Synchronizing zoom settings between QwtPlotZoomer and QwtPlotMagnifier for a touch

[bob2oneil]

Synchronizing zoom settings between QwtPlotZoomer and QwtPlotMagnifier for a touchscreen interfaceplot.jpg

I have an application which uses a class derived from
QwtPlot. It is for a touchscreen interface, so separation
of events associated with a mouse (such as left, middle, or
right buttons or scroller) do not apply. See the attached
screenshot for some idea of the layout and control buttons for the plot.

I need to provide support for three types of zooming, roughly
analogous in behavior of QwtPlotZoomer, QwtPlotPanner, and
QwtPlotMagnifier.

There are three operating modes for zooming based on the control
button selected from the UI, which are from left to right in the
screenshot:

Zoom Mode
Pan Mode
Zoom Extent
Zoom In
Zoom Out

The zoom "mode" is selected by either the Zoom or Pan mode buttons
which are mutually exclusive. These seem to work fine, I can zoom
in with the rectangle and pan with the mouse. The setEnabled methods
on these classes allows for mutually exclusive behavior.

Now my question has to do with how to implement zoom in and zoom out
in concert with the existing zoom value from using the zoom rectangle,
that is keeping these settings somewhat synchronized.

How can I determine the zoom factor that is currently active as a
function of using the zoom rectangle as the beginning point or scale
factor to use when selecting either Zoom In or Zoom Out?
I need to be able to zoom in and out from this active setting.

Since the rescale() method of QwtPlotMagnifier is private, I assume
I need to derive from this class and create a method for my implementation,
and perhaps customize its behavior.

Anybody try something like this or have any general direction?

[Uwe]

How can I determine the zoom factor that is currently active as a function of using the zoom rectangle as the beginning point or scale factor to use when selecting either Zoom In or Zoom Out?

The zoomer has the zoom stack, what is the list of rectangles in the zoom history. The zoom base ( the first entry in this list ) is your "beginning point". If you want to have the rectangles of the panning in the zoom stack ( zoom forward/backward ) too you have to manipulate the stack manually.

Don't use QwtPlotMagnifier as long as you are not interested in its wheel and mouse handling. Instead implement your own scaling using QwtPlot::axisScaleDiv() and QwtPlot::setAxisScale(). It makes sense to copy the code of QwtPlotMagnifier::rescale to your application - it doesn't depend on the rest of the class.

Uwe

[bob2oneil]

Hi Uwe, I have a rescale copied from the QwtPlotMagnifier class in place. My implementation for ZoomIn and ZoomOut are as follows. My intention was to keep a scale factor, and basically change by 10% for zoom in/out. What I am noticing in practice with this approach, is that the zoom does not behave as I would expect. For example, if I zoom in 4 times, and zoom out 4 times, I am not often back to where I begin. This is exclusive of the zoom rectangle implementation. Perhaps this approach is not the best. Any thoughts on improving this approach?

void Plot::zoomIn()
{
    m_scaleFactor -= .1;
    if (m_scaleFactor < 0)
        m_scaleFactor = 0;
 
    rescale(m_scaleFactor);
}
 
void Plot::zoomOut()
{
    m_scaleFactor += .1;
    //if (m_scaleFactor > 1.0)
    //    m_scaleFactor = 1.0;
 
    rescale(m_scaleFactor);
}

void Plot::zoomIn()
{
    m_scaleFactor -= .1;
    if (m_scaleFactor < 0)
        m_scaleFactor = 0;

    rescale(m_scaleFactor);
}

void Plot::zoomOut()
{
    m_scaleFactor += .1;
    //if (m_scaleFactor > 1.0)
    //    m_scaleFactor = 1.0;

    rescale(m_scaleFactor);
}

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[bob2oneil]

I have a working implementation inspired by Uwe's QMagnifier code base as follows where the zoom factor correspond to the keyFactor in the magnifier class. My personal thanks to Uwe for all of the fine work done on QWT. This type of generous contributions of time and effort to the broader Qt community is often not recognized enough. I can only imagine the amount of many hours/years devoted to Qwt and that of all the support in this forum on Qwt.

void Plot::zoomIn()
{
    rescale(m_zoomFactor);
}
 
void Plot::zoomOut()
{
    rescale(1.0 / m_zoomFactor);
}

void Plot::zoomIn()
{
    rescale(m_zoomFactor);
}

void Plot::zoomOut()
{
    rescale(1.0 / m_zoomFactor);
}

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