var water_width = water_img.width;
                        var water_height = water_img.height;
                        if(img_width > 400 && img_width > water_width && water_width > 4) {//这里限制了水印添加的条件，可自行修改！
                                var x =  img_width - water_width - 16;
                                var y = img_height - water_height - 16;
                               
                                // 参数参考：https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/drawImage
                                ctx.globalAlpha = 0.3; // 水印透明度
                                ctx.beginPath();
                                ctx.drawImage(water_img, 0, 0, water_width, water_height, x, y, water_width, water_height);        // 将水印图像绘制到canvas上
                                ctx.closePath();
                                ctx.save();
                        }