HTML Canvas: Tests if .shadowOffsetX or .shadowOffsetY context properties are inverted (Android bug)

shadowtransform.js

/**
 * Usually shadowOffsetX and shadowOffsetY work as expected, but Android
 * browser has a known issue where it inverts shadowOffsetY.
 * (See http://code.google.com/p/android/issues/detail?id=16025)
 * So, we test for it, creating an offset object for x and y
 */
var shadowTransform = (function() {
	// create a 3x3 canvas and put a 1px square in the middle
	var canvas = document.createElement('canvas');
	canvas.height = canvas.width = 3;
	// -1 means shadow SHOULD cast to top left pixel
	var offset = -1;
	
	var cc = canvas.getContext('2d');
	cc.shadowOffsetX = offset;
	cc.shadowOffsetY = offset;
	cc.shadowBlur = 1;
	cc.shadowColor = 'red';
	cc.fillRect(1, 1, 1, 1);

	var data = cc.getImageData(0, 0, 3, 3).data;
	var transform = { x: 1, y: 1 };
	// depending on where the shadow is cast, adjust transform
	switch(255) {
		case data[32]:
			transform.x = transform.y = -1;
			//alert('x is bad, y is bad');
			break;
		case data[24]:
			transform.y = -1;
			//alert('y is bad');
			break;
		case data[8]:
			transform.x = -1;
			//alert('x is bad');
			break;
		case data[0]:
			//alert('correct');
			break;
	};
	
	return transform;

})();