[gdal-dev] Cropping a raster by a polygon - only nodata outside polygon wanted

[Even Rouault]

Selon mathieu <mathieu.rajerison at gmail.com>:

> The problem is that I don't know want an alpha channel because some of my
> users work on MapInfo and MapInfo doesn't deal with alpha channels..

Really ??? Tell them to use QGIS instead. It does support alpha channel.

Otherwise try to find a nodata value that isn't a valid pixel value. But I'd
note that the nodata support in TIFF is a GDAL extended TIFF tag, so I wouldn't
be surprised if MapInfo doesn't support it either.

>
> 2015-01-14 11:39 GMT+01:00 Even Rouault-2 [via OSGeo.org] <
> ml-node+s1560n5181432h49 at n6.nabble.com>:
>
> > Mathieu,
> >
> > If 0 is a valid value at some locations of your raster, then you cannot
> > use it
> > as the nodata value. Use an alpha band instead for example by using
> > -dstalpha
> > instead of -dstnodata 0
> >
> > Even
> >
> > > Hello,
> > >
> > > I crop a 3 band raster by a polygon with the following formula :
> > > /gdalwarp -of GTiff -cutline polygon.shp -cl polygon input.tif
> > output.tif
> > > /
> > > But pixels outside my polygon are assigned the 0 value. I'd like them to
> > > have nodata
> > >
> > > If I put the -dstnodata 0 option :
> > > /gdalwarp -of GTiff -cutline polygon.shp -cl polygon input.tif
> > output.tif
> > > -dstnodata 0/, all the pixels in each of the 3 bands that contain the 0
> > > value will be turned to nodata, even if they are contained in my
> > polygon.
> > >
> > > Hence, 127 0 222 will become 127 nodata 222
> > > or 0 127 222 will become nodata 127 222
> > >
> > > How to put the nodata value only outside my polygon ?
> > >
> > > Thanks a lot in advance !
> > >
> > > Mathieu
> > >
> > >
> > >
> > > --
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