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Re: Bug report: Allow CFF based OT fonts with missing map table


From: suzuki toshiya
Subject: Re: Bug report: Allow CFF based OT fonts with missing map table
Date: Sat, 8 Jun 2024 21:15:34 +0900
User-agent: Mozilla Thunderbird

Dear Derek,

Thank you for reminding the origin of the discussion.

Regards,
mpsuzuki

On 2024/06/08 5:27, Derek B. Noonburg wrote:
I just wanted to clarify that I'm not actually requesting anything. The
email that started this thread was from Honnesh Ramachandra, who
suggested a patch.

My use of FreeType (in Xpdf) is very specific to PDF. In particular,
PDF files specify a bunch of extra encoding information outside of the
embedded font, so in some cases the OpenType cmap table is not used. My
point here is that I already have a lot of code that checks for various
situations with the PDF encoding info, missing tables in fonts, etc.,
and it's PDF-specific, so not anything that I would expect FreeType to
fix.

I've never encountered an OpenType font with head and CFF tables, but
without a cmap table. If it comes up, I don't think it would be too hard
to extend my code to check for that case and extract the CFF font (since
I'm already checking for a missing head table).

In any case, I'll be happy to help with PDF construction/manipulation
-- just let me know if you need anything.

- Derek


On Fri, 7 Jun 2024 11:06:58 +0900
suzuki toshiya <mpsuzuki@hiroshima-u.ac.jp> wrote:

Dear Adam, Derek,

Adam, thank you for reminding that SFNT-wrapped PostScript fonts
on Apple platforms, it sounds reasonable attitude to handle such
incorrectly(?) embedded CFF font.

Derek, my understanding of current situation is:

1) currently FreeType simply raises an error for "head + CFF"
stream without special information. Even if a FreeType client
is capable to process such stream, the client has to catch all
errors and re-investigate the content of the stream. It is
"duplicating" of the font parsing.

2) the option best for the PDF rendering is "dealing with
head + CFF stream as simple CFF stream, ignoring head table".

3) another option better than current is "raising special
error indicating 'CFF is included, but other essential
tables for CFF OpenType are missing' and let the FreeType
client decide how deal such special case.

Is it correct?

I would try to make an artificial example by adding "head"
table to its internal CFF streams, to investigate how existing
PDF viewers handle such. I would ask how such PDF set the
font descriptor metadata to "head + CFF" stream in later.

I'm quite sorry for that I'm too busy to recover my PDF
playground in this month, I hope this issue is not immediate
to be resolved within a few days.

Regards,
mpsuzuki

On 2024/06/06 4:40, Adam Twardoch (List) wrote:


My own focus is on PDF files. The PDF spec says that fonts declared
as OpenType should be OpenType, and the files I'm looking at
violate the spec. (PDF also supports standalone CFF fonts, so it's
relatively little extra work for me to pull the CFF blob out of
these bogus "OpenType" fonts, and handle it as a CFF font.)

Oh yes, you're right there. However, this is a very general
problem. Even though OpenType is a Microsoft trademark, Microsoft
had done a very poor job in explaining/defining what is and what
ISN'T an "OpenType font".

Windows had been announcing OpenType fonts with a glyf table but
without a DSIG table as "TrueType" for many years, even though
TrueType is an Apple trademark. So many end-users for a long time
associated CFF-based OpenType with "OpenType".

And PDF has its own notion of font format "branding", and then
there's a question of how certain apps like Acrobat present these.

I lost count of the different variants of fonts that "can exist" in
a PDF a log time ago. I guess a Type42 with a variable CFF2 table
is also theoretically possible :) Or with just CBDT, without glyf
and CFFx.

One thing for sure: on "desktop", certain fonts work out they
don't, but they need to be kind of "complete". But PDF has this
flurry of "partial font resources" which makes it extra-complex.

A.









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