function trimCanvas(canvas) {
    var context = canvas.getContext("2d");

    var imgWidth = canvas.width;
    var imgHeight = canvas.height;

    var imgData = context.getImageData(0, 0, imgWidth, imgHeight).data;

    // get the corners of the relevant content (everything that's not white)
    var cropTop = scanY(true, imgWidth, imgHeight, imgData);
    var cropBottom = scanY(false, imgWidth, imgHeight, imgData);
    var cropLeft = scanX(true, imgWidth, imgHeight, imgData);
    var cropRight = scanX(false, imgWidth, imgHeight, imgData);

    // + 1 is needed because this is a difference, there are n + 1 pixels in
    // between the two numbers inclusive
    var cropXDiff = (cropRight - cropLeft) + 1;
    var cropYDiff = (cropBottom - cropTop) + 1;

    // get the relevant data from the calculated coordinates
    var trimmedData = context.getImageData(cropLeft, cropTop, cropXDiff, cropYDiff);

    // set the trimmed width and height
    canvas.width = cropXDiff;
    canvas.height = cropYDiff;
    // clear the canvas
    context.clearRect(0, 0, cropXDiff, cropYDiff);
    // place the trimmed data into the cleared canvas to create
    // a new, trimmed canvas
    context.putImageData(trimmedData, 0, 0);
    return canvas; // for chaining
}

// returns the RGBA values of an x, y coord of imgData
function getRGBA(x, y, imgWidth, imgData) {
    return {
        red: imgData[(imgWidth * y + x) * 4],
        green: imgData[(imgWidth * y + x) * 4 + 1],
        blue: imgData[(imgWidth * y + x) * 4 + 2],
        alpha: imgData[(imgWidth * y + x) * 4 + 3]
    }
}

function getAlpha(x, y, imgWidth, imgData) {
    return getRGBA(x, y, imgWidth, imgData).alpha;
}

// finds the first y coord in imgData that is not white
function scanY(fromTop, imgWidth, imgHeight, imgData) {
    var offset = fromTop ? 1 : -1;
    var firstCol = fromTop ? 0 : imgHeight - 1;

    // loop through each row
    for(var y = firstCol; fromTop ? (y < imgHeight) : (y > -1); y += offset) {
        // loop through each column
        for(var x = 0; x < imgWidth; x++) {
            // if not white, return col
            if(getAlpha(x, y, imgWidth, imgData)) {
                return y;
            }
        }
    }

    // the whole image is white already
    return null
}

// finds the first x coord in imgData that is not white
function scanX(fromLeft, imgWidth, imgHeight, imgData) {
    var offset = fromLeft ? 1 : -1;
    var firstRow = fromLeft ? 0 : imgWidth - 1;

    // loop through each column
    for(var x = firstRow; fromLeft ? (x < imgWidth) : (x > -1); x += offset) {
        // loop through each row
        for(var y = 0; y < imgHeight; y++) {
            // if not white, return row
            if(getAlpha(x, y, imgWidth, imgData)) {
                return x;
            }
        }
    }

    // the whole image is white already
    return null;
}