invTransform not working

[ppp]

Hello,
I am trying to work out how the invTransform function works. But I guess I misunderstood something.

Here is what I tried already:

#include <QApplication>
 
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
 
int main( int argc, char** argv ){
 
	QApplication app( argc, argv );
 
	QwtPlot plot;
	QwtPlotCurve curve;
	QVector<QPointF> list;
 
	list.append( QPointF(2,1.5) );
	list.append( QPointF(3,5) );
	list.append( QPointF(4,3.4) );
 
	curve.setSamples( list );
	curve.attach( &plot );
 
	qDebug() << "invTransform(): " << plot.invTransform( curve.xAxis(), 3 );
	qDebug() << "invTransform(): " << plot.canvasMap( QwtPlot::xBottom ).invTransform( list[1].x() );
 
	plot.show();
 
	qDebug() << "invTransform(): " << plot.invTransform( curve.xAxis(), 3);
 
	return app.exec();
}

#include <QApplication>

#include <qwt_plot.h>
#include <qwt_plot_curve.h>

int main( int argc, char** argv ){

	QApplication app( argc, argv );

	QwtPlot plot;
	QwtPlotCurve curve;
	QVector<QPointF> list;

	list.append( QPointF(2,1.5) );
	list.append( QPointF(3,5) );
	list.append( QPointF(4,3.4) );

	curve.setSamples( list );
	curve.attach( &plot );

	qDebug() << "invTransform(): " << plot.invTransform( curve.xAxis(), 3 );
	qDebug() << "invTransform(): " << plot.canvasMap( QwtPlot::xBottom ).invTransform( list[1].x() );

	plot.show();

	qDebug() << "invTransform(): " << plot.invTransform( curve.xAxis(), 3);

	return app.exec();
}

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The Console outputs:

invTransform():  3
invTransform():  3
invTransform():  -3

invTransform():  3
invTransform():  3
invTransform():  -3

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Can someone please tell me why it's not working?!
I appreciate it.

[Uwe]

You have to call replot first - otherwise you don't have a valid scale and mapping between scale and paint device coordinates will never work.

Uwe

[ppp]

Well, if I add

plot.replot();

plot.replot();

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right before all the invTransform() functions the console outputs:

invTransform():  1.97826
invTransform():  1.97826
invTransform():  1.96078

invTransform():  1.97826
invTransform():  1.97826
invTransform():  1.96078

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It should output a five. Those 1.9something looks to me like the lowerBound...

[Uwe]

invTransform maps from widget to scale coordinates and a value of 3 is close to the beginning on an interval that is about [0, width() ]. So no surprise, that you are close to the lower bound.

Uwe

[ppp]

OK. And how can I get the right return value?
I need to find out what's the y value to a corresponding x value...

[Uwe]

OK. And how can I get the right return value?
I need to find out what's the y value to a corresponding x value...

By recalling what you have learned in school - this is basic maths completely unrelated to Qwt ( f.e see QLineF::pointAt() if you don't know how to calculate a point on a line ).
You could also have a look at the curvetracker example, what does something similar.

Uwe

[ppp]

I fixed my problem with the idea like in the curvetracker example without invTransform.
Thank you.