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

[Andrea Aime]

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

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