Qwt Interpolation

[Mr_Cloud]

Hi,

I am aware that Qwt features bilinear interpolation as a visualisation feature. It looks really nice, especially in array plotting, but can it also be accessed and modified or saved as an array? And can the user specify the number of iterations in the interpolation?

Thank you.


Regards,
Mr_Cloud

[Uwe]

I am aware that Qwt features bilinear interpolation as a visualisation feature. It looks really nice, especially in array plotting, but can it also be accessed and modified or saved as an array? And can the user specify the number of iterations in the interpolation?

You can call QwtMatrixData::value() from application code too and of course you can do with the result whatever you want - f.e to store it in a array.
But even if the API is not o.k. for you - there is not much mystery behind this algo.

Uwe

[Mr_Cloud]

You can call QwtMatrixData::value() from application code

Yes of course, but that just returns values from the input matrix I assigned in the first place. I tried returning the valueMatrix after doing setResampleMode(), but that still has the same matrix values with no interpolation data. If I wish to plot a simple [1,2;3,4] matrix, is the interp data between these points available to the user in Qwt?

Thanks.


Regards,
Mr_Cloud

[Uwe]

Yes of course, but that just returns values from the input matrix I assigned in the first place.

No in BilinearInterpolation mode it interpolates, when you ask for coordinates inside of its bounding rectangle. Qwt is open source - check the code when you don't believe it.

Uwe

[Mr_Cloud]

Dear Uwe, could you please give a brief example on how to retrieve interpolated data?

Thanks in advance.


Regards,
Mr_Cloud

[Uwe]

double x1 = matrixRasterData.interval( Qt::XAxis ).minValue();
double x2 = matrixRasterData.interval( Qt::XAxis ).maxValue();
double y1 = matrixRasterData.interval( Qt::YAxis ).minValue();
double y2 = matrixRasterData.interval( Qt::YAxis ).maxValue();
 
if ( x2 < x1 )
    qSwap( x1, x2 );
if ( y2 < y1 )
    qSwap( y1, y2 );
 
const double dx = ( x2 - x1 ) / n;
const double dy = ( y2 - y1 ) / m;
 
for ( double x = x1; x <= x2; x += dx )
{
    for ( double y = y1; y <= y2; y += dy )
    {
        const double valueXY = matrixRasterData( x, y );
        ...
    }
}

double x1 = matrixRasterData.interval( Qt::XAxis ).minValue();
double x2 = matrixRasterData.interval( Qt::XAxis ).maxValue();
double y1 = matrixRasterData.interval( Qt::YAxis ).minValue();
double y2 = matrixRasterData.interval( Qt::YAxis ).maxValue();

if ( x2 < x1 )
    qSwap( x1, x2 );
if ( y2 < y1 )
    qSwap( y1, y2 );

const double dx = ( x2 - x1 ) / n;
const double dy = ( y2 - y1 ) / m;

for ( double x = x1; x <= x2; x += dx )
{
    for ( double y = y1; y <= y2; y += dy )
    {
        const double valueXY = matrixRasterData( x, y );
        ...
    }
}

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HTH,
Uwe

[Mr_Cloud]

Brilliant! I see what you mean now Uwe, QwtMatrixData::value() accepts double x and double y as parameters! Thank you, it did help indeed.


Regards,
Mr_Cloud

[Uwe]

x,y are plot coordinates - not an index for a element of the matrix.

Uwe

PS: I have fixed the code snippet in my previous answer - the increments in the loop need to be dx and dy

[Mr_Cloud]

Yep, I realized what you meant and got the code working well. Now I can specify the number of iterations in my output matrix by simply varying dx and dy. Thanks!