Question

I don’t understand the term/noun ‘label image’ in Matlab, and what’s the normal usage of it? Details would be appreciated. Thanks in advance.

Answer 1

I assume you are referring to the label image or label matrix produced by functions such as bwlabel, or bwconncomp and labelmatrix.

The label matrix takes each connected component, that is each separate region of ON pixels, in a binary image and assigns it a unique integer “label.”

For example, the binary image given by this matrix:

1 1 1 0 0 0 0
1 1 1 0 0 0 0
1 1 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 1 1 1
0 0 0 0 1 1 1

Would become this label matrix:

1 1 1 0 0 0 0
1 1 1 0 0 0 0
1 1 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0 0 2 2 2
0 0 0 0 2 2 2

These are very useful functions for dealing with individual regions in binary images. I often use patterns like:

labels = bwlabel(binaryImage);
for i=1:max(max(labels))
    regionMask = labels == i;
    %# do something with regionMask
end

or

bwcc = bwconncomp(binaryImage);
bwprops = regionprops(bwcc);
for i=length(bwprops)
    regionMask = imcrop(binaryImage,bwprops(i).BoundingBox);
    %# do something with regionMask
end