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How can i get object distance using camera/depth/image_raw

asked 2016-10-20 06:31:43 -0500

rasoo gravatar image

updated 2016-10-21 08:56:15 -0500

Hello all, I am using the ros topic /camera/depth/image_raw. It publish sensor_msgs::image data with depth information. I can get the image but I do not exactly how can i get depth information. I need to what is the minimum distance of any pixel infront of the camera. I actually do not want to use the topic /camera/depth/points to measure the distance, as it is said /camera/depth/image_raw provides depth infromation. Can anyone please share some suggestion? I am using ros indigo, kinect, freenect.launch, opencv. Can anyone suggest me which library function is appropriate for me here

void callback(const sensor_msgs::Image::ConstPtr& msg){

     if( == 0)
         img_previous = *msg;
         img_primary = *msg;
         {img_previous = img_current;}
    img_current = *msg;

     cout<<"------------------------header of msg :---------:"<< msg->header.seq <<"\n";
    cout<<"------------------------current image info :---------:"<<<<"\n";
    cout<<"------------------------Height :---------:"<<img_current.height<<"\n";
    cout<<"------------------------Width :---------:" <<img_current.width<<"\n";

    cout<<"------------------------previous image info :---------:"<<<<"\n";
    cout<<"------------------------Height :---------:"<<img_previous.height<<"\n";
    cout<<"------------------------Width :---------:" <<img_previous.width<<"\n";

        //Convert from the ROS image message to a CvImage suitable for working with OpenCV for processing
            cv_bridge::CvImagePtr cv_ptr_prev, cv_ptr_cur;
            cv::Mat image_cur, image_pre, differenceImage;
            //thresholded difference image (for use in findContours() function)
            cv::Mat thresholdImage;


                //Always copy, returning a mutable CvImage
                //OpenCV expects color images to use BGR channel order.
                cv_ptr_cur = cv_bridge::toCvCopy(msg, enc::TYPE_32FC1);//cv_bridge::toCvCopy(img_current, enc::BGR8);
                cv_ptr_prev = cv_bridge::toCvCopy(img_previous, enc::TYPE_32FC1);//TYPE_16UC1

                image_cur = cv_ptr_cur->image;
                image_pre = cv_ptr_prev->image;
                int count_diff=0;
                bool motion_detected = false;

                //perform frame differencing with the sequential images.
                //threshold intensity image at a given sensitivity value
                cv::threshold(differenceImage,thresholdImage, 20,255,cv::THRESH_BINARY);
                cout<<"~~~~~~~~~~~~IMP~~~~~~~~~~difference image info :---------:"<<thresholdImage.rows<<"\n";
                cout<<"~~~~~~~~~~~~~IMP~~~~~~~~~difference image info :---------:"<<thresholdImage.cols<<"\n";
                for(int j=0; j< thresholdImage.rows; ++j)
                    //const double* Mi = differenceImage.ptr<double>(j);
                    for(int i=0; i< thresholdImage.rows; ++i)
                        //cout<<"diff img pixel val : "<<Mi[i]<<"\n";
                        //double depth =<double>(cv::Point(j,i));
                        //cout<<"diff img depth val : "<<depth<<"\n";
                        cout<<"+++++ i: "<<i <<"------ j : "<< j<<"++++++++++++++++++++++++++++++++++++++++\n";
                        cout<<"current  image    : "<<<cv::Vec3b>(j,i)       << "cur img depth val : "<<<double>(cv::Point(j,i))<<"\n";
                        cout<<"previous  image   : "<<<cv::Vec3b>(j,i)       << "pre img depth val : "<<<double>(cv::Point(j,i))<<"\n";
                        cout<<"abs-diff  image   : "<<<cv::Vec3b>(j,i) << "dif img depth val : "<<<double>(cv::Point(j,i))<<"\n";
                        cout<<"threshholöd image : "<<<cv::Vec3b>(j,i ) << "thr img depth val : "<<<double>(cv::Point(j,i))<<"\n";


                for(int j=0; j<image_cur.rows; ++j)
                    if(count_diff > 150)
                        for(int i=0; i<image_cur.cols; ++i)
                            int depth =<short int>(cv::Point(j,i));
                            if(depth < 0.6)
                                cout<<"##############something within the range infront of the camera...########################\n";
                                if(<cv::Vec3b>(j,i) !=<cv::Vec3b>(j,i))
                                    cout<<"i : "<<i<<" ... j ...
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why absdiff method is giving some such wierd value?

rasoo gravatar image rasoo  ( 2016-10-21 08:58:15 -0500 )edit

2 Answers

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answered 2016-10-20 10:42:10 -0500

sloretz gravatar image

That depends on the image encoding. Check the value of "encoding" on the message being published.

If it's "32FC1" then the depth data for each pixel is a 32bit float where each value is the distance in meters from the camera. In c++ you can get the depth data from this encoding by casting to an array of floats.

//sensor_msgs::Image image;
if (image.encoding == sensor_msgs::image_encodings::TYPE_32FC1)
    const float * depth_array = reinterpret_cast<const float *>(&([0]));
    //depth_array[0], depth_array[1], ..., depth_aray[image.height * image.width - 1]
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Comments[0] -- does it means that the depth information for the zero-th pixel ?

rasoo gravatar image rasoo  ( 2016-10-20 13:00:10 -0500 )edit[0] would give you the first byte (of 4 if encoding is 32FC1) of the zero-th pixel. depth_array[0] gives you the depth of the zero-th pixel. Pixels are in row-major order.

sloretz gravatar image sloretz  ( 2016-10-20 13:15:25 -0500 )edit

Also, the package cv_bridge has some methods for converting between ros messages and opencv types. You may find these tutorials useful.

sloretz gravatar image sloretz  ( 2016-10-20 13:27:26 -0500 )edit

Thanks for your response, but I am having some problem to understand. I added partial code in my original post. Can you please recheck it for me? I target is 1. movement detection by image compare 2. only detect when any object is within the range(0.6m).

rasoo gravatar image rasoo  ( 2016-10-21 07:47:57 -0500 )edit

the Topic /camera/depth/image is of TYPE_32FC1 and the Topic /camera/depth/image_raw is of TYPE_16UC1. now if need to know the depth information of (i,j)th pixel then what should i do?

rasoo gravatar image rasoo  ( 2016-10-23 21:23:03 -0500 )edit

answered 2021-06-25 01:31:57 -0500

askkvn gravatar image

updated 2021-06-25 01:33:13 -0500

My answer with complete code (c++) :

Code funtionality:

  • Subscribed to /camera/depth/image_raw rostopic,
  • Convert ROS image to OpenCV image
  • Get image dimension
  • Get global max and min value,
  • Get depth value at specific point,
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Hey, how do I do in python ?

Siddarth09 gravatar image Siddarth09  ( 2021-12-13 07:50:29 -0500 )edit

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Asked: 2016-10-20 06:31:43 -0500

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Last updated: Jun 25 '21