Spline interpolation behaves "weird"

[Uwe]

The problem should be related to the polygon clipping, that is enabled by default. As the cutting point between curve and canvas replaces the previous point you get a different polynomial.
If I'm right you should be able to work around this problem by:

m_curve->setPaintAttribute( QwtPlotCurve::ClipPolygons, false );

m_curve->setPaintAttribute( QwtPlotCurve::ClipPolygons, false );

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Could you please modify the code in QwtPlotCurve:: drawLines, so that the fitting is done before the spline interpolation ?

However a proper fix is not so easy as this reordering results in ( potentially many ) more points that go into both algos and could be a real performance issue for QwtWeedingCurveFitter.

Uwe

[mwh]

The workaround helped to fix the problem, thank you.

(I do not know where and how exactly to patch the QwtPlotCurve::drawLines method... it helped to turn polygon clipping off in our code when we switch to spline interpolation.)

I am well aware that both, the workaround and a proper fix have an impact on the performance, but since a spline interpolation by definition is slower than a linear interpolation I do not consider this a real problem for us (other qwt users may of course object to this option).

[Uwe]

I added a bug report with an idea how to "avoid" the problem: https://sourceforge.net/p/qwt/bugs/369/

Uwe

[mwh]

Sounds good.