How to make this be a loop?

Hi, I want to make the function deal with a lot of images,

but I got errors when trying to add the loop.

The original one is:

imx = 'Images\20\IMG_4596.JPG',
Noisex = NoiseExtractFromImage(imx,2);
Noisex = WienerInDFT(Noisex,std2(Noisex));
 
% The optimal detector (see publication "Large Scale Test of Sensor Fingerprint Camera Identification")
Ix = double(rgb2gray(imread(imx)));
C = crosscorr(Noisex,Ix.*Fingerprint);
detection = PCE(C)

And I add a loop like this

file_path = 'Images\20\';
img_path_list = dir(strcat(file_path,'*.jpg'));
img_num = length(img_path_list);
if img_num > 0 
for j = 1:img_num 
image_name = img_path_list(j).name;
I = imread(strcat(file_path,image_name));
fprintf('%d %d %s\n',j,img_num,strcat(file_path,image_name));
imx(j).name = strcat(file_path,image_name);
Noisex{j} = NoiseExtractFromImage(imx(j).name,2);
Noisex(j) = WienerInDFT(Noisex(j),std2(Noisex(j)));
 
% The optimal detector (see publication "Large Scale Test of Sensor Fingerprint Camera Identification")
Ix(j) = double(rgb2gray(imread(imx(j).name)));
C(j) = crosscorr(Noisex(j),Ix(j).*Fingerprint);
detection(j) = PCE(C(j))
 
end
end

But I get an error :

Unable to perform assignment because brace indexing is not supported for variables of this type.

Error in Example (line 39)

Noisex{j} = NoiseExtractFromImage(imx(j).name,2);

Does anyone know how to make it right?

I'm new to matlab,

Thank you so much!

[Yixn Chen]

Hey did you ever fix this? If yes do you mind sharing how :D

Answer 1

[Jan]

Start with simplifying your code:

file_path     = 'Images\20\';
img_path_list = dir(fullfile(file_path,'*.jpg'));  % Safer than STRCAT
img_num       = length(img_path_list);
for j = 1:img_num 
  image_name = img_path_list(j).name;
  image_file = fullfile(file_path,image_name);
  I          = imread(image_file);
  fprintf('%d %d %s\n', j, img_num, image_file);
  Noisex{j} = NoiseExtractFromImage(image_file, 2);
  Noisex(j) = WienerInDFT(Noisex(j), std2(Noisex(j)));
 
  % The optimal detector (see publication "Large Scale Test of Sensor Fingerprint Camera Identification")
  Ix(j) = double(rgb2gray(I));
  C(j)  = crosscorr(Noisex(j),Ix(j) .* Fingerprint);
  detection(j) = PCE(C(j))
end

Now this looks strange:

 Noisex{j} = NoiseExtractFromImage(image_file, 2);
 Noisex(j) = WienerInDFT(Noisex(j), std2(Noisex(j)));

You use curly and round parentheses for the same variable. NoiseExtractFromImage and WienerInDFT do not belong to Matlab's toolbox functions, so I cannot guess, what the outputs are. Please check this.

double(rgb2gray(I)) replies a matrix. Then assignign the output to the scalar Ix(j) should fail. Do you want a cell array? Then use curly braces: Ix{j} .

Prefer to pre-allocate the results before the loop. This is more efficient and the types of the variables are well defined.