Agenda+ topic for WG call on May 18

Topic: WOFF2 handling of simple glyph flags (See
https://github.com/google/woff2/issues/117 for details.)

 

Background: 

Simple glyph flags define the format of x/y coordinate vectors for each
point of the glyph outline. In WOFF2, the pre-processing step converts both
the flags and the coordinates themselves into different data streams. Six
different flags (bits 0-5) can be set to define on/off-curve point and the
format for each set of point coordinates, flags can be shared among multiple
point coordinates. 

 

The bit 6 flag (OVERLAP_SIMPLE) is different, it can only be set once on the
first flag of the glyph. If set, it indicates that contours of the glyph
_may_ overlap. The OpenType spec doesn't require this flag be set, and some
(but not all) rasterizers would ignore this bit. Bit7 is reserved / unused.
(See description of simple glyph flags for details:
https://docs.microsoft.com/en-us/typography/opentype/spec/glyf)

 

Some years ago (in 2011-2014 timeframe, I wasn't able to find any specific
references to the discussion about flags) a decision was made that since
flag bit6 is not required and bit 7 is reserved, they can simply be omitted
when the source data is encoded, and set to '0' upon reconstruction of glyph
data. Apparently, this has caused the loss of data for flag bit 6 in some
fonts that have glyphs with overlapping contours where this bit 6 is set,
and it's an issue for some rasterizers that still rely on flag bit 6 setting
to deal with contour overlaps.

 

Proposed solution:

WOFF2Header (https://www.w3.org/TR/WOFF2/#woff20Header) has Uint16 field
reserved. One of these 16 bits can be redefined to be used as a flag
indicating presence of the new data stream, which can be used to encode
overlap flag (bit 6) values.

 

Since bit 6 flag can only be set once per glyph, a new numGlyphs-long bit
array can be defined and appended in the end of the WOFF2 transformed 'glyf'
table (https://www.w3.org/TR/WOFF2/#glyf_table_format) to encode overlap
flag values for simple glyphs. (We are in good shape for composite glyph
flags since they are part of the compositeStream[]). I believe this change
would be 100% backward compatible - legacy WOFF2 decoders would not care
about reserved bit values and would not expect/read an additional bit array
in the end of the table.

 

Spec changes:

Due to the fact that this would be a minor non-breaking specification change
that is clearly needed to fix a bug / design flaw, I believe we can utilize
an errata process (https://www.w3.org/2020/Process-20200915/#errata) for the
purpose of revising a Recommendation introducing substantive changes
(https://www.w3.org/2020/Process-20200915/#revised-rec-substantive). Once
spec changes are drafted, we'd issue a Call for Review of the Proposed
Changes (https://www.w3.org/2020/Process-20200915/#last-call-review) that
can be rolled into the existing WOFF2 Recommendation upon approval.

 

Thank you,

Vlad