CODEWITHSUNDEEP
arraysmatlabmultiplication
Apastrix
Apastrix

Reputation: 114

Matlab wrong in array multiplication?

I have this simple program:

% Read Image:
I=imread('Bureau.bmp');

% calculate Hist: 
G=  unique(I); % Calculate the different gray values
Hist= zeros(size(G)); % initialize an array with the same size as G

% For each different gray value, loop all the image, and each time you find
% a value that equals the gray value, increment the hist by 1
for j=1:numel(G) 
  for i= 1:numel (I)

     if G(j)== I(i)
         Hist(j)=Hist(j)+1;
     end

  end        
end

Now look at this multiplication: 

>> G(2)

ans =

    1

>> Hist(2)

ans =

   550

>> Hist(2)*G(2)

ans =

  255

And it's giving me 255 not only for the index 2, but for any combination of indexes!

Upvotes: 1

Views: 355

Answers (1)

Bentoy13
Bentoy13

Reputation: 4956

Two things for your problem.

First, here is the reason of your problem of multiplication: different types. I and so Gare of type uint8. H is of type double. When you perform the multiplication, Matlab seems to use the most restrictive type, so here uint8. So the result of Hist(2)*G(2) is of type uint8, comprised between 0 and 255.

Second: please DON'T compute an histogram this way. Matlab has numerous functions for that (hist and histc for the most common ones), so please read the doc and use it instead of creating your own code. If you want nevertheless write your own function (learning purpose), this code is far too slow. You go through the image about 256 times, it is useless. Instead of that, a classic way would be:

Hist = zeros(1,256);
for i=1:numel(I)
    Hist(int32(I(i))+1) = Hist(int32(I(i))+1)+1
end

You use directly the value of the current pixel (+1 because index starts at 1 in Matlab) to access the corresponding slot of your histogram. Also, you must cast the pixel value to int32, to avoid the problem of value 255 (with uint8 variables, 255+1=0).

I don't want here to be pedantic, but Matlab comes with thousands of functions (without mentioning the dozens of toolboxes) and a very well-written doc, so please read it and use every suitable you can find inside, that's the best advice I could give to anybody who starts learning Matlab.

Upvotes: 3

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