Why imwrite command changes some pixel automatically?

I run following code

then result is

Why saved image is different than image which I provided?

Answer 1

[Guillaume]

That has nothing to do with imwrite and everything to do with the image format you've used for saving the image.

Go read the wikipedia article about JPEG compression to learn more about it. In particular the first line of that section says: "JPEG uses a lossy form of compression".

If you care about the exact value of the pixels of your image do not use jpg for storing your images, use one of the non-lossy file format. One of the more versatile is png.

[Enkhamgalan Boldbaatar]

Thanks a lot for Mr. Guillaume.

[Image Analyst]

I second that. I use PNG whenever possible. It's rapidly becoming the new de facto standard. You should start using it also. By the way, you can also "Vote" for (like I did), as well as Accept, the answer to give Guillaume even more reputation points