<?php

**
 * Estimate image sharpnes based on color
 * similarity in pixels in a grid.
 *
 * @param Imagick $img
 * @return float
 */
function GetImageSharpness($img)
{

    $w = $img->getImageWidth();
    $h = $img->getImageHeight();
    $grid = $w / 9;
    $maximum = 0;

    for ($x = $grid / 2; $x <= $w - $grid / 2; $x += $grid) {

        $pixel = $img->getImagePixelColor($x, $grid / 2)->getColor();
        $r = $pixel['r'];
        $g = $pixel['g'];
        $b = $pixel['b'];
        $v0 = floor(0.35 * $r + 0.50 * $g + 0.15 * $b);
        for ($y = $grid / 2 + 1; $y <= $h - $grid / 2; $y++) {
            $pixel = $img->getImagePixelColor($x, $y)->getColor();
            $r = $pixel['r'];
            $g = $pixel['g'];
            $b = $pixel['b'];
            $v = floor(0.35 * $r + 0.50 * $g + 0.15 * $b);
            if ($maximum < abs($v - $v0)) {
                $maximum = abs($v - $v0);
                $xmax = $x;
                $ymax = $y;
            }
            $v0 = $v;
        }
    }

    for ($y = $grid / 2; $y <= $h - $grid / 2; $y += $grid) {
        $pixel = $img->getImagePixelColor($grid / 2, $y)->getColor();
        $r = $pixel['r'];
        $g = $pixel['g'];
        $b = $pixel['b'];
        $v0 = floor(0.35 * $r + 0.50 * $g + 0.15 * $b);
        for ($x = $grid / 2 + 1; $x <= $w - $grid / 2; $x++) {
            $pixel = $img->getImagePixelColor($x, $y)->getColor();
            $r = $pixel['r'];
            $g = $pixel['g'];
            $b = $pixel['b'];
            $v = floor(0.35 * $r + 0.50 * $g + 0.15 * $b);
            if ($maximum < abs($v - $v0)) {
                $maximum = abs($v - $v0);
                $xmax = $x;
                $ymax = $y;
            }
            $v0 = $v;
        }
    }

    $maxv = 0;
    $minv = 255;
    for ($x = $xmax - 4; $x <= $xmax + 4; $x++) {
        for ($y = $ymax - 4; $y <= $ymax + 4; $y++) {
            $pixel = $img->getImagePixelColor($x, $y)->getColor();
            $r = $pixel['r'];
            $g = $pixel['g'];
            $b = $pixel['b'];
            $v = floor(0.35 * $r + 0.50 * $g + 0.15 * $b);
            if ($v > $maxv) $maxv = $v;
            if ($v < $minv) $minv = $v;
        }
    }

    return round(($maximum / (15 + $maxv - $minv)) * 27000 / 255, 2);
}