The AAT `ankr` anchor point table is an auxiliary table for `kerx`,
used to store anchor overrides in case the glyph itself does not
supply the needed anchors as control points. Among the fonts that
come pre-installed with MacOS 10.12.6, `ankr` is used by a handful
of non-Latin fonts such as “Myanmar MN”, “Devanagari Sangam MN”,
and “Arial HB”.
https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6ankr.html
Other metamorphosis types are not yet supported and will raise an error
upon decompilation. The TTX tool catches the error and continues to emit
a hexdump of the table contents, just as before this change.
Interestingly, this can handle the examples from the AAT specification
(which are part of the unit tests), and also most AAT fonts on my disk.
However, some other AAT fonts such as Apple Chancery cannot be decompiled.
The failure seems to be a general problem with how fonttools decompiles
AAT lookups of format 4, and unrelated to this present change.
In format 2 and 6, AAT lookups contain a binary search header with
the number of elements in the lookup table. Before this change, the
element count would also include the special trailing end-of-table
value that is required by the font format specification. However,
the binary search header should only count the actual elements
without the trailer.
Also, in the examples from the AAT specification, the special
end-of-table entries use 0xFFFF for glyph ID keys, and zeroes
for the filler values. Before this change, we had filled the
values with 0xFF bytes.
Before this change, the code assumed that all values of AAT lookups
get internally represented as strings, which is correct for GlyphID
values but not generally the case.
Also renaming the XML element from `Substitution` to `Lookup`
because AAT lookups have other uses beyond glyph substitutions.
Before this change, the decoder would silently remove "redundant" values
when decompiling AAT lookups. However, it is perfectly valid for a lookup
to map a glyph ID to itself, and also not all AAT lookups have glyph IDs as
their value range.
With AAT, the same lookup data structure can be used for various
types of values. In the morx table, the values are glyph IDs or
glyph classes, which both are encoded as 16-bit unsigned integers.
In other AAT tables, however, the values can be different data types
with different encodings. By passing a `valueWriter` callback and
explicit `valueSize`, we prepare for eventually templatizing
the building of AATLookups.
Also, assert that the called writer wrote the exact number of bytes
that was predicted when figuring out what format should be used for
encoding an AATLookup.
On some Python builds, Unicode characters above U+FFFF get escaped
to two question marks ?? instead of just one ?. This is not a real
problem, but for testing it is a nuisance when the results depend
on the platform.
This class had been added in the `morx` branch, and I wanted
to merge it into master. While writing tests for it, I noticed
that `otConverters` has meanwhile been extended by an `UInt8`
converter. Therefore, only adding a test for the existing
implementation.
I've got some changes in the `morx` branch which I'd like to merge
into master. So I'm writing this unit test as an example how to
test `otConverter` code.
When the tests are run with `python setup.py test` (e.g. from the ./run-tests.sh
script), we can't have setuptools install pre-compiled wheels for extra test
requirements (e.g. Brotli, unicodedata2).
Instead of failing, it's better that we skip the tests that have such extra
requirements.
NOTE: The recommended way to run the full test suite is by using tox
(like we do on the CI), or by installing pytest and the rests of requirements.txt,
and then run `pytest` command directly.
