Re: Paint.Arc draw direction

[Lee <t.lee.davidson@xxxxxxxxx>]

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.


--
Lee

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