Unable to find the number of pixels on the boundary of a ROI

Hi,

I want to find the number of pixels on the boundary of a ROI. I also want to find the number of pixels inside the ROI. I tried the regionprops method but it is throwing me an error message. Any suggestions would be appreciated.

myFolder = 'D:\regionGrowing_MLT\newim\Segmentation Results';
% Check to make sure that folder actually exists.  Warn user if it doesn't.
if ~isfolder(myFolder)
    errorMessage = sprintf('Error: The following folder does not exist:\n%s\nPlease specify a new folder.', myFolder);
    uiwait(warndlg(errorMessage));
    myFolder = uigetdir(); % Ask for a new one.
    if myFolder == 0
         % User clicked Cancel
         return;
    end
end
% Get a list of all files in the folder with the desired file name pattern.
filePattern = fullfile(myFolder, '*.jpg'); % Change to whatever pattern you need.
theFiles = dir(filePattern);
for k = 1 : length(theFiles)
    baseFileName = theFiles(k).name;
    fullFileName = fullfile(theFiles(k).folder, baseFileName);
    fprintf(1, 'Now reading %s\n', fullFileName);
    % Now do whatever you want with this file name,
    % such as reading it in as an image array with imread()
    imageArray = imread(fullFileName);
    imageArray=imbinarize(imageArray);
    s = regionprops(imageArray,'Area','Perimeter');
    s(k,:)=s;
 
end

The following error message is displayed:

Unable to perform assignment because the size

of the left side is 1-by-1 and the size of

the right side is 5-by-1.

Error in im3d (line 24)

s(k,:)=s;

Answer 1

[Image Analyst]

Try this:

perimImage = bwperim(imageArray);
numPerimPixels = nnz(perimImage);

or

boundaries = bwboundaries(imageArray);
numPerimPixels = 0;
for k = 1 : length(boundaries)
    thisBoundary = boundaries{k};
    numPerimPixels = numPerimPixels + size(thisBoundary, 1);
end

[Warid Islam]

Hi @Image Analyst,

That worked. If I want to find the area of my ROI, is bwarea the appropriate way to proceed?

[Image Analyst]

bwarea gives an area weighted by the shape of the boundary, while regionprops() or nnz() is strictly a pixel count. Just depends on how you want it.

for this image

0 1

1 1

What is the area? Is it 3 pixels? Or is it half a pixel? Neither is wrong - it's just how you interpret "area". Do you consider a pixel like a little block or tile? Or are you going from pixel center to pixel center.

What is the length of the 1 region here:

0 1 1 1 0

Is it 3? Or is it 2 (going center to center)?