How to find the entropy value of an RGB image after histogra equalization in Y plane.?

I am finding entropy value of an RGB image after histogram processing in Y plane, as follows:

i % the original image

y1=rgb2ycbcr(i); y=y1(:,:,1);cb=y1(:,:,2);cr=y1(:,:,3); he1=histeq(y); r1=cat(3,he1,cb,cr); r1=ycbcr2rgb(r1);
g1=rgb2gray(r1); e1=entropy(g1):

Now I followed the procedure:

imwrite(r1,'temp1.jpg'); i2=imread('temp1.jpg'); g2=rgb2gray(i2); e2=entropy(g2)

But now e1 and e2 are different. Why it is so?

Answer 1

[Guillaume]

How much different?

Do you know that the jpg format is a lossy compression format? Every time you save an image as jpg, some of the pixel values are slightly changed.

If you use a non-lossy compression format such as png, does the entropy change?

[arun kumar]

Thank you so much Guillaume. Now I saved the image with 'jpg' extension in 'lossless' mode. Now getting the same values of e1 and e2. Even with 'png' also I can get same values.

Thank you once again