how to restore the watermark set with Imagick::steganoImage() - php

Question

So I used this Imagemagick library wrapper written in PHP and came to know about this function called steganoImage() which simply says that it will hide a watermark inside the image. But problem is documentation doesn't state how to restore that image back. I checked all the other functions too, didn't find anything. I would like to have an Imagick solution for this.

Answer 1

Before you spend to much time on this you need the original dimension you used to create the secret image to retrieve it. So if the image is cropped, resized or format changed you probably will not be able to retrieve the message.
For your information this is a good site for Imagick examples: https://phpimagick.com/Imagick/queryFormats Although they do not mention steganoImage()
Looking on the web I found the code below ( Codesearch ) which adapted should work for IMagick:
#!/usr/bin/perl
use Image::Magick;
#
# Hide an image within an image
#
$watermark=Image::Magick->new;
$watermark->ReadImage('smile.gif');
($width, $height)=$watermark->Get('width','height');
#
# Hide image in image.
#
$image=Image::Magick->new;
$image->ReadImage('model.gif');
$image->SteganoImage(image=>$watermark,offset=>91);
$image->Write('model.png');
$image->Write('win:');
#
# Extract image from image.
#
$size="$width" . "x" . "$height" . "+91";
$stegano=Image::Magick->new(size=>$size);
$stegano->ReadImage('stegano:model.png');
$stegano->Write('stegano.gif');
$stegano->Write('win:');

Answer 2

The answer provided by Bonzo is correct. An example in PHP's Imagick would look very similar.
$image = new Imagick('rose:');
$watermark = new Imagick('label:Hello World!');
// The decoding process must "know" about the watermarks size, and starting
// pixel offset.
define('STEGANO_OFFSET', 64); // Secret offset
define('STEGANO_WIDTH', $watermark->getImageWidth());
define('STEGANO_HEIGHT', $watermark->getImageHeight());
$stegano = $image->steganoImage($watermark, STEGANO_OFFSET);
$stegano->writeImage('output.png');
To decode the original watermark, define the width, height, and offset of the hidden image before reading the file.
$decoded = new Imagick();
$decoded->setSizeOffset(STEGANO_WIDTH, STEGANO_HEIGHT, STEGANO_OFFSET);
$decoded->readImage('STEGANO:output.png');
$decoded->writeImage('decoded.png');