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

[Jukka Rahkonen]

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, 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-with-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? ECW driver does not seem to have any creation option for that but
perhaps it is not necessary.

-Jukka-