MATLAB: How to remove a white background and only keep certain objects in a binary image on MATLAB

crackimage processingImage Processing Toolbox

I would like the remove the white pixels in the background, but somehow keep the white lines. How would I go about doing that.
Sorry I didn't mean to remove the question. I was trying to comment on my phone, but I must have unknowingly modified the question.

Best Answer

Justin - are you still there? Perhaps you're still struggling over this so I made a complete and fancy demo for you. Here it is:
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 25;
%===============================================================================

% Get the name of the image the user wants to use.
baseFileName = 'concrete_inverted.png';
% Get the full filename, with path prepended.
folder = []; % Determine where demo folder is (works with all versions).
fullFileName = fullfile(folder, baseFileName);
%===============================================================================
% Read in a demo image.
grayImage = imread(fullFileName);
% Get the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
if numberOfColorChannels > 1
  % It's not really gray scale like we expected - it's color.
  % Use weighted sum of ALL channels to create a gray scale image.
  grayImage = rgb2gray(grayImage);
  % ALTERNATE METHOD: Convert it to gray scale by taking only the green channel,
  % which in a typical snapshot will be the least noisy channel.
  % grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the image.
subplot(2, 2, 1);
imshow(grayImage, []);
axis on;
axis image;
caption = sprintf('Original Gray Scale Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo();
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
drawnow;
% Binarize the image by thresholding.
mask = grayImage > 128;
% For some reason, the top two lines of his image are all white.  Blacken those 2 lines:
mask(1:2, :) = false;
% Display the mask image.

subplot(2, 2, 2);
imshow(mask);
axis on;
axis image; % Make sure image is not artificially stretched because of screen's aspect ratio.


title('Binary Image Mask', 'fontSize', fontSize);
drawnow;
% Get rid of small blobs.
mask = bwareaopen(mask, 500);
% Label the image.
labeledImage = bwlabel(mask);
% Find the areas and perimeters
props = regionprops(labeledImage, 'Area', 'Perimeter');
allAreas = [props.Area];
sortedAreas = sort(allAreas, 'descend')
allPerimeters = [props.Perimeter];
% Compute circularities
circularities = allPerimeters .^ 2 ./ (4 * pi * allAreas)
sortedC = sort(circularities, 'descend')
% Keep conly blobs that are nowhere close to circular or compact.
minAllowableCircularity = 10;
keeperIndexes = find(circularities >= minAllowableCircularity);
mask = ismember(labeledImage, keeperIndexes);
% Display the mask image.
subplot(2, 2, 3);
imshow(mask);
axis on;
axis image; % Make sure image is not artificially stretched because of screen's aspect ratio.
title('Intermediate Cleaned Mask', 'FontSize', fontSize);
drawnow;
% Get rid of black islands (holes) in struts without filling large black areas.
subplot(2, 2, 4);
mask = ~bwareaopen(~mask, 1000);
imshow(mask);
axis on;
axis image; % Make sure image is not artificially stretched because of screen's aspect ratio.
title('Final Cleaned Mask', 'FontSize', fontSize);
drawnow;
I think the results look pretty good, like what I think you're expecting. Don't you? If not, let me know what I could do to improve it. Basically the same original crack lines are in the image (no change in exterior shape), except that the small black islands in them are filled in, all all the little isolated white island are removed. Is that what you want? There are a few parameters/variables in there, with obvious names, that you can adjust if you want to. Please get back to me.
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