qwt-interest — Qwt Q&A

Re: incremental updates

[Giacomo S. <gia...@el...>]

I see.

Indeed, I added points in the middle of a spectrum data and redrew from 
there on.

Evidently, as you say, the last points are drawn on top of a dirty 
area... that makes sense.

With regards

Giacomo.

El 22/12/25 a las 8:08 a. m., Uwe Rathmann via qwt-interest escribió:
> On 12/19/25 2:24 PM, Giacomo S. wrote:
>
>> I am using QwtPlotDirectPainter to incrementally update segments of 
>> curves when new data arrives.
>
> It draws lines on top of the existing content - that's all. It does 
> not redraw any dirty parts.
>
> Uwe
>
>
>
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> qwt-interest mailing list
> qwt...@li...
> https://lists.sourceforge.net/lists/listinfo/qwt-interest

Re: incremental updates

[Uwe R. <Uwe...@ti...>]

On 12/19/25 2:24 PM, Giacomo S. wrote:

> I am using QwtPlotDirectPainter to incrementally update segments of 
> curves when new data arrives.

It draws lines on top of the existing content - that's all. It does not 
redraw any dirty parts.

Uwe

Fw: Accessing Qwt objects added in a Qt .ui file

[Grace Yu <gr...@ga...>]

The link error is gone after I added QWT_DLL in C/C++, Preprocessor, Preprocessor Definitions. But the returned value (ui_plot) in the following code is nullptr.

QwtPlot *ui_plot = findChild<QwtPlot *>("qwtPlot");

________________________________
From: Grace Yu <gr...@ga...>
Sent: Friday, December 19, 2025 1:43 PM
To: qwt...@li... <qwt...@li...>
Subject: Accessing Qwt objects added in a Qt .ui file

Hi,

I have a question regarding the Qwt designer plugin that is built with Qt 6.10. Once the plugin dll is generated at C:\Qwt-6.3.0\plugins\designer and also copied to the path of "C:\Qt\6.10.1\msvc2022_64\plugins\designer", I am able to add a QwtPlot widget in a .ui file through the Qt designer. The .ui file can be loaded by QUiLoader().load() successfully. A QwtPlot is in the main window when it come ups. But the following line of code always causes a link error when it tries to access the QwtPlot object

QwtPlot *ui_plot = findChild<QwtPlot *>("qwtPlot"); //"qwtPlot" is the QwtPlot object name given in the Qt designer.

The link error is following
[cid:ac739314-cd39-49a6-b94d-ca4ce6d1938d]

Seems I can only edit the ui within the designer but cannot access the corresponding object in the code. Anything I should check or correct?

Thanks!

Accessing Qwt objects added in a Qt .ui file

[Grace Yu <gr...@ga...>]

Hi,

I have a question regarding the Qwt designer plugin that is built with Qt 6.10. Once the plugin dll is generated at C:\Qwt-6.3.0\plugins\designer and also copied to the path of "C:\Qt\6.10.1\msvc2022_64\plugins\designer", I am able to add a QwtPlot widget in a .ui file through the Qt designer. The .ui file can be loaded by QUiLoader().load() successfully. A QwtPlot is in the main window when it come ups. But the following line of code always causes a link error when it tries to access the QwtPlot object

QwtPlot *ui_plot = findChild<QwtPlot *>("qwtPlot"); //"qwtPlot" is the QwtPlot object name given in the Qt designer.

The link error is following
[cid:ac739314-cd39-49a6-b94d-ca4ce6d1938d]

Seems I can only edit the ui within the designer but cannot access the corresponding object in the code. Anything I should check or correct?

Thanks!

Re: incremental updates

[Giacomo S. <gia...@el...>]

adding several debug messages in the code I see that I invoke

d->directPainter->drawSeries(c, data->size() - cnt - 1, data->size());

but within the reimplemented method:

void QuPlotCurve::drawCurve(QPainter *p, int style,
                             const QwtScaleMap &xMap,
                             const QwtScaleMap &yMap,
                             const QRectF &canvasRect,
                             int from, int to) const


from is consistent but to is one less than expected:

QuTrendPlot.refresh: partial draw from 42 to 49   (where i invoke 
directPainter->drawSeries )
QuPlotCurve: drawing from 42 to 48 (data size is 49)    (from = 42, to = 
48 not 49! )


With regards

Giacomo.



El 19/12/25 a las 2:24 p. m., Giacomo S. escribió:
> Hi everyone,
>
> I am using QwtPlotDirectPainter to incrementally update segments of 
> curves when new data arrives.
>
> I make sure the X and Y scales are wide enough to host the new (x,y), 
> but still I see artifacts until a full replot() takes place (e.g. by 
> resizing the window).
>
> Am I missing something?
>
> With regards
>
> Giacomo
>
>
>
> _______________________________________________
> qwt-interest mailing list
> qwt...@li...
> https://lists.sourceforge.net/lists/listinfo/qwt-interest

incremental updates

[Giacomo S. <gia...@el...>]

Hi everyone,

I am using QwtPlotDirectPainter to incrementally update segments of 
curves when new data arrives.

I make sure the X and Y scales are wide enough to host the new (x,y), 
but still I see artifacts until a full replot() takes place (e.g. by 
resizing the window).

Am I missing something?

With regards

Giacomo

qwt and Qt Widgets desginer

[Max Y. <max...@gm...>]

How integrated is qwt into the Qt Widgets designer used from Visual 
Studio?  When I run the stand alone Qt Widgets Designer I see a section 
with Qwt Widgets, but running from VS2022 using Qt tools, I don't see 
this section.

What did I miss?

Max

Re: QwtPlot::replot()

[Uwe R. <Uwe...@ti...>]

On 12/1/25 11:54 AM, David C. Partridge wrote:

> I’m using QwtPlotSpectrogram with QwtMatrixRasterData to plot some
> data that is quite large (about 9600 x 6400).

The resolution of the image to be calculated depends on the resolutions
of your plot canvas ( size * devicePixelRatio ) and your data: the
smaller one is used.

So the size of the matrix should not matter as long as the canvas 
resolution is below.

To very what is going on have a look at the imageSize parameter of
QwtPlotSpectrogram::renderImage.

The rasterview example shows a situation, where the resolution of the 
data is below the one of the canvas. For NearestNeighbour the imageSize 
is here 4x4 as QwtMatrixRasterData does not provide more values.

> Is there any way that you can change the code to use the 
> QFutureWatcher::finished signal to drive the repaint/update
> **after** the time-consuming calculations are complete?

The image composition is running over all pixels ( imageSize ) and
requests a value from the raster data - probably downsampling in your
situation. It divides the image into tiles filling each in its own thread.

When using QwtMatrixRasterData the performance of QwtRasterData::value
depends on the ResampleMode. As you are downsampling NearestNeighbour
should be the right choice - what is a lookup in a QVector with some
range checks.

You could have a look at the spectrogram example where the time for the
image composition is displayed on the status bar. When resizing the
window you see the effect of the canvas size.

The QwtRasterData::value implementation in the example is lighter than
the one from QwtMatrixRasterData - but you see what to expect.

HTH,
Uwe

QwtPlot::replot()

[David C. P. <dav...@pe...>]

I'm using QwtPlotSpectrogram with QwtMatrixRasterData to plot some data that
is quite large (about 9600 x 6400).

 

When I invoke QwtPlot::replot() the Qt event processing is blocked for a
long time.

 

Is there any way that you can change the code to use the
QFutureWatcher::finished signal to drive the repaint/update *after* the
time-consuming calculations are complete?

 

I tried to work out what I could change to achieve that, but I'm not
familiar enough with the Qwt code to see where it would need to be changed.

 

Thanks,

David

 

Re: Ensuring aspect ratio is preserved in QwtPlotSpectrogram

[Uwe R. <Uwe...@ti...>]

Attach the rescaler after the first replot - otherwise it gets 
initialized with invalid scale intervals.

HTH,
Uwe

RE: Ensuring aspect ratio is preserved in QwtPlotSpectrogram

[David C. P. <dav...@pe...>]

Hmm I added this to my class definition:

 

                              QwtPlotRescaler* rescaler{ nullptr };

and the following to the class ctor:

                              rescaler = new
QwtPlotRescaler(qualityPlot->canvas());

 
rescaler->setRescalePolicy(QwtPlotRescaler::Expanding);

 
rescaler->setAspectRatio(static_cast<double>(width) /
static_cast<double>(height));

and then in the code that populates the Spectrogram:

                                             //

                                             // Update the color map with
FWHM values from the interpolated grid

                                             //

 
rasterData->setValueMatrix(QVector<double>(zgFWHM.cbegin(), zgFWHM.cend()),
static_cast<int>(xg.size()));

                                             rescaler->rescale();

 

                                             qualityPlot->replot();

but I get this odd result:



which clearly isn't as intended - what am I doing wrong?

 

Thanks

David

 

-----Original Message-----
From: David C. Partridge <dav...@pe...> 
Sent: 24 November 2025 09:31
To: 'List for both Qwt users and developers'
<qwt...@li...>
Cc: 'Uwe Rathmann' <Uwe...@ti...>
Subject: RE: Ensuring aspect ratio is preserved in QwtPlotSpectrogram

 

> <https://qwt.sourceforge.io/class_qwt_plot_rescaler.html>
https://qwt.sourceforge.io/class_qwt_plot_rescaler.html

 

Many thanks

David

 

 

 

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RE: Ensuring aspect ratio is preserved in QwtPlotSpectrogram

[David C. P. <dav...@pe...>]

>https://qwt.sourceforge.io/class_qwt_plot_rescaler.html

Many thanks
David

Re: Ensuring aspect ratio is preserved in QwtPlotSpectrogram

[Uwe R. <Uwe...@ti...>]

On 11/22/25 5:09 PM, David C. Partridge wrote:
> The data for which I am displaying the spectrogram is extracted from an 
> image that has an aspect ratio of 3:2
> 
> I want the spectrogram plot to display the plot with the spectrogram 
> having the same aspect ratio as the original image.

https://qwt.sourceforge.io/class_qwt_plot_rescaler.html

Ensuring aspect ratio is preserved in QwtPlotSpectrogram

[David C. P. <dav...@pe...>]

The data for which I am displaying the spectrogram is extracted from an
image that has an aspect ratio of 3:2

 

I want the spectrogram plot to display the plot with the spectrogram having
the same aspect ratio as the original image.

 

But it was stretched horizontally:

 



 

I tried setting the major step size for both axes to 500, but that didn't
quite work:

 



 

Thanks, David

RE: How to invert Y axis?

[David C. P. <dav...@pe...>]

Thanks Uwe, 

I knew it had to be simple - I just couldn't work it out from the
documentation.

Thanks again, David 

Re: How to invert Y axis?

[Uwe R. <Uwe...@ti...>]

On 11/22/25 1:59 PM, David C. Partridge wrote:

> However, I want the Y axis to be inverted so 0 is at the top and 3500 at 
> the bottom (and of course with the plot flipped vertically).

     plot->setAxisScale( QwtPlot::yleft, 3500, 0 );

or when using the autoscaler:

     plot_>axisScaleEngine( QwtPlot::yLeft )->setAttribute(
	QwtScaleEngine::Inverted );

HTH,
Uwe

How to invert Y axis?

[David C. P. <dav...@pe...>]

I've managed to create spectrogram chart with interpolated data:

 



 

However, I want the Y axis to be inverted so 0 is at the top and 3500 at the
bottom (and of course with the plot flipped vertically).

 

How can I do that please?

 

Thank you, David 

 

RE: Building against Qt 6.10.0 fails

[David C. P. <dav...@pe...>]

The code is not being built with "Treat warnings as errors" this is a Qt
change to force these to be errors (previously they were warnings).

-----Original Message-----
From: Uwe Rathmann via qwt-interest <qwt...@li...> 
Sent: 21 November 2025 11:43
To: List for both Qwt users and developers
<qwt...@li...>
Cc: Uwe Rathmann <Uwe...@ti...>
Subject: Re: Building against Qt 6.10.0 fails

On 11/21/25 12:24 PM, David C. Partridge wrote:

> 1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(135,17): error C4996: 
> 'QDateTime::toTimeSpec': Use toTimeZone instead

Should be warnings only - unless you have set your compiler options to fail
on those warnings.

So they should be no showstopper - nevertheless I will fix them.

HTH,
Uwe


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Re: Building against Qt 6.10.0 fails

[Uwe R. <Uwe...@ti...>]

On 11/21/25 12:24 PM, David C. Partridge wrote:

> 1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(135,17): error C4996: 
> 'QDateTime::toTimeSpec': Use toTimeZone instead

Should be warnings only - unless you have set your compiler options to 
fail on those warnings.

So they should be no showstopper - nevertheless I will fix them.

HTH,
Uwe

Building against Qt 6.10.0 fails

[David C. P. <dav...@pe...>]

1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(135,17): error C4996:
'QDateTime::toTimeSpec': Use toTimeZone instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(160,17): error C4996:
'QDateTime::toTimeSpec': Use toTimeZone instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(178,13): error C4996:
'QDateTime::setTimeSpec': Use setTimeZone() instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(182,15): error C4996:
'QDateTime::toTimeSpec': Use toTimeZone instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(280,17): error C4996:
'QDateTime::QDateTime': Pass QTimeZone instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date.cpp(656,32): error C4996:
'QDateTime::QDateTime': Pass QTimeZone instead

1>qwt_dyngrid_layout.cpp

1>qwt_event_pattern.cpp

1>qwt_graphic.cpp

1>qwt_interval.cpp

1>qwt_interval_symbol.cpp

1>D:\Github\DSS\qwt-6.3.0\qwt_date_scale_draw.cpp(276,12): error C4996:
'QDateTime::setOffsetFromUtc': Use setTimeZone() instead

1>qwt_knob.cpp

1>qwt_legend.cpp

1>qwt_legend_data.cpp

1>qwt_legend_label.cpp

1>qwt_magnifier.cpp

1>qwt_math.cpp

1>qwt_matrix_raster_data.cpp

1>qwt_null_paintdevice.cpp

1>D:\Github\DSS\qwt-6.3.0\qwt_date_scale_engine.cpp(1119,12): error C4996:
'QDateTime::setOffsetFromUtc': Use setTimeZone() instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date_scale_engine.cpp(1281,12): error C4996:
'QDateTime::setOffsetFromUtc': Use setTimeZone() instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date_scale_engine.cpp(1306,23): error C4996:
'QDateTime::QDateTime': Pass QTimeZone instead

1>D:\Github\DSS\qwt-6.3.0\qwt_date_scale_engine.cpp(1313,12): error C4996:
'QDateTime::setOffsetFromUtc': Use setTimeZone() instead

1>qwt_painter.cpp

 

 

 

Re: Sparse data for contour plot

[Uwe R. <Uwe...@ti...>]

On 11/14/25 7:12 PM, Uwe Rathmann via qwt-interest wrote:

> The implemented interpolation modes
> ( QwtMatrixRasterData::ResampleMode ) are for situations where the
> resolution of the data is below the resolution of the output device.

So for example, when having an area of 1000x1000 and you pass a 10x10 
matrix with valid values:

the resolution of the data would be below ( each data pixel covers 
100x100 ) and you can f.e enable QwtMatrixData::BilinearInterpolation.

In case the valid pixels of the sparse data are equidistant this could 
be done.

HTH,
Uwe

Re: Sparse data for contour plot

[Uwe R. <Uwe...@ti...>]

On 11/14/25 6:15 PM, David C. Partridge wrote:

> I want to create a contour plot from the data that I do have.

Qwt does not invent any values - it is displaying what is given. And a 
NaN value is interpreted as a gap ( areas where you don't have values ).

The implemented interpolation modes
( QwtMatrixRasterData::ResampleMode ) are for situations where the
resolution of the data is below the resolution of the output device.

In Qwt the term "contour" is used like in matlab: https://
www.mathworks.com/help/matlab/ref/contour.html. Not
what your posted screenshot showed.

> There was a post by David Stranz from Sierra Analytics about using
> Gaussian "splatting" with Qwt to interpolate non-uninform/sparse
> data with QwtPlotSpectrogram on the QtCentre forum, but
> unfortunately the code he posted there doesn't compile with Qwt
> 6.3.0.
This should be an operation on your input matrix - not sure why this has 
to be dependent on Qwt. Why not using the algo and passing the result to 
QwtPlotSpecrogram ?

Uwe

RE: Sparse data for contour plot

[David C. P. <dav...@pe...>]

I have no idea what your question means.  The data is the size I told you.
One pixel on the plot at its default size will be minute (smaller that a
screen pixel as it is mapped from the data array which is large to a small
screen area.

So, a data point can be anywhere in that 5202x3464 area (so I guess that's
the bounding rectangle) or are you asking what the default QwtPlot rectangle
is? If so that's about 740x500

-----Original Message-----
From: Uwe Rathmann via qwt-interest <qwt...@li...> 
Sent: 14 November 2025 11:48
To: qwt...@li...
Cc: Uwe Rathmann <Uwe...@ti...>
Subject: Re: Sparse data for contour plot

On 11/14/25 12:42 PM, David C. Partridge wrote:

> I want the data interpolated (ignoring the Nan values). The actual 
> display window is relatively small but can be enlarged.

Please answer my question: what is the bounding rectangle of the data and
what is the size of one pixel - in plot coordinates ?

Uwe


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Re: Sparse data for contour plot

[Uwe R. <Uwe...@ti...>]

On 11/14/25 12:42 PM, David C. Partridge wrote:

> I want the data interpolated (ignoring the Nan values). The actual
> display window is relatively small but can be enlarged.

Please answer my question: what is the bounding rectangle of the data 
and what is the size of one pixel - in plot coordinates ?

Uwe

RE: Sparse data for contour plot

[David C. P. <dav...@pe...>]

I want the data interpolated (ignoring the Nan values).   The actual display
window is relatively small but can be enlarged.

D.

-----Original Message-----
From: Uwe Rathmann via qwt-interest <qwt...@li...> 
Sent: 14 November 2025 10:18
To: List for both Qwt users and developers
<qwt...@li...>
Cc: Uwe Rathmann <Uwe...@ti...>
Subject: Re: Sparse data for contour plot

On 11/14/25 10:06 AM, David C. Partridge wrote:

> That size is the size of the image from which the data comes and
valuesToPlot is therefore 5202x3464 in size.

You have to think in terms of plot coordinates - the boundaries of axes.

So your spectrogram shows data from [0-5202] and [0-3464]. But what is the
size ( in plot coordinates ) of the area that is covered by a valid value ?

Uwe


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