how can I crop a sector of a circle on an image in python using scikit image

Question

I am using scikit image for python 3.5. I'd like to crop a sector of a circle and save it as a different image.

All I have is center of the circle(cx,cy), radius r, 2 coordinates(x1,y1),(x2,y2) on the perimeter of the circle, and the equations of 2 lines of a sector.

If it is entire circle, i can use the circle equation and blacken the rest of the image but as it is a sector, I am facing a problem.

Answer 1

Using the drawing functions from skimage.draw, you can construct a circle and a polygon and intersect the two to obtain a slice:

import numpy as np
from skimage import data, draw
import matplotlib.pyplot as plt

# Random image
image = np.random.random((200, 200))

# --- coordinate specification

r0, c0 = 100, 70  # circle center (row, column)
R = 100  # circle radius

theta0 = np.deg2rad(20)  # angle #1 for arc
theta1 = np.deg2rad(40)  # angle #2 for arc

# Above, I provide two angles, but you can also just give the two
# coordinates below directly

r1, c1 = r0 - 1.5 * R * np.sin(theta0), c0 + 1.5 * R * np.cos(theta0)  # arc coord #1
r2, c2 = r0 - 1.5 * R * np.sin(theta1), c0 + 1.5 * R * np.cos(theta1)  # arc coord #2

# --- mask calculation

mask_circle = np.zeros(image.shape[:2], dtype=bool)
mask_poly = np.zeros(image.shape[:2], dtype=bool)

rr, cc = draw.circle(r0, c0, R, shape=mask_circle.shape)
mask_circle[rr, cc] = 1

rr, cc = draw.polygon([r0, r1, r2, r0],
                      [c0, c1, c2, c0], shape=mask_poly.shape)
mask_poly[rr, cc] = 1

mask = mask_circle & mask_poly

plt.imshow(mask, cmap='gray')
plt.show()