Ros下使用OpenCV对图像进行处理

Ros中的图像信息是sensor_msgs/Image的格式，而OpenCV 中图像是cv:Mat格式。所以从Ros中获取的图像不能直接用OpenCV进行处理，需要进行格式转化。还好，Ros中提供了进行ROS和OpenCV之间图像信息格式转化的接口CvBridge

下面附上Ros官网提供的源码：

#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <cv_bridge/cv_bridge.h>
#include <sensor_msgs/image_encodings.h>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>

static const std::string OPENCV_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_)
{
// Subscrive to input video feed and publish output video feed
image_sub_ = it_.subscribe("/camera/image_raw", 1,
&ImageConverter::imageCb, this);
image_pub_ = it_.advertise("/image_converter/output_video", 1);

cv::namedWindow(OPENCV_WINDOW);
}

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

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

// Draw an example circle on the video stream
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));

// Update GUI Window
cv::imshow(OPENCV_WINDOW, cv_ptr->image);
cv::waitKey(3);

// Output modified video stream
image_pub_.publish(cv_ptr->toImageMsg());
}
};

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

本段代码源自：
http://wiki.ros.org/cv_bridge/Tutorials/UsingCvBridgeToConvertBetweenROSImagesAndOpenCVImages
下面来解释一下里面的代码

namespace cv_bridge {

class CvImage
{
public:
std_msgs::Header header;
std::string encoding;
cv::Mat image;
};

typedef boost::shared_ptr<CvImage> CvImagePtr;
typedef boost::shared_ptr<CvImage const> CvImageConstPtr;

}

cv_bridge里定义了类CvImage，类中包含了ros的头文件，OpenCV图像信息和编码。我们可以用这个类来进行两种图像格式的转换

cv_bridge里提供了以下两种函数来将ros中的sensor_msgs::image转换为Cvimage

// Case 1: Always copy, returning a mutable CvImage
CvImagePtr toCvCopy(const sensor_msgs::ImageConstPtr& source,
const std::string& encoding = std::string());
CvImagePtr toCvCopy(const sensor_msgs::Image& source,
const std::string& encoding = std::string());

// Case 2: Share if possible, returning a const CvImage
CvImageConstPtr toCvShare(const sensor_msgs::ImageConstPtr& source,
const std::string& encoding = std::string());
CvImageConstPtr toCvShare(const sensor_msgs::Image& source,
const boost::shared_ptr<void const>& tracked_object,
const std::string& encoding = std::string());

toCvCopy复制ROS消息中的图像数据，可以对返回的cvimage进行自由修改。

toCvShare用来进行数据共享，安全地共享ROS消息中包含的数据。也就是说如果想使用opencv进行图像处理的话，使用toCvCopy

使用toImageMsg()成员函数可以将opencv图像转换成ROS图像信息

class CvImage
{
sensor_msgs::ImagePtr toImageMsg() const;

// Overload mainly intended for aggregate messages that contain
// a sensor_msgs::
9d81
Image as a member.
void toImageMsg(sensor_msgs::Image& ros_image) const;
};

如要进行图像处理时，直接对cv_ptr->image进行处理即可

如要对kinect2中获得的图像rgb信息进行处理，将节点订阅中的/camera/image_raw换成kinect2发布的信息即可