[Goal] sensor_msgs::Image 를 cv::Mat & cv::Mat 을 sensor_msgs::Image 로 변환

[Convert sensor_msgs::Image to cv::Mat]

Reference Site: https://github.com/HKUST-Aerial-Robotics/VINS-Fusion/blob/be55a937a57436548ddfb1bd324bc1e9a9e828e0/vins_estimator/src/rosNodeTest.cpp#L48

Code example

  cv::Mat sensorMsg2cvMat(const sensor_msgs::ImageConstPtr &img_msg)
  {
      cv_bridge::CvImageConstPtr ptr;
      if (img_msg->encoding == "8UC1")
      {
          sensor_msgs::Image img;
          img.header = img_msg->header;
          img.height = img_msg->height;
          img.width = img_msg->width;
          img.is_bigendian = img_msg->is_bigendian;
          img.step = img_msg->step;
          img.data = img_msg->data;
          img.encoding = "mono8";
          ptr = cv_bridge::toCvCopy(img, sensor_msgs::image_encodings::MONO8);
      }
      else
          ptr = cv_bridge::toCvCopy(img_msg, sensor_msgs::image_encodings::MONO8);
        
      cv::Mat img = ptr->image.clone();
  		cv::cvtColor(img,img,cv::COLOR_BGRA2BGR);
        
      return img;
  }

[Convert cv::Mat to sensor_msgs::Image]

Reference Site: https://stackoverflow.com/questions/27080085/how-to-convert-a-cvmat-into-a-sensor-msgs-in-ros

Code example

  sensor_msgs::Image cvMat2sensorMsg(cv::Mat image, std_msgs::Header header)
  {
  	cv_bridge::CvImage img_bridge;
  	sensor_msgs::Image img;
        
  	img_bridge = cv_bridge::CvImage(header, sensor_msgs::image_encodings::RGB8, image);
  	img_bridge.toImageMsg(img);
        
  	return img;
  }

[Others]

How to return an empty cv::Mat in a function for c++?

Reference Site: https://stackoverflow.com/questions/59113391/how-to-return-an-empty-mat-in-a-function-for-c

To change return following as:

  cv::Mat sensorMsg2cvMat(const sensor_msgs::Image &img_msg)
  {
  	// ***** fill in your code ***** //
  	cv_bridge::CvImageConstPtr ptr;
  	ptr = cv_bridge::toCvCopy(img_msg, sensor_msgs::image_encodings::MONO8);
  	cv::Mat img = ptr->image.clone();
            
  	// Case 1. return empty image 
  	return cv::Mat{};
            
  	// Case 2. return black image same size as img_msg
  	return cv::Mat{img.size(), img.type(), cv::Scalar{0,0,0}};
  }

[Error]

OpenCV Error: Assertion failed (_image.type() == CV_8UC3) in apply
- Reference Site: https://stackoverflow.com/questions/37764838/bilateral-filter-error-assertion-failed-src-type-cv-8uc1-src-type
- Code change following as (이미지 filter 문제):
```
  cv::cvtColor(img, img, cv::COLOR_BGRA2BGR);
```

**OpenCV Error: Assertion failed (scn == 3 scn == 4) in cvtColor**
- Reference Site: http://daplus.net/python-열린-cv-오류-215-scn-3-cvtcolor-함수의-scn-4/
- Reference Site: https://snowdeer.github.io/ros2/2018/01/04/ros2-opencv-camera-image-subscriber/
- Code change following as (이미지 channel 문제):
```
  // Error Code Line
  ptr = cv_bridge::toCvCopy(img, sensor_msgs::image_encodings::MONO8);
            
  // Modfied following as:
  ptr = cv_bridge::toCvCopy(img_msg, sensor_msgs::image_encodings::BGRA8);
```

[Convert] sensor_msgs ↔ cv::Mat

[Goal] sensor_msgs::Image 를 cv::Mat & cv::Mat 을 sensor_msgs::Image 로 변환