Imagick::steganoImage() Unable to show the watermark - imagick

Question

I'm trying to use Imagick::steganoImage with an example image from wikimedia commons.
If I try to show the decoded image of the watermark, I always obtain the 1 pixel image you can see below, whatever image I choose as source.
Anyone may help me to understand why and how I can sort this out?
<?
header("Content-Type: image/png");
// Create a new imagick object
$image = new Imagick('https://upload.wikimedia.org/wikipedia/commons/thumb/1/16/Frostedbubble2.jpg/640px-Frostedbubble2.jpg');
$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');
$decoded = new Imagick();
$decoded->setSizeOffset(STEGANO_WIDTH, STEGANO_HEIGHT, STEGANO_OFFSET);
$decoded->readImage('STEGANO:output.png');
$decoded->writeImage('decoded.png');
// Show the output
$decoded->setImageFormat('png');
echo $decoded->getImageBlob();
?>
I tried also the code at this page https://www.geeksforgeeks.org/php-imagick-steganoimage-function/ and the geeksforgeeks image is shown correctly but the stegano image appear as totally black.
<?php
// Create a new imagick object
$imagick = new Imagick(
'https://media.geeksforgeeks.org/wp-content/uploads/geeksforgeeks-13.png');
// Create another Imagick object containing watermark
$watermark = new Imagick('label:This is my secret.');
// Hide $watermark inside $imagick
$imagick = $imagick->steganoImage($watermark, 64);
// Write image to the local folder
$imagick->writeImage('output.png');
// Set the offset
$imagick->setSizeOffset($watermark->getImageWidth(),
$watermark->getImageHeight(), 64);
// Read the encoded image and extract secret
$imagick->readImage('STEGANO:output.png');
// Show the output
$imagick->setImageFormat('png');
header("Content-Type: image/png");
echo $imagick->getImageBlob();
?>

Answer 1

Looks like new Imagick('LABEL:Hello World!'); is no longer enough for a simple label. You'll need to set the size of the canvas, and the point size BEFORE reading the label.
define('STEGANO_OFFSET', 64); // Secret offset
define('STEGANO_WIDTH', 88);
define('STEGANO_HEIGHT', 14);
$watermark = new Imagick();
$watermark->setSize(STEGANO_WIDTH, STEGANO_HEIGHT);
$watermark->setPointSize(16);
$watermark->readImage('LABEL:Hello World!');
$stegano = $image->steganoImage($watermark, STEGANO_OFFSET);
I can't speak when/why this changed, but if you run the following...
convert 'label:Hello World!' decoded.png
... you would have the same image previously extracted from STEGANO: