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fitellipse not fitting

asked 2017-01-04 19:41:28 -0500

image detective gravatar image

updated 2017-01-04 23:42:48 -0500

I get this result when fitting an ellipse to a contour (the contour is green, the fitted ellipse in blue) image description

Here's the original image :

image description

Why doesn't it fit?

Here is the code :

----------
img1 = cv2.imread(r'limb_edge_fl28_iss3628.jpg')

gray1 = cv2.cvtColor(img1, cv2.COLOR_BGR2GRAY)

# Threshold the images
thresh1 = cv2.threshold(gray1, 127, 255, cv2.THRESH_BINARY)[1]

contours = cv2.findContours(thresh1.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)[1]

cnt1 = contours[0]

# draw contours found
cv2.drawContours(img1, [cnt1], 0, green, 2)

# Fit an ellipse to the contours
ellipse = cv2.fitEllipse(cnt1)


# Draw the ellipse on the image
cv2.ellipse(img1,ellipse,(255,255,0),2)

cv2.imshow("ellipse fit 1", img1)
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Comments

1

I'm getting the same results. I'll have a look at it tomorrow. I tried skipping the contour and simply adding every point on the line, but still the same.

Tetragramm gravatar imageTetragramm ( 2017-01-05 00:15:21 -0500 )edit

May be related to this bug! https://github.com/opencv/opencv/issu...

Balaji R gravatar imageBalaji R ( 2017-01-05 00:50:11 -0500 )edit

Is there another free Python Function/library that I could use to fit an ellipse?

image detective gravatar imageimage detective ( 2017-01-05 09:03:30 -0500 )edit

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answered 2017-01-05 20:25:56 -0500

Tetragramm gravatar image

updated 2017-01-05 21:09:17 -0500

Ok, I found the paper that the algorithm comes from, and there are some differences. Hewing closer to the original algorithm fixes the problem, at least for this example.

Namely, there is a comment in fitEllipse

// A - C signs inverted as proposed by APP

then three lines with negatives. If those negatives are removed, you get the correct answer.

This also fixes #4515 and #6544. That makes me happy, so I'll run the tests and commit the changes.

EDIT: Hmm, it is failing some tests though. More work to do.

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@Tetragram Are you saying that we should change these lines

Ad[i*5] = -(double)p.x * p.x; // A - C signs inverted as proposed by APP
Ad[i*5 + 1] = -(double)p.y * p.y;
Ad[i*5 + 2] = -(double)p.x * p.y;

to this

Ad[i*5] = (double)p.x * p.x; // A - C signs inverted as proposed by APP
Ad[i*5 + 1] = (double)p.y * p.y;
Ad[i*5 + 2] = (double)p.x * p.y;
Balaji R gravatar imageBalaji R ( 2017-01-06 04:35:27 -0500 )edit

If yes then the Results are pretty bad than before! I guess This is not helping!

Balaji R gravatar imageBalaji R ( 2017-01-06 04:53:13 -0500 )edit

That is what I was saying. It does fix the sample, and it fixed the test cases in bug #4515 and 6544. It apparently does not work for all scenarios though. I am still working on it and will see if I can get something working for all test cases.

Tetragramm gravatar imageTetragramm ( 2017-01-06 17:45:24 -0500 )edit

Tetragramm, any news on the fitellipse fix? Thanks

image detective gravatar imageimage detective ( 2017-01-11 18:45:51 -0500 )edit

They say no news is good news. In this case however, it's just frustrating. 3 papers, 1 algorithm. And I still can't get the output of the math to match the constraints given in the papers. I'm setting up to use matlab to make sure it isn't the library I'm using.

I am still working on it though.

Tetragramm gravatar imageTetragramm ( 2017-01-11 19:13:16 -0500 )edit

Thanks for the update!

image detective gravatar imageimage detective ( 2017-01-11 19:23:05 -0500 )edit
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Asked: 2017-01-04 19:41:28 -0500

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Last updated: Jan 05 '17