How to store data in an image with php? - php

Question

I have tried to use the imagick library to create two functions like this:
function storeCoordinatesImage($img_path, $coordinates){
$im = new imagick($img_path);
$im->setImageProperty("coords", $coordinates);
$im->writeImage($img_path);
}
function getCoordinatesImage($img_path){
$im = new imagick($img_path);
return $im->getImageProperty("coords");
}
If I run:
if(!storeCoordinatesImage("I.jpg", "hi")) echo "fal";
echo getCoordinatesImage("I.jpg");
Nothing is returned.
But if I run:
$im = new imagick($img_path);
$im->setImageProperty("coords", "hello");
echo $im->getImageProperty("coords");
it returns "hello".
So it must be some issue with writing to the image? Although none of these functions are returning false. (i.e they are all working)

Answer 1

Use an image's profile payload to store arbitrary data. Although a JSON payload stored in an image comment (i.e. JPG_COM) tag seems the quick-n-easy, several competing technologies exist for this propose. The most popular being exif, but I would recommend xmp.
eXtensible Metadata Platform
In my opinion, xmp seems over engineered, but does offer a platform to ensure all vendor proprietary information is separated via XML namespace.
Wikipedia has a great overview, and Adobe's white papers (pdf) do a great job outlining the "does-n-don'ts" for a vendor to implement.
ImageMagick doesn't handle anything outside of read/write profile payloads, so you would be responsible for implementing a XML manager.
For example...
// A minimal XMP tempalte
$XMP_BASE='<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMPTk 2.8">'
.'<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"></rdf:RDF>'
.'</x:xmpmeta>';
$xmp = new DOMDocument();
$xmp->loadXML($XMP_BASE);
// Create a <rdf:Descriptiom> & <Coords> DOM element.
foreach($xmp->getElementsByTagName('RDF') as $node) {
$coords = $xmp->createElement('Coords', 'hello');
$description = $xmp->createElement('rdf:Description');
$description->setAttribute('about', '');
$description->appendChild($coords);
$node->appendChild($description);
}
$img = new Imagick('rose:');
// Write profile to image.
$img->setImageProfile('xmp', $xmp->saveXML());
$img->writeImage('output.jpg');
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
$img2 = new Imagick('output.jpg');
$xmp2 = new DOMDocument();
// Read profile from image.
$xmp2->loadXML($img2->getImageProfile('xmp'));
// Grab `Coords' value
foreach($xmp2->getElementsByTagName('Coords') as $coords) {
print $coords->textContent . PHP_EOL;
}
//=> "hello"
And you can verify with the identify utility.
identify -verbose output.jpg | grep Coords
#=> Coords: hello
Of course, you would also need to implement a XML DOM "merge" method in the event that an image already contains a XMP profile, and you don't wish to overwrite existing data.

Answer 2

As Ben mentioned this is not possible. Instead you can add a "comment":
function storeCommentImage($img_path, $coordinates){
$im = new imagick($img_path);
$im->commentImage($coordinates);
return $im->writeImage($img_path);
}
function getCommentImage($img_path){
$im = new imagick($img_path);
return $im->getImageProperty("comment");
}

Answer 3

Seems like you can't persist that data for jpegs: https://github.com/ImageMagick/ImageMagick/issues/55#issuecomment-157114261
Maybe try with a png?