SmartSquareFilter.php

<?php

class SmartSquareFilter extends AbstractImagickFilter
{
    protected function doApply(\Imagick $im)
    {
        $target = $im->getImageGeometry();

        if ($target['width'] === $target['height']) {
            // return early if already square
            return;
        }

        if ($target['width'] > $target['height']) {
            // landscape
            $target['width'] = $target['height'];
        } else {
            // portrait
            $target['height'] = $target['width'];
        }

        // how different entropy must be in order for it to matter
        $threshold = 0.5;

        // focus on this percentage of center of each slice
        $focus = 0.5;

        // the max size of each slice
        $maxSlice = 10;

        while (($diff = $im->getImageWidth() - $target['width']) > 0) {
            $width = $im->getImageWidth();
            $height = $im->getImageHeight();
            $slice = min($diff, $maxSlice);

            $left = clone $im;
            $left->cropImage($slice, $height * $focus, 0, ($height - ($height * $focus)) / 2);

            $right = clone $im;
            $right->cropImage($slice, $height * $focus, $width - $slice, ($height - $height * $focus) / 2);

            $diff = self::entropy($right) - self::entropy($left);

            if ($diff > $threshold) {
                $im->cropImage($width - $slice, $height, $slice, 0);
            } elseif ($diff < -$threshold) {
                $im->cropImage($width - $slice, $height, 0, 0);
            } elseif (rand(0, 1)) {
                $im->cropImage($width - $slice, $height, floor($slice / 2), 0);
            } else {
                $im->cropImage($width - $slice, $height, ceil($slice / 2), 0);
            }

            $im->setImagePage(0, 0, 0, 0);
        }

        while (($diff = $im->getImageHeight() - $target['height']) > 0) {
            $width = $im->getImageWidth();
            $height = $im->getImageHeight();
            $slice = min($diff, $maxSlice);

            $top = clone $im;
            $top->cropImage($width * $focus, $slice, ($width - $width * $focus) / 2, 0);

            $bottom = clone $im;
            $bottom->cropImage($width * $focus, $slice, ($width - $width * $focus) / 2, $height - $slice);

            $diff = self::entropy($top) - self::entropy($bottom);

            if ($diff > $threshold) {
                $im->cropImage($width, $height - $slice, 0, 0);
            } elseif ($diff < -$threshold) {
                $im->cropImage($width, $height - $slice, 0, $slice);
            } elseif (rand(0, 1)) {
                $im->cropImage($width, $height - $slice, 0, floor($slice / 2));
            } else {
                $im->cropImage($width, $height - $slice, 0, ceil($slice / 2));
            }

            $im->setImagePage(0, 0, 0, 0);
        }
    }

    private static function entropy(\Imagick $im)
    {
        $im = clone $im;

        // convert to grayscale
        $im->setImageType(\Imagick::IMGTYPE_GRAYSCALE);

        // calculate total number of pixels
        $area = $im->getImageWidth() * $im->getImageHeight();

        // use the histogram to calculate entropy
        $entropy = 0;
        foreach ($im->getImageHistogram() as $pixel) {
            if ($count = $pixel->getColorCount()) {
                $perc = $count / $area;
                $entropy += $perc * log($perc, 2);
            }
        }

        return $entropy * -1;
    }
}