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

[mathieu]

Is there a simple way to find one or several pixel value none of the bands
use ?

Mathieu

2015-01-14 12:06 GMT+01:00 Even Rouault-2 [via OSGeo.org] <
ml-node+s1560n5181443h90 at n6.nabble.com>:

> Selon mathieu <[hidden email]
> <http:///user/SendEmail.jtp?type=node&node=5181443&i=0>>:
>
> > 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] <
> > [hidden email] <http:///user/SendEmail.jtp?type=node&node=5181443&i=1>>:
>
> >
> > > 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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