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It appears that connectedComponents does not separate components that are divided by a single pixel dividing line.

This is an issue when trying to obtain region stats from an image segmented with the watershed method (ultimately I want to use connectedComponentsWithStats). Since connectedComponents does not handle already labeled images, I binarize the image by setting the background and dividing lines as 0 and the labeled regions as 255. The resulting binary image (segmented_bin) has regions separated by a single-pixel-wide line.

Running connectedComponents on this image returns only 2 regions: the background and an aggregate of the foreground regions.

Steps to reproduce

Here is a sample code to reproduce the issue

```python import cv2

[...] # Prepare markers as in https://docs.opencv.org/3.4/d3/db4/tutorial_py_watershed.html

segmented = cv2.watershed(img,markers)

segmented_bin = segmented.copy() segmented_bin[segmented < 2] = 0 # -1 is dividing regions, no 0s, 1 is background segmented_bin[segmented > 1] = 255 # all above 1 are distinct regions num_labels, label_image = cv2.connectedComponents(segmented_bin.astype('uint8'), 8, cv2.CV_16U, cv2.CCL_GRANA) ```

Executing this code returns num_labels = 2 instead of 27.

It appears that connectedComponents does not separate components that are divided by a single pixel dividing line.

This is an issue when trying to obtain region stats from an image segmented with the watershed method (ultimately I want to use connectedComponentsWithStats). Since connectedComponents does not handle already labeled images, I binarize the image by setting the background and dividing lines as 0 and the labeled regions as 255. The resulting binary image (segmented_bin) has regions separated by a single-pixel-wide line.

Running connectedComponents on this image returns only 2 regions: the background and an aggregate of the foreground regions.

Steps to reproduce

Here is a sample code to reproduce the issue

import cv2
 
cv2

[...] # Prepare markers as in https://docs.opencv.org/3.4/d3/db4/tutorial_py_watershed.html
 
https://docs.opencv.org/3.4/d3/db4/tutorial_py_watershed.html

segmented = cv2.watershed(img,markers)
 
cv2.watershed(img,markers)

segmented_bin = segmented.copy()
segmented_bin[segmented < 2] = 0 # -1 is dividing regions, no 0s, 1 is background
segmented_bin[segmented > 1] = 255 # all above 1 are distinct regions
num_labels, label_image = cv2.connectedComponents(segmented_bin.astype('uint8'), 8, cv2.CV_16U, cv2.CCL_GRANA)
```