Question

I am making a C++ application that uses opencv and zeromq. And I am experiencing some problems when trying to send a cv::Mat object (CV_8UC3 ) over a zmq tcp socket.

Here is the updated code sample:

#include <iostream>
#include <zmq.hpp>
#include <pthread.h>
#include <opencv/cv.h>
#include <opencv/highgui.h>

using namespace std;

int main()
{
    zmq::context_t ctx( 1 );
    zmq::socket_t mysocket( ctx, ZMQ_PUSH );
    mysocket.bind( "tcp://lo:4050" );

    cv::VideoCapture capture( CV_CAP_ANY );
    capture.set( CV_CAP_PROP_FRAME_WIDTH, 640 );
    capture.set( CV_CAP_PROP_FRAME_HEIGHT, 480 );

    cv::Mat3b frame;

    capture >> frame; //First one is usually blank
    capture >> frame;
    capture >> frame;

    cv::Mat3b clonedFrame( 480, 640, CV_8UC3 );
    frame.copyTo( clonedFrame );

    cout << "Original:" << endl
         << "Address of data:\t" << &frame.data << endl
         << "Size:\t\t\t" << frame.total() * frame.channels() << endl << endl;

    cout << "Cloned:" << endl
         << "Address of data:\t" << &clonedFrame.data << endl
         << "Size:\t\t\t" << clonedFrame.total() * clonedFrame.channels() << endl << endl;

    cout << "Gap between data:\t" << &clonedFrame.data - &frame.data << endl;

    unsigned int frameSize = frame.total() * frame.channels();

    zmq::message_t frameMsg( frame.data, frameSize, NULL, NULL );
    zmq::message_t clonedFrameMsg( clonedFrame.data, frameSize, NULL, NULL );

    cv::imshow( "original", frame );
    cv::imshow( "cloned", clonedFrame );


    cvWaitKey( 0 );

    if( frame.isContinuous() )
    {
        cout << "Sending original frame" << endl;
        mysocket.send( frameMsg, 0 ); //This works
        cout << "done..." << endl;
    }

    cvWaitKey( 0 );

    if( clonedFrame.isContinuous() )
    {
        cout << "Sending cloned frame" << endl;
        mysocket.send( clonedFrameMsg, 0 ); //This fails
        cout << "done..." << endl;
    }

    return EXIT_SUCCESS;
}

The latter send() makes zmq fail some assertion.
output:

Original:
Address of data:    0xbfdca480
Size:           921600

Cloned:
Address of data:    0xbfdca4b8
Size:           921600

Gap between data:   14
Sending original frame
done...
Sending cloned frame
Bad address
done...
nbytes != -1 (tcp_socket.cpp:203)

Why does clone() mess up the pointer, and how can I solve this?

Any help is appreciated.

Edit 2012-05-25:
Updated the code sample.
I can send the original frame by giving one of the following pointers to the message constructor: frame.ptr(), frame.data, frame.datastart, frame.at(). They all work for the original, but none for the constructor.
As you can see, the address space between the two datapointers is small. Shouldn’t it bee at least frameSize?

//John

Answer 1

Ok… So I ended up making my own copy function

It looks like this:

struct frameData_t
{
    unsigned char *data;
    size_t size;
};

struct frameData_t *copyMatData( cv::Mat3b &indata )
{
    struct frameData_t *ret = new struct frameData_t;

    ret->size = indata.total() * indata.channels();
    ret->data = new unsigned char[ret->size];

    int datapos = 0;
    for( int channel = 0; channel < indata.channels(); channel++ )
    {
        for( int row = 0; row < indata.rows; row++ )
        {
            const cv::Vec3b *srcrow = indata[row];
            for( int col = 0; col < indata.cols; col++, datapos++ )
            {
                ret->data[datapos] = srcrow[col][channel];
            }
        }
    }

    return ret;
}

And i use it like this:

struct frameData_t *clonedFrame = copyMatData( frame );
zmq::message_t frameMsg( frame.data, frameSize, NULL, NULL );
mysocket.send( clonedFrameMsg, 0 );

If anyone knows of a better way, please let me know.

//John