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convexHull clockwise parameter inverted ?

asked 2016-10-14 06:53:34 -0500

HDM gravatar image

Perhaps I'm misunderstanding the 3rd parameter to convexHull ? :

clockwise - Orientation flag. If it is true, the output convex hull is oriented clockwise. Otherwise, it is oriented counter-clockwise. The assumed coordinate system has its X axis pointing to the right, and its Y axis pointing upwards.

When I run the following code:

#include <iostream>
#include <vector>

#include <opencv2/core/base.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/calib3d.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>

int main(int argc, char **argv)
{
   std::vector<cv::Point2i> points, hull_points;
   points.push_back(cv::Point2i(114, 22));
   points.push_back(cv::Point2i(329, 245));
   points.push_back(cv::Point2i(37, 98));
   points.push_back(cv::Point2i(43, 272));
   points.push_back(cv::Point2i(120, 150));
   cv::convexHull(points, hull_points, false);
   cv::Mat img(350, 460, CV_8UC3, cv::Scalar::all(0));   
   int no = 1;
   for (auto it = hull_points.begin(); it != hull_points.end(); ++it)
   {
      cv::Point2i &pt = *it;
      cv::circle(img, pt, 3, cv::Scalar(0, 255, 0), 1, cv::LINE_AA);      
      std::stringstream ss;
      ss << no++ << ":(" << pt.x << "," << pt.y << ")";
      cv::putText(img, ss.str(), cv::Point2i(pt.x+1, pt.y-1), CV_FONT_HERSHEY_COMPLEX_SMALL, 1.0, cvScalar(0, 255, 255), 1, CV_AA);
      std::cout << ss.str() << " ";
   }
   std::cout << std::endl;
   cv::imwrite("img.png", img);      
}

I get the following image:

image description

which appears to be clockwise unless its supposed to be traversed in descending order.

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answered 2016-10-14 07:16:49 -0500

updated 2016-10-14 07:44:33 -0500

Ah the good old issue of coordinate system orientation.

  • clockwise means moving from x axis towards the y axis if you consider a upstraight coordinate system, so Y running from bottom to top.
  • however OpenCV uses the top left corner as origin and has a downwards pointing y axis.
  • this means to achieve this you need to warp the movement over the x axis.
  • and thus your rotation is actually clockwise in the OpenCV coordinate system.

I know it is sometimes a bit sketchy because coordinate systems are used in different constellations inside OpenCV, but now you know why you get this result.

VISUAL EXPLANATION

image description

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However the documkentation explicitly states:

The assumed coordinate system has its X axis pointing to the right, and its Y axis pointing upwards.

HDM gravatar imageHDM ( 2016-10-14 07:21:33 -0500 )edit

YES for defining the concept of counter clockwise and clockwise. Probably the explanation should be more precise.

StevenPuttemans gravatar imageStevenPuttemans ( 2016-10-14 07:33:33 -0500 )edit
1

Yes, I assumed they meant the coordinate system used for the hull.

Thanks for the reply.

HDM gravatar imageHDM ( 2016-10-14 07:35:28 -0500 )edit

You can use this graphics too

LBerger gravatar imageLBerger ( 2016-10-14 11:57:29 -0500 )edit
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Asked: 2016-10-14 06:53:34 -0500

Seen: 158 times

Last updated: Oct 14 '16