Trouble using imhist in image

Hello! I want to see the histogram of my image, but I don't know why, it doesn't show.

Here's my code:

I=imread('D7.jpg');
imshow(I);
imhist(I);

The error is :" Error using imhist

Expected input number 1, I or X, to be two-dimensional.

Error in imhist>parse_inputs (line 278)

validateattributes(a,

{'double','uint8','int8','logical','uint16','int16','single','uint32',

'int32'}, ...

Error in imhist (line 60)

[a, n, isScaled, top, map] = parse_inputs(varargin{:});

Error in Process (line 3)

imhist(x);".

My image is 1024x995x3 uint8.

Thank you very much

Answer 1

[Image Analyst]

You're passing in a color image and imhist() is not set up for that. If you want 3 histograms, extract each color channel independently,

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

like in the attached demo, and histogram each color channel one at a time. Or if you don't care which color channel, then just lump them all together with (:)

imhist(rgbImage(:));

[Margarida Moura]

Thank you for the answer, but it's not working...

Here is my image, attached.

[Image Analyst]

It does work. In fact here is the code:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 16;

%===============================================================================
% Get the name of the image the user wants to use.
baseFileName = 'index.jpg';
folder = pwd;
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

%=======================================================================================
% Read in demo image.
rgbImage = imread(fullFileName);
% Get the dimensions of the image.
[rows, columns, numberOfColorChannels] = size(rgbImage)

% Display image.
subplot(2, 4, 1);
imshow(rgbImage, []);
impixelinfo;
axis on;
caption = sprintf('Original Color Image\n%s', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.

% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0.05 1 0.95]);
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
drawnow;

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

% Display the color channel images.
subplot(2, 4, 2);
imshow(redChannel);
caption = sprintf('Red Channel Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
axis on;
subplot(2, 4, 3);
imshow(greenChannel);
caption = sprintf('Green Channel Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
axis on;
subplot(2, 4, 4);
imshow(blueChannel);
caption = sprintf('Blue Channel Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
axis on;
drawnow;

% See if all three channels are identical. 
% If so, it's a gray scale image in a color image format.
subplot(2, 4, 5:8);
if isequal(redChannel, greenChannel) && isequal(redChannel, blueChannel)
	% It's gray scale.
	imhist(redChannel); % Just take any one of them
else
	redCounts = imhist(redChannel);
	greenCounts = imhist(greenChannel);
	blueCounts = imhist(blueChannel);
	gls = 0 : 255;
	plot(gls, redCounts, 'r-', 'LineWidth', 2);
	hold on;
	plot(gls, greenCounts, 'g-', 'LineWidth', 2);
	plot(gls, blueCounts, 'b-', 'LineWidth', 2);
	xlabel('Gray Levels', 'FontSize', 16);
	ylabel('Counts', 'FontSize', 16);
	title('All 3 color histograms', 'FontSize', 16);
end
grid on;

The reason that the color histograms are different, is because you unwisely chose to save the image in JPEG format. I believe the image should really be grayscale. Don't use JPG format when doing image analysis.