How can I detect black color and mark bounding box from overall gray image?

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Alexai
Alexai 2022-2-23
编辑： Image Analyst 2022-2-24
I wanna detect black color from gray image and mark like this way. How can I detect like this. please know me..

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KALYAN ACHARJYA
KALYAN ACHARJYA 2022-2-23
Foremost segment that Black ROI regions, then apply the following-
#1 It can be done using regionprops
https://in.mathworks.com/help/images/ref/regionprops.html
#2 To add the bounding box use rectangle function.
https://in.mathworks.com/help/matlab/ref/rectangle.html
Please try, it is absolutely possible.

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Image Analyst
Image Analyst 2022-2-23
What does "detect" mean to you. You can easily threshold it, like find pixels darker than, say, 20 by doing
mask = grayImage < 20;
Now what? What do you want to do now that it's been detected. Maybe get its Area and Centroid?
props = regionprops(mask, 'Area', 'Centroid');
You'll want to look at my Image Segmentation Tutorial for a quick start lesson:
Image Analyst's File Exchange
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Alexai
Alexai 2022-2-24
Thank you so much but I want to know more. can you know me how can I detect a lot of bounding box like this photo.
Image Analyst
Image Analyst 2022-2-24
编辑：Image Analyst 2022-2-24
You never tried my tutorial, did you? OK, so here it is adapted for your image:
% Demo by Image Analyst
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 = 22;
markerSize = 40;
%--------------------------------------------------------------------------------------------------------
% READ IN IMAGE
folder = pwd;
baseFileName = 'Screenshot_20220224-200026_Samsung Notes.jpg';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
% The file doesn't exist -- didn't find it there in that folder.
% Check the entire search path (other folders) for the file by stripping off the folder.
fullFileNameOnSearchPath = baseFileName; % No path this time.
if ~exist(fullFileNameOnSearchPath, 'file')
% Still didn't find it. Alert user.
errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
uiwait(warndlg(errorMessage));
return;
end
end
rgbImage = 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(rgbImage)
if numberOfColorChannels > 1
% It's not really gray scale like we expected - it's color.
% Extract the blue channel.
grayImage = rgbImage(:, :, 3);
else
grayImage = rgbImage;
end
%--------------------------------------------------------------------------------------------------------
% Display the image.
subplot(2, 2, 1);
imshow(grayImage);
impixelinfo;
axis('on', 'image');
title('Original Gray Scale Image', 'FontSize', fontSize, 'Interpreter', 'None');
hold on
% Maximize window.
g = gcf;
g.WindowState = 'maximized'
drawnow;
%--------------------------------------------------------------------------------------------------------
% Get a histogram
subplot(2, 2, [2, 4]);
imhist(grayImage);
grid on;
title('Histogram of Original Gray Scale Image', 'FontSize', fontSize, 'Interpreter', 'None');
%--------------------------------------------------------------------------------------------------------
% Take the histogram
counts = histcounts(grayImage);
%--------------------------------------------------------------------------------------------------------
% Set thresholds for each level.
lowThreshold = 0;
% Get the triangle threshold for the upper threshold
highThreshold = 150;
% Interactively and visually set a threshold on a gray scale image.
% https://www.mathworks.com/matlabcentral/fileexchange/29372-thresholding-an-image?s_tid=srchtitle
% [lowThreshold, highThreshold] = threshold(lowThreshold, highThreshold, grayImage)
% Draw threshold line over histogram.
xline(highThreshold, 'Color', 'r', 'LineWidth', 2);
%--------------------------------------------------------------------------------------------------------
% Binarize the image to get a mask.
mask = grayImage >= lowThreshold & grayImage <= highThreshold;
% Display mask image.
subplot(2, 2, 3);
imshow(mask);
hold on;
impixelinfo;
axis('on', 'image');
drawnow;
caption = sprintf('Mask with Threshold = %d gray levels', highThreshold);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
% Take the blob closest to the center of the image.
props = regionprops(mask, 'Centroid', 'BoundingBox');
for k = 1 : length(props)
xCentroid = props(k).Centroid(1);
yCentroid = props(k).Centroid(2);
rectangle('Position', props(k).BoundingBox, 'EdgeColor', 'r', 'LineWidth', 2);
str = sprintf('Centroid = (%.2f, %.2f)', xCentroid, yCentroid);
text(xCentroid, yCentroid, str, 'Color', 'r', 'FontSize', 14, 'FontWeight','bold');
end
% Tell the user
message = sprintf('Done!\n')
uiwait(helpdlg(message))

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