[gdal-dev] What does "per dataset alpha" mean?

[Even Rouault]

Andrea,

This relates to https://trac.osgeo.org/gdal/wiki/rfc15_nodatabitmask . 
Basically in the GDAL abstraction, each band can have its (possibly own 
dedicated) mask band. The flag "PER_DATASET ALPHA" means that the mask band is 
in fact shared by all bands, and it is an alpha band (some formats may have 
only a mask with binary 0/255 values). A "classic" alpha channel to say things 
otherwise. 

Like in the case of this TIFF file with an "unassoc-alpha" extrasample channel.

Even

> Hi,
> I'm looking at a geotiff file that has the following band information:
> 
> Band 1 Block=256x8 Type=Byte, ColorInterp=Red
>   Mask Flags: PER_DATASET ALPHA
> Band 2 Block=256x8 Type=Byte, ColorInterp=Green
>   Mask Flags: PER_DATASET ALPHA
> Band 3 Block=256x8 Type=Byte, ColorInterp=Blue
>   Mask Flags: PER_DATASET ALPHA
> Band 4 Block=256x8 Type=Byte, ColorInterp=Alpha
> 
> I'm not sure what a "mask with per dataset alpha" means.... if I had to
> venture a guess,
> it looks like each band has its own private extra information as a sidecar
> 8 bit band.
> (but if it's the case, why have a alpha band, with its own per dataset
> alpha?).
> 
> tiffinfo also just reports 4 samples per pixel:
> > tiffinfo /tmp/gwc_test.tiff
> 
> TIFFReadDirectory: Warning, Unknown field with tag 33550 (0x830e)
> encountered.
> TIFFReadDirectory: Warning, Unknown field with tag 33922 (0x8482)
> encountered.
> TIFFReadDirectory: Warning, Unknown field with tag 34735 (0x87af)
> encountered.
> TIFFReadDirectory: Warning, Unknown field with tag 34737 (0x87b1)
> encountered.
> TIFF Directory at offset 0x8 (8)
>   Image Width: 2048 Image Length: 1096
>   Bits/Sample: 8
>   Sample Format: unsigned integer
>   Compression Scheme: LZW
>   Photometric Interpretation: RGB color
>   Extra Samples: 1<unassoc-alpha>
>   Samples/Pixel: 4
>   Rows/Strip: 1
>   Planar Configuration: single image plane
>   Tag 33550: 0.008343,0.008343,0.000000
>   Tag 33922:
> 0.000000,0.000000,0.000000,492999.995534,5878229.148718,0.000000
>   Tag 34735:
> 1,1,0,7,1024,0,1,1,1025,0,1,1,1026,34737,22,0,2049,34737,7,22,2054,0,1,9102
> ,3072,0,1,32632,3076,0,1,9001 Tag 34737: WGS 84 / UTM zone 32N|WGS 84|
>   Predictor: none 1 (0x1)
> 
> Also wondering, if this "per dataset alpha" a GDAL specific extension, or
> is it part of the TIFF standard?
> Looking at the spec it might be related to the ExtraSamples tag, but I'm
> understanding how
> it related with the above band interpretation.
> 
> Cheers
> Andrea

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com