Histogram Equalization Results

Question

I am trying to code histogram equalization by my self, but the results are different from the built in function in matlab histeq . Any suggestions why is this happening?. I attached the code and the results.

%% Histogram Equalization
close all
clear all
%%
I = imread('Iris_virginica.jpg');
grayI = rgb2gray(I);
[m,n] = size(grayI);
L = 256;
[counts,x] = imhist(grayI);
myCDF = cumsum(counts)/(m*n);
equalizedI = (L-1)*myCDF(double(grayI)+1);
equalizedI = uint8(equalizedI);
histMyOut = imhist(equalizedI);
builtInEqualizationI = histeq(grayI);
histBuiltInEqu = imhist(builtInEqualizationI);
%%
figure
subplot(1,3,1), stem(x,counts), title('Histogram');
subplot(1,3,2), stem(x,myCDF), title('Commulative Distribution');
subplot(1,3,3), stem(x,imhist(equalizedI)), title('Equalized');

figure
subplot(1,3,1),imshow(grayI), title('Input image');
subplot(1,3,2), imshow(equalizedI), title('Equalized image (mine)');
subplot(1,3,3), imshow(builtInEqualizationI), title('Equalized image (matlab)');

figure
subplot(1,3,1), stem(x,counts), title('Histogram');
subplot(1,3,2), stem(x,histMyOut), title('Equalized Histogram (mine)');
subplot(1,3,3), stem(x,histBuiltInEqu), title('Equalized Histogram (matlab)');

difference = abs(equalizedI-builtInEqualizationI);
figure, imshow(difference,[]);

Answer 1

histeq by default uses 64 bins to equalize your histogram. You are using the default implementation of histeq . Try doing:

builtInEqualizationI = histeq(grayI, 256);

... to specify 256 bins for equalization as your manual code is using that many bins for the equalization. The second parameter overrides the default and informs histeq to use that many bins to perform the equalization.