CODEWITHSUNDEEP
pythonimageopencvimage-processingpolygon
VICTOR
VICTOR

Reputation: 1942

Opencv detect quadrilateral in Python

I am using Python 2.7 and OpenCV 3.0. I am doing a project to detecting the car license plate.

I am now checking the number of vertex of the contours. If there are 4 vertex(number of elements in approx), then it is more likely to be a rectangle/ parallelogram/ quadrilateral .

(cnts, _) = cv2.findContours(edged.copy(), cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
cnts=sorted(cnts, key = cv2.contourArea, reverse = True)[:10]

# loop over our contours
for c in cnts:
    peri = cv2.arcLength(c, True)
    approx = cv2.approxPolyDP(c, 0.02 * peri, True)
    if len(approx) == 4 and ratio(approx):
        cv2.drawContours(image, [approx], -1, (0,255,0), 3)

And I got two quadrilaterals with array.

enter image description here

However, you can see, there is an irregular polygon. This is the array:

[[[209 198]]

 [[466  94]]

 [[259 153]]

 [[247   1]]]

I am asking for how can I omitted the irregular quadrilateral. Thank you

Upvotes: 3

Views: 9831

Answers (2)

user1270710
user1270710

Reputation: 629

Note: If you know how many sides your polynomial is supposed to have, for example if you are looking for convex quadrilaterals in an image, then you only need call:

hull = cv2.convexHull(approx, returnPoints = False)

And then you only need to check that the len(hull) == num_expected_sides, in the quadrilateral case:

len(cv2.convexHull(approx, returnPoints = False)) == 4

Upvotes: 1

Alessandro Jacopson
Alessandro Jacopson

Reputation: 18593

As suggested by https://stackoverflow.com/users/4606294/user89161 in his comment Opencv detect quadrilateral in Python maybe you can add a test on your approx, something like:

hull = cv2.convexHull(approx,returnPoints = False)
defects = cv2.convexityDefects(approx,hull)

sum_of_defects=0
for i in range(defects.shape[0]):
    s,e,f,d = defects[i,0]
    sum_of_defects=sum_of_defects+d

if sum_of_defects <= threshold:
    print "approx is convex"
else:
    print "approx is not convex"

and you have to properly choose threshold, I will start with threshold=3 or something like that. Theoretically threshold should be zero.

Upvotes: 2

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