problem after divide image

Hello all .. This code to divide the image into blocks, each block 4 * 4 pixels and then find the average of the values of pixels in each block and then put the new values in the image matrix (a) The problem : after storage and display picture shows me a white image why? and Storage in the matrix are not taken properly The value of the average for the first block stored in the last position of the new matrix (a)

if true
  I = imread('4.png');

 Range_Image=rgb2gray(imread('4.png'));
 [m n]=size(Range_Image); Nbrx = floor(m./4);
 Nbry = floor(n./4); Nd=1;
 for i=1:Nbrx 
     for j=1:Nbry
 Mat=Range_Image((i-1)*4+1:i*4,(j-1)*4+1:j*4);
 sumpixel=sum(Mat(:));
 avg= floor(sumpixel/16);
 a(i,j)=avg;
% subplot(Nbrx,Nbry,Nd); imshow(Mat)
% Nd=Nd+1; 
end 
 end

end

Answer 1

[Iain]

Sounds like either your number format is saturating.

Cast the image as double or single before averaging.

Alternatively use "imagesc" instead of imshow.

[wissa amer]

thank you soo much ..

[Iain]

To cast it as double:

dbl_version = double( Range_Image );

Answer 2

[Image Analyst]

uint8 images clip at 255 so it you add up 16 uint8 pixels, chances are they will sum to more than 255 and so it will clip at 255. You need to cast to single or double first. Then, if you want values in the same range, you need to divide by 16. Either way, if you want to display it you'll have to use [] as the second argument to imshow(). You can cast back to uint8 if you want the averages to be uint8, like if you want to save the small images or something.

[wissa amer]

So what should to do .. I am a beginner in MATLAB.

[wissa amer]

imshow(a,[]) i do that.. rigth or wrong

[Image Analyst]

That's right. But like I said, you need to make sure a is of the right type to handle a sum, like uint16 or double, or single.