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Object detection using OpenCV

asked 2018-07-21 10:55:37 -0500

Dylan gravatar image

updated 2018-07-21 12:38:45 -0500

Hello everyone,

I'm trying to do pattern recognition from some images and it's not working. I have my drone with a bottom camera, and the camera is connected to a ROS node. Using cv_bridge I converted the image from a ROS format to an OpenCV format, but what I have to do now is to detect if a determined pattern (learned previously) is in the image.

In the ROS node it appears a different image each 0.02 second (I can change the frequency). The pattern will stay on the floor, so what can be done is to detect the variance of the image (for example, the floor or the sea will have low variance, and the pattern that I set on the floor will have high variance (for example, a pattern like a chess table)).

This is the code that I'm using (

#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_;

    : 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);



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

    // 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);

    // Output modified video stream

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

but that code just prints the image and puts a red circle on the upper left corner, so what has to be modified is this part:

// 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));

I would like if you can help me. This is just the beginning of the project, then I have to take decisions in order to what is seen, and so on. If you don't have a code that does what I need, please provide me some links or books or whatever that can help me (I've been 2 days trying to make this work and I couldn't).


So I need your help in the OpenCV part (if you know about ROS it would be much better, but I can try to do the translation between OpenCV and ROS): imagine that a different image come to your program each 0.02 seconds and you ... (more)

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unfortunately, noone can solve ROS problems from here ;(

we can help you if you maybe more focus on the opencv part.

then, "object detection" is far too broad, to be useful. can you narrow it down ?

maybe also showing an example image would be helpful.

berak gravatar imageberak ( 2018-07-21 12:06:11 -0500 )edit

Thanks for your answer. I edited the main post, so I hope it's more clear now :)

Dylan gravatar imageDylan ( 2018-07-21 12:39:16 -0500 )edit

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answered 2018-07-21 12:53:55 -0500

berak gravatar image

maybe aruco markers might solve your problem ?

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Oh yes. I think this is the solution. Have you ever worked with that?

Dylan gravatar imageDylan ( 2018-07-21 13:26:29 -0500 )edit

me, not much. imho, you should play with the samples and ask more questions.

and, -- are you sure, you even need ROS here ?

berak gravatar imageberak ( 2018-07-21 22:31:22 -0500 )edit

Thanks. I will use that samples and I hope it works. I need ROS because I need to program a real drone, and I'm using a ROS simulator (Gazebo) to make all the tests and everything. That's why I said that the pattern recognition part is very important but it's just the beginning. Thanks!

Dylan gravatar imageDylan ( 2018-07-22 00:24:17 -0500 )edit
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Asked: 2018-07-21 10:55:37 -0500

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Last updated: Jul 21 '18