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

mathieu mathieu.rajerison at gmail.com
Wed Jan 14 08:30:13 PST 2015 

Ok thanks a lot for the answer!
Le 14 janv. 2015 13:25, "Even Rouault-2 [via OSGeo.org]" <
ml-node+s1560n5181459h86 at n6.nabble.com> a écrit :

> Selon mathieu <[hidden email]
> <http:///user/SendEmail.jtp?type=node&node=5181459&i=0>>:
>
> > Is there a simple way to find one or several pixel value none of the
> bands
> > use ?
>
> Analyzing the histograms is the closest way I can think of.
>
> With "gdalinfo -hist in.tif" for example (but it is admitedly not
> completely
> trivial to find the index from the output)
> Or programatically with GetDefaultHistogram():
>
> http://www.gdal.org/classGDALRasterBand.html#afeffe2ed8bfdc3d84200a57068ad0945
>
> >
> > Mathieu
> >
> > 2015-01-14 12:06 GMT+01:00 Even Rouault-2 [via OSGeo.org] <
> > [hidden email] <http:///user/SendEmail.jtp?type=node&node=5181459&i=1>>:
>
> >
> > > 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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