Convert panned() parameters to plot values

[TheBadger]

Hi,

I am using a QwtPlotPanner to pan a plot.
I connect to the QwtPlotPanner::panned(int,int) signal and in the slot I want to convert the dx value to the corresponding value on the plot.

What I am doing:
I click the plot and pan it by 5 units (by checking the axis values)
Then in my slot I debug a few values as follow:


double dxInv = plot->invTransform(QwtPlot::xBottom, dx);
qDebug() << "dx : " << dx;
qDebug() << "dxInv: " << dxInv;

and the output is:


dx : -81
dxInv: 49.768

If I pan 10 units I get:


dx : -159
dxInv: 49.8353


What should I do to actually get the panned amount in the coordinates of the axis (5 in the first example and 10 in the second)?

Regards,

[Uwe]

You could have a look at the implementation of QwtPlotPanner::moveCanvas ( the slot, that is connected to the panned signal ).

Uwe

[TheBadger]

Some more investigation:
My code


const QwtScaleMap map = d->plot->canvasMap( QwtPlot::xBottom );
qDebug() << "<<<<<<<<<<<<<<<<<";
qDebug() << "Left (p): " << d->plot->canvas()->rect().left();
qDebug() << "Right (p): " << d->plot->canvas()->rect().width();
qDebug() << "Left : " << d->plot->invTransform(QwtPlot::xBottom, 0);
qDebug() << "Right: " << d->plot->invTransform(QwtPlot::xBottom, d->plot->canvas()->rect().width());
qDebug() << "dx (p): " << dx;
qDebug() << "dx : " << d->plot->invTransform(QwtPlot::xBottom, fabs(dx));


Output after moving 5 units:


<<<<<<<<<<<<<<<<<
Left (p): 0
Right (p): 502
Left : 14.6537
Right: 45.3258
dx (p): -81
dx : 19.6028


So my question is, what is dx?
The (limited) documentation (http://qwt.sourceforge.net/class_qwt_panner.html#ae9ce78e6f9ae73317af29b2dc5d f7372) says "Offset in horizontal direction", my question is offset from what?

My deductions:
- If left is 0 and right is 502 in pixels(?)
- In my plot axis, left is 14 and right is 45
- So moving 5 units in the units of my plot should be about (+/-)17 pixels(?) in either direction
- Why is the dx then -81, where does that value come from?

[Uwe]

The panned signal is derived from QwtPanner, that doesn't know about plots and plot coordinates at all. So dx, dy are obviously widget ( = canvas ) coordinates: the offset between the mouse press/release positions ( see QMouseEvent ).

Note that Qwt is open source, meaning, that you always have the option to look into the implementation.

Uwe

[TheBadger]

Thanks Uwe, I have looked at the implementation but could not learn anything.

I use the code from there as follow:


const double p1 = map.transform( d->plot->axisScaleDiv( QwtPlot::xBottom ).lowerBound() );
const double p2 = map.transform( d->plot->axisScaleDiv( QwtPlot::xBottom ).upperBound() );

double d1, d2;
d1 = map.invTransform( p1 - dx );
d2 = map.invTransform( p2 - dx );
qDebug() << "d1: " << d1;
qDebug() << "d2: " << d2;

Moving again 5 units in 1 direction I get:


d1: 19.9083
d2: 49.9083

and in the other direction 5 units:


d1: 0.214057
d2: 30.2141


I am still not sure how those values map to my plots...

[Uwe]

In case of a linear scale, this is probably what you want:


const QwtScaleMap map = plot->canvasMap( QwtPlot::xBottom );
double xOff = map.invTransform( dx ) - map.invTransform( 0 );


Uwe

[TheBadger]

In case of a linear scale, this is probably what you want:


const QwtScaleMap map = plot->canvasMap( QwtPlot::xBottom );
double xOff = map.invTransform( dx ) - map.invTransform( 0 );


Uwe

Well that was simple enough :D

Thank you very much for the help, it is what I was looking for.
My problem was that I should have used the map to do the transform, and not the plot.

Apart from the struggles now and again, I have to say very well done on this library.

Regards,