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

[Tomaka, Jacek]

Jukka, Even, 
Here is an example of ECW v2 with opacity. 
https://trac.osgeo.org/gdal/ticket/6028

Regards.
Jacek Tomaka
-----Original Message-----
From: gdal-dev-bounces at lists.osgeo.org [mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of Even Rouault
Sent: Wednesday, 8 July 2015 8:08 PM
To: gdal-dev at lists.osgeo.org
Cc: Jukka Rahkonen
Subject: Re: [gdal-dev] Transparency mask (alpha band) in ECW

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-ec
> w-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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