How to use OpenCV's connectedComponentsWithStats in Python?

PythonOpencvConnected Components

Python Problem Overview


I am looking for an example of how to use OpenCV's connectedComponentsWithStats() function in Python. Note this is only available with OpenCV 3 or newer. The official documentation only shows the API for C++, even though the function exists when compiled for Python. I could not find it anywhere online.

Python Solutions


Solution 1 - Python

The function works as follows:

# Import the cv2 library
import cv2
# Read the image you want connected components of
src = cv2.imread('/directorypath/image.bmp')
# Threshold it so it becomes binary
ret, thresh = cv2.threshold(src,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)
# You need to choose 4 or 8 for connectivity type
connectivity = 4  
# Perform the operation
output = cv2.connectedComponentsWithStats(thresh, connectivity, cv2.CV_32S)
# Get the results
# The first cell is the number of labels
num_labels = output[0]
# The second cell is the label matrix
labels = output[1]
# The third cell is the stat matrix
stats = output[2]
# The fourth cell is the centroid matrix
centroids = output[3]

Labels is a matrix the size of the input image where each element has a value equal to its label.

Stats is a matrix of the stats that the function calculates. It has a length equal to the number of labels and a width equal to the number of stats. It can be used with the OpenCV documentation for it:

> Statistics output for each label, including the background label, see > below for available statistics. Statistics are accessed via > stats[label, COLUMN] where available columns are defined below. > > - cv2.CC_STAT_LEFT The leftmost (x) coordinate which is the inclusive start of the bounding box in the horizontal direction. > - cv2.CC_STAT_TOP The topmost (y) coordinate which is the inclusive start of the bounding box in the vertical direction. > - cv2.CC_STAT_WIDTH The horizontal size of the bounding box > - cv2.CC_STAT_HEIGHT The vertical size of the bounding box > - cv2.CC_STAT_AREA The total area (in pixels) of the connected component

Centroids is a matrix with the x and y locations of each centroid. The row in this matrix corresponds to the label number.

Solution 2 - Python

I have come here a few times to remember how it works and each time I have to reduce the above code to :

_, thresh = cv2.threshold(src,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)
connectivity = 4  # You need to choose 4 or 8 for connectivity type
num_labels, labels, stats, centroids = cv2.connectedComponentsWithStats(thresh , connectivity , cv2.CV_32S)

Hopefully, it's useful for everyone :)

Solution 3 - Python

Adding to Zack Knopp answer, If you are using a grayscale image you can simply use:

import cv2
import numpy as np

src = cv2.imread("path\\to\\image.png", 0)
binary_map = (src > 0).astype(np.uint8)
connectivity = 4 # or whatever you prefer

output = cv2.connectedComponentsWithStats(binary_map, connectivity, cv2.CV_32S)

When I tried using Zack Knopp answer on a grayscale image it didn't work and this was my solution.

Attributions

All content for this solution is sourced from the original question on Stackoverflow.

The content on this page is licensed under the Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license.

Content TypeOriginal AuthorOriginal Content on Stackoverflow
QuestionZack KnoppView Question on Stackoverflow
Solution 1 - PythonZack KnoppView Answer on Stackoverflow
Solution 2 - PythonDan ErezView Answer on Stackoverflow
Solution 3 - PythonBarel LevyView Answer on Stackoverflow