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MATLAB CODE:Local Histogram equalization


For every pixel, based on the neighbor hood value the histogram equalization is done. Here I used 3 by 3 window matrix for explanation. By changing the window matrix size, the histogram equalization can be enhanced. By changing the values of M and N the window size can be changed in the code given below.

Steps to be performed:





MATLAB CODE:


A=imread('tire.tif');
figure,imshow(A);
Img=A;
     
  
%WINDOW SIZE
M=10;
N=20;


mid_val=round((M*N)/2);

%FIND THE NUMBER OF ROWS AND COLUMNS TO BE PADDED WITH ZERO
in=0;
for i=1:M
    for j=1:N
        in=in+1;
        if(in==mid_val)
            PadM=i-1;
            PadN=j-1;
            break;
        end
    end
end
%PADDING THE IMAGE WITH ZERO ON ALL SIDES
B=padarray(A,[PadM,PadN]);

for i= 1:size(B,1)-((PadM*2)+1)
    
    for j=1:size(B,2)-((PadN*2)+1)
        cdf=zeros(256,1);
        inc=1;
        for x=1:M
            for y=1:N
  %FIND THE MIDDLE ELEMENT IN THE WINDOW          
                if(inc==mid_val)
                    ele=B(i+x-1,j+y-1)+1;
                end
                    pos=B(i+x-1,j+y-1)+1;
                    cdf(pos)=cdf(pos)+1;
                   inc=inc+1;
            end
        end
                      
        %COMPUTE THE CDF FOR THE VALUES IN THE WINDOW
        for l=2:256
            cdf(l)=cdf(l)+cdf(l-1);
        end
            Img(i,j)=round(cdf(ele)/(M*N)*255);
     end
end
figure,imshow(Img);
figure,
subplot(2,1,1);title('Before Local Histogram Equalization'); imhist(A);
subplot(2,1,2);title('After Local Histogram Equalization'); imhist(Img);



















After Local Histogram Equalization

Histogram equalization of an Image:
 http://angeljohnsy.blogspot.com/2011/04/matlab-code-histogram-equalization.html

like button Like "IMAGE PROCESSING" page

11 comments:

Anonymous said... Reply to comment

How can this code be modified to perhaps have a window of 7x7

Anonymous said... Reply to comment

How can this code be modified to have another "window" other than 3x3?

Aaron Angel said... Reply to comment

@Anonymous
I have modified the code so that you can change the window size of your convenience.

rushabh said... Reply to comment

@Aaron Angel

if i have an image of size 112X92 and i want to apply histogram based image processing what appropriate size of window should i give??

rushabh said... Reply to comment

if i have an image of size 112X92 an i want to apply histogram based image processing
what appropriate window size should i give?

Aaron Angel said... Reply to comment

@rushabh
Use a large window with mxn values in odd number.

kumail abbas said... Reply to comment

SALAM...can anyone plz help me on MATLAB Code: Global Histogram Equalization on this page..i too need it..plz help me as early as u can..lotttt of thanxxx

Aaron Angel said... Reply to comment

@kumail abbas
Check this link

http://angeljohnsy.blogspot.com/2011/04/matlab-code-histogram-equalization.html

shubham matlab said... Reply to comment

how can this code be extended to oriented local histogram equalisation
sir,
please help me
i m providing you some data
OLHE is similar to local histogram
equalization (LHE), but it captures the orientation of edges
while LHE does not. We begin with a brief review on LHE. For
each pixel on an image, we perform the histogram equalization
on the local w -by- h window centering on this pixel using
f ( x ) = (round(cd f ( x )) − min(cd f))*(L-1)/(w · h − cd f)
min

where x is the pixel intensity value, cd f ( x ) is the cumulative
distribution function of the histogram of the pixel intensities in
the w -by- h window, cd f
min is the minimum intensity in this
window, andL is the desired number of output gray levels.
Typically a square window is used, and we definek ≡ w = h .
We call the center of the k -by- k window the anchor . For LHE,
the anchor point is the pixel to be processed itself. For thewhole image, each pixel repeats the above operation and uses
f ( x ) to get its new intensity value. Fig. 1 illustrates LHE.
We define the generalized LHE operator as
L
ξ,η
k
( I W ∗ H
) = I


W ∗ H
(2)
where ξ,η is the relative position of the anchor point to
the pixel to be processed, I W ∗ H
is the input image whose
dimension is W -by- H ,andI


W ∗ H
is the histogram-equalized
image with the same dimension. The typical LHE which uses
the k -by- k local window can be denoted as L
0 , 0
k
, since the
anchor point is exactly the pixel to be processed itself.
If the pixel to be processed is brighter than all the neigh-boring pixels around it, it will have a large intensity value
after the local histogram equalization, and vice versa. We can
make LHE ‘oriented’ by changing anchor positions. Fig. 2
shows nine LHE operators using 3-by-3 windows. The eight
operators with { ξ,η } other than{ 0 , 0 } are ‘oriented’, and they
are dubbed as the Oriented Local Histogram Equalization
operators (OLHE operators) in this paper. The following
gives the formal definition of the OLHE operators:
O

k
≡ L
(
( k − 1 )
2
,
− ( k − 1 )
2
)
k
, O

k
≡ L
( 0 ,
− ( k − 1 )
2
)
k
,
O

k
≡ L
(
− ( k − 1 )
2
,
− ( k − 1 )
2
)
k
, O
−→
k
≡ L
(
( k − 1 )
2
, 0 )
k
,
O
←−
k
≡ L
(
− ( k − 1 )
2
, 0 )
k
, O

k
≡ L
(
( k − 1 )
2
,
( k − 1 )
2
)
k
,
O

k
≡ L
( 0 ,
( k − 1 )
2
)
k
, O

k
≡ L
(
− ( k − 1 )
2
,
( k − 1 )
2
)
k
(3)
where k is an odd number. Note that according to our
definition, there will always be exactly eight OLHE opera-tors no matter what the value of k is. Given an image I ,
OLHE produces 8 images, which areO

k
( I ) , O

k
( I ) , O

k
( I ) ,
O
−→
k
( I ) , O

k
( I ) , O
←−
k
( I ) O

k
( I ) and O

k
( I ) . The 8 images
are referred to as the OLHE images.

Memoona Umar said... Reply to comment

hey can any one please help me with code for enhancement of an image of a finger because I have to extract the fingerprint from it but cant get past binzrization cz the change intensity of the pattern in image is very low so it just gives me a black screen please help.

Unknown said... Reply to comment

Great job, but i have a question.

Why on i and j you used i= 1:size(B,1)-((PadM*2)+1) and j=1:size(B,2)-((PadN*2)+1) instead of use i= 1:size(B,1) and j=1:size(B,2), for the lest elements from the right can participate on the calculation of the probability of density?

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