Plotting RGB Histograms of Colored Images

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Good day,
As part of my cloud tracking project, I am required to analyze an image's RGB component. From my understanding, the function imhist() is only valid for intensity images [0 - 255] so I am unsure of how should I proceed to analyze the individual RGB components.
I would really appreciate if any of you experts can point me in the right direction.
Best Regards
Guan Zhao
07/10/2012

Accepted Answer

Image Analyst
Image Analyst on 7 Oct 2012
Edited: Image Analyst on 7 Oct 2012
Guan Zhao, see my demo:
%----------------------------------------------------------
% Program to read in all the RGB color images in a folder and
% display the histograms of each color channel.
%, Feb. 2011
%----------------------------------------------------------
function RGB_Histogram_Demo()
% Change the current folder to the folder of this m-file.
if(~isdeployed)
cd(fileparts(which(mfilename)));
end
clc; % Clear command window.
close all; % Close all figure windows except those created by imtool.
imtool close all; % Close all figure windows created by imtool.
workspace; % Make sure the workspace panel is showing.
fontSize = 16;
try
% Read in standard MATLAB color demo images.
% Construct the folder name where the demo images live.
imagesFolder = fullfile(matlabroot, '\toolbox\images\imdemos');
if ~exist(imagesFolder, 'dir')
% That folder didn't exist. Ask user to specify folder.
message = sprintf('Please browse to your image folder');
button = questdlg(message, 'Specify Folder', 'OK', 'Cancel', 'OK');
drawnow; % Refresh screen to get rid of dialog box remnants.
if strcmpi(button, 'Cancel')
return;
else
imagesFolder = uigetdir();
if imagesFolder == 0
% Exit if uer clicked Cancel.
return;
end
end
end
% Read the directory to get a list of images.
filePattern = [imagesFolder, '\*.jpg'];
jpegFiles = dir(filePattern);
filePattern = [imagesFolder, '\*.tif'];
tifFiles = dir(filePattern);
filePattern = [imagesFolder, '\*.png'];
pngFiles = dir(filePattern);
filePattern = [imagesFolder, '\*.bmp'];
bmpFiles = dir(filePattern);
% Add more extensions if you need to.
imageFiles = [jpegFiles; tifFiles; pngFiles; bmpFiles];
% Bail out if there aren't any images in that folder.
numberOfImagesProcessed = 0;
numberOfImagesToProcess = length(imageFiles);
if numberOfImagesToProcess <= 0
message = sprintf('I did not find any JPG, TIF, PNG, or BMP images in the folder\n%s\nClick OK to Exit.', imagesFolder);
uiwait(msgbox(message));
return;
end
% Create a figure for our images.
figure;
set(gcf, 'Position', get(0,'Screensize')); % Maximize figure.
set(gcf,'name','Image Analysis Demo','numbertitle','off')
% Preallocate arrays to hold the mean intensity values of all the images.
redChannel_Mean = zeros(numberOfImagesToProcess, 1);
greenChannel_Mean = zeros(numberOfImagesToProcess, 1);
blueChannel_Mean = zeros(numberOfImagesToProcess, 1);
% We'll be skipping monochrome and indexed images
% and just looking at true color images.
% Keep track of how many we actually look at.
numberOfImagesToProcess2 = numberOfImagesToProcess;
% Loop though all images, calculating and displaying the histograms.
% and then getting the means of the Red, green, and blue channels.
for k = 1 : numberOfImagesToProcess
% Read in this one file.
baseFileName = imageFiles(k).name;
fullFileName = fullfile(imagesFolder, baseFileName);
rgbImage = imread(fullFileName);
% Check to see that it is a color image (3 dimensions).
% Skip it if it is not true RGB color.
if ndims(rgbImage) < 3
% Skip monochrome or indexed images.
fprintf('Skipped %s. It is a grayscale or indexed image.\n', baseFileName);
% Decrement the number of images that we'll report that we need to look at.
numberOfImagesToProcess2 = numberOfImagesToProcess2 - 1;
continue;
end
% If we get to here, it's a true color image.
subplot(3, 3, 1);
imshow(rgbImage, []);
[rows columns numberOfColorBands] = size(rgbImage);
% Create a title for the image.
caption = sprintf('Original Color Image\n%s\n%d rows by %d columns by %d color channels', ...
baseFileName, rows, columns, numberOfColorBands);
% If there are underlines in the name, title() converts the next character to a subscript.
% To avoid this, replace underlines by spaces.
caption = strrep(caption, '_', ' ');
title(caption, 'FontSize', fontSize);
drawnow; % Force it to update, otherwise it waits until after the conversion to double.
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Red image:
subplot(3, 3, 4);
imshow(redChannel, []); % Display the image.
% Compute mean
redChannel_Mean(k) = mean(redChannel(:));
caption = sprintf('Red Image. Mean = %6.2f', redChannel_Mean(k));
title(caption, 'FontSize', fontSize);
% Compute and display the histogram for the Red image.
pixelCountRed = PlotHistogramOfOneColorChannel(redChannel, 7, 'Histogram of Red Image', 'r');
% Green image:
subplot(3, 3, 5);
imshow(greenChannel, []); % Display the image.
% Compute mean
greenChannel_Mean(k) = mean(greenChannel(:));
caption = sprintf('Green Image. Mean = %6.2f', greenChannel_Mean(k));
title(caption, 'FontSize', fontSize);
% Compute and display the histogram for the Green image.
pixelCountGreen = PlotHistogramOfOneColorChannel(greenChannel, 8, 'Histogram of Green Image', 'g');
% Blue image:
subplot(3, 3, 6);
imshow(blueChannel, []); % Display the image.
numberOfImagesProcessed = numberOfImagesProcessed + 1;
% Compute mean
blueChannel_Mean(k) = mean(blueChannel(:));
caption = sprintf('Blue Image. Mean = %6.2f', blueChannel_Mean(k));
title(caption, 'FontSize', fontSize);
% Compute and display the histogram for the Blue image.
pixelCountBlue = PlotHistogramOfOneColorChannel(blueChannel, 9, 'Histogram of Blue Image', 'b');
% Plot all three histograms on the same plot.
subplot(3, 3, 2:3);
lineWidth = 2;
hold off;
plot(pixelCountRed, 'r', 'LineWidth', lineWidth);
hold on;
grid on;
plot(pixelCountGreen, 'g', 'LineWidth', lineWidth);
plot(pixelCountBlue, 'b', 'LineWidth', lineWidth);
title('All the Color Histograms (Superimposed)', 'FontSize', fontSize);
% Set the x axis range manually to be 0-255.
xlim([0 255]);
% Prompt user to continue, unless they're at the last image.
if k < numberOfImagesToProcess
promptMessage = sprintf('Currently displaying image #%d of a possible %d:\n%s\n\nDo you want to\nContinue processing, or\nCancel processing?',...
numberOfImagesProcessed, numberOfImagesToProcess2, baseFileName);
button = questdlg(promptMessage, 'Continue?', 'Continue', 'Cancel', 'Continue');
if strcmp(button, 'Cancel')
break;
end
end
end
% Crop off any unassigned values:
redChannel_Mean = redChannel_Mean(1:numberOfImagesProcessed);
greenChannel_Mean = greenChannel_Mean(1:numberOfImagesProcessed);
blueChannel_Mean = blueChannel_Mean(1:numberOfImagesProcessed);
% Print to command window
fprintf(1, ' Filename, Red Mean, Green Mean, Blue Mean\n');
for k = 1 : length(redChannel_Mean)
baseFileName = imageFiles(k).name;
fprintf(1, '%24s %6.2f, %6.2f, %6.2f\n', ...
baseFileName, redChannel_Mean(k), greenChannel_Mean(k), blueChannel_Mean(k));
end
if numberOfImagesProcessed == 1
caption = sprintf('Done with demo!\n\nProcessed 1 image.\nCheck out the command window for the results');
else
caption = sprintf('Done with demo!\n\nProcessed %d images.\nCheck out the command window for the results', numberOfImagesProcessed);
end
msgbox(caption);
catch ME
errorMessage = sprintf('Error in function RGB_Hist_Demo.\n.\n\nError Message:\n%s', ME.message);
uiwait(warndlg(errorMessage));
end
%----------------------------------------------------------
% Plots a bar chart of the histogram of the color channel.
function pixelCount = PlotHistogramOfOneColorChannel(oneColorChannel, subplotNumber, caption, color)
try
% Let's get its histogram into 256 bins.
[pixelCount grayLevels] = imhist(oneColorChannel, 256);
subplot(3, 3, subplotNumber);
bar(grayLevels, pixelCount, 'FaceColor', color);
title(caption, 'FontSize', 16);
grid on;
% Set the x axis range manually to be 0-255.
xlim([0 255]);
catch ME
errorMessage = sprintf('Error in function PlotHistogramOfOneColorChannel.\n.\n\nError Message:\n%s', ME.message);
uiwait(warndlg(errorMessage));
end
return;
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