'Pixellate' a graphic based on mesh gradients

pixels.php

<?php
define('BOX_WIDTH', 45);
define('BOX_MARGIN', 5);

$image = imagecreatefromjpeg('callanish.jpg');
$width = imagesx($image);
$height = imagesy($image);

$s = new CairoImageSurface(CairoFormat::ARGB32, $width, $height);
$c = new CairoContext($s);
$c->setSourceRGB(0, 0, 0);
$c->rectangle(0, 0, $width, $height);
$c->fill();

function getColourAt($image, $x, $y) {
  $colour = imagecolorat($image, $x, $y);
	$r = $colour >> 16;
	$g = ($colour >> 8) & 255;
	$b = $colour & 255;

	$r = ($r == 0) ? 0 : 1 / (255 / $r);
	$g = ($g == 0) ? 0 : 1 / (255 / $g);
	$b = ($b == 0) ? 0 : 1 / (255 / $b);

	return array($r, $g, $b);
}

function roundRect(CairoContext $c, $x, $y, $width, $height, $aspect, $radius) {
	$radius = $radius / $aspect;
	$degrees = M_PI / 180.0;

	$c->newSubPath();
	$c->arc($x + $width - $radius, $y + $radius, $radius, -90 * $degrees, 0 * $degrees);
	$c->arc($x + $width - $radius, $y + $height - $radius, $radius, 0 * $degrees, 90 * $degrees);
	$c->arc($x + $radius, $y + $height - $radius, $radius, 90 * $degrees, 180 * $degrees);
	$c->arc($x + $radius, $y + $radius, $radius, 180 * $degrees, 270 * $degrees);
	$c->closePath();
}

for ($x = 0; $x < ($width - BOX_WIDTH); $x += BOX_WIDTH + BOX_MARGIN) {
	for ($y = 0; $y < ($height - BOX_WIDTH); $y += BOX_WIDTH + BOX_MARGIN) {

		$colour1 = getColourAt($image, $x, $y);
		$colour2 = getColourAt($image, $x + BOX_WIDTH, $y);
		$colour3 = getColourAt($image, $x + BOX_WIDTH, $y + BOX_WIDTH);
		$colour4 = getColourAt($image, $x, $y + BOX_WIDTH);

		$pattern = new CairoMeshPattern();
		$pattern->beginPatch();
		$pattern->moveTo($x, $y);
		$pattern->lineTo($x + BOX_WIDTH, $y);
		$pattern->lineTo($x + BOX_WIDTH, $y + BOX_WIDTH);
		$pattern->lineTo($x, $y + BOX_WIDTH);
		$pattern->setCornerColorRGB(0, $colour1[0], $colour1[1], $colour1[2]);
		$pattern->setCornerColorRGB(1, $colour2[0], $colour2[1], $colour2[2]);
		$pattern->setCornerColorRGB(2, $colour3[0], $colour3[1], $colour3[2]);

		$pattern->setCornerColorRGB(3, $colour4[0], $colour4[1], $colour4[2]);
		$pattern->endPatch();

		$c->setSource($pattern);
		roundRect($c, $x, $y, BOX_WIDTH, BOX_WIDTH, 1.0, BOX_WIDTH / 5);
		$c->fill();
	}
}

$s->writeToPng('output.png');