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c++geometryangle

Calculating point in arc

I am trying to calculate the co-ordinates for the red lines shown in the image.

I've written the following:

    QPoint clsLOFmimic::ptCalcEndPt(QPoint* pptOrigin
                                   ,int intRadius
                                   ,float fltAngle) {
        double dblRadians = ((double)fltAngle) * (M_PI / 180.0);
        return QPoint(((double)intRadius * cos(dblRadians)) + pptOrigin->x()
                      ,((double)intRadius * sin(dblRadians)) + pptOrigin->y());
    }

However the orientation of the calculated end point isn't correct. 'pptOrigin' is the co-ordinates at the bottom left where the lines intersect.

The goal is to calculate the correct end points to reflect the lines shown in the diagram. X increases from left to right and Y increases from top to bottom.

enter image description here

Solution

  • If y is increasing downward, you'll have to apply a minus sign to the sin() function. The trig functions work as you would expect when x increases to the right and y increases upwards.