Calculates the "busiest" square area of an image to use for a thumbnail.

squarize.php

<?php

/**
 * An example of how to use the `squarize()` function below.
 */

$path_input = 'input.png';
$path_output = 'output.png';
$thumb_w = 100;
$thumb_h = 100;

$input = imagecreatefrompng($path_input);
$output = imagecreatetruecolor($thumb_w, $thumb_h);

list($crop_x, $crop_y, $crop_w, $crop_h) = squarize($source);

imagecopyresampled($output, $input, 0, 0, $crop_x, $crop_y, $thumb_w, $thumb_h, $crop_w, $crop_h);
imagepng($output, $path_output, 9);

imagedestroy($input);
imagedestroy($output);


/**
 * Calculates the "busiest" square area of an image to use for a thumbnail.
 *
 * Samples a grid of pixels from an image, calculates the standard deviation of the RGB values of
 * those pixels, and returns information about the biggest square area of the image with the largest
 * standard deviation of colors (i.e. the "busiest" part of the image).
 *
 * @param resource $img    A GD image link resource.
 * @param integer  $width  The width of the source image (optional).
 * @param integer  $height The height of the source image (optional).
 * @param integer  $pixels The distance between each pixel to sample (default: 10).
 *
 * @return array An array containing the x-coordinate of the top left corner of the crop area, the
 *               y-coordinate of the top left corner of the crop area, the width of the crop area,
 *               and the height of the crop area.
 */
function squarize($img, $width = null, $height = null, $pixels = 10)
{
    if ($width === null || $height === null) {
        $width = imagesx($img);
        $height = imagesy($img);
    }

    $rgb = array();

    for ($x = 0; $x < $width; $x += $pixels) {
        for ($y = 0; $y < $height; $y += $pixels) {
            $color = imagecolorat($img, $x, $y);
            $rgb[$x][$y] = array(($color >> 16) & 0xFF, ($color >> 8) & 0xFF, $color & 0xFF);
        }
    }

    $dev_max = 0;
    $diff = abs($height - $width);
    $x_bound = array(0, $width);
    $y_bound = array(0, $height);
    $count = pow((int)(min($height, $width) / $pixels), 2);

    for ($i = 0, $r = 0, $g = 0, $b = 0, $dev = 0; $i < $diff; $i += $pixels) {
        if ($height > $width) {
            $y_bound = array($i, $width + $i);
        } elseif ($height < $width) {
            $x_bound = array($i, $height + $i);
        }

        for ($x = $x_bound[0]; $x < $x_bound[1]; $x += $pixels) {
            for ($y = $y_bound[0]; $y < $y_bound[1]; $y += $pixels) {
                $r += $rgb[$x][$y][0];
                $g += $rgb[$x][$y][1];
                $b += $rgb[$x][$y][2];
            }
        }

        $r /= $count;
        $g /= $count;
        $b /= $count;

        for ($x = $x_bound[0]; $x < $x_bound[1]; $x += $pixels) {
            for ($y = $y_bound[0]; $y < $y_bound[1]; $y += $pixels) {
                $tmp = ($rgb[$x][$y][0] - $r) * ($rgb[$x][$y][0] - $r);
                $tmp += ($rgb[$x][$y][1] - $g) * ($rgb[$x][$y][1] - $g);
                $tmp += ($rgb[$x][$y][2] - $b) * ($rgb[$x][$y][2] - $b);
                $dev += sqrt($tmp);
            }
        }

        $dev /= $count;

        if ($dev > $dev_max) {
            $dev_max = $dev;
            $crop_x = $x_bound[0];
            $crop_y = $y_bound[0];
        }
    }

    $crop_w = min($width, $height);
    $crop_h = $crop_w;

    return array($crop_x, $crop_y, $crop_w, $crop_h);

}//end squarize()