Convert panned() parameters to plot values

[TheBadger]

Hi,

I am using a QwtPlotPanner to pan a plot.
I connect to the QwtPlotPanner:anned(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;

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

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and the output is:

dx   :  -81 
dxInv:  49.768

dx   :  -81 
dxInv:  49.768

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If I pan 10 units I get:

dx   :  -159 
dxInv:  49.8353

dx   :  -159 
dxInv:  49.8353

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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));

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));

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Output after moving 5 units:

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

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

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So my question is, what is dx?
The (limited) documentation 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;

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;

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Moving again 5 units in 1 direction I get:

d1:  19.9083 
d2:  49.9083

d1:  19.9083 
d2:  49.9083

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and in the other direction 5 units:

d1:  0.214057 
d2:  30.2141

d1:  0.214057 
d2:  30.2141

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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 );

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

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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 );

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

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Uwe

Well that was simple enough

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,