how to force cv2.ellipse to draw a clockwise circular arc?

Question

cv2.ellipse seems to have a mind of its own about which way it interprets start-and end angles. I want to draw a circular arc from 300° to 130° in the clockwise direction, i.e. like this:

with this code:

import numpy as np
import cv2

center = (200, 200) # x,y
axes = (100, 100) # first, second
angle = 0. # clockwise, first axis, starts horizontal
for i in range( 330,130,-1):
  image = np.zeros((400, 400, 3)) # creates a black image
  image = cv2.ellipse(image, center, axes, angle, 0., 360, (0,0,255))
  image = cv2.ellipse(image, center, axes, angle, 330, i, (0,255,0))
  cv2.imshow("image", image)
  cv2.waitKey(5)

cv2.waitKey(0)
cv2.destroyAllWindows()

I got this instead, with the green segment being drawn counter-clockwise from 330 to 130 instead of clockwise.

The code generates the red circle first and then overlays the green arc, but it demonstrates the problem.

Answer 1

Note: You should have probably mentioned that your code comes from [SO]: Understanding the ellipse parameters in Open CV using Python (@api55's answer).

• Listing [OpenCV.Docs]: Drawing Functions - ellipse()

• Everything is a bit complicated since the display xOy system is flipped vertically ((+∞ on) Y axis is pointing down) from what we are used to (on paper), the origin is the top left corner instead of the bottom left one (and the 1^st quadrant is corresponding to the 4^th).

Given a circle, and 2 points on it A and B, where each point name is also the degrees (A, B ∈ [0, 360)), there are 2 ways to move from A to B (on the circle), as there are 2 arches. Choosing which arch to take has the following implications:

1. How much to move (distance):

   • Arch 1 (let this be the shorter): D1 = abs(B - A) - as distance (even in angles) is always positive

   • Arch 2: D2 = 360 - D1 - the remainder of the circle

2. Direction to move to (CCW, CW). This translates to the angle increment being added / subtracted to / from the initial value (A).
   Here, I must also mention that moving from B to A is like moving from A to B but in the opposite direction

The 2 things I mentioned above (combined with the fact that CCW is the default direction), translate into the 2 if clauses in the modified version of your code (below). Maybe the conditions are not very obvious (that's why I tried expanding them in the commented lines - code has same effect), but I couldn't find a nicer version (yet).
Anyway, it should work for any angles and directions.

code00.py:

#!/usr/bin/env python

import sys

import numpy as np
import cv2


def main(*argv):
    center = (200, 200)
    axes = (100, 100)
    angle = 0.
    start = 330
    end = 130
    cw = 0 #len(sys.argv) > 1  # @TODO cfati: ClockWise?
    if start < end:
        steps = 360 + start - end
    else:
        steps = start - end
    if cw:
        steps = 360 - steps
        cw_factor = 1
    else:
        cw_factor = -1
    """
    # The above if conditions, expanded (same effect)
    if start < end:
        if cw:
            steps = end - start
            cw_factor = 1
        else:
            steps = 360 + start - end
            cw_factor = -1
    else:
        if cw:
            steps = 360 - start + end
            cw_factor = 1
        else:
            steps = start - end
            cw_factor = -1
    """
    step = 1
    base_color = (0, 0, 255)
    active_color = (0, 255, 0)
    image = np.zeros((400, 400, 3))
    for i in range(1, steps, step):
        cv2.ellipse(image, center, axes, angle, 0, 360, base_color)
        cv2.ellipse(image, center, axes, angle, start, start + cw_factor * i, active_color)
        cv2.imshow("image", image)
        cv2.waitKey(1)

    print("Close drawing window to exit...")
    cv2.waitKey(0)
    cv2.destroyAllWindows()


if __name__ == "__main__":
    print(
        "Python {:s} {:03d}bit on {:s}\n".format(
            " ".join(elem.strip() for elem in sys.version.split("\n")),
            64 if sys.maxsize > 0x100000000 else 32,
            sys.platform,
        )
    )
    rc = main(*sys.argv[1:])
    print("\nDone.\n")
    sys.exit(rc)