Detect ellipse parameters from a given elliptical mask

Question

I'm following the tutorial from the skimage website to detect edges from a mask and then get the ellipse perimeters.

However, when I run this code I get this:

I've reduced the threshold in the hough_ellipse function to 100 because that's the highest value that's working (which at times doesn't work with other masks) and min_size to 10 (for the same reason). I have no experience with with image processing or computer vision and have not found any other way to get the perimeters from large number of masks.

Answer 1

Here is how to do that with fitting to an ellipse in Python/OpenCV.

• Read the input
• Convert to gray
• Threshold
• Fit ellipse to shape
• Draw ellipse on input image
• Save results

Input:

import cv2
import numpy as np

# read image
img = cv2.imread('ellipse_shape.png')
hh, ww = img.shape[:2]

# convert to grayscale
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)

# threshold to binary and invert
thresh = cv2.threshold(gray, 252, 255, cv2.THRESH_BINARY)[1]

# fit ellipse
# note: transpose needed to convert y,x points from numpy to x,y for opencv
points = np.column_stack(np.where(thresh.transpose() > 0))
hull = cv2.convexHull(points)
((centx,centy), (width,height), angle) = cv2.fitEllipse(hull)
print("center x,y:",centx,centy)
print("diameters:",width,height)
print("orientation angle:",angle)

# draw ellipse on input image
result = img.copy()
cv2.ellipse(result, (int(centx),int(centy)), (int(width/2),int(height/2)), angle, 0, 360, (0,0,255), 2)

# show results
cv2.imshow('image', img)
cv2.imshow('thresh', thresh)
cv2.imshow('result', result)
cv2.waitKey(0)
cv2.destroyAllWindows()

# save results
cv2.imwrite('ellipse_shape_fitted.png', result)

Resulting ellipse:

Ellipse Data:

center x,y: 291.0881042480469 337.10638427734375
diameters: 176.3456573486328 207.72769165039062
orientation angle: 125.05526733398438