Detecting blobs on image with OpenCV

Question

I want to get some descriptors for each white area on image to filter that areas and operate with them separately. How can I do it?

I have read How to use OpenCV SimpleBlobDetector and http://www.learnopencv.com/blob-detection-using-opencv-python-c/ but still can't get any result with my simple image.

Here is my code in python

img = cv2.imread("map.jpg", cv2.IMREAD_GRAYSCALE)
params = cv2.SimpleBlobDetector_Params()
params.blobColor = 255
params.filterByColor = True
params.minArea = 16
params.filterByArea = True
detector = cv2.SimpleBlobDetector_create(params)
keypoints = detector.detect(255 - img)
len(keypoints)
# 0

OpenCV 3.1.0

Image is grayscaled.

UPD: Code updated following comment by @api55

you need to pass the blob params, setting the blobColor to 255 and filter by color to true. By default it tries to find dark blobs in white background. Or you could invert the colors of the image — api55
– api55, Commented May 11, 2016 at 12:33
@api55 I have updated code (in post too) but still no result — Askhat Nuriddinov
– Askhat Nuriddinov, Commented May 11, 2016 at 12:43
Try changing also the minArea parameter to something small, like 16 (4x4 pixel blob approx). In the tutorial, it says that filter by color is not working (not sure what version you have).... you can try inverting the image colors — api55
– api55, Commented May 11, 2016 at 12:51

Askhat Nuriddinov · Accepted Answer · 2016-05-12 16:53:42Z

1

I want to get some descriptors for each white area on image to filter that areas and operate with them separately. How can I do it?

My goal could be reached with sklearn.measure.label. This function returns a numpy array with the same shape and labels for each connected area.

But anyway it is still not clear why SimpleBlobDetector from OpenCV doesn't work.

answered May 12, 2016 at 16:53

Askhat Nuriddinov

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1 Comment

Chandra Over a year ago

Try skimage blob detection. scikit-image.org/docs/dev/auto_examples/features_detection/….

Guido · Accepted Answer · 2017-01-07 01:22:24Z

0

I was having the same problem. I had to remove the fiterbyarea parameter:

params.filterByArea = False

answered Jan 7, 2017 at 1:22

Guido

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