How to display a single connected component out of many using `regionprops` - matlab

Question

When using the function regionprops in MATLAB, there is an option to extract the binary image of each connected component. The size of the binary image is reduced to the size of the connected component. I don't want the size of the binary image to reduce. I want the size of the binary image to retain its original size while only showing the selected connected component at its corresponding location in the original image size. How can I extract the connected component in the original image size?

Answer 1

Simply create a blank image that's the same size as your original image and instead of extracting the image per blob, extract the actual pixel locations with reference to the original image for each blob then populate the blank image by setting these locations to binary true in this blank image. Use the PixelIdxList attribute from regionprops to obtain the column major locations of your desired component, then use these to set the output image at these same locations to true.
Assuming your regionprops structure is stored in S and you want to extract out the kth component and the original image is stored in A, do the following:
% Allocate blank image
out = false(size(A, 1), size(A, 2));
% Run regionprops
S = regionprops(A, 'PixelIdxList');
% Determine which object to extract
k = ...; % Fill in ID here
% Obtain the indices
idx = S(k).PixelIdxList;
% Create the mask to be the same size as the original image
out(idx) = true;
imshow(out); % Show the final mask
If you have multiple objects and want to create this mask that is the original size of the image separately per object, you can use a for loop to do that for you:
% Run regionprops
S = regionprops(A, 'PixelIdxList');
% For each blob...
for k = 1 : numel(S)
out = false(size(A, 1), size(A, 2)); % Allocate blank image
% Extract out the kth object's indices
idx = S(k).PixelIdxList;
% Create the mask
out(idx) = true;
% Do your processing with out ...
% ...
end