[gdal-dev] GDALWarp API and paletted images

[Even Rouault]

Selon Thomas Sevaldrud <thomas at silentwings.no>:

> Great, thanks!
>
> I tried this, but got only a red image as result, so I guess only the first
> channel was used.
>
> This is the relevant code, where _ds is the input paletted data set
>
> vrtDS = vrtDriver->Create("", origW, origH, 3, GDT_Byte, NULL);
> double geoTransform[6];
> _ds->GetGeoTransform(geoTransform);
> vrtDS->SetGeoTransform(geoTransform);
> GDALSetProjection(vrtDS, _srcProjectionWKT.c_str());
>
> GDALRasterBand* srcBand = _ds->GetRasterBand(1);
>
> vrtDS->AddBand(GDT_Byte, NULL);
> vrtDS->AddBand(GDT_Byte, NULL);
> vrtDS->AddBand(GDT_Byte, NULL);

You don't need the above 3 AddBand() since you called Create() with 3 already

> VRTSourcedRasterBand* vrtRBand =
> (VRTSourcedRasterBand*)vrtDS->GetRasterBand(1);
> VRTSourcedRasterBand* vrtGBand =
> (VRTSourcedRasterBand*)vrtDS->GetRasterBand(2);
> VRTSourcedRasterBand* vrtBBand =
> (VRTSourcedRasterBand*)vrtDS->GetRasterBand(3);
> vrtRBand->AddComplexSource(srcBand, -1, -1, -1, -1, -1, -1, -1, -1, 0.0,
> 1.0, VRT_NODATA_UNSET, 1);
> vrtGBand->AddComplexSource(srcBand, -1, -1, -1, -1, -1, -1, -1, -1, 0.0,
> 1.0, VRT_NODATA_UNSET, 2);
> vrtBBand->AddComplexSource(srcBand, -1, -1, -1, -1, -1, -1, -1, -1, 0.0,
> 1.0, VRT_NODATA_UNSET, 3);
>
> Is there anything I have missed in the setup here?

Looks good otherwise. Did you check that you configured properly the warper to
use the 3 bands of the VRT ?
>
> - Thomas
>
>
>
> On Tue, Jun 9, 2015 at 1:44 PM, Even Rouault <even.rouault at spatialys.com>
> wrote:
>
> > Thomas,
> > >
> > > I'm using the GDALWarp api from C++ to reproject and cut various input
> > > images. In general this works very well for my purposes, except that for
> > > paletted images I have to use NearestNeighbour resampling,
> > >
> > > I would like to use Bilinear or higher order resampling, and wonder if
> > > there is any way to expand a paletted image to true RGB during the warp
> > > process, so I can use these resampling methods.
> >
> > No, you must use an intermediate step before.
> >
> > >
> > > Alternatively, is there a way to do this conversion (from code) before
> > the
> > > warp without having to run through the entire image in memory?
> >
> > You can use a in-memory VRT dataset to do on-the-fly expansion to RGB.
> >
> > With the AddComplexSource() method of VRTSourcedRasterBand class:
> >
> >     CPLErr         AddComplexSource( GDALRasterBand *poSrcBand,
> >                                      int nSrcXOff=-1, int nSrcYOff=-1,
> >                                      int nSrcXSize=-1, int nSrcYSize=-1,
> >                                      int nDstXOff=-1, int nDstYOff=-1,
> >                                      int nDstXSize=-1, int nDstYSize=-1,
> >                                      double dfScaleOff=0.0,
> >                                      double dfScaleRatio=1.0,
> >                                      double dfNoDataValue =
> > VRT_NODATA_UNSET,
> >                                      int nColorTableComponent = 0);
> >
> > Set nColorTableComponent to 1,2,3 for respectively R,G,B. This is what
> > gdal_translate will do if you use "gdal_translate -of VRT -expand rgb
> > pct.tif
> > out.vrt"
> >
> > Even
> >
> > --
> > Spatialys - Geospatial professional services
> > http://www.spatialys.com
> >
>


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