how to convert rgb pixel value into single value in matlab? this is my code

 clear all
close all
a=imread('C:\Users\shankar\Documents\MATLAB\filtering\image.jpg');
[m n]=size(a);
t=155;
for i=1:m
for j=1:n
if a(i,j)<t
  b(i,j)=0;
else
  b(i,j)=1;
end
end
end
subplot(2,2,1),imshow(a),title('original image'),
subplot(2,2,2),imshow(b),title('thresholded image'),
xlabel(sprintf('threshold value is %g',t))

Answer 1

[Image Analyst]

Never do this with a jpg image:

[m n]=size(a);

Why not? See http://blogs.mathworks.com/steve/2011/03/22/too-much-information-about-the-size-function/ So do this instead

[rows, columns, numberOfColorChannels] = size(a);

Then you can do skip the loops and threshold like this:

binaryImage = a < t;

Here's a more full blown demo using a demo image. Just copy and paste:

clc;    % Clear the command window.
% clearvars;
% close all;  % Close all figures (except those of imtool.)
workspace;  % Make sure the workspace panel is showing.
fontSize = 20;
format compact;

%===============================================================================
% Read in a  gray scale demo image.
folder = fileparts(which('cameraman.tif')); % Determine where demo folder is (works with all versions).
baseFileName = 'onion.png';
% 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.
  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
grayImage = imread(fullFileName);
% Get the dimensions of the image.  
% numberOfColorBands should be = 1.
[rows, columns, numberOfColorChannels] = size(grayImage);
if numberOfColorChannels > 1
  % It's not really gray scale like we expected - it's color.
  % Convert it to gray scale by taking only the green channel.
  grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the image.
subplot(2, 2, 1);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize, 'Interpreter', 'None');

% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% 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') 

% Let's compute and display the histogram.
[pixelCount, grayLevels] = imhist(grayImage);
subplot(2, 2, 2); 
bar(grayLevels, pixelCount); % Plot it as a bar chart.
grid on;
title('Histogram of original image', 'FontSize', fontSize, 'Interpreter', 'None');
xlabel('Gray Level', 'FontSize', fontSize);
ylabel('Pixel Count', 'FontSize', fontSize);
xlim([0 grayLevels(end)]); % Scale x axis manually.

% Threshold the image
t = 155;
binaryImage = grayImage < t;
% Display the image.
subplot(2, 2, 4);
imshow(binaryImage, []);
title('Binary Image', 'FontSize', fontSize, 'Interpreter', 'None');

[Nivetha m]

useful answer for me.

[Nivetha m]

how will you calculate MAE,SNR for above code?

[Image Analyst]

For the mean absolute difference, you'd do

mad = mean2(abs(double(image1)-double(image2)));

For SNR, you can use the psnr() function. From the help:

peaksnr = psnr(A,ref) calculates the peak signal-to-noise ratio for the image A, with the image ref as the reference. A and ref must be of the same size and class.

Answer 2

[Azzi Abdelmalek]

b=rgb2gray(a)
imshow(b)

[Nivetha m]

thank you, very useful answer...

[Nivetha m]

how will you calculate MAE,SNR for above code?