Check each pixel for a specific color

Question

Shahd Ewawi 2013-3-27

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I want to check the value of each pixel in image to see if the image is black and i get this error " Attempted to access l(0,0); index must be a positive integer or logical."

 for i=0:m
    for j=0:n
       if l(i,j)==0
          count=count+1;
           end
       end 
Does anyone know the best way to do this?

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Answer 1

Doug Hull 2013-3-27

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编辑：Image Analyst 2013-3-27

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MATLAB uses ones based indexing. There is no 0,0 element. Try starting at 1,1.

A better way is like this:

a = [11 22 33; 22 11 44; 11 11 11]
a==11  %returns binary matrix
nnz(a==11) %counts the number of 11's in the matrix

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Shahd Ewawi 2013-3-27

Thank you :)

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Answer 2

Ahmed A. Selman 2013-3-27

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Try

I=imread('imageHere');
[a,b,c]=size(I); % change is needed
for i=1:a
for j=1:b
F(i,j)=all(I(i,j)==0);
end 
end

everywhere you got 0 in the color picture you'll get a value of one in the logical matrix F. If your image was already black-white, replace and use

[a,b]=size(I);

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Image Analyst 2013-3-27

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all() is unnecessary because you're looking at just a single element. Anyway, the whole thing can be done in one line like Doug showed:

F = I==0;

The 2D matrix F will be true (1) wherever (the badly-named) I is zero.

Ahmed A. Selman 2013-3-28

I agree, the bit of code is written to the whole image while the needed is (perhaps) for a single pixel. Yet, the original question is to detect single pixel _for a _specific color. That is quite odd. Regards.

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