Qwt Knobs

[eto]

Hi,
I'm developing the interface for an oscilloscope instrument; I would like to use knobs for gain and timescale selections, but the current knobs does not have the required capabilities.

I'm going to develop myself the required stuff, if needed, but before I write here to ask for suggestions.
I need a knob with only a set of predefined values in a wide scale range: lets say 1, 2 and 5 as fixed multiplier. I even would like that the scale labels were with the unit specified (as 5us, or 1mV).
Maybe I can create a my version of QwtRoundScaleDraw, to fix this last point, and I can write a specialized QwtLog10ScaleEngine for the 125 scale. But I need that the knob only selects values in the scale (1, 2 or 5 x scale factor).

Any idea?

Alessandro

[Uwe]

You don't need to implement a scale engine, when you know your ticks in advance. Simply assign them using "knob->setScale(const QwtScaleDiv &)".

If you need an equidistant scale use a linear scale even if you don't have equidistant values and translate the values of the linear scale into appropriate labels.

Uwe

[eto]

Currently I managed it differently, which may not be the best way.
I use QwtLog10ScaleEngine, and than I do setRange (log10 (min), log10 (max), 0.33). Then I intercept the valueChanged signal and fix there the value
to the close 1,2,5 tick.
The only think I don't know is how to reissue the specialized valueChanged signal; now I'm emitting a different signal (which is not really clean). But I didn't see anywhere any doc about polymorphism in signals.

From what I saw the QwtScaleDiv only regards the scale, not the range: this way when I move the knob it can be set to values not matching any tick in the scale. And this is bad, for me.

[Uwe]

From what I saw the QwtScaleDiv only regards the scale, not the range: this way when I move the knob it can be set to values not matching any tick in the scale.

Well, there is this mysterious QwtAbstractSlider::getScrollMode(). You can try to derive from QwtKnob and overload this method somehow returning QwtAbstractSlider::ScrPage.

But to be honest: I didn't write these methods myself and havn't seen this code for years. So I'm not sure if this is really a possible solution.

Uwe

[eto]

In any case now I fixed as I told, intercepting valueChanged and outputing the right value.

Now I tried to implement the other point, using text label:

class knob125_scale: public QwtRoundScaleDraw {
  public:
    void drawLabel(QPainter *painter, double value) const {
 
      QwtText label(decade_helper::format (value));
      label.setRenderFlags(0);
      label.setLayoutAttribute(QwtText::MinimumLayout);
      label.textSize(painter->font ()); 
 
      if (label.isEmpty ()) return;
 
      const int tval = map ().transform (value);
      double startAngle = map ().p1 ();
      if ((tval > startAngle + 359 * 16) || (tval < startAngle - 359 * 16)) return;
 
      double radius = QwtRoundScaleDraw::radius ();
      if (hasComponent(QwtAbstractScaleDraw::Ticks) || hasComponent(QwtAbstractScaleDraw::Backbone)) radius += spacing (); 
 
      if (hasComponent(QwtAbstractScaleDraw::Ticks)) radius += majTickLength ();
 
      const QSize sz = label.textSize (painter->font ());
      const double arc = tval / 16.0 / 360.0 * 2 * M_PI;
 
      const int x = center ().x () + qRound ((radius + sz.width () / 2.0) * sin (arc));
      const int y = center ().y () - qRound ((radius + sz.height () / 2.0) * cos (arc));
 
      const QRect r(x - sz.width () / 2, y - sz.height () / 2, sz.width (), sz.height ());
      label.draw(painter, r);
    }
};

class knob125_scale: public QwtRoundScaleDraw {
  public:
    void drawLabel(QPainter *painter, double value) const {

      QwtText label(decade_helper::format (value));
      label.setRenderFlags(0);
      label.setLayoutAttribute(QwtText::MinimumLayout);
      label.textSize(painter->font ()); 

      if (label.isEmpty ()) return;

      const int tval = map ().transform (value);
      double startAngle = map ().p1 ();
      if ((tval > startAngle + 359 * 16) || (tval < startAngle - 359 * 16)) return;

      double radius = QwtRoundScaleDraw::radius ();
      if (hasComponent(QwtAbstractScaleDraw::Ticks) || hasComponent(QwtAbstractScaleDraw::Backbone)) radius += spacing (); 

      if (hasComponent(QwtAbstractScaleDraw::Ticks)) radius += majTickLength ();

      const QSize sz = label.textSize (painter->font ());
      const double arc = tval / 16.0 / 360.0 * 2 * M_PI;

      const int x = center ().x () + qRound ((radius + sz.width () / 2.0) * sin (arc));
      const int y = center ().y () - qRound ((radius + sz.height () / 2.0) * cos (arc));

      const QRect r(x - sz.width () / 2, y - sz.height () / 2, sz.width (), sz.height ());
      label.draw(painter, r);
    }
};

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Which is a cut/paste from the corrisponding method of QwtRoundScaleDraw, using decade_helper which is a tool to substitute zeroes with scale (0.03 becomes 30m) .
This solution is not very clean, but unfortunatelly there is no simple way to have ticks with other than doubles...
It works but I have some points with the total widget size.

[eto]

Ops I didn't see the cpuplot example... thanks to the other thread

Now I reimplemented it in the right way

class knob125_scale: public QwtRoundScaleDraw {
  public:
    virtual QwtText label(double v) const {
      return QwtText(decade_helper::format (v));
    }
};

class knob125_scale: public QwtRoundScaleDraw {
  public:
    virtual QwtText label(double v) const {
      return QwtText(decade_helper::format (v));
    }
};

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Good.