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

[Even Rouault]

Le mercredi 08 juillet 2015 12:46:27, Jukka Rahkonen a écrit :
> Hi,
> 
> Nodata values with lossy compression like JPEG, ECW, or JPEG2000 do not
> really work. I noticed that newest ECW SDK versions (>= 4) have support for
> using a transparency mask:
> 
> http://blog.hexagongeospatial.com/help-ecw-speckled-edges/
> 
> "Using the ECWJP2 SDK terminology, a transparency mask is implemented as an
> opacity band called AllOpacity."
> 
> 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

Driver: ECW/ERDAS Compressed Wavelets (SDK 5.0)
Files: ../gdrivers/data/stefan_full_rgba_ecwv3_meta.ecw
Size is 162, 150
Coordinate System is `'
Metadata:
  CLOCKWISE_ROTATION_DEG=0.000000
  COLORSPACE=RGB
  COMPRESSION_DATE=2013-04-04T09:20:03Z
  COMPRESSION_RATE_ACTUAL=3.165093
  COMPRESSION_RATE_TARGET=20
  FILE_METADATA_ACQUISITION_DATE=2012-09-12
  FILE_METADATA_ACQUISITION_SENSOR_NAME=Leica ADS-80
  FILE_METADATA_ADDRESS=2 Abbotsford Street, West Leederville WA 6007 
Australia
  FILE_METADATA_AUTHOR=Unknown
  FILE_METADATA_CLASSIFICATION=test gdal image
  FILE_METADATA_COMPANY=ERDAS-QA
  FILE_METADATA_COMPRESSION_SOFTWARE=python2.7/GDAL v1.10.0.0/ECWJP2 SDK 
v5.0.0.0
  FILE_METADATA_COPYRIGHT=Intergraph 2013
  FILE_METADATA_EMAIL=support at intergraph.com
  FILE_METADATA_TELEPHONE=+61 8 9388 2900
  VERSION=3
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0,  150.0)
Upper Right (  162.0,    0.0)
Lower Right (  162.0,  150.0)
Center      (   81.0,   75.0)
Band 1 Block=256x256 Type=Byte, ColorInterp=Red
  Description = Red
  Overviews: arbitrary
  Mask Flags: PER_DATASET ALPHA 
Band 2 Block=256x256 Type=Byte, ColorInterp=Green
  Description = Green
  Overviews: arbitrary
  Mask Flags: PER_DATASET ALPHA 
Band 3 Block=256x256 Type=Byte, ColorInterp=Blue
  Description = Blue
  Overviews: arbitrary
  Mask Flags: PER_DATASET ALPHA 
Band 4 Block=256x256 Type=Byte, ColorInterp=Alpha
  Description = AllOpacity
  Overviews: arbitrary

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)

Even

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