Interpolation of data in Qwt Raster Matrix Data

[Uwe]

Am I missing a point here and next neighbour interpolation in QwtRasterMatrixData isn't used for smoothing ... ?

Next neighbour is not interpolating between values by definition: see https://en.wikipedia.org/wiki/Bicubic_interpolation ( the rasterview example shows exactly the data given in the article ).

There is only one algo implemented, that interpolates between data point: bilinear interpolation ( see QwtMatrixRasterData::BilinearInterpolation ).
For more advanced methods you would have to overload QwtRasterData::value() and write some code - but this is usually a trade of between performance and the quality of the interpolation.

Uwe