How do I remove a white background and only keep

To remove white backgroud you need to follow this:

%===============================================================================
baseFileName = 'concrete_inverted.webp';
folder = []; % Determine where demo folder is (works with all versions).
fullFileName = fullfile(folder, baseFileName);

%===============================================================================
grayImage = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(grayImage)
if numberOfColorChannels > 1
  grayImage = rgb2gray(grayImage);
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(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
drawnow;

mask = grayImage > 128;
mask(1:2, :) = false;
subplot(2, 2, 2);
imshow(mask);
axis on;
axis image;
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;