Qwt WaterfallPlot (QwtPlotSpectrogram) disappears on window resize with specific frequency ranges

[Joe von Habsburg]

Hi,
I'm using QwtPlotSpectrogram with QwtMatrixRasterData to display a waterfall plot. I'm experiencing a strange rendering issue that only occurs with specific frequency ranges combined with window resize operations.
Steps to reproduce:

Display waterfall with range 0–3000 MHz → works fine in normal and maximized window

Change range to 0–2900 MHz in normal window → waterfall disappears

Maximize the window → waterfall reappears

Restore to normal size → waterfall disappears again
Change range back to 0–3000 MHz → waterfall reappears in normal window and stays visible through resize
Change range to 0–2800 MHz in normal window → works fine, waterfall visible in both normal and maximized
Change range to 0–2900 MHz while maximized → restore to normal size → waterfall disappears

What I've tried:

Calling invalidateCache() + replot() on resize
Calling raster->setValueMatrix() again on resize
Using QTimer::singleShot to delay resize

My setup:

QwtPlotSpectrogram + QwtMatrixRasterData
Matrix width changes dynamically based on frequency range
Data is QVector<double>, updated in real-time

[Joe von Habsburg]

I found the problem

Problem was that I think :
I receive 65536 point but my widget width only 2000px and 32.768(33) point equal 1px, I lost the peak value there. That is my opinion and I solved. If you are think problem is different. I listen.

Windows 11
Qt 6.9.x
Qwt-6.3.0

[SGaist]

Hi,

Which version of Qt are you using ?
On which platform ?
If Linux:

• Which distribution ?
• Which window manager ?

Can you please provide a minimal compilable example that shows this behaviour ?

[Joe von Habsburg]

I found the problem

Problem was that I think :
I receive 65536 point but my widget width only 2000px and 32.768(33) point equal 1px, I lost the peak value there. That is my opinion and I solved. If you are think problem is different. I listen.

Windows 11
Qt 6.9.x
Qwt-6.3.0