how to output the OpenCV image out of the callback function?

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Hi, I am subscribing a videostream of compressed image and i knew how to convert it into OpenCV image and was able to show it already. But i want to output(like return value) the OpenCV image from the callback function.How should I do it?

void imageCallback_L(const sensor_msgs::CompressedImageConstPtr&msg)
     {
            try
            {
            cv::imshow("imgCallBack",cv::imdecode(cv::Mat(msg->data),1));
            cv::waitKey(1);
            ROS_INFO("receving fisheye_left_image");
            }
            catch(cv_bridge::Exception&e)
            {
            ROS_ERROR("cv_bridge exception: %s",e.what());
            }   
    }


int main(int argc, char** argv)
{
    ros::init(argc,argv,"CompressedImage");
    ros::NodeHandle nh;
    ros::Subscriber image_sub;
    std::string image_topic="/sensors/camera/fisheye_left/image_raw/compressed";
    image_sub=nh.subscribe(image_topic,10,imageCallback_L);

    while (ros::ok())
    {
        ros::spinOnce();
    }

    return 0;
}