"Index exceeds matrix dimensions" error during performing RGB channel separation

The purpose of my code is -

1. scan all image from a folder
2. calculate entropy for each of RGB channel
3. export data to a xlsx file

my code :

folder = 'F:\images\';
filePattern = fullfile(folder, '*.png');
myFiles = dir(filePattern); % the folder inwhich ur images exists

for k = 1 : length(myFiles)
  fullFileName = fullfile(folder, myFiles(k).name);
  I= fullFileName;
Red = I(:,:,1);
Green = I(:,:,2);
Blue = I(:,:,3);

%I = I(:); % Vectorization of RGB values
p = imhist(Red); % Histogram
p(p == 0) = [ ];% remove zero entries in p
p = p ./ numel(I); % normalize p so that sum(p) is one.
Er(k) = round(-sum(p.*log2(p)),3);

p = imhist(Blue); % Histogram
p(p == 0) = [ ];% remove zero entries in p
p = p ./ numel(I); % normalize p so that sum(p) is one.
Eb(k) = round(-sum(p.*log2(p)),3);

xlswrite('myfile.xlsx', [Er(k),Eb(k)], 'Sheet 1');
end

But i am getting following error:

Index exceeds matrix dimensions.

Error in test1 (line 9)
Green = I(:,:,2);

I couldn't solve this problem. Can anyone help me please?

Answer 1

[Simon Chan]

According to your code shown above, variable I is only a file name in the beginning of the for loop.

Have you do imread or some other command to read the images?

for k = 1 : length(myFiles)
  fullFileName = fullfile(folder, myFiles(k).name);
  I= imread(fullFileName);                                          % I added imread
Red = I(:,:,1);
Green = I(:,:,2);
Blue = I(:,:,3);

[rakib mostafiz]

it solves the error. But the data on xlsx file are overwritten. After completion, i only get one image data! can you help me with it?

[Simon Chan]

You can write the data to an excel file after the for loop as follows:

xlswrite('myfile.xlsx', [Er;Eb], 'Sheet 1'); 

If all data are numeric, you may use writematrix as follows (also after the for loop):

writematrix([Er;Eb],'myfile.xlsx');
       

[rakib mostafiz]

All of the data are placed side by side column. Can it be done by two column only?

[Simon Chan]

Doing transpose:

xlswrite('myfile.xlsx', [Er',Eb'], 'Sheet 1'); 

or

writematrix([Er',Eb'],'myfile.xlsx');

Answer 2

[Yongjian Feng]

Most likely your image is just a grey scale one. Only one channel.

[rakib mostafiz]

Those image are colorful. Moreover, if i perform this operation on a single image rather than looping out all image from folder, it works just fine! But when i try to read each image and perform this operation, it showing that error. I really dont understand the problem here