Calculate image variance in Imagick - php

Question

How can I calculate the variation of an image in Imagick? I already converted the image in greyscale, but through native functions I can only get Mean and Standard Deviation.

Answer 1

My approach is generating two noise images and calculate the Euclidian difference between both RGB pixel color values.
This will certainly look better with real images for example of a webcam.
$getRandomNoiseImage = function ($x, $y, $steps): Imagick {
$draw = new ImagickDraw();
$draw->setResolution($x, $y);
$draw->setStrokeWidth(1);
$pixel = new ImagickPixel();
for ($i = 0; $i < $x; $i += $steps) {
for ($j = 0; $j < $y; $j += $steps) {
$color = join(',', [rand(0, 255), rand(0, 255), rand(0, 255)]);
$pixel->setColor("rgb($color)");
$draw->setFillColor($pixel);
$draw->setStrokeColor($pixel);
$draw->rectangle($i, $j, $i + $steps, $j + $steps);
}
}
$im = new Imagick();
$im->newImage($x, $y, new ImagickPixel('black'));
$im->drawImage($draw);
return $im;
};
$width = 64;
$height = 64;
$im1 = $getRandomNoiseImage($width, $height, 16);
$im2 = $getRandomNoiseImage($width, $height, 16);
$draw = new ImagickDraw();
$draw->setResolution($width, $height);
$draw->setStrokeWidth(1);
$pixel = new ImagickPixel();
for ($i = 0; $i < $width; $i++) {
for ($j = 0; $j < $height; $j++) {
$c1 = $im1->getImagePixelColor($i, $j)->getColor(0);
$c2 = $im2->getImagePixelColor($i, $j)->getColor(0);
$color = [
sqrt(abs($c2['r']**2 - $c1['r']**2)),
sqrt(abs($c2['g']**2 - $c1['g']**2)),
sqrt(abs($c2['b']**2 - $c1['b']**2)),
];
$pixel->setColor(sprintf('rgb(%d,%d,%d)', ...$color));
$draw->setFillColor($pixel);
$draw->setStrokeColor($pixel);
$draw->rectangle($i, $j, $i + 1, $j + 1);
}
}
$im3 = new Imagick();
$im3->newImage($width, $height, new ImagickPixel('black'));
$im3->drawImage($draw);
$im1->writeImage('image1.png');
$im2->writeImage('image2.png');
$im3->writeImage('image3.png');