CODEWITHSUNDEEP
opencv
user1036908
user1036908

Reputation: 861

efficient way of counting pixels in a contour

Given a contour, what is the efficient way of counting the pixels that belong inside the contour?

I tried using opencv function
cv::pointPolygonTest()

but the function is highly inefficient and takes a lot of time for bigger contours.
Please advice

Upvotes: 1

Views: 6820

Answers (3)

Joe Aspara
Joe Aspara

Reputation: 1197

If contourArea doesn't give you expected result, you can follow this approach:

  1. Copy the contour in a new empty Mat

  2. Apply countNonZero on the new submatrix and you will get the number of non-black pixel

        //1. Copy the contour in a new empty Mat
    Rect cRect = boundingRect(*it); //it is an iterator for your contours vector
    Mat subImg = dilatedImg(cRect);
    double cArea = countNonZero(subImg);

Note: if you doesn't have a binary image, you may need to threshold it before to count only the right pixels.

Upvotes: 1

Barney Szabolcs
Barney Szabolcs

Reputation: 12514

I think you could have a simple recursive traversal that fills the area started from a single contour point. It will be efficient if it goes col-first I think so you can use the row iterator.

If you care about efficiency the most, you can perhaps unroll the recursion into loops and do for-unlooping.

Upvotes: 0

Sam
Sam

Reputation: 20058

I suppose you have extracted the contours with the findContours() function.

Then, you can use the contourArea() directly.

cout << Contour area is << contourArea(contours[k]) ;

Upvotes: 1

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