Matlab code: Histogram equalization without using histeq function

              It is the re-distribution of gray level values uniformly. Let’s consider a 2 dimensional image which has values ranging between 0 and 255.

MATLAB CODE:

GIm=imread('tire.tif');



numofpixels=size(GIm,1)*size(GIm,2);



figure,imshow(GIm);

title('Original Image');

HIm=uint8(zeros(size(GIm,1),size(GIm,2)));

freq=zeros(256,1);

probf=zeros(256,1);

probc=zeros(256,1);

cum=zeros(256,1);

output=zeros(256,1);



%freq counts the occurrence of each pixel value.

%The probability of each occurrence is calculated by probf.



for i=1:size(GIm,1)

    for j=1:size(GIm,2)

        value=GIm(i,j);

        freq(value+1)=freq(value+1)+1;

        probf(value+1)=freq(value+1)/numofpixels;

    end

end



sum=0;

no_bins=255;



%The cumulative distribution probability is calculated. 

for i=1:size(probf)

   sum=sum+freq(i);

   cum(i)=sum;

   probc(i)=cum(i)/numofpixels;

   output(i)=round(probc(i)*no_bins);

end



for i=1:size(GIm,1)

    for j=1:size(GIm,2)

            HIm(i,j)=output(GIm(i,j)+1);

    end

end

figure,imshow(HIm);

title('Histogram equalization');

%The result is shown in the form of a table

figure('Position',get(0,'screensize'));

dat=cell(256,6);



for i=1:256

dat(i,:)={i,freq(i),probf(i),cum(i),probc(i),output(i)};   

end



   columnname =   {'Bin', 'Histogram', 'Probability', 'Cumulative histogram','CDF','Output'};

columnformat = {'numeric', 'numeric', 'numeric', 'numeric', 'numeric','numeric'};

columneditable =  [false false false false false false];

t = uitable('Units','normalized','Position',...

            [0.1 0.1 0.4 0.9], 'Data', dat,...

            'ColumnName', columnname,...

            'ColumnFormat', columnformat,...

            'ColumnEditable', columneditable,...

            'RowName',[]); 

    subplot(2,2,2); bar(GIm);

    title('Before Histogram equalization');

    subplot(2,2,4); bar(HIm);

    title('After Histogram equalization');

To find the histogram of an Image:  http://angeljohnsy.blogspot.com/2011/06/histogram-of-image.html


Local histogram equalization of an Image:  http://angeljohnsy.blogspot.com/2011/06/local-histogram-equalization.html

                              

Here is a simple Version of Histogram Equalization MATLAB CODE:

%Read a grayscale Image or a matrix mxn

A=imread('tire.tif');



figure,imshow(A);



%Specify the bin range[0 255]

bin=255;

%Find the histogram of the image.

Val=reshape(A,[],1);

Val=double(Val);

I=hist(Val,0:bin);



%Divide the result by number of pixels

Output=I/numel(A);



%Calculate the Cumlative sum

CSum=cumsum(Output);





%Perform the transformation S=T(R) where S and R in the range [ 0 1]

HIm=CSum(A+1);



%Convert the image into uint8

HIm=uint8(HIm*bin);



figure,imshow(HIm);