<?php
/* (C) 2008 by Julian von Mendel (prog@derjulian.net)
 * License: LGPL3
 * $LastChangedBy: jvm $
 * $LastChangedDate: 2008-04-03 14:04:24 +0000 (Thu, 03 Apr 2008) $
 * $Revision: 5 $
 *
 * Todo: generalize
 */

require_once("image.php");

class image_sobel extends imagealgorithmmask {
	/* sobel mask */
	public static $gx = array(
			array(-1, 0, 1),
			array(-2, 0, 2),
			array(-1, 0, 1));
	public static $gy = array(
			array(1, 2, 1),
			array(0, 0, 0),
			array(-1, -2, -1));
    protected $colors = true;
    protected $alpha = array();
    protected $maxbrightness = false;

    public function setmaxbrightness($yes) {
        $this->maxbrightness = $yes;
        return $this;
    }

    public function setcolors($yes) {
        $this->colors = $yes;
        return $this;
    }

    public function getalpha() {
        return $this->alpha;
    }

	public function algorithm($x, $y) {
		if (($y == 0 || $y == $this->image->height() - 1) ||
				($x == 0 || $x == $this->image->width() - 1))
			return 0;

		$sumx = 0;
		$sumy = 0;

		for($i = -1; $i <= 1; $i++) { for($j = -1; $j <= 1; $j++) {
			if (self::$gx[$i + 1][$j + 1] == 0 &&
					self::$gy[$i + 1][$j + 1] == 0)
				continue;
			
			$intensity = $this->image->getpixel($x + $i, $y + $j, false);
            $intensity = reset($intensity);
			$sumx += $intensity * self::$gx[$i + 1][$j + 1];
			$sumy += $intensity * self::$gy[$i + 1][$j + 1];
		}}
		
		$sum = sqrt(pow($sumx, 2) + pow($sumy, 2));

        $alpha = atan2($sumy, $sumx) * 180 / M_PI;
        $alpha = round($alpha / 45) * 45;
        if ($alpha < 0)
            $alpha += 180;
        $alpha %= 180;

        if ($y == 0)
            $this->alpha[$x] = array();
        $this->alpha[$x][$y] = $alpha;

        if ($sum < $this->threshold)
            return 0;

        if ($this->maxbrightness)
            $sum = 255;

        if (!$this->colors)
            return $sum;

        switch ($alpha) {
            case 0:
                return array("red" => $sum, "green" => $sum, "blue" => 0);
            case 45: /* red */
                return array("red" => $sum, "green" => 0, "blue" => 0);
            case 90: /* blue */
                return array("red" => 0, "green" => 0, "blue" => $sum);
            case 135: /* green */
               return array("red" => 0, "green" => $sum, "blue" => 0);
            default:
                throw new AlgorithmException("Impossible.");
        }
	}
}