QwtPointMapper toImage Can't Use Threads?

[nilocm]

Hopefully I'm missing something, but it looks to me like QwtPointMapper::toImage() will never use threads.

I'm trying to plot a large number of points using a QwtPlotCurve with the Dots style. While stepping through the code in debug I noticed that QwtPointMapper::toImage skipped the QFuture/QtConcurrent code based on a preprocessor check, "#if QWT_USE_THREADS".

I don't see any way for this check to pass. The only define for QWT_USE_THREADS that I can find is at the top of qwt_point_mapper.cpp :

#if !defined(QT_NO_QFUTURE)
#define QWT_USE_THREADS 0
#endif

#if !defined(QT_NO_QFUTURE)
#define QWT_USE_THREADS 0
#endif

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If QT_NO_QFUTURE isn't defined then QWT_USE_THREADS = 0, and if QT_NO_QFUTURE is defined then QWT_USE_THREADS is not defined. Either way it seems that a subsequent check, "#if QWT_USE_THREADS", will fail.

Am I misinterpreting the logic?

[Uwe]

No you are right. Of course this setting was nonsense and I fixed it in all branches >= 6.1.

QwtPointMapper::toImage() is relevant for scatter plots, where the values are displayed by a dot of 1 pixel. In this situation multithreading is possible and can improve the performance of the render cycle significantly.

Uwe