Returns a copy of a canvas element with surrounding transparent space removed

trim_canvas.js

var trimCanvas = (function() {
    function rowBlank(imageData, width, y) {
        for (var x = 0; x < width; ++x) {
            if (imageData.data[y * width * 4 + x * 4 + 3] !== 0) return false;
        }
        return true;
    }

    function columnBlank(imageData, width, x, top, bottom) {
        for (var y = top; y < bottom; ++y) {
            if (imageData.data[y * width * 4 + x * 4 + 3] !== 0) return false;
        }
        return true;
    }

    return function(canvas) {
        var ctx = canvas.getContext("2d");
        var width = canvas.width;
        var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
        var top = 0, bottom = imageData.height, left = 0, right = imageData.width;

        while (top < bottom && rowBlank(imageData, width, top)) ++top;
        while (bottom - 1 > top && rowBlank(imageData, width, bottom - 1)) --bottom;
        while (left < right && columnBlank(imageData, width, left, top, bottom)) ++left;
        while (right - 1 > left && columnBlank(imageData, width, right - 1, top, bottom)) --right;

        var trimmed = ctx.getImageData(left, top, right - left, bottom - top);
        var copy = canvas.ownerDocument.createElement("canvas");
        var copyCtx = copy.getContext("2d");
        copy.width = trimmed.width;
        copy.height = trimmed.height;
        copyCtx.putImageData(trimmed, 0, 0);

        return copy;
    };
})();