php background triangle pattern generator

bggen

<?php

if (empty($_GET['w']) || empty($_GET['h']) || empty($_GET['tw'])) {
	die('w=width&h=height&tw=trianglewidth');
}

$w = $_GET['w'];
$h = $_GET['h'];
$tw = $_GET['tw'];

$sx = $tw;
$sy = $sx * 2 / sqrt(3);

$im = @imagecreate($w, $h) or die("Cannot Initialize new GD image stream");
if (!isset($_GET['d'])) header("Content-Type: image/png");
imagefilledrectangle($im, 0, 0, $w, $h, imagecolorallocatealpha($im, 90, 90, 90, 0));

$_m = 127 / 255;
$c = array(
	imagecolorallocatealpha($im, 255, 255, 255, (255 - 27) * $_m),
	imagecolorallocatealpha($im, 255, 255, 255, (255 - 36) * $_m),
	imagecolorallocatealpha($im, 255, 255, 255, (255 - 45) * $_m),
	imagecolorallocatealpha($im, 255, 255, 255, (255 - 43) * $_m),
	imagecolorallocatealpha($im, 255, 255, 255, (255 - 18) * $_m),
);
$cc = count($c) - 1;
$bc = imagecolorallocatealpha($im, 255, 255, 255, (255 - 41) * $_m);

for ($i = 0; $i < $w / $sx; $i++) {
	$z = 0;
	for ($j = -$sy; $j < $h; $j += $sy / 2) {
		$z = $z ^ 1;
		$points = array(
			$i * $sx, $j,
			$i * $sx + $sx, $j + $sy / 2,
			$i * $sx, $j + $sy,
		);
		if ($i % 2 == 0) {
			for ($k = 0; $k < 3; $k++) {
				$points[$k * 2 + 1] += $sy / 2;
			}
		}
		if ($z % 2 == 0) {
			$points[0] += $sx;
			$points[2] -= $sx;
			$points[4] += $sx;
		}
		imagefilledpolygon($im, $points, 3, $c[rand(0, $cc)]);
		imageline($im, $points[0], $points[1], $points[2], $points[3], $bc);
		imageline($im, $points[2], $points[3], $points[4], $points[5], $bc);
	}
}

imagepng($im);
imagedestroy($im);