<?php

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

// header("Content-type: image/png");

$invert = FALSE;

/* TODO:

- Error checking
- Automated color checking (white on black, e.g.)

*/

$fill_hex = $_GET['fill'];
if ($fill_hex == "") $fill_hex = "000000";

$src_img = imagecreatefromfile($_GET['url']);
if ($src_img === false) {

      header('HTTP/1.1 500 Internal Server Error');
      echo 'Internal error';
}
imagealphablending($src_img, FALSE);
imagesavealpha($src_img, TRUE);

/* copy source image to one with a white background to get rid of any alpha already in the image. */
/* this obviously fails if it's a white transparent png, e.g. */

$mask_x = imagesx($src_img);
$mask_y = imagesy($src_img);

$mask_img = @imagecreatetruecolor($mask_x,$mask_y);
imagealphablending($mask_img, TRUE); // important to layer pngs
imagesavealpha($mask_img, TRUE);
imagefill( $mask_img, 0, 0, imagecolorallocate( $mask_img, 255, 255, 255 ) );
imagecopy($mask_img, $src_img, 0, 0, 0, 0, $mask_x, $mask_y);
imagedestroy($src_img);

/* convert to greyscale */
imagefilter($mask_img, IMG_FILTER_GRAYSCALE);

/* invert if necessary */
/* check upper left corner: if white, then we want that to be transparent, so invert */
$alpha = imagecolorsforindex( $mask_img, imagecolorat( $mask_img, 0, 0) );
if ($alpha['red'] = 255) $invert = true;
if ($invert)
	imagefilter($mask_img, IMG_FILTER_NEGATE);

/* final $img_mask is 128-bit greyscale image, where black will be 100% transparent and white will be 100% opaque */

$img = @imagecreatetruecolor($mask_x,$mask_y);
imagesavealpha($img, TRUE);
imagealphablending($img, FALSE);
imagefill( $img, 0, 0, imagecolorallocatealpha( $img, 255, 255, 255, 127 ) );

for ($x=0; $x<$mask_x; $x++) {
	for ($y=0; $y<$mask_y; $y++) {
		$alpha = imagecolorsforindex( $mask_img, imagecolorat( $mask_img, $x, $y ) );
		$alpha = 127 - floor( $alpha[ 'red' ] / 2 );
		imagesetpixel($img, $x, $y, imagecolorallocatehexalpha($img, $fill_hex, $alpha));
	}
}

imagepng($img);

imagedestroy($mask_img);
imagedestroy($img);

/* functions */

function imagecreatefromfile( $filename ) {
	 return imagecreatefromstring(file_get_contents($filename));
}

function imagecolorallocatehexalpha ($img, $hex, $alpha) {
	$int = hexdec($hex);
	$color = array("red" => 0xFF & ($int >> 0x10),
               "green" => 0xFF & ($int >> 0x8),
               "blue" => 0xFF & $int);
	return imagecolorallocatealpha($img, $color['red'],$color['green'],$color['blue'],$alpha);
}

?>