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 _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
