Grafika\Gd\Image::histogram PHP Method

    public function histogram($slice = null)
    {
        $gd = $this->getCore();
        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();
        $aBin = array();
        for ($y = $sliceY; $y < $sliceY + $sliceH; $y++) {
            for ($x = $sliceX; $x < $sliceX + $sliceW; $x++) {
                $rgb = imagecolorat($gd, $x, $y);
                $a = $rgb >> 24 & 0x7f;
                // 127 in hex. These are binary operations.
                $r = $rgb >> 16 & 0xff;
                $g = $rgb >> 8 & 0xff;
                $b = $rgb & 0xff;
                if (!isset($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);
    }