How to create QWT plot curve with gaps?

[kiriM]

I need to draw a line curve, which has "gaps" (or "holes").
Yes, I can create several plot curves, which won't be connected each other, but is there a more elegant solution?
Have I to override QwtPlotCurve?

[john_god]

I never used qwt, but i did this with opengl in the following way:

- wrote a function to find the zeros of the function (note that the gaps of f(x) are the zeros of 1/f(x))
- create a class with a array of x and y points to store the graph values, and a min and max value (the limits of the segment)
-create a array of that class (each element of the array will be a segment)
- put the array in a for and draw the graph

So I guess there is no elegant way, just use the segments, and if you find another away let us know

[Uwe]

Have I to override QwtPlotCurve?

Yes, QwtPlotCurve::drawLines().

If you don't need any optimizations its simply iterating over the points, translating ( QwtScaleMap ) them into pixel positions and painting them.

Uwe

[kiriM]

Thanks, guys!
I'll override drawLines().
But it's desirable to implement this feature in future versions of QwtPlot )
I'm surprised that nobody in this forum didn't run into this problem. I think it looks ugly when plot in missed points has a dip to X axis (when missed Y value is zero, for example).

[kiriM]

OK, I've created a QwtPlotCurve descendant, QwtPlotGappedCurve, with
virtual void draw(QPainter *p, const QwtScaleMap &xMap,
const QwtScaleMap &yMap, int from, int to) const
overridden and additional data member, gapValue.

Everybody interested in can see attachment.

Uwe, maybe it will be useful to include this class into Qwt package?

[Fractal]

Thanks, KiriM.

I had to implement some tweaks to get this to work in qwt-6.0.0 because the draw() function is different in 6.0.0.

I overrode QwtPlotCurve::drawSeries(), included the QRectF of the canvas in it's constructor, and used the d_deries->samples(i).y() data-values in the comparison to "gapValue_"

////////////////////////////////////////////////////////////////////////////////
void QwtPlotGappedCurve::drawSeries(QPainter *painter, const QwtScaleMap &xMap,
                                                          const QwtScaleMap &yMap, const QRectF &canvRect, int from, int to) const
{
	if ( !painter || dataSize() <= 0 )
		return;
 
	if (to < 0)
		to = dataSize() - 1;
 
	int i = from;
	while (i < to)
	{
        // If data begins with missed values,
        // we need to find first non-missed point.
            double y = d_series->sample(i).y();
            while ((i < to) && (y == gapValue_))
            {
                ++i;
                y = d_series->sample(i).y();
            }
        // First non-missed point will be the start of curve section.
            int start = i;
            y = d_series->sample(i).y();
        // Find the last non-missed point, it will be the end of curve section.
            while ((i < to) && (y != gapValue_))
            {
                ++i;
                y = d_series->sample(i).y();
            }
        // Correct the end of the section if it is at missed point
            int end = (y == gapValue_) ? i - 1 : i;
 
        // Draw the curve section
            if (start <= end)
                    QwtPlotCurve::drawSeries(painter, xMap, yMap, canvRect, start, end);
	}
}
 
////////////////////////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////////////////////////
void QwtPlotGappedCurve::drawSeries(QPainter *painter, const QwtScaleMap &xMap,
                                                          const QwtScaleMap &yMap, const QRectF &canvRect, int from, int to) const
{
	if ( !painter || dataSize() <= 0 )
		return;

	if (to < 0)
		to = dataSize() - 1;

	int i = from;
	while (i < to)
	{
        // If data begins with missed values,
        // we need to find first non-missed point.
            double y = d_series->sample(i).y();
            while ((i < to) && (y == gapValue_))
            {
                ++i;
                y = d_series->sample(i).y();
            }
        // First non-missed point will be the start of curve section.
            int start = i;
            y = d_series->sample(i).y();
        // Find the last non-missed point, it will be the end of curve section.
            while ((i < to) && (y != gapValue_))
            {
                ++i;
                y = d_series->sample(i).y();
            }
        // Correct the end of the section if it is at missed point
            int end = (y == gapValue_) ? i - 1 : i;

        // Draw the curve section
            if (start <= end)
                    QwtPlotCurve::drawSeries(painter, xMap, yMap, canvRect, start, end);
	}
}

////////////////////////////////////////////////////////////////////////////////

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[EliGri]

Thank you Fractal and KiriM. This subclass works great with Qwt6.0.2.

[Momergil]

Hello!

I'm trying to make this subclass work in my Qt 5.1.0 + Qwt 6.1.0 project, but without success. I added the change suggested by Fractal above, but the compiler gives me the error that "d_series" is private.

Any guess how to correct this?

Thanks,

Momergil

[Momergil]

Hello!

Since no one managed to help me, I created a new class able to do this in a different way:

//.h
#define QWTPLOTCURVESPECIAL_DEFAULT   -1
 
class QwtPlotCurveSpecial : public QwtPlotCurve
{
public:
    QwtPlotCurveSpecial(const QString &title = QString::null );
 
    void setPenList(QList<QPen>& pen);
    void setPosList(QList<char>& pos);
    virtual void drawLines(QPainter *p, const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &canvasRect, int from, int to) const;
 
private:
    QList<QPen> v_pen;
    QList<char> v_pos;
};
 
//cpp
QwtPlotCurveSpecial::QwtPlotCurveSpecial(const QString& title)
{    
    setTitle(title);
}
 
void QwtPlotCurveSpecial::setPenList(QList<QPen>& pen)
{
    v_pen = pen;
}
 
void QwtPlotCurveSpecial::setPosList(QList<char>& pos)
{
    v_pos = pos;
}
 
void QwtPlotCurveSpecial::drawLines(QPainter *p, const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &canvasRect, int from, int to) const
{
    for (int counter = 1; counter < v_pos.size(); counter++)
    {
        if (v_pos.at(counter) == -1)
            continue;
 
        p->setPen(v_pen[v_pos[counter]]);
 
        QwtPlotCurve::drawLines(p,xMap,yMap,canvasRect,counter-1,counter);
    }
}

//.h
#define QWTPLOTCURVESPECIAL_DEFAULT   -1

class QwtPlotCurveSpecial : public QwtPlotCurve
{
public:
    QwtPlotCurveSpecial(const QString &title = QString::null );

    void setPenList(QList<QPen>& pen);
    void setPosList(QList<char>& pos);
    virtual void drawLines(QPainter *p, const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &canvasRect, int from, int to) const;

private:
    QList<QPen> v_pen;
    QList<char> v_pos;
};

//cpp
QwtPlotCurveSpecial::QwtPlotCurveSpecial(const QString& title)
{    
    setTitle(title);
}

void QwtPlotCurveSpecial::setPenList(QList<QPen>& pen)
{
    v_pen = pen;
}

void QwtPlotCurveSpecial::setPosList(QList<char>& pos)
{
    v_pos = pos;
}

void QwtPlotCurveSpecial::drawLines(QPainter *p, const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &canvasRect, int from, int to) const
{
    for (int counter = 1; counter < v_pos.size(); counter++)
    {
        if (v_pos.at(counter) == -1)
            continue;

        p->setPen(v_pen[v_pos[counter]]);

        QwtPlotCurve::drawLines(p,xMap,yMap,canvasRect,counter-1,counter);
    }
}

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As you can see, in order to use this class, one have to crate a QList<QPen> with all the pen he want to use in the draw (useful for another situation, which is when somebody wants to draw a line with multiple colors) and a QList<char> containing which of the QPen he want to be used for each point. If hits QList<char> contains a -1 value, this is interpreted as not wanting to draw that point, so nothing is draw in that point.

Notice that its required that the QList<char> list has the same value as the samples set in setSamples(...), otherwise one would have a failure there.


Momergil

[osiris81]

Hi,

I'm using QWT 6.1.x.

just had the same problem and used KiriM's solution patched with Fractal's code and had the same errors as Momergil ("d_series" is private). I found out that by just replacing 'd_series->sample' with 'sample' it works.
If anyone needs a working solution with QWT >= 6.1.x, please use the attached files for the QwtPlotGappedCurve.

Cheers and thanks to kiriM and Fractal

Edit:

Since the value 0.0 is a valid value in my case I needed another value that marks a gap. The double value NAN is not possible because NAN != NAN. Since I'm not a fan of curious magic numbers I simply use 1.0/0.0 (inf) as the gap value