...I'm purposefully not using the CodeIgniter image library, but if that can help in this situation, I am open to that idea.

Answer 1

This seems to be a limitation in the ImageMagick library and the JPEG image format. The short version is that only properties named 'comment' are persisted by the ImageMagick library for JPG images. See the code below for an example.

TBH I think the sensible thing to do would be to store your meta information separately from the image file. ie take the image file name, append ".json" and store the calculated data in there.

It will work, will avoid re-saving the images if they haven't been modified and generally be a more robust solution than storing it in the image file.

$propertyNames = [
    "comment",
    "anything_else"
];

$formats = [
    'jpg',
    'png'
];

foreach ($formats as $format) {
    foreach ($propertyNames as $propertyName) {
        $imagick = new Imagick('./LittleRobin.jpg');
        $imagick->setImageProperty($propertyName, "Modified value");
        $imagick->setFormat($format);
        $imagick->writeImage("./testModified.".$format);
        $imagick2 = new Imagick("./testModified.".$format);
        printf(
            "After reloading '%s' property '%s' is:%s\n",
            $format,
            $propertyName,
            var_export($imagick2->getImageProperty($propertyName), true)
        );
    }
}

Output is:

After reloading 'jpg' property 'comment' is:'Modified value'
After reloading 'jpg' property 'anything_else' is:false
After reloading 'png' property 'comment' is:'Modified value'
After reloading 'png' property 'anything_else' is:'Modified value'