qt coordinate system

[qtlearner123]

Hi all,

i downloaded a code from Qtapplications.org website for a manometer. it is a very lengthy code so i am not posting the entire code. I do not have a very clear concept in coordinate system so some body please explain the following coordinates and how can i relate to the picture i am drawing (manometer). i am pasting parts of the code

painter.translate(width() / 2, height() / 2);
 painter.scale(side / 335.0, side / 335.0);
 
static const int scaleTriangle[6] = { -6,141,6,141,0,129 };
 
 painter.drawEllipse(-152,-152,304,304);

painter.translate(width() / 2, height() / 2);
 painter.scale(side / 335.0, side / 335.0);

static const int scaleTriangle[6] = { -6,141,6,141,0,129 };

 painter.drawEllipse(-152,-152,304,304);

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my main doubt is the first two lines what width and height actually mean and why it is getting divided by 2 and then 335 and 335.
Most of the code contains the dimensions like above and i don't understand those. please somebody explain me little bit and then direct me to the proper documentation.

thanks in advance,
regards,
satya