Histograms too different compared to how they should be - matlab

Question

I have a black/white image I0 (512 x 512) to which I have to remove the first k pixel and calculate the histogram of the resulting image.
Let me explain: I have to build the histogram of the image I0 without considering the first k pixels.
This is my code:
k = 8;
% compute the histogram of the entire image I0
[vecComp, histComp] = histKtoEnd(I0, 0, k);
% compute the histogram of the image I0 without the first k pixel
[vecWithoutKPixel, histWithoutKPixel] = histKtoEnd(I0, k, k);
where:
function [vecWithoutKPixel, hist] = histKtoEnd(image, k, colorDepth)
% image to row vector
imageVec = reshape(image.', [], 1);
l = length(imageVec);
% I "delete" the first k pixel
vecWithoutKPixel = imageVec((k+1) : l-1);
vecWithoutKPixel(end+1) = imageVec(l);
% inizialization
hist = zeros(1, 2^colorDepth);
% create the vector of occurrences
for i = 0 : (2^colorDepth - 1)
grayI = (vecWithoutKPixel == i);
hist(1, i+1) = sum(grayI(:));
end
end
To display the two histograms do:
subplot(1, 2, 1);
bar(0:2^k-1, histComp, 'r');
title('Histogram of the entire image');
axis([minColor, maxColor, 0, numberOfPixels]);
subplot(1, 2, 2);
bar(0:2^k-1, histWithoutKPixel, 'r');
title('Histogram of the image without the first k pixels');
axis([minColor, maxColor, 0, numberOfPixels]);
and I get:
As you can see, the histograms are very different, yet should differ very little since the difference is only 8 pixels.
Where am I wrong?
Also in warkspace the newly created variables have these dimensions:
vecComp -> 262144 x 1 uint8
histComp -> 1 x 256 double
vecWithoutKPixel -> 65528 x 1 uint8
histWithoutKPixel -> 1 x 256 double
This is very strange. I should have:
vecComp -> 262144 x 1
vecWithoutKPixel -> 262136 x 1
Could someone help me?
Thank you
I'm working with DICOM images and the command info = dicominfo(filename) get
size = info.FileSize; % 262582;
colorType = info.ColorType; % grayscale
So I don't think that the problem is the image.
If I put a brakpoint on line vecWithoutKPixel(end+1) = imageVec(l); I get that imageVec is 262144 x 1 uint8, and:
function [vecWithoutKPixel, hist] = histKtoEnd(image, k, colorDepth)
% image to row vector
imageVec = reshape(image.', [], 1);
l = length(imageVec);
size(imageVec) % 262144 x 1
% I "delete" the first k pixel
vecWithoutKPixel = imageVec((k+1) : l-1);
vecWithoutKPixel(end+1) = imageVec(l);
% inizialization
hist = zeros(1, 2^colorDepth);
% create the vector of occurrences
for i = 0 : (2^colorDepth - 1)
grayI = (vecWithoutKPixel == i);
hist(1, i+1) = sum(grayI(:));
end
end
If I change the commands vecWithoutKPixel = imageVec((k+1) : l-1);
vecWithoutKPixel(end+1) = imageVec(l); with vecWithoutKPixel = imageVec((k+1) : l); I get that vecWithoutKPixel = [].

Answer 1

Creating a 'random-image' using I0=randi(255,100,100) and then running your code will produce a plot like this:
I guees this plot looks exactly like it should and the dimensions of the variables are correct as well:
vecComp 10000x1 double
vecWithoutKPixel 9992x1 double
Thus our problem is not actually your code, but your image. Your image is not really gray level valued which has been mentioned by others before. But instead of casting some variables to uint8 like #Richard suggested, you should cast your image to double. If your image is rgb valued use I0=double(rgb2gray(I0)); or if it is just uint8 for some other reason use I0=double(I0) and then pass it to the function. Hope this helps.

Answer 2

I believe the problem is that the image data is originally in uint8 format and gets converted to double at some point. Therefore, the line
grayI = (vecWithoutKPixel == i);
in histKtoEnd probably only works with the uint8 data ( since you compare the data with an integer).
Try adding the line
vecWithoutKPixel = uint8(vecWithoutKPixel);
to the histKtoEnd function:
function [vecWithoutKPixel, hist] = histKtoEnd(image, k, colorDepth)
% image to row vector
imageVec = reshape(image.', [], 1);
l = length(imageVec);
% I "delete" the first k pixel
vecWithoutKPixel = imageVec((k+1) : l-1);
vecWithoutKPixel(end+1) = imageVec(l);
% inizialization
hist = zeros(1, 2^colorDepth);
% create the vector of occurrences
for i = 0 : (2^colorDepth - 1)
vecWithoutKPixel = uint8(vecWithoutKPixel); % Cast to integer
grayI = (vecWithoutKPixel == i);
hist(1, i+1) = sum(grayI(:));
end
end