Question

I saw an example that uses the following to create a point that will be used to centre a window in Qt:

x = (screenWidth – WIDTH) / 2;

y = (screenHeight – HEIGHT) / 2;

Provided that screenWidth and screenHeight are found using the width() and height() functions respectively of QDesktopWidget.

How does the preceding code centre the window? Yes, I know it centres the window, but couldn’t understand it from a calculation point of view.

Thanks.

Answer 1

First calculate the total amount of ‘extra’ horizontal space around your window:

extra_space = screenWidth - your_window_width

now, spread that space at left and right:

left_space + right_space = extra_space

the space at both sides should be the same:

right_space = left_space
==>  2 * left_space = extra_space
==>  2 * left_space = screenWidth - your_window_width
==>  left_space = (screenWidth - your_window_width) / 2

that’s your x. The same goes for the y coordinate.