Relatively to other things you paint (with that painter), it does not move.
i don't understand what is above sentence mean. Plz explain me. What ever i have drawn using that painter all are changing.

A common "trick" is to first move the origin with translate() and then paint whatever you want relative to your origin, that way if you scale (*after* that translation) only the size will increase, but the origin will remain.
that mean we translate to center of window every time constantly are changed to every time