Histogram Equalization yielding unexpected results

Question

I have a series of images with decreasing brightness that I would like to try to correct with histogram equalization. I applied histeq to some test data to learn how the function works

% Image that I would like to apply histogram equalization to
C = gallery('wilk',21);
figure, imagesc(C)

E = histeq(C);
figure, imagesc(E);

However, when I look at the output of histeq, I get a result that only has two unique values: 0.873 and 1.000. How come the output doesn't span the whole range of the input? I would expect there to be more than two unique values in the output.

Answer 1

According to the documentation for histeq, if the input is of type double or single it is expected to be in the range: [0, 1].

Intensity values in the appropriate range: [0, 1] for images of class double, [0, 255] for images of class uint8, and [0, 65535] for images of class uint16.

Your data is not normalized and is of type double,

whos C

%  Name       Size            Bytes  Class     Attributes
%
%  C         21x21             3528  double    

[min(C(:)), max(C(:))]
%   0   10

You will need to normalized it first. You can use mat2gray to do this:

E = histeq(mat2gray(C));