OpenCV get pixels on an ellipse

Question

I'm trying to get the pixels of an ellipse from an image.

For example, I draw an ellipse on a random image (sample geeksforgeeks code):

import cv2
path = r'C:\Users\Rajnish\Desktop\geeksforgeeks\geeks.png'
image = cv2.imread(path)
window_name = 'Image'
center_coordinates = (120, 100)
axesLength = (100, 50)
angle = 0
startAngle = 0
endAngle = 360
color = (0, 0, 255)
thickness = 5
image = cv2.ellipse(image, center_coordinates, axesLength,
           angle, startAngle, endAngle, color, thickness)
cv2.imshow(window_name, image)

It gives output like below:

Now, I want to get the pixel value of boundary line of ellipse. If it is possible I would like to get the pixel of ellipse using cv2.ellipse() back as an array of coordinates.
Can anyone help me with this please.

Answer 1

There is no direct OpenCV way probably to get these points of the ellipse but you can extract your points via indirect way like this:

mask = cv2.inRange(image, np.array(color), np.array(color))
contour = cv2.findContours(mask, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)[-2][0]

contour will store the outer points of your red ellipse. Here, what I have done is created a mask image of the ellipse and found the externalmost contour's points that is the required thing.

Answer 2

If you want to obtain points (locations) on an ellipse, you can use ellipse2Poly() function.

---

If the argument type of ellipse2Poly() is inconvenient, calculating by yourself is most convenient way.

This sample code is C ++, but what calculated is clear.

//Degree -> Radian
inline double RadFromDeg( double Deg ){ return CV_PI*Deg/180.0; }

//Just calculate points mathematically.
//  Arguments are same as cv::ellipse2Poly (alothough ellipse parameters is cv::RotateRect).
void My_ellipse2Poly(
    const cv::RotatedRect &EllipseParam,
    double StartAngle_deg,
    double EndAngle_deg,
    double DeltaAngle_deg,
    std::vector< cv::Point2d > &DstPoints
)
{
    double Cos,Sin;
    {
        double EllipseAngleRad = RadFromDeg(EllipseParam.angle);
        Cos = cos( EllipseAngleRad );
        Sin = sin( EllipseAngleRad );
    }

    //Here, you will be able to reserve the destination vector size, but in this sample, it was omitted.
    DstPoints.clear();

    const double HalfW = EllipseParam.size.width * 0.5;
    const double HalfH = EllipseParam.size.height * 0.5;
    for( double deg=StartAngle_deg; deg<EndAngle_deg; deg+=DeltaAngle_deg )
    {
        double rad = RadFromDeg( deg );
        double u = cos(rad) * HalfW;
        double v = sin(rad) * HalfH;
        double x = u*Cos + v*Sin + EllipseParam.center.x;
        double y = u*Sin - v*Cos + EllipseParam.center.y;
        DstPoints.emplace_back( x,y );
    }
}