Re: Paint.Arc draw direction

[Claus Dietrich <claus.dietrich@xxxxxxxxxx>]

Am 17.02.26 um 22:41 schrieb Lee:
> Thank you for your response, Claus.
>
> I don't know exactly what Paint.Scale() does, but if I use 
> "Paint.Scale(1, 1)" the arc is drawn clockwise.
>
> I have looked at the code for the Paint component and the 
> documentation for Qt's QPainterPath class; specifically for 
> QPainterPath::arcTo [https://doc.qt.io/qt-6/qpainterpath.html#arcTo].
>
> The Qt documentation does indeed state that arcs are drawn 
> counter-clockwise and clockwise if the angles (which includes the arc 
> length in degrees) are negative.
>
> However, the Paint.Arc() method in the component (at lines 911,912 of 
> /gb.qt4/src/cpaint_impl.cpp) reverses the sign of the angles:
>     angle = - angle;
>     length = - length;
>
> Perhaps Paint.Scale(1, -1) effectively makes that angles negative 
> which would make them be drawn clockwise, but Paint.Arc reverses the 
> signs again making the arc be drawn counter-clockwise. Unfortunately, 
> transformation matrices are above my pay grade 😉
>
> I have to conclude that the Wiki documentation follows the Qt 
> documentation, but since the component reverses the signs of the 
> angles, the Wiki documentation is also reversed. I would like further 
> confirmation before I "correct" the Wiki and risk actually making it 
> incorrect.

The documentation of cairo actually tells the opposit of the Gambas wiki 
says:

https://www.cairographics.org/manual/cairo-Paths.html#cairo-arc

So I am assuming that the Gambas documentatiion is wrong.

Best regards, Claus