Help understanding QWT Contour Plot

[Uwe]

1. So given the scenario of my problem (having only about 20 data points in real world), what I should do is implement some sort of contour algorithm like CONREC or Perlin Noise to fill a large enough array of data points based on my original 20 data points.

A contour plot shows border line between areas with values above/below a threshold. But a point is no area and 20 points somewhere in space will never result in a useful contour plot ! This has nothing to do with Qwt or the algorithm that is used for contouring. So before you continue better try to find out where the areas are you want to display and if your points are raster data at all !

2. Or if I when created my database use the screen coordinates for each of my data point instead of the real world coordinates then the sizes and coordinates here are the widget coordinates. Correct?

No.

3. My covered area may change in size from time to time. But when I try to reset the bounding rect (after it's been initialized to some size), nothing happens. I did it like this:

myRasterData->setBoundingRect(QwtDoubleRect(-new_xlen/2, -new_ylen/2, new_xlen, new_ylen);
myPlot->replot();

The image of the spectrogram is cached. If you change something relevant behind the back of the spectrogram item you have to invalidate the cache ( spectrogram->invalidateCache() ) before you replot.

Uwe

[jwieland]

Hi Uwe,

I almost figure it out now! There still some problem I need your help with: Since our app is constantly monitoring data for changes and update the contour plot whenever it does, we need to update the plot very quite often. However, when I updated the data and try to call replot() to update the plot, nothing happen! The plot is still showing old data.

What am I missing that results in this plot not refreshing? What is the proper way to do this?

Thank you so much for your help.

[Uwe]

However, when I updated the data and try to call replot() to update the plot, nothing happen! The plot is still showing old data.

Read my previous answer.

Uwe

[jwieland]

Hi Uwe,

Sorry but that didn't work. Taking the spectrogram example and modified it, here is what I did:
SpectrogramData( ... ) : QwtRasterData( ... )), ... {
...
val = 0.2;
}


virtual double value(double x, double y) const
{
// do some calculation here to figure data at x, y
...

return val;
}

double val;
...


And then I tried changing the value of val variable and call the invalidateCache() before replot() to no avail!

Any idea what could went wrong?

[Uwe]

You forgot to adjust SpectrogramData::copy(). Note, that when this operation does a deep copy you need to rassign ( spectrogram->setData()) your data whenever you change the original.

Uwe

[jwieland]

Thanks, Uwe. You are a life saver!!!

It all works out now. wonderful!@

[OverTheOCean]

jwieland,

could you post your code to plot contours of yours data ???

thanks,

Michael