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

I wanna detect black color from gray image and mark like this way. How can I detect like this. please know me..

Answer 1

[KALYAN ACHARJYA]

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.

Answer 2

[Image Analyst]

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

[yanqi liu]

img = imread('https://ww2.mathworks.cn/matlabcentral/answers/uploaded_files/904390/Notes_220223_224815.jpg');
mask = imdilate(bwareafilt(~im2bw(img, 100/255),1), strel('square', 39));
props = regionprops(mask, 'BoundingBox');
figure; imshow(img, []); 
hold on; rectangle('position', props.BoundingBox, 'EdgeColor', 'g', 'LineWidth', 2)

[Alexai]

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]

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))