scale in QwtSlider and QwtPlot

[mastupristi]

I have a QwtSlider with range (for example) [2, 17]

When it is drawn the scale is "unclear":
qwtSliderScaleExample.png

the uppoer and lower value are not explicitly shown.

similar problem is on QwtPlot scale.

Can I change the scale to show the upper and lower value?

best regards
max

[Uwe]

The boundaries of a scale and the position of all ticks are defined by a QwtScaleDiv object. Tick labels are painted for major ticks.
A scale division can be assigned manually from application code or can be aligned/calculated by a QwtScaleEngine from its boundaries and a step size. A scale engine implements 2 algorithms:

• aligning the boundaries
  see QwtScaleEngine::autoScale()
• finding the ticks
  see QwtScaleEngine::divideScale()

Each type of scale engine offers parameters that have an impact on its algorithm.

In case of a slider calculating the scale div manually ( QwtAbstractScale::setScale( const QwtScaleDiv & ) ) might be an option, but in case of plot axes the application code often doesn't know about the scales - f.e. when your plot offers any type of navigation ( zooming, panning ... ) - and you need to have an algo for calculating boundaries and ticks.

In case of QwtSlider most relevant methods can be found in its base class QwtAbstractScale.

HTH,
Uwe

[mastupristi]

I have inherit a QwtScaleEngine in this way:

class boundScaleEngine : public QwtLinearScaleEngine
{
public:
 
    virtual QwtScaleDiv divideScale( double x1, double x2,
        int numMajorSteps, int numMinorSteps,
                                     double stepSize = 0.0 ) const;
};
 
QwtScaleDiv boundScaleEngine::divideScale( double x1, double x2,
        int numMajorSteps, int numMinorSteps,
                                     double stepSize) const
{
    QwtScaleDiv sd = QwtLinearScaleEngine::divideScale(x1, x2, numMajorSteps, numMinorSteps, stepSize);
 
    QList<double> ticks[QwtScaleDiv::NTickTypes];
 
    ticks[QwtScaleDiv::MajorTick] = sd.ticks(QwtScaleDiv::MajorTick);
    ticks[QwtScaleDiv::MediumTick] = sd.ticks(QwtScaleDiv::MediumTick);
    ticks[QwtScaleDiv::MinorTick] = sd.ticks(QwtScaleDiv::MinorTick);
 
    QwtInterval interval = sd.interval();
    double upper,lower;
 
    if(x1>x2)
    {
        upper = x1;
        lower = x2;
    }
    else
    {
        upper = x2;
        lower = x1;
    }
 
    if(ticks[QwtScaleDiv::MajorTick].indexOf(lower) == -1)
        ticks[QwtScaleDiv::MajorTick].prepend(lower); // add the lower bound tick if it is not present
    if(ticks[QwtScaleDiv::MajorTick].indexOf(upper) == -1)
        ticks[QwtScaleDiv::MajorTick].append(upper); // add the upper bound tick if it is not present
 
    return QwtScaleDiv(interval, ticks);
}

class boundScaleEngine : public QwtLinearScaleEngine
{
public:

    virtual QwtScaleDiv divideScale( double x1, double x2,
        int numMajorSteps, int numMinorSteps,
                                     double stepSize = 0.0 ) const;
};

QwtScaleDiv boundScaleEngine::divideScale( double x1, double x2,
        int numMajorSteps, int numMinorSteps,
                                     double stepSize) const
{
    QwtScaleDiv sd = QwtLinearScaleEngine::divideScale(x1, x2, numMajorSteps, numMinorSteps, stepSize);

    QList<double> ticks[QwtScaleDiv::NTickTypes];

    ticks[QwtScaleDiv::MajorTick] = sd.ticks(QwtScaleDiv::MajorTick);
    ticks[QwtScaleDiv::MediumTick] = sd.ticks(QwtScaleDiv::MediumTick);
    ticks[QwtScaleDiv::MinorTick] = sd.ticks(QwtScaleDiv::MinorTick);

    QwtInterval interval = sd.interval();
    double upper,lower;

    if(x1>x2)
    {
        upper = x1;
        lower = x2;
    }
    else
    {
        upper = x2;
        lower = x1;
    }

    if(ticks[QwtScaleDiv::MajorTick].indexOf(lower) == -1)
        ticks[QwtScaleDiv::MajorTick].prepend(lower); // add the lower bound tick if it is not present
    if(ticks[QwtScaleDiv::MajorTick].indexOf(upper) == -1)
        ticks[QwtScaleDiv::MajorTick].append(upper); // add the upper bound tick if it is not present

    return QwtScaleDiv(interval, ticks);
}

To copy to clipboard, switch view to plain text mode 

I used the boundScaleEngine class in both QwtSlider and QwtPlot, and it seems to work properly, but I don't know if there is some pitfalls.
what is your opinion?

bets regards
max

[Uwe]

I used the boundScaleEngine class in both QwtSlider and QwtPlot, and it seems to work properly, but I don't know if there is some pitfalls.
what is your opinion?

You have to take care of layout issues: the tick labels at the borders might overlap, when being too close to its neighbors.

Uwe