When I change the resolution of the screen, my screen turns off and on again (but just for a short period of time), and then off and then on again and finally it has the resolution I wanted.
But why does it need to flicker? Why does it need to turn off and on again, probably even 2 times?
I've seen this on old monitors as well as new ones and I don't quite get why this is necessary when changing the resolution.
Does anyone know?
The monitor and graphics card are negotiating a certain resolution and refresh rate. There are a couple things that could happen.
The monitor has to reset with the new settings. It will turn off and then turn on again with the new settings.
When changing resolution, the graphics card will briefly stop sending a signal. The monitor detects this loss of signal, and goes into standby mode. When the graphics card starts sending the new resolution, the monitor comes back on.
If you are running multiple displays, the graphics card may be also switching between which displays are active. If the monitor has multiple inputs, it may be switching between them when it detects the loss of signal from the graphics card.
It totally depends on the combination of the hardware, the drivers and the software that handles the GUI environment (the so-called display manager) and finally the application software that is displaying stuff .
Each of these can (but not necessarily has to) force a re-draw (with the flicker) of the display.
Typically (in Windows) the first flicker happens when the display driver reconfigures the hardware for the new resolution.
In many cases the video-chip has to stop displaying. And then, 1 or more display-frames later, start producing output in the new format.
The monitor will see a brief disconnect/reconnect of the video-card and re-adjusts itself accordingly (see fooot's answer too).
In most cases the content of the display memory is not longer valid either, so that get's cleared too. Usually to a black screen.
Then in a second step Windows itself redraws all the display-components at the new resolution. As Windows doesn't know what is present in the display-buffer at this point (it has no way of knowing fur sure whether or not the video-driver blanked the video-buffer) this usually starts with a fresh blank screen in the default background color. Then Windows adds the task-bar, wall-paper, etc.
This may cause some more visual flicker.
And finally running applications may do an additional re-draw to re-adjust themselves to the new display size. (Especially if they are running maximized/full-screen or snapped to the borders of the display.)
To summarize: Some flicker is video-hardware related, some flicker is software redraw related.
Last but not least:
Some people are far more sensitive to this sort of thing than others.
It is sometimes (literally) "in the eye of the beholder".
Some people hardly notice, others find it irritating.
