Moving TimeScaleDraw in real time

[Uwe]

For the QwtPlotMarker, since the position is always in middle of the graph canvas, I could use a custom QLabel that sit there and doesn't move..

Sure, but for this particular use case this doesn't make much sense as all what needs to be done is a plot item that draws a vertical line in the center of the canvas:

class MarkerItem: public QwtPlotItem
{
public:
    virtual void draw( QPainter *painter,
        const QwtScaleMap &, const QwtScaleMap &,
        const QRectF &canvasRect ) const
    {
        painter->setPen( ... );
        painter->drawLine( canvasRect.center.x(), canvasRect.top(),  canvasRect.center.x(), canvasRect.bottom() );
    }
};

class MarkerItem: public QwtPlotItem
{
public:
    virtual void draw( QPainter *painter,
        const QwtScaleMap &, const QwtScaleMap &,
        const QRectF &canvasRect ) const
    {
        painter->setPen( ... );
        painter->drawLine( canvasRect.center.x(), canvasRect.top(),  canvasRect.center.x(), canvasRect.bottom() );
    }
};

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Using widget overlays makes sense when the content of the overlay is moving fast and you want to avoid replots.

But before using just any widget for such a use case you should have a look at QwtWidgetOverlay, what is tailored for this situation. In opposite to regular widgets it calculates a widget mask to avoid full repaints ( = flushing the backing store ) of the plot canvas, when its content changes. In a powerful environment you won't see the difference, but f.e. when running the itemeditor remote X11 ( or on slow embedded devices with QWS ) example you will see a major difference.

Uwe

[Maximus2]

Hey Uwe,

Just wanted to update this.
I can move the graph in real-time successfully as shown here :
https://www.youtube.com/watch?v=I7C-GyJBzog

The graph is not lagging when the width of the graph is big enough and I'm happy with the performance.
The only thing that I would like to improve is when the width of the graph get smaller (windows resize or smaller resolution), the bottom axis get laggier as the graph get smaller. Not sure why this happen, seems to be related to the bottom axis getting too small. Maybe I need to put a minimumWidth on the graph or find another elegant solution for smaller graph.

On a side note, i'm using 6.1 inside Qt 5.4 and all is working fine, I had to modify qwt_transform.h and qwt_transform.cpp so it can compile under 5.4

qwt_transform.h:

static const double LogMin;
static const double LogMax;
//QT_STATIC_CONST double LogMin;
//QT_STATIC_CONST double LogMax;

static const double LogMin;
static const double LogMax;
//QT_STATIC_CONST double LogMin;
//QT_STATIC_CONST double LogMax;

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qwt_transform.cpp:

const double QwtLogTransform::LogMin = 1.0e-150;
const double QwtLogTransform::LogMax = 1.0e150;
//QT_STATIC_CONST_IMPL double QwtLogTransform::LogMin = 1.0e-150;
//QT_STATIC_CONST_IMPL double QwtLogTransform::LogMax = 1.0e150;

const double QwtLogTransform::LogMin = 1.0e-150;
const double QwtLogTransform::LogMax = 1.0e150;
//QT_STATIC_CONST_IMPL double QwtLogTransform::LogMin = 1.0e-150;
//QT_STATIC_CONST_IMPL double QwtLogTransform::LogMax = 1.0e150;

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Thanks for Qwt i'm enjoying it,
Max

[Uwe]

This has been fixed in the 6.1 SVN branch some time ago. There will be a Qwt 6.1.2 as soon as I succeeded to build Qt 5.4.0 on my box - unfortunately building Qt 5.4.0 is far from being an easy one.

Uwe

[Uwe]

O.k done: Qwt 6.1.2 is available see https://sourceforge.net/projects/qwt/files/qwt/6.1.2

Uwe