I'm trying to find centroids of four green circles in image_transport video. I'm planning to convert to OpenCV image with cv_bridge. Then, with OpenCV, extract green channel (with cv::Split) and apply a threshold.

For this I need cvCreateImage, cvGetSize and cvSplit, like this:

cv_ptr = cv_bridge::toCvCopy(original_image, enc::BGR8); this works

cv_ptr_red = cvCreateImage( cvGetSize(cv_ptr), 8, 1); this does not work

cv_ptr_green = cvCreateImage( cvGetSize(cv_ptr), 8, 1); this does not work

cv_ptr_blue = cvCreateImage( cvGetSize(cv_ptr), 8, 1); this does not work

These lines that doesn't work give me the following error:

cannot convert ‘cv_bridge::CvImage’ to ‘const CvArr* {aka const void*}’ for argument ‘1’ to ‘CvSize cvGetSize(const CvArr*)’

What am I missing?

Update on error

I'm sorry for so many basic questions, but this C++ is way more complicated then Matlab.

It does compile using the cv_ptr_green->image but the it gives me an error when running it. I reduced the code to try to find out where it was happening and now my code only has:

cv_bridge::CvImagePtr cv_ptr_green;
cv_ptr = cv_bridge::toCvCopy(original_image, enc::BGR8);
cv_ptr_green->image.create(cv_ptr->image.size(), CV_8UC1);

And the error I get when running (compiling is OK):

ardrone_visualservo: /usr/include/boost/smart_ptr/shared_ptr.hpp:418: T* boost::shared_ptr<t>::operator->() const [with T = cv_bridge::CvImage]: Assertion `px != 0' failed.

Aborted (core dumped)

If I understood what it tried to say, it's complaining about the -> operator usage with cv_bridge::CvImage (which is the type of my cv_ptr_green object).

Any ideas how to fix this?

Updating using alternative function cv::mixChannels

I found that maybe I can use cv::mixChannels() to extract only the green channel (as I actually need) with

    // cv_ptr[1] -> cv_ptr_green[1]
    int from_to[] = {1,1};
    cv::mixChannels( cv_ptr, 1, cv_ptr_green, 1, from_to, 1);

but it also gives me an error

path/to/cpp/file.cpp: In function ‘void imageCallback(const ImageConstPtr&)’:

path/to/cpp/file.cpp:62:58: error: no matching function for call to ‘mixChannels(cv_bridge::CvImagePtr&, int, cv_bridge::CvImagePtr&, int, int [2], int)’

path/to/cpp/file.cpp:62:58: note: candidates are: /opt/ros/fuerte/include/opencv2/core/core.hpp:2137:17: note: void cv::mixChannels(const cv::Mat, size_t, cv::Mat, size_t, const int*, size_t)

/opt/ros/fuerte/include/opencv2/core/core.hpp:2137:17: note: no known conversion for argument 1 from ‘cv_bridge::CvImagePtr {aka boost::shared_ptr<cv_bridge::cvimage>}’ to ‘const cv::Mat*’

/opt/ros/fuerte/include/opencv2/core/core.hpp:2139:17: note: void cv::mixChannels(const std::vector<cv::mat>&, std::vector<cv::mat>&, const int*, size_t)

/opt/ros/fuerte/include/opencv2/core/core.hpp:2139:17: note: candidate expects 4 arguments, 6 provided

/opt/ros/fuerte/include/opencv2/core/core.hpp:2141:19: note: void cv::mixChannels(cv::InputArrayOfArrays, cv::InputArrayOfArrays, const std::vector<int>&)

Complete code

    #include <ros/ros.h>
    //Use image_transport for publishing and subscribing to images in ROS
    #include <image_transport/image_transport.h>
    //Use cv_bridge to convert between ROS and OpenCV Image formats
    #include <cv_bridge ...