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fitEllipse - angle of resulting rotated rect

asked 2014-08-11 08:43:51 -0600

Alan Kazbekov gravatar image

updated 2014-08-11 14:13:29 -0600

It seems that fitEllipse realization miss the result RotatedRect angle computing in some cases.

...
box.center.x = (float)rp[0] + c.x;
box.center.y = (float)rp[1] + c.y;
box.size.width = (float)(rp[2]*2);
box.size.height = (float)(rp[3]*2);
if( box.size.width > box.size.height )
{
    float tmp;
    CV_SWAP( box.size.width, box.size.height, tmp );
    box.angle = (float)(90 + rp[4]*180/CV_PI);
}
if( box.angle < -180 )
    box.angle += 360;
if( box.angle > 360 )
    box.angle -= 360;

return box;

If box.size.width <= box.size.height the angle of rotated rect would be indefinite. Please correct me, if I am wrong.

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Comments

1

Thank you for reporting, it seems that you're right - would be nice if you could fix it and make a pull-request!

Guanta gravatar imageGuanta ( 2014-08-11 14:50:49 -0600 )edit
1

Indeed, take a look at the how to contribute page. It would be nice if you provided a fix for this.

StevenPuttemans gravatar imageStevenPuttemans ( 2014-08-12 05:54:37 -0600 )edit

2 answers

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2

answered 2015-12-06 16:32:27 -0600

to understand and test i tried to implement the code below. May be it will be helpful for some one who want to fix

#include <iostream>
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"

using namespace cv;
using namespace std;

void process( Mat src );

int main()
{
    Mat clk(640,640,CV_8UC3);

    Point cent(320,320);
    Point perim(320,0);

    float sec_angle=270;

    while(1)
    {
        if(sec_angle>360)sec_angle=sec_angle-360;

        perim.x =  (int)round(cent.x + 200 * cos(sec_angle * CV_PI / 180.0));
        perim.y =  (int)round(cent.y + 200 * sin(sec_angle * CV_PI / 180.0));
        line( clk,cent,perim, Scalar(0,255,255), 8 );

        process( clk );
        putText( clk, format("%f",sec_angle), perim, FONT_HERSHEY_PLAIN, 2, Scalar(0,0,255) );

        imshow("Clock",clk);
        clk.setTo(0);

        sec_angle ++;

        char c=waitKey();
        if(c==27)break;
    }
    return 0;
}

void process( Mat src )
{
    Mat gray;
    cvtColor( src, gray, COLOR_BGR2GRAY );
    gray = gray >  127;

    vector<vector<Point> > contours;

    findContours(gray.clone(), contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

    RotatedRect _minAreaRect;

    for (size_t i = 0; i < contours.size(); ++i)
    {
        _minAreaRect = minAreaRect( Mat(contours[i]) );
        Point2f pts[4];
        _minAreaRect.points(pts);

        double angle = atan2(pts[0].y - pts[1].y,pts[0].x - pts[1].x) * 180.0 / CV_PI;
        putText( src, format("minAreaRect : %f, %f",_minAreaRect.angle, angle), Point(30,30), FONT_HERSHEY_PLAIN, 2, Scalar(0,0,255) );

        for( int j = 0; j < 4; j++ )
            line(src, pts[j], pts[(j+1)%4], Scalar(0, 255, 0), 2, LINE_AA);

        _minAreaRect = fitEllipse( Mat(contours[i]) ) ;
        _minAreaRect.points(pts);
        angle = atan2(pts[0].y - pts[1].y,pts[0].x - pts[1].x) * 180.0 / CV_PI;
        putText( src, format("fitEllipse : %f, %f",_minAreaRect.angle, angle ), Point(30,70), FONT_HERSHEY_PLAIN, 2, Scalar(0,0,255) );

        for( int j = 0; j < 4; j++ )
            line(src, pts[j], pts[(j+1)%4], Scalar(0, 0, 255), 1, LINE_AA);

    }
}
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1

answered 2015-11-16 11:47:03 -0600

matman gravatar image

updated 2015-11-16 12:12:53 -0600

I want to dig this old thread because this bug isn't fixed until now. I think changing this piece of code to

...
box.center.x = (float)rp[0] + c.x;
box.center.y = (float)rp[1] + c.y;
box.size.width = (float)(rp[2]*2);
box.size.height = (float)(rp[3]*2);
box.angle = (float)(rp[4]*180/CV_PI);

if( box.size.width > box.size.height )
{
    float tmp;
    CV_SWAP( box.size.width, box.size.height, tmp );
    box.angle += 90;
}

if( box.angle < -180 )
    box.angle += 360;
if( box.angle > 180 )
    box.angle -= 360;

return box;

should fix it. And I think the angle range should be between -180 and +180 degrees (or even between -90 and +90 degrees?), so I changed the last condition, too.

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Comments

1

Again I would ask if you could provide a PR with the fix. It wil make sure that a possible fix for the problem gets inside the source code!

StevenPuttemans gravatar imageStevenPuttemans ( 2015-11-17 01:57:30 -0600 )edit

@matman i tested your proposed fix. but i think it is not correct.

sturkmen gravatar imagesturkmen ( 2015-12-07 10:51:18 -0600 )edit
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Asked: 2014-08-11 08:43:51 -0600

Seen: 1,088 times

Last updated: Dec 06 '15