[Convert] sensor_msgs ↔ cv::Mat

[Goal]

• sensor_msgs::Image 를 cv::Mat & cv::Mat 을 sensor_msgs::Image 로 변환

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[Reference Site]

[ http://library.isr.ist.utl.pt/docs/roswiki/cv_bridge(2f)Tutorials(2f)UsingCvBridgeToConvertBetweenROSImagesAndOpenCVImages.html ]

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[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 ]

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• 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;
}

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[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 ]

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• 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;
}

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[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 ]

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• • 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}};
}

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[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);

 

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• OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor
  • Reference Site
    • [ http://daplus.net/python-%EC%97%B4%EB%A6%B0-cv-%EC%98%A4%EB%A5%98-215-scn-3-cvtcolor-%ED%95%A8%EC%88%98%EC%9D%98-scn-4/ ]
      [ 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);