Redrawing a single row of a QwtPlotSpectrogram

[Uwe]

How can I buffer the previous image? I can keep up with a QImage object "buffer_image" within my derived spectrogram class, but I can't change it in my overloaded renderImage since renderImage is a constant function.

No, see: http://www.highprogrammer.com/alan/rants/mutable.html. Of course you can also do a const_cast for your this pointer.

Uwe