[gdal-dev] Transparency mask (alpha band) in ECW

Even Rouault even.rouault at spatialys.com
Wed Jul 8 05:07:48 PDT 2015

Le mercredi 08 juillet 2015 14:01:47, Jukka Rahkonen a écrit :
> Even Rouault <even.rouault <at> spatialys.com> writes:
> > > GDAL ECW driver does not have support for this. I wonder if it could be
> > > doable and if I should make a feature request. I also wonder if GDAL
> > > can decode ECW files which have transparency mask correctly.
> > 
> > Jukka,
> > 
> > I'm a bit confused by the above. If you look to the bottom of the page
> > you mentio,n you'll see a gdalinfo report with an alpha band.
> > 
> > And I've just tried with a test file of the autotest suite:
> > 
> > gdalinfo ../gdrivers/data/stefan_full_rgba_ecwv3_meta.ecw
> ....
> > This is a ECWv3 file. I don't have an example with ECWv2 and the
> > "appended" mask, but I believe the driver should also likely be able to
> > read/write this (but I haven't a ECWv2 image with alpha handy)
> Hi Even,
> I apologize,

No problem

> I do not use ECW myself nowadays but I was reading this
> gis.stackexchange question which deals actually with FME
> http://gis.stackexchange.com/questions/153490/how-to-convert-tif-to-ecw-wit
> h-fme-maintaining-a-certain-value?noredirect=1#comment225797_153490
> Then I read our ECW manual page http://www.gdal.org/frmt_ecw.html which
> does not talk anything about nodata, alpha, transparency etc. I just
> thought that we can't handle nodata any better than FME but obviously we
> can. I do not have SDK license so I could not test ECW output myself.
> Is the ECW transparency band created automatically if user has Read/Write
> SDK >= 4.0 and source data has either alpha channel or nodata set in
> GeoTIFF?

As far as I understand the code, yes that should be automatic if the 4th band 
of the source raster is advertized as an Alpha channel.

> ECW driver does not seem to have any creation option for that but
> perhaps it is not necessary.
> -Jukka-
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