How to correctly publish and receive cv::Mat using cv_bridge? (C++)

asked 2017-02-06 01:51:13 -0600

CocoJambo gravatar image

updated 2017-02-06 03:14:45 -0600

Hey everyone,

I am having troubles converting a cv::Mat to a sensor_msgs/Image, publishing it and doing the reverse operation to regain the original cv::Mat. The imshow() function shows me the correct image(white-gray-black image with numbers ranging from 0 to 1) before sending, but shows a seemingly random and black-white(binary) image after receiving it. Normalizing the matrix with cv::normalize() before displaying it does not do the trick either.

So I would be happy if you could check out my code below or give me a working sample code from one of your projects :). There is the fear that the mistake lies outside the shown code, but I don't think you want to read all my nodes.

Extract publisher code:

cv::Mat out_mat;
// fill mat....
std::cout << "out_mat.type() = " << out_mat.type() << "\n"; // prints out_mat.type() = 5
cv::namedWindow("out_mat", CV_WINDOW_NORMAL);
cv::resizeWindow("out_mat", 500, 500);
cv::imshow("out_mat", out_mat); // shows correct image
cv_bridge::CvImage cvi_mat;
cvi_mat.encoding = sensor_msgs::image_encodings::MONO8;
cvi_mat.image = out_mat;
cvi_mat.toImageMsg(container); // container of type sensor_msgs/Image

Extract subscriber code:

// convert image msg to cv mat
cv_bridge::CvImagePtr img;
img = cv_bridge::toCvCopy(container, sensor_msgs::image_encodings::MONO8); // container of type sensor_msgs/Image
cv::Mat mat_received = img->image;
cv::Mat mat;
std::cout << "mat type: " << mat.type() << "\n"; // prints mat_c type: 5
cv::namedWindow("Grid", CV_WINDOW_NORMAL);
cv::resizeWindow("Grid", 500, 500);
cv::imshow("Grid", mat); // shows false/random (binary black-white) image

Every help is appreciated.

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For anyone wondering: As I only needed to send the matrix I created a different rosmsg containing an array where I wrote the values of the matrix. After receiving the array, I convert it back to a matrix. Therefore I don't have to deal with encodings and stuff.

CocoJambo gravatar imageCocoJambo ( 2017-02-06 07:33:01 -0600 )edit

Not sure but maybe is the problem is on the matrix type conversion (I suggest to use the CV definition instead the int value!). Did you try to show your mat_received image instead the converted one?

afranceson gravatar imageafranceson ( 2017-02-07 05:28:55 -0600 )edit