Grafika\Imagick\Image::histogram PHP Method

    public function histogram($slice = null)
    {
        if (null === $slice) {
            $sliceX = 0;
            $sliceY = 0;
            $sliceW = $this->getWidth();
            $sliceH = $this->getHeight();
        } else {
            $sliceX = $slice[0][0];
            $sliceY = $slice[0][1];
            $sliceW = $slice[1][0];
            $sliceH = $slice[1][1];
        }
        $rBin = array();
        $gBin = array();
        $bBin = array();
        $aBin = array();
        // Loop using image
        $pixelIterator = $this->getCore()->getPixelIterator();
        foreach ($pixelIterator as $y => $rows) {
            /* Loop through pixel rows */
            if ($y >= $sliceY and $y < $sliceY + $sliceH) {
                foreach ($rows as $x => $px) {
                    /* Loop through the pixels in the row (columns) */
                    if ($x >= $sliceX and $x < $sliceX + $sliceW) {
                        /**
                         * @var $px \ImagickPixel */
                        $pixel = $px->getColor();
                        $r = $pixel['r'];
                        $g = $pixel['g'];
                        $b = $pixel['b'];
                        $a = $pixel['a'];
                        if (!isset($rBin[$r])) {
                            $rBin[$r] = 1;
                        } else {
                            $rBin[$r]++;
                        }
                        if (!isset($gBin[$g])) {
                            $gBin[$g] = 1;
                        } else {
                            $gBin[$g]++;
                        }
                        if (!isset($bBin[$b])) {
                            $bBin[$b] = 1;
                        } else {
                            $bBin[$b]++;
                        }
                        if (!isset($aBin[$a])) {
                            $aBin[$a] = 1;
                        } else {
                            $aBin[$a]++;
                        }
                    }
                }
            }
        }
        return array('r' => $rBin, 'g' => $gBin, 'b' => $bBin, 'a' => $aBin);
    }