python - How to remove small connected objects using OpenCV

Question

I use OpenCV and Python and I want to remove the small connected object from my image.

I have the following binary image as input:

The image is the result of this code:

dilation = cv2.dilate(dst,kernel,iterations = 2)
erosion = cv2.erode(dilation,kernel,iterations = 3)

I want to remove the objects highlighted in red:

How can I achieve this using OpenCV?

What criteria did you use to determine which objects to highlight? Why those particular 7 and any of the other similarly sized or smaller objects? — Dan Mašek
nn i want to remove all smalls object and this 7 its just example for object how i want to remove it — Zahra
@DanMašek I want to use the surface of the object as a criteria — Zahra
@DanMašek i know this method but i need to use a method without contour :( — Zahra
@Zahra Then why didn't you mention that fact (and specify that requirement) that in your question? — Dan Mašek

Soltius Soltius · Accepted Answer · 2017-03-15T14:11:18

How about with connectedComponentsWithStats :

#find all your connected components (white blobs in your image)
nb_components, output, stats, centroids = cv2.connectedComponentsWithStats(img, connectivity=8)
#connectedComponentswithStats yields every seperated component with information on each of them, such as size
#the following part is just taking out the background which is also considered a component, but most of the time we don't want that.
sizes = stats[1:, -1]; nb_components = nb_components - 1

# minimum size of particles we want to keep (number of pixels)
#here, it's a fixed value, but you can set it as you want, eg the mean of the sizes or whatever
min_size = 150  

#your answer image
img2 = np.zeros((output.shape))
#for every component in the image, you keep it only if it's above min_size
for i in range(0, nb_components):
    if sizes[i] >= min_size:
        img2[output == i + 1] = 255

Output :

python - How to remove small connected objects using OpenCV

2 Answers