Prepares extensibility for the eventual case of supporting multi-planar
image layouts that are to be written into a multi-planar buffer as
indicated by the caller in some way (possibly not via read_*_bytes but a
method taking a wrapper around the output buffer instead). Through the
struct we can later add an indicator that can be used by the caller to
decide between methods of fetching the pixel data as appropriate.
There are two kinds of directories: those that are image directories
directly in the TIFF file's main sequence, and directories that follow
the structure but represent auxiliary data. The method will link the
directory into the main sequence but those images have additional
requirement on their IFDs which is not clear to the caller. We make that
explicit and also provide a way to write auxiliary directories.
This currently avoids buffering the collection, but not buffering them
individually where we still format it into a byte vector.
The main drive here is making the DirectoryEncoder less dependent on the
values. That is, it should be concerned with the Entry representation of
values only. A few decoders expect all values to strictly follow the
image directory and the data layout of such files should be controllable
by the user. This could be achieved by encoding dummy entries which are
later corrected once the actual item is known by seeking back and
writing the directory anew.
By no longer assuming values to be available on finishing a directory we
also prepare an interface that encodes an existing directory.
This is just the implementation details. In the DirectoryEncoder we did
not differentiate between different directories that should go into the
main image chain and other directories. This is now possible by storing
the prior offset as an explicit Option<NonZeroU64>.
Additionally, the whole encoder did not allow any intermediate data
between its last directory and new directories on the chain. The offset
of the relevant `next` field was not stored explicitly but derived from
the current stream position at the moment of starting the directory.
This would not allow any padding data or data written outside the
returned `DirectoryEncoder` object.
This relaxed both requirements by turning the offsets into attributes.
Having the value reader itself take care of the bigtiff and limit
semantics is a little cleaner, with the TagReader being responsible
for the interface of grabbing specific types of IFD values.
This allows us to recombine the reader and other file data in a Decoder
with any IFD, whether that of an image or an extension IFD as indicated
by an IFD pointer such as Exif and GPS.
The previous strategy could not keep track of all cycles such as
reflexive ones at the very start. Additionally, it would fail to be able
to iterate after seeking since it was overly cautious and verified the
next IFD position before allowing the read of a current IFD.
The new implementation is furthermore proof against having multiple
independent chains of IFD which may occur when iterating SubIFDs for
thumbnails. (Note that we do not verify the tree-structure, only a
freedom from cycles).
Aurelia Molzer