Calculate bounding box for content on an HTML5 Canvas

contentBoundingBox.js

const contextBoundingBox = (ctx: CanvasRenderingContext2D, alphaThreshold: number) => {
	const effectiveAlphaThrh = alphaThreshold || 15;
	const w = ctx.canvas.width;
	const h = ctx.canvas.height;
	const data = ctx.getImageData(0, 0, w, h).data;
	let x: number;
	let y: number;
	let minX: number;
	let minY: number;
	let maxX: number;
	let maxY: number;
​
	o1: {
		for (y = h; y--;) {
		for (x = w; x--;) {
			if (data[(w * y + x) * 4 + 3] > effectiveAlphaThrh) {
				maxY = y;
				break o1;
			}
		}
		if (!maxY) {
			return;
		}
	}
​
	o2: {
		for (x=w;x--;) {
			for (y=maxY+1;y--;)      if (data[(w*y+x)*4+3]>effectiveAlphaThrh){ maxX=x; break o2 }
	o3: for (x=0;x<=maxX;++x) for (y=maxY+1;y--;)      if (data[(w*y+x)*4+3]>effectiveAlphaThrh){ minX=x; break o3 }
	o4: for (y=0;y<=maxY;++y) for (x=minX;x<=maxX;++x) if (data[(w*y+x)*4+3]>effectiveAlphaThrh){ minY=y; break o4 }
	return {x:minX,y:minY,maxX:maxX,maxY:maxY,w:maxX-minX,h:maxY-minY};
}