How to get luminance of an image?

Hello,

I have a directory of images and I want to group and save the images by luminance. However, I'm not sure how to calculate the luminance of an image. How would i go about this?

Thank you!

Answer 1

[Image Analyst]

Code samples in the FAQ:

https://matlab.fandom.com/wiki/FAQ#How_can_I_process_a_sequence_of_files.3F

So, try this if you have PNG files, otherwise change the extension to whatever you want.

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 = 15;

% Specify the folder where the files live.
myFolder = pwd; % or 'C:\Users\yourUserName\Documents\My Pictures';
% Check to make sure that folder actually exists.  Warn user if it doesn't.
if ~isfolder(myFolder)
	errorMessage = sprintf('Error: The following folder does not exist:\n%s\nPlease specify a new folder.', myFolder);
	uiwait(warndlg(errorMessage));
	myFolder = uigetdir(); % Ask for a new one.
	if myFolder == 0
		% User clicked Cancel
		return;
	end
end
% Get a list of all files in the folder with the desired file name pattern.
filePattern = fullfile(myFolder, '*.png'); % Change to whatever pattern you need.
theFiles = dir(filePattern);
for k = 1 : length(theFiles)
	baseFileName = theFiles(k).name;
	fullFileName = fullfile(theFiles(k).folder, baseFileName);
	fprintf(1, 'Now reading %s\n', fullFileName);
	% Now do whatever you want with this file name,
	% such as reading it in as an image array with imread()
	imageArray = imread(fullFileName);
	imshow(imageArray);  % Display image.
	caption = sprintf('#%d of %d : %s', k, length(theFiles), theFiles(k).name);
	title(caption, 'FontSize', fontSize);
	drawnow; % Force display to update immediately.
	[rows, columns, numberOfColorChannels] = size(imageArray);
	if numberOfColorChannels == 3
		theMeans(k) = mean2(rgb2gray(imageArray));
	else
		theMeans(k) = mean2(imageArray);
	end
end
cla reset;
bar(theMeans);
grid on;
title('Image Means', 'FontSize', fontSize);
xlabel('Image Number', 'FontSize', fontSize);
ylabel('Mean Gray Level', 'FontSize', fontSize);
fprintf('Done!  Thanks Image Analyst!\n');

Answer 2

[Jonas]

concerning luminance have a look into this post

do you also need help in writing a loop for image loading/analysis?