In particular we avoid mistaking the absence of this tag for an absence
of subsampling. The specification instead requires it to mean half the
samples in both directions—a mode that we do not support yet. Instead of
returning erroneous samples or an error in the data stream (which would
appear too short) we return an error when asked to layout the read
buffer for these images.
We previously handed it a wrapper that was supposedly type-safe but
really internally cast the buffer to bytes. That was rather pointless as
the `_bytes` method could always be used instead. We replace the
semantics by a utility which allocates an appropriate buffer with the
internal limits, wrapping the exact sequence identified previously in
the example as a pattern.
By adding the type interpretation to `BufferLayoutPreference` (simply
computed from the same fields that are trivial to fetch in
`ReadoutLayout` with the rest) we can allocate an appropriate
`DecoderResult` without any `Decoder` instance. We still want a Limits
instance though. This lets a caller read _typed_ data from the `_bytes`
methods which perform less allocations. It also lets them read multiple
planes in a more convenient interface. See, `read_image` will keep its
planar treatment bug for a little bit so you don't accidentally break
during an upgrade.
This also implies less code is being monomorphized, just a bonus.
This is well-defined even if we add sub-sampling support later on. We
can adjust the layout type accordingly with factors per each plane. Note
that the layout, in theory, does not depend on the index of the unit if
we describe all planes for a multi-planar layout. The specification
requires that the subsampling factors divide the tile sizes. However we
have to handle non-planar images here and this leaves us open to other
potential parameters.
We currently have one type in decoder/image.rs and another in
decoder/mod.rs which each contributes part of the computation for laying
out the output buffer. Confusingly though they rely on shared
implementation details such as determining the number of channels that
are written and their bit depth. We should have one shared method and
resulting struct instead.
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 idea is to allow codegen to exploit specific known constants. It
seems that LLVM by itself will not generate split loop blocks by an
input argument that is a loop constant. Whereas we know, from PNG, that
some constants yield much better code. In this case
coef[i] = coef[i] + coef[i - n]
This has a destructive dependency chain for `n = 1`, should never get an
argument of 0 and is almost embarrassingly parallel in SIMD if n >= 8
where we can increase the amount of data loaded at once for each
independent loop iteration.
By splitting the loop we make the compiler apply independent
optimization passes to each of case, then have a fallback for things we
did not cover with vectorized possibilities.
Currently, when getting the LZWStatus::NoProgress from the underlying
weezl library, tiff checks that no input bytes were read and no
output bytes were written. Although this is according to what the
weezl library mentions in their doc-comment, it's not what the real
result always looks like.
Our fuzzer found input that triggered exactly this case, where the LZW
reader is able to write some output byte before hitting the NoProgress
status.
This change removes the assert, as the resulting action is anyways to return
an error that can be handled by the caller.
A. Molzer