Try this:
% Computes average histogram of all images in a folder.
% Demo by Image Analyst, March, 2021.
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
clearvars;
workspace; % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 16;
fprintf('Beginning to run %s.m ...\n', mfilename);
myFolder = pwd;
filePattern = fullfile(myFolder, '*.jpg') ;
theFiles = dir(filePattern);
numberOfBins = 256;
numberOfFiles = length(theFiles);
allPixelCounts = zeros(1, numberOfBins);
for k = 1 : numberOfFiles
% Get the input filename.
baseFileName = theFiles(k).name;
fullFileName = fullfile(myFolder, baseFileName);
% Read in the image
theImage = imread(fullFileName);
% Look at red channel, or entire image (if it's already grayscale).
theImage = im2double(theImage(:,:,1));
% Display image
subplot(2, 1, 1);
imshow(theImage, []);
drawnow;
caption = sprintf('#%d of %d : %s', k, numberOfFiles, baseFileName);
title(caption, 'Interpreter', 'none');
% Get the histogram.
[thesePixelCounts, edges] = histcounts(theImage,numberOfBins); %nbins= 512
allPixelCounts = allPixelCounts + thesePixelCounts;
subplot(2, 1, 2);
bar(allPixelCounts, 1);
grid on;
drawnow;
end
subplot(2, 1, 2);
% Get average
allPixelCounts = allPixelCounts / numberOfFiles;
bar(allPixelCounts, 1);
grid on;
title('Average Histogram');
xlabel('Gray Level');
ylabel('Pixel Count');
grayLevel = 0 : (numberOfBins - 1);
% Optional. Plot lines from tip of bar to tip of bar.
% hold on;
% plot(grayLevel, allPixelCounts, 'r.-', 'LineWidth', 1);
% hold off;
fprintf('Done running %s.m\n', mfilename);
