base64图片裁剪空白区域

/**
     * 裁剪空白区域
     * @param base64
     * @param callback
     */
    function tailor(base64,callback=null) {
        var img = new Image();//创建图片对象
        img.src = base64;
        img.onload = function () {
            var c = document.createElement('canvas');//创建处理画布对象
            var ctx = c.getContext('2d');
            c.width = img.width;
            c.height = img.height;
            ctx.drawImage(img, 0, 0);//绘制
            var imgData = ctx.getImageData(0, 0, c.width, c.height).data;//读取图片数据
            var lOffset = c.width, rOffset = 0, tOffset = c.height, bOffset = 0;
            for (var i = 0; i < c.width; i++) {
                for (var j = 0; j < c.height; j++) {
                    var pos = (i + c.width * j) * 4
                    if (imgData[pos] == 255 || imgData[pos + 1] == 255 || imgData[pos + 2] == 255 || imgData[pos + 3] == 255) {
                        bOffset = Math.max(j, bOffset); // 找到有色彩的最下端
                        rOffset = Math.max(i, rOffset); // 找到有色彩的最右端
                        tOffset = Math.min(j, tOffset); // 找到有色彩的最上端
                        lOffset = Math.min(i, lOffset); // 找到有色彩的最左端
                    }
                }
            }
            lOffset++;
            rOffset++;
            tOffset++;
            bOffset++;
            var x = document.createElement("canvas");//创建处理后画布对象
            x.width = rOffset - lOffset;
            x.height = bOffset - tOffset;
            var xx = x.getContext("2d");
            xx.drawImage(img, lOffset, tOffset, x.width, x.height, 0, 0, x.width, x.height);//绘制
            if (callback){
                callback(x.toDataURL());
            }

        }
    }