Opencv ellipse() bug?

Question

cv.ellipse( img, center, axes, angle, startAngle, endAngle, color[, thickness[, lineType[, shift]]] ) 

The startAngle is the starting angle of the elliptic arc in degrees.If we set angele=30 and startAngle=60,the angle of the starting line of the ellipse may be 30+60=90.Is that right? But this is not the case. My code:

img2 = np.zeros((500, 500))
img2 = cv2.ellipse(img2, (250, 250), (120, 40), 30, 60, 180, 255, -1)
cv2.imshow('2', img2)
cv2.waitKey(0)

Lets look at the result, the angle of the starting line(red line) is obviously not 90 degrees.

enter image description here

Why?Is it a bug?

Answer 1

I got it. startAngle is not the starting angle based on the ellpise.It is based on the circumcircle of that ellipse.Lets look at the following figure

It does a mapping operation.The blue curve is the output.