ROS custom message with sensor_msgs/Image issue with subscriber - ros

Question

I have a custom message -
sensor_msgs/Image im
float32 age
string name
I could successfully write a publisher for this message and it seem to run ok. However, I have an issue with the subscriber.
#include <ros/ros.h>
#include <custom_msg/MyString.h>
#include <custom_msg/MyImage.h>
#include <image_transport/image_transport.h>
#include <sensor_msgs/Image.h>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/opencv.hpp>
#include <cv_bridge/cv_bridge.h>
void custom_image_rcvd( const custom_msg::MyImage& msg )
{
ROS_INFO_STREAM( "msg::: Name:"<< msg.name << " Age:"<< msg.age );
cv::Mat im = cv_bridge::toCvShare( msg.im, "bgr8" )->image ;
cv::imshow("viewz", im );
cv::waitKey(30);
}
int main( int argc, char ** argv )
{
ros::init(argc, argv, "custom_subscriber");
ros::NodeHandle nh;
ros::Subscriber sub2 = nh.subscribe( "custom_image", 2, custom_image_rcvd );
ros::spin();
}
When I try to catkin_make this I get the following error.
/home/eeuser/ros_workspaces/HeloRosProject/src/custom_msg/subsc.cpp: In function ‘void custom_image_rcvd(const MyImage&)’:
/home/eeuser/ros_workspaces/HeloRosProject/src/custom_msg/subsc.cpp:22:57: error: no matching function for call to ‘toCvShare(const _im_type&, const char [5])’
/home/eeuser/ros_workspaces/HeloRosProject/src/custom_msg/subsc.cpp:22:57: note: candidates are:
/opt/ros/hydro/include/cv_bridge/cv_bridge.h:198:17: note: cv_bridge::CvImageConstPtr cv_bridge::toCvShare(const Image&, const boost::shared_ptr<const void>&, const string&)
/opt/ros/hydro/include/cv_bridge/cv_bridge.h:198:17: note: no known conversion for argument 2 from ‘const char [5]’ to ‘const boost::shared_ptr<const void>&’
/opt/ros/hydro/include/cv_bridge/cv_bridge.h:171:17: note: cv_bridge::CvImageConstPtr cv_bridge::toCvShare(const ImageConstPtr&, const string&)
/opt/ros/hydro/include/cv_bridge/cv_bridge.h:171:17: note: no known conversion for argument 1 from ‘const _im_type {aka const sensor_msgs::Image_<std::allocator<void> >}’ to ‘const ImageConstPtr& {aka const boost::shared_ptr<const sensor_msgs::Image_<std::allocator<void> > >&}’
make[2]: *** [custom_msg/CMakeFiles/subscribe.dir/subsc.cpp.o] Error 1
make[1]: *** [custom_msg/CMakeFiles/subscribe.dir/all] Error 2
make: *** [all] Error 2
Invoking "make" failed
What I can make out is that msg.im is of the type _im_type aka. sensor_msgs::Image_<ContainerAllocator>. I cannot seem to understand this part.
How can I retrieve my image correctly from this custom message?

Answer 1

You have to take a closer look on the signature of toCvShare. It can be read from the error message, that this function has two overloads:
cv_bridge::CvImageConstPtr cv_bridge::toCvShare(const Image&, const boost::shared_ptr<const void>&, const string&)
and
cv_bridge::CvImageConstPtr cv_bridge::toCvShare(const ImageConstPtr&, const string&)
So the function either expects an Image plus a pointer to some object (first case) or a ImageConstPtr (second case). You are, however, only passing a Image, so this matches neither of the two options.
If I understand the API documentation correctly, the second argument in the first case is expected to be a pointer to the message that contains the image. Try the following code:
void custom_image_rcvd(const custom_msg::MyImageConstPtr& msg)
{
ROS_INFO_STREAM("msg::: Name:" << msg->name << " Age:" << msg->age);
cv::Mat im = cv_bridge::toCvShare(msg->im, msg, "bgr8")->image;
cv::imshow("viewz", im);
cv::waitKey(30);
}
Note that I changed the call of toCvShare as well as the signature of the callback.