1-bit Atkinson dither in PHP with GD

Atkinson dither in PHP

<?php

$img = imagecreatefromjpeg('./delayclose.jpg');
imagefilter($img, IMG_FILTER_GRAYSCALE); 
$width = imagesx($img);
$height = imagesy($img);
$img_arr = array();

// Parse image (can be combined with dither stage, but combining them is slower.)

for($y=0; $y < $height; $y++){
    for($x=0; $x < $width; $x++){
        $img_arr[$x][$y] = imagecolorat($img, $x, $y);
    }
}


// make a b/w output image.
$output = imagecreate($width, $height);
$black = imagecolorallocate($output, 0, 0, 0); //background color.
$white = imagecolorallocate($output, 0xff, 0xff, 0xff);

// Dither image with Atkinson dither

/* Atkinson Error Diffusion Kernel:

1/8 is 1/8 * quantization error.

+-------+-------+-------+-------+
|       | Curr. |  1/8  |  1/8  |
+-------|-------|-------|-------|
|  1/8  |  1/8  |  1/8  |       |
+-------|-------|-------|-------|
|       |  1/8  |       |       |
+-------+-------+-------+-------+

*/

for($y=0; $y < $height; $y++){
    for($x=0; $x < $width; $x++){
        $old = $img_arr[$x][$y];
        if($old > 0xffffff*.5){ // This is the b/w threshold. Currently @ halfway between white and black.
            $new = 0xffffff;
            imagesetpixel($output, $x, $y, $white); // Only setting white pixels, because the image is already black.
        }else{
            $new = 0x000000;
        }
        $quant_error = $old-$new;
        $error_diffusion = (1/8)*$quant_error; //I can do this because this dither uses 1 value for the applied error diffusion.
        //dithering here.
        $img_arr[$x+1][$y] += $error_diffusion;
        $img_arr[$x+2][$y] += $error_diffusion;
        $img_arr[$x-1][$y+1] += $error_diffusion;
        $img_arr[$x][$y+1] += $error_diffusion;
        $img_arr[$x+1][$y+1] += $error_diffusion;
        $img_arr[$x][$y+2] += $error_diffusion;
    }
}

// plop out a png of the dithered image.

Header("Content-type: image/png");
imagepng($output, NULL, 9); //to print to screen

?>