[gdal-dev] VRT KernelFilteredSource and overviews

[Even Rouault]

On mercredi 4 octobre 2017 11:11:28 CEST Rahkonen Jukka (MML) wrote:
> Hi,
> 
> I was playing with kernel filters in VRT and using this really applies
> sharpen filter on-the-fly when looking the image at native resolution with
> QGIS:
> 
> <VRTRasterBand dataType="Byte" band="1">
>     <ColorInterp>Red</ColorInterp>
>     <KernelFilteredSource>
>       <SourceFilename relativeToVRT="1">image.tif</SourceFilename>
>       <SourceBand>1</SourceBand>
>       <SourceProperties RasterXSize="12000" RasterYSize="12000"
> DataType="Byte" BlockXSize="12000" BlockYSize="1" /> <SrcRect xOff="0"
> yOff="0" xSize="12000" ySize="12000" />
>       <DstRect xOff="0" yOff="0" xSize="12000" ySize="12000" />
>         <Kernel normalized="0">
>           <Size>3</Size>
>            <Coefs>0 -1 0 -1 5 -1 0 -1 0</Coefs>
>   </Kernel>
>     </KernelFilteredSource>
>   </VRTRasterBand>
> 
> However, filter is not applied to overviews which means that image looks
> different when I zoom out. Is this the intended behavior?

I guess not from a user point of view. But looking at the code, I see:

/* -------------------------------------------------------------------- */
/*      For now we don't support filtered access to non-full            */
/*      resolution requests. Just collect the data directly without     */
/*      any operator.                                                   */
/* -------------------------------------------------------------------- */
    if( nBufXSize != nXSize || nBufYSize != nYSize )
    {
        return VRTComplexSource::RasterIO( nXOff, nYOff, nXSize, nYSize,
                                           pData, nBufXSize, nBufYSize,
                                           eBufType, nPixelSpace, nLineSpace,
                                           psExtraArg );
    }

(which means that over-sampling will also not apply the kernel)


So seems the issue is "known". Probably that documenting this limitation should be done for 
now.
Doc source is in frmts/vrt/vrt_tutorial.dox

Even


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