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How to convert a matrix to a Gray scale image?

Asked by vikas on 1 Mar 2012

i need a matrix(with values between 0 to 255) to be converted to a gray scale image. I have tried using the function mat2gray but the problem with this function is that it scales down the pixel values of the image within the range 0 to 1 .Although the correct image is being displayed the exact values(0-255) of the matrix are not being reflected in the image.My requirement is that the pixels of the image must have the same values as that of the corresponding matrix.I just want to know if there is any alternative solution to this problem. kindly help.

0 Comments

vikas

5 Answers

Answer by Image Analyst on 7 Dec 2012

Your matrix is already a gray scale image. To display it use this:

imshow(yourMatrix, []);

The [] lets it scale the min value to 0 and the max value to 255. If you don't want that scaling, use

imshow(yourMatrix, [0 255]);

This won't do any scaling, so that if your min is at 100, it will show up with gray level 100, not at gray level 0 as in the [] case.

The reason you were seeing all white is that if the class of the image is single or double, then MATLAB expects the image to be in the range 0-1. 0 goes to 0 and 1 goes to 255 upon display. Because your values were all floating point values more than 1, they all got clipped at 255. You can use im2double() or gray2mat() to scale your image to the range 0-1, or just use [] with imshow() and not worry about it - that's what I do, it's the easiest way.

One final way is to just cast your image to uint8 before calling imshow:

imshow(uint8(yourImage), [0 255]);

Please run and understand this demo:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;
% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'pout.tif';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
	% File doesn't exist -- didn't find it there.  Check the search path for it.
	fullFileName = baseFileName; % No path this time.
	if ~exist(fullFileName, '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
grayImage = imread(fullFileName);
% Get the dimensions of the image.  
% numberOfColorBands should be = 1.
[rows columns numberOfColorBands] = size(grayImage);
% Display the original gray scale image.
subplot(2, 2, 1);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Give a name to the title bar.
set(gcf,'name','Demo by ImageAnalyst','numbertitle','off') 
% Create a double-class image in the range 0-255
dblImage = double(grayImage);
% Display double image scaling min to 0 and max to 255.
subplot(2, 2, 2);
imshow(dblImage, []);
title('Double Grayscale Image', 'FontSize', fontSize);
% Display double image without intensity scaling.
subplot(2, 2, 3);
imshow(dblImage, [0 255]);
title('Double Grayscale Image', 'FontSize', fontSize);
% Create a double-class image in the range 0-1
dblImage2 = im2double(grayImage);
% Display double image scaling min to 0 and max to 255.
subplot(2, 2, 4);
imshow(dblImage2, []);
title('Double Grayscale Image in range 0-1', 'FontSize', fontSize);

3 Comments

Image Analyst on 7 Dec 2012

Oh, sorry, I just realized that Jurgen replied to a very old thread, and that this thread is not current. Oh well, maybe it will help explain things to someone.

Brandon on 18 Apr 2013

Helped me out!

Image Analyst on 18 Apr 2013

Great. Glad it helped you. Vikas still has not accepted any answer so I guess it's still an open issue for him.

Image Analyst
Answer by Jonathan Sullivan on 1 Mar 2012
M = randi(255,[1e3 1e3]); % Your Matrix
image(M);
colormap('gray')

2 Comments

vikas on 1 Mar 2012

By doing it this way the image is not being displayed(i am just getting a white image).But when i used the function impixelregion to view the pixel values i could see the values in the range 0-255.However this didn't match the original matrix.

Walter Roberson on 1 Mar 2012

should be

colormap(gray)

Jonathan Sullivan
Answer by Walter Roberson on 1 Mar 2012

Is your original matrix RGB with distinct color planes? If so then there is no way to make it grayscale and preserve the original pixel values.

If your original matrix is RGB on which the color planes are all identical, then take any one of the color planes.

1 Comment

vikas on 1 Mar 2012

The matrix basically contains values from 0 to 255. So if i try to convert it to an image it must be a gray image.

Walter Roberson
Answer by yang on 1 Mar 2012

THE matrix[M,N],[M,N,3],[M,N,O]?

1 Comment

vikas on 1 Mar 2012

[M,N]

yang
Answer by Jurgen on 6 Dec 2012

Its probably double. So do im2uint8(A) or uint8(A) if A is the matrix. Also use imshow instead of image if you can.

0 Comments

Jurgen

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