Query a frame from a video passed as a message by ROS to OpenCV

edit

hi everybody, I'm trying to extract a frame from a video passed by a ROS node to OpenCV. Obviously I made the necessary conversion (via cv_bridge) exactly as suggested in ros.org/wiki/cv_bridge/Tutorials/UsingCvBridgeToConvertBetweenROSImagesAndOpenCVImages .

namespace enc = sensor_msgs::image_encodings;
static const char WINDOW[] = "Image window";
class ImageConverter
{
ros::NodeHandle nh_;
image_transport::ImageTransport it_;
image_transport::Subscriber image_sub_;
image_transport::Publisher image_pub_;

public:
ImageConverter()
: it_(nh_)
{
image_pub_ = it_.advertise("out", 1);
image_sub_ = it_.subscribe("in", 1, &ImageConverter::imageCb, this);

cv::namedWindow(WINDOW);
} 

~ImageConverter()
{
cv::destroyWindow(WINDOW);
}

void imageCb(const sensor_msgs::ImageConstPtr& msg)
{
cv_bridge::CvImagePtr cv_ptr;
try
{
  cv_ptr = cv_bridge::toCvCopy(msg, enc::BGR8);
}
catch (cv_bridge::Exception& e)
{
  ROS_ERROR("cv_bridge exception: %s", e.what());
  return;
}

if (cv_ptr->image.rows > 60 && cv_ptr->image.cols > 60)
  cv::circle(cv_ptr->image, cv::Point(50, 50), 10, CV_RGB(255,0,0));

cv::imshow(WINDOW, cv_ptr->image);
cv::waitKey(3);

image_pub_.publish(cv_ptr->toImageMsg());
}
};

int main(int argc, char** argv)
{
ros::init(argc, argv, "image_converter");
ImageConverter ic;
ros::spin();
return 0;
}

So I don't have to do a cvCapture operation and now the video data (cv_ptr->image) are in the form cv::Mat . if I write the following code:

IplImage* frame_act;
frame_act = cvQueryFrame(cv_ptr->image);

i receive this error:

error: cannot convert ‘cv::Mat’ to ‘CvCapture*’ for argument ‘1’ to ‘IplImage* cvQueryFrame(CvCapture*)’

have you same ideas in order to solve this small problem? thank you very much for your help!