Creating a labelmatrix from a non-binary matrix for regionprops

I have an RGB image that I've imported into MATLAB using imread(), and I have code that has converted each cells value from RGB values into a single value (e.g. going from [1;2;1] to [34]). I'm using this to record unique labels for each cell, which correspond to labels of object's I've segmented previously. A small section of the matrix would like this:

0 0  0  52 52 52 0  0  0
0 0  52 52 31 31 31 0  0
0 52 52 0  0  31 31 31 31
0 0  0  0  0  31 31 0  0

I'm currently trying to run regionprops on the matrix with single values, thinking it would automatically interpret the matrix as a labelmatrix. This doesn't seem to be the case though as the 'label' property is not present in the output. I have areas in my matrix where objects are touching each other, and thus converting the single matrix to binary and running regionprops on that will lead to unwanted merging of objects.

Is there a way to get regionprops to recongize my matrix as a labelmatrix as-is? Or at least convert my matrix into a labelmatrix?

Answer 1

[Githin George]

Hello,

I understand you are having a matrix with unique values of labels for each point and would like to use this matrix as a ‘labelmatrix’ for running the “regionprops” function.

A ‘single’ valued labelmatrix is a supported argument for the “regionprops” function but the label property will be present in the output only if the input matrix L is a ‘categorical’ labelmatrix as described in the MATLAB documentation for “regionprops” function.

https://www.mathworks.com/help/images/ref/regionprops.html#buoixjn-1-stats

As a workaround, you could modify the following code snippet to add a custom label property to the “regionprops” output for each label or convert your matrix into a ‘categorical’ based on the label information you already have.

% let L be your labelMatrix 
lVal = unique(L); 
baseMask = zeros(size(L)); 

regProps = {}; 
for ii=2:numel(unique(lVal)) 
    idx = find(L == lVal(ii)); 
    MaskTemp = baseMask; 
    MaskTemp(idx) = 1; 
    % You could add logic for custom labeling Here 
    regProps{ii-1} = regionprops(MaskTemp,"basic"); 
    % 
end 

I hope this helps.