Please look over this demo and see how you use ismember to select objects bigger than 95 pixels, and how you use just normal indexing/assignment to set all those objects to 254 in a single line of code. This is an adaptation of my Image Segmentation tutorial at my File Exchange: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
imtool close all; % Close all imtool figures.
workspace; % Make sure the workspace panel is showing.
fontSize = 20;
% Create sample image.
originalImage = propsSynthesizeImage;
subplot(2, 2, 1);
imshow(originalImage);
impixelinfo; % Allow cursor to show gray levels.
title('Synthetic Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Threshold the image to get a binary image (only 0's and 1's) of class "logical."
thresholdValue = 190;
binaryImage = originalImage > thresholdValue; % Bright objects will be the chosen if you use >.
subplot(2, 2, 2);
imshow(binaryImage);
title('Binary Image', 'FontSize', fontSize);
% Label the image.
[labeledImage numberOfOriginalBlobs] = bwlabel(binaryImage, 8); % Label each blob so we can make measurements of it
% Get all the blob properties. Can only pass in originalImage in version R2008a and later.
blobMeasurements = regionprops(labeledImage, originalImage, 'all');
% Now I'll demonstrate how to select certain blobs based using the ismember function.
% Let's say that we wanted to find only those blobs
% with an area greater than 95 pixels.
allBlobAreas = [blobMeasurements.Area];
% Get a list of the blobs that meet our criteria and we need to keep.
allowableAreaIndexes = allBlobAreas > 95; % Take the large objects.
keeperIndexes = find(allowableAreaIndexes);
% Extract only those blobs that meet our criteria, and
% eliminate those blobs that don't meet our criteria.
% Note how we use ismember() to do this.
keeperBlobsImage = ismember(labeledImage, keeperIndexes);
% Re-label with only the keeper blobs kept.
[areasGT95Image numberOfBlobs] = bwlabel(keeperBlobsImage, 8); % Label each blob so we can make measurements of it
% Now we're done. We have a labeled image of blobs that meet our specified criteria.
subplot(2, 2, 3);
% Convert the labeled image back to a binary image.
binary95 = areasGT95Image > 0;
imshow(binary95, []);
title('"Keeper" blobs larger than 95 pixels', 'FontSize', fontSize);
% Make those pixels have a value of 254 in the original image.
outputImage = originalImage; % Make a copy
outputImage(binary95) = 254; % Change the copy.
% Display the final output image.
subplot(2, 2, 4);
imshow(outputImage, []);
title('Original blobs larger than 95 pixels = 254', 'FontSize', fontSize);
