Re: Paint.Arc draw direction

[Benoît Minisini <benoit.minisini@xxxxxxxxxxxxxxxx>]

Le 17/02/2026 à 22:41, Lee a écrit :
> On 2/17/26 2:03 PM, Claus Dietrich wrote:
> > Hi lee
> >
> > Am 16.02.26 um 16:10 schrieb Lee:
> > > Just to confirm, am I misunderstanding the documentation, or is
> > > it in need of correction?
> >
> > I typically draw circles with following transformation matrix:
> >
> > Paint.Translate($iDrawCenterX, $iDrawCenterY) Paint.Scale(1, -1) 
> > Paint.Arc(0, 0, $iRadius, 0, Rad(90))
> >
> > With this, arc is drawn as documented. Could it be, that the 
> > documentation was inadvertently written under the application of
> > such transformation matrix? From my point of view the sentence "If
> > _Length_ is negative, then the arc is drawn clockwise." is wrong
> > and I suggest to remove it from the documentation.
> >
> > Best regards, Claus
>
> 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.

It's the wiki that is false. The implementation actually follows Cairo,
which does the contrary of Qt, i.e. it draws arcs clockwise if the
angles are positive, and counter-clockwise otherwise.

Regards,

--
Benoît Minisini.