Trims the surrounding transparent pixels from a canvas

trim-canvas.js

('HTMLCanvasElement' in this) && (function () {
	HTMLCanvasElement.prototype.trim = function (opts) {
		opts = opts || {};

		var
		element = this,
		bound = {
			top: null,
			left: null,
			right: null,
			bottom: null
		},
		ctx = element.getContext('2d'),
		newctx = document.createElement('canvas').getContext('2d'),
		pixels = ctx.getImageData(0, 0, element.width, element.height),
		l = pixels.data.length,
		i, x, y;

		for (i = 0; i < l; i += 4) {
			if (pixels.data[i + 3] !== 0) {
				x = (i / 4) % element.width;
				y = ~~((i / 4) / element.width);
		
				if (bound.top === null) {
					bound.top = y;
				}
				
				if (bound.left === null) {
					bound.left = x; 
				} else if (x < bound.left) {
					bound.left = x;
				}
				
				if (bound.right === null) {
					bound.right = x; 
				} else if (bound.right < x) {
					bound.right = x;
				}
				
				if (bound.bottom === null) {
					bound.bottom = y;
				} else if (bound.bottom < y) {
					bound.bottom = y;
				}
			}
		}

		var
		trimmedHeight = Math.max(bound.bottom - bound.top, 1),
		trimmedWidth = Math.max(bound.right - bound.left, 1),
		trimmedData = ctx.getImageData(bound.left, bound.top, trimmedWidth, trimmedHeight),
		offsetX = 0,
		offsetY = 0;

		if (opts.minAspectRatio && opts.minAspectRatio > (trimmedWidth / trimmedHeight)) {
			var trimmedWidthNew = trimmedHeight * opts.minAspectRatio;
			offsetX = (trimmedWidthNew - trimmedWidth) / 2;
			trimmedWidth = trimmedWidthNew;
		}
		else if (opts.maxAspectRatio && opts.maxAspectRatio < (trimmedWidth / trimmedHeight)) {
			var trimmedHeightNew = trimmedWidth / opts.maxAspectRatio;
			offsetY = (trimmedHeightNew - trimmedHeight) / 2;
			trimmedHeight = trimmedHeightNew;
		}

		if (opts.minWidth && opts.minWidth > trimmedWidth) {
			var trimmedWidthNew = opts.minWidth;
			offsetX += (trimmedWidthNew - trimmedWidth) / 2;
			trimmedWidth = trimmedWidthNew;
		}

		if (opts.minHeight && opts.minHeight > trimmedHeight) {
			var trimmedHeightNew = opts.minHeight;
			offsetY += (trimmedHeightNew - trimmedHeight) / 2;
			trimmedHeight = trimmedHeightNew;
		}

		newctx.canvas.width = trimmedWidth;
		newctx.canvas.height = trimmedHeight;
		newctx.putImageData(trimmedData, offsetX, offsetY);

		return newctx.canvas;
	};
})();