Calculate bounding box of arbitrary pixel-based drawing

Question

Given a contiguous drawing of arbitrary pixels (e.g. on an HTML5 Canvas) is there any algorithm for finding the axis-aligned bounding box that is more efficient than simply

Answer 1

I dislike the current answer. Here's my code that I plugged into OP website. It's much faster in firefox and chrome.

The idea is check all pixels on x axis to see if there's a hit on the Y axis. If so update Y and increase X so we can scan for max X

function contextBoundingBox(ctx,alphaThreshold){
    if (alphaThreshold===undefined) alphaThreshold = 15;
    var w=ctx.canvas.width,h=ctx.canvas.height;
    var data = ctx.getImageData(0,0,w,h).data;

    let minX=w;
    let maxX=0
    let minY=h
    let maxY=0
    for(let y=0; y