scale widget with major tick in the middle

[dlipsa]

Hi,
I am trying to have a ScaleWidget that has a Major tick in the middle of
the interval for the scale. What is the best way to achieve this?

I tried:



QwtLinearScaleEngine scaleEngine;
scaleEngine.setAttribute (QwtScaleEngine::IncludeReference);
scaleEngine.setAttribute (QwtScaleEngine::Floating);
scaleEngine.setReference ((interval.minValue () + interval.maxValue
()) / 2);
scaleEngine.setMargins (0, 0);
scaleWidgetColorBar->setScaleDiv (
scaleEngine.transformation (),
scaleEngine.divideScale (
interval.minValue (), interval.maxValue (), 3, 10));


However, IncludeReference and Floating attributes don't seem to have any
effect.

Thanks a lot,
Dan

[Uwe]

I am trying to have a ScaleWidget that has a Major tick in the middle of the interval for the scale.

QList<double> ticks[QwtScaleDiv::NTickTypes];
....
ticks[QwtScaleDiv::MajorTick] += interval.minValue () + interval.maxValue()) / 2);
 
plot->setAxisScaleDiv(..., QwtScaleDiv(interval, ticks);

QList<double> ticks[QwtScaleDiv::NTickTypes];
....
ticks[QwtScaleDiv::MajorTick] += interval.minValue () + interval.maxValue()) / 2);

plot->setAxisScaleDiv(..., QwtScaleDiv(interval, ticks);

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HTH,
Uwe

[dlipsa]

Hi Uwe,
Thanks for your answer.
I am trying to use a a QwtScaleWidget by itself, without a plot. I have tried something similar with what you sugested, but that did not produce any ticks.


Here is the code I have. The version using a LinearScaleEngine produces major ticks but the middle one is not centered. The commented code did not produce any ticks.

Do you have any ideas or suggestions?
Thanks a lot!

QwtLinearScaleEngine scaleEngine;
scaleEngine.setAttribute (QwtScaleEngine::IncludeReference);
scaleEngine.setAttribute (QwtScaleEngine::Floating);
scaleEngine.setReference (
(interval.minValue () + interval.maxValue ()) / 2);
scaleEngine.setMargins (0, 0);
scaleWidgetColorBar->setScaleDiv (
scaleEngine.transformation (),
scaleEngine.divideScale (
interval.minValue (), interval.maxValue (), 3, 10));

/*
QwtScaleDiv scaleDiv;
QwtValueList tickList;
tickList.append ((interval.minValue () + interval.maxValue ()) / 2);
scaleDiv.setInterval (interval);
scaleDiv.setTicks (QwtScaleDiv::MajorTick, tickList);
scaleWidgetColorBar->setScaleDiv (
new QwtScaleTransformation (QwtScaleTransformation::Linear), scaleDiv);
*/
setupBlueRedColorMap (&m_colorMap);
scaleWidgetColorBar->setColorMap (interval, m_colorMap);

[Uwe]

The scale engine calculates a QwtScaleDiv, but of course you have to do something with it:

...
QwtScaleDiv scaleDiv = scaleEngine.divideScale(...);
QwtValueList majorTicks = scaleDiv.ticks[QwtScaleDiv::MajorTick];
majorTicks += interval.minValue () + interval.maxValue()) / 2);
scaleDiv.setTicks(QwtScaleDiv::MajorTick, majorTicks);
scaleWidgetColorBar->setScaleDiv (..., scaleDiv);

...
QwtScaleDiv scaleDiv = scaleEngine.divideScale(...);
QwtValueList majorTicks = scaleDiv.ticks[QwtScaleDiv::MajorTick];
majorTicks += interval.minValue () + interval.maxValue()) / 2);
scaleDiv.setTicks(QwtScaleDiv::MajorTick, majorTicks);
scaleWidgetColorBar->setScaleDiv (..., scaleDiv);

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Of course you can compose your QwtScaleDiv object manually without using a scale engine as well.

Uwe

[dlipsa]

Great. Thanks a lot!
The following code does exactly what I want: get a scale with the min, max and middle of the interval marked.

QwtLinearScaleEngine scaleEngine;
QwtScaleDiv scaleDiv = scaleEngine.divideScale (
interval.minValue (), interval.maxValue (), 0, 0);
QwtValueList majorTicks;
majorTicks += (interval.minValue () + interval.maxValue()) / 2;
majorTicks += interval.minValue ();
majorTicks += interval.maxValue ();
scaleDiv.setTicks(QwtScaleDiv::MajorTick, majorTicks);
scaleWidgetColorBar->setScaleDiv (scaleEngine.transformation (), scaleDiv);


Do you have any suggestions on how to get rid of the scale engine? It seems strange to use the scale engine just to get no division. However that was
the only was I could get divisions on the scaleDiv. Thanks again!
Dan

[dlipsa]

Hi Uwe,
Another related question is: How do tell the LinearScaleEngine to start the minor ticks in the middle of the scale?
I thought QwtScaleEngine::IncludeReference should work, but unfortunately it did not work fo me. (see the above code)

Thanks a lot!