Small screen resolution causes black rectangle around

Question

The resolution of my monitor is 1920*1080.

But if I change it to smaller, for example 1600*900.

Then the monitor only shows the screen in the center, and leaves black rectangles around.

########################
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####                ####
####     Screen     ####
####                ####
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It is like displaying 1600*900 screen in a 1920*1080.

The size of everything is still the same.

What is wrong with my PC?

This happened earlier today because I played some low resolution legacy game.

Answer 1

Unlike CRTs, LCDs have a fixed "native resolution", determined by the number of physical pixels; in your case, 1920x1800. Most monitors will stretch a low-res picture to the monitor's native resolution, but this causes loss of quality (it's impossible to have 1/5 of a pixel black and 4/5 white), so some monitors have an option to just center the picture instead. Take a look inside the monitor's configuration menu, press all the buttons you can find, etc. (It is possible that the monitor does not have a "stretch" mode at all, although I would guess this is very rare.)