Re: Help with understanding QwtRasterData needed

On Wednesday 16 April 2008 15:54, Matthias Pospiech wrote:

> In my problem the data however has to be delivered to the plot as a
> double array (2D Array).

QwtRasterData ( similar to QwtData ) is the link between the application data 
and the QwtRasterItem ( guess here a spectrogram ), that is responsible for 
displaying the data. 
When the raster item needs to render its image it calculates the position of 
each pixel and asks the raster data object for a value at this position. This 
value is mapped into a color using the value range, that is also provided by 
the data object. 

You need to implement a raster data object, that can access your 2d array:

a) YourRasterData::range()

If you have a color map from blue to red you also need to define which value 
means blue and which means red. Values above or below are mapped to the 
borders of the color map. Range is what we call "Wertebereich" in German.

b) YourRasterData::value()

Here you have to return the value at a specific position. Usually your 2D 
array will have a different raster, so you have to do some resampling here ( 
f.e. next neighbour ).

The following methods are optional, but might speedup the process of rendering 
a lot:

c) YourRasterData::initRaster()

The item tells the data object in advance about the raster it is going to 
render. F.e. if you have a huge number of samples in a file, you can resample 
and load it into memory here.

d) YourRasterData::discardRaster()

The item tells the data object, that the rendering of the image is complete. 
So you can do some cleanups here.

e) YourData::rasterHint

Here you can return the resolution of your 2D matrix and limit the resolution 
of the image, that is rendered. ( There is no need to render an image in a 
higher resolution than the data has ). On screen this is not so important, 
but for printing ( f.e 1200dpi on a A0 plotter ) it can be substantial.

Uwe

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