Why this problem has no forum.
It is interesting question of image processing
but I could'n find in internet similar description.
After some thinking 20-string prog was created.

Please give me your idea.
naxart@yandex.ru
Thanks.

It's is enough to solve the problem on each row
for it's [i,i+21] segments

I'll try to do this.
Thanks.

I've broke my brain trying to do it that way :D But no way, couldn't find a stable solution not based on luck and meeting TL for any test.

Finally I have AC. With error diffusion :)

My solution is fully random-based. Just O(M*N), and the most time-consuming part is reading/writing...

Mine is too.