<p dir="ltr">Ok thanks a lot for the answer! </p>
<div class="gmail_quote">Le 14 janv. 2015 13:25, "Even Rouault-2 [via OSGeo.org]" <<a href="/user/SendEmail.jtp?type=node&node=5181520&i=0" target="_top" rel="nofollow" link="external">[hidden email]</a>> a écrit :<br type="attribution"><blockquote style='border-left:2px solid #CCCCCC;padding:0 1em' class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Selon mathieu <<a href="http:///user/SendEmail.jtp?type=node&node=5181459&i=0" rel="nofollow" link="external" target="_blank">[hidden email]</a>>:
<br><br>> Is there a simple way to find one or several pixel value none of the bands
<br>> use ?
<br><br>Analyzing the histograms is the closest way I can think of.
<br><br>With "gdalinfo -hist in.tif" for example (but it is admitedly not completely
<br>trivial to find the index from the output)
<br>Or programatically with GetDefaultHistogram():
<br><a href="http://www.gdal.org/classGDALRasterBand.html#afeffe2ed8bfdc3d84200a57068ad0945" rel="nofollow" link="external" target="_blank">http://www.gdal.org/classGDALRasterBand.html#afeffe2ed8bfdc3d84200a57068ad0945</a><br><div><div class='shrinkable-quote'><br>>
<br>> Mathieu
<br>>
<br>> 2015-01-14 12:06 GMT+01:00 Even Rouault-2 [via OSGeo.org] <
<br>> <a href="http:///user/SendEmail.jtp?type=node&node=5181459&i=1" rel="nofollow" link="external" target="_blank">[hidden email]</a>>:
<br>>
<br>> > Selon mathieu <[hidden email]
<br>> > <<a href="http:///user/SendEmail.jtp?type=node&node=5181443&i=0" rel="nofollow" link="external" target="_blank">http:///user/SendEmail.jtp?type=node&node=5181443&i=0</a>>>:
<br>> >
<br>> > > The problem is that I don't know want an alpha channel because some of
<br>> > my
<br>> > > users work on MapInfo and MapInfo doesn't deal with alpha channels..
<br>> >
<br>> > Really ??? Tell them to use QGIS instead. It does support alpha channel.
<br>> >
<br>> > Otherwise try to find a nodata value that isn't a valid pixel value. But
<br>> > I'd
<br>> > note that the nodata support in TIFF is a GDAL extended TIFF tag, so I
<br>> > wouldn't
<br>> > be surprised if MapInfo doesn't support it either.
<br>> >
<br>> > >
<br>> > > 2015-01-14 11:39 GMT+01:00 Even Rouault-2 [via OSGeo.org] <
<br>> > > [hidden email] <<a href="http:///user/SendEmail.jtp?type=node&node=5181443&i=1" rel="nofollow" link="external" target="_blank">http:///user/SendEmail.jtp?type=node&node=5181443&i=1</a>>>:
<br>> >
<br>> > >
<br>> > > > Mathieu,
<br>> > > >
<br>> > > > If 0 is a valid value at some locations of your raster, then you
<br>> > cannot
<br>> > > > use it
<br>> > > > as the nodata value. Use an alpha band instead for example by using
<br>> > > > -dstalpha
<br>> > > > instead of -dstnodata 0
<br>> > > >
<br>> > > > Even
<br>> > > >
<br>> > > > > Hello,
<br>> > > > >
<br>> > > > > I crop a 3 band raster by a polygon with the following formula :
<br>> > > > > /gdalwarp -of GTiff -cutline polygon.shp -cl polygon input.tif
<br>> > > > output.tif
<br>> > > > > /
<br>> > > > > But pixels outside my polygon are assigned the 0 value. I'd like
<br>> > them to
<br>> > > > > have nodata
<br>> > > > >
<br>> > > > > If I put the -dstnodata 0 option :
<br>> > > > > /gdalwarp -of GTiff -cutline polygon.shp -cl polygon input.tif
<br>> > > > output.tif
<br>> > > > > -dstnodata 0/, all the pixels in each of the 3 bands that contain
<br>> > the 0
<br>> > > > > value will be turned to nodata, even if they are contained in my
<br>> > > > polygon.
<br>> > > > >
<br>> > > > > Hence, 127 0 222 will become 127 nodata 222
<br>> > > > > or 0 127 222 will become nodata 127 222
<br>> > > > >
<br>> > > > > How to put the nodata value only outside my polygon ?
<br>> > > > >
<br>> > > > > Thanks a lot in advance !
<br>> > > > >
<br>> > > > > Mathieu
<br>> > > > >
<br>> > > > >
<br>> > > > >
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