bwlabel changes the number of pixels in blobs

Hey

I have a logical image containing 3 components as seen here:

I then use the bwlabel function on the image and get the following image:

I do not understand why bwlabel adds pixels to one of the components. I use bwlabel with the default settings.

Answer 1

[Image Analyst]

Your image is not a binary image. It's an RGB image. bwlabel returns a labeled image as a double, not a binary image as you showed in your second image. So I have no idea what your original image really was and what code you used to produce the labeled image. This is what I did:

rgbImage = imread('bwlabel.png');
class(rgbImage)

ans = 'uint8'

size(rgbImage)

ans = 1×3

496 852 3

subplot(2, 2, 1);
imshow(rgbImage, []);
title('Original RGB Image')
axis('on', 'image');
impixelinfo
if size(rgbImage, 3) == 3
    fprintf('Converting RGB image to logical/binary image.\n')
    rgbImage = logical(rgbImage(:, :, 2));
	subplot(2, 2, 2);
	imshow(rgbImage, []);	
	title('Now converted to gray scale Image')
	impixelinfo
	axis('on', 'image');
end

Converting RGB image to logical/binary image.

% Crop off the frame with a gray level of 240.
rgbImage = rgbImage(9:478, 13:834);
% Label the image.
[labeledImage, numBlobs] = bwlabel(rgbImage);
class(labeledImage)

ans = 'double'

fprintf('Found %d blobs in the binary image', numBlobs);

Found 3 blobs in the binary image

subplot(2, 2, 3);
imshow(labeledImage, []);
impixelinfo
title('Labeled RGB Image')
axis('on', 'image');

As you can see from my code above, the labeled image has no more non-zero pixels than your original image.

[DGM]

I think it's important to note where exactly the distinction is occurring. When calling bwlabel() with any image that's not logical-class, it will be cast as logical before labeling. This is equivalent to

bw = bw ~= 0;

... which is rarely a good threshold value if you're trying to binarize an image.

The lesson? Make sure your image is satisfactorily binarized before feeding it to bwlabel(), bwconncomp(), etc. Otherwise, bwlabel() will make it binary with complete disregard for the content.