These samples are normal bits in the encoded (e.g. compressed) stream of
each tile or row of the image. However, when considering the photometric
interpretation of an image these samples should be discarded. They are
only relevant for multiband.
For the most part we previously rejected these images due to a mismatch
between sample count and the color as indicated by the interpretation.
Only, for alpha we never really verified if the extra channel is indeed
alpha or something unrelated. We don't have a perfect solution as
GeoTiff motivates us having a MultiBand pseudo-color output
configuration that has no related photometric interpretation and borrows
the usual gray tone instead. We can only really distinguish this from a
gray-alpha image through looking at the values of the ExtraSamples
vector. Then again we do not yet support GrayAlpha anyways.
As a simplification for the discarding process we only support full-byte
extra samples and require all samples to be the same bit-depth, not only
those which we return. This is blessed by Tiff's recommendations for
baseline decoding as well as libtiff.
The test files in this directory mostly have dimension 157x151.
The naming convention is
photometric-channels-bits.tiff
minisblack-1c-16b.tiff
minisblack-1c-8b.tiff
miniswhite-1c-1b.tiff
palette-1c-1b.tiff
palette-1c-4b.tiff
palette-1c-8b.tiff
rgb-3c-16b.tiff
rgb-3c-8b.tiff
logluv-3c-16b.tiff: logluv compression/photometric interp
minisblack-2c-8b-alpha.tiff: grey+alpha
BMP files (anchient BMPv2 since v3 does not work):
palette-1c-8b.bmp
rgb-3c-8b.bmp
GIF files (anchient GIF '87 since modern GIF does not work):
palette-1c-8b.gif
PNM files:
minisblack-1c-8b.pgm
miniswhite-1c-1b.pbm
rgb-3c-8b.ppm
A. Molzer
8517bb81db
