I have tried the coding. But i don't understand what does mean idx = Igray == 0? Can someone explain it to me.

if true
  % code
endSeparate to RGB channel
Ir = imgcrop(:,:,1);
Ig = imgcrop(:,:,2);
Ib = imgcrop(:,:,3);

% Extract the background (black) region
Igray = rgb2gray(imgcrop);
idx = Igray == 0;

% Calculate average RGB of the region
Rave = uint8(mean(Ir(~idx)));
Gave = uint8(mean(Ig(~idx)));
Bave = uint8(mean(Ib(~idx)));

Answer 1

[Ameer Hamza]

This code is calculating the average of RGB channels while excluding the pixels where the intensity level is zero.

The line

idx = Igray == 0;

find all the pixels in grayscale image Igray, where the intensity is 0. It will return a logical matrix idx. Since only those pixels are required where intensity level is not 0, therefore we invert the logical matrix using ~idx,

Ir(~idx)

will return all the required pixels and then using mean() find the average. See Logical Indexing here.

[syafini alias]

Okay. I got it. Thanks for the explanation. But if i don't want to extract the background and use the code below, why i didn't get only one value for average RGB.

 Rave = uint8(mean(Ir));
Gave = uint8(mean(Ig));
Bave = uint8(mean(Ib));
disp([Rave,Gave,Bave]);

[Walter Roberson]

Use mean2 to get the mean over the entire array. mean() takes the mean over the first nonscalar dimension (that is typically the rows)

[Ameer Hamza]

As @Walter mentioned, mean() will do average along first nonscalar dimensions. In case of 2D matrix, it will average all the columns. Either use mean2() or convert to a single column before taking average like this

Rave = uint8(mean(Ir(:)));
Gave = uint8(mean(Ig(:)));
Bave = uint8(mean(Ib(:)));
disp([Rave,Gave,Bave]);

or take mean() twice

Rave = uint8(mean(mean(Ir)));
Gave = uint8(mean(mean(Ig)));
Bave = uint8(mean(mean(Ib)));
disp([Rave,Gave,Bave]);

all will produce same output.

[syafini alias]

Okay. Thank you.

Answer 2

[KSSV]

You need to read about logical indexing in MATLAB.

idx = Igray == 0;

The above line gives the indices of pixel zero's in the matrix data Igray.

[syafini alias]

Sorry. I still don't get it. What use of ~idx in Rave?

Rave = uint8(mean(Ir(~idx)));