RE: DSIG and other issues

Behdad wrote:
Sure.  But I don't see why you would want to make a DSIG exception in the file format when it essentially saves nothing (5 bytes max?).  Perhaps add it back to the known-tables list.

Yes, we can do that as well, although doing so would also require to redefine the 6-bit long field of the flags that are currently allocated to known table tags. Right now, we have exactly 63 known tables and the last entry is reserved for arbitrary tags.
So, we do have multiple options to consider:

-        Use extra flag bit to extend known table tags and add DSIG there,

-        Or, keep the flag reserved and encode DSIG as an arbitrary flag, if present and
- remove DSIG signatures and explicitly encode empty DSIG as part of the WOFF2 file, or
- remove DSIG table completely and only encode its presence in the table directory (similar to ‘loca’, which will serve as an indication for decoder to insert empty DSIG table in the output file), or

-        use a flag bit (e.g. set with the last table directory entry) to indicate the need to insert an empty DSIG.

I don’t have any strong preference to any of those options and their combination but since we do have choices - we need to make one.

Thank you,
Vlad


From: Behdad Esfahbod [mailto:behdad@google.com]
Sent: Wednesday, May 21, 2014 6:28 PM
To: Levantovsky, Vladimir
Cc: David Kuettel; Chris Lilley; w3c-webfonts-wg (public-webfonts-wg@w3.org)
Subject: Re: DSIG and other issues

On Wed, May 21, 2014 at 4:20 PM, Levantovsky, Vladimir <Vladimir.Levantovsky@monotype.com<mailto:Vladimir.Levantovsky@monotype.com>> wrote:
In the scenario that Behdad proposed we would have to keep both the DISG entry in table directory and the empty table as part of the font data. Removing the DSIG table and inserting the empty DSIG table when the font is decoded is a possible alternative option, we would only need to record the presence of the DISG in the original file (we do have reserved flags that can be used for this) to let the decoder know when the empty one needs to be inserted.

Sure.  But I don't see why you would want to make a DSIG exception in the file format when it essentially saves nothing (5 bytes max?).  Perhaps add it back to the known-tables list.



Vlad

On Wednesday, May 21, 2014 5:32 PM David Kuettel wrote:
On Wed, May 21, 2014 at 2:18 PM, Behdad Esfahbod <behdad@google.com<mailto:behdad@google.com>> wrote:
> My recommendation is that the spec be changed, to recommend that if
> the original font had a DSIG table, then all signatures in the DSIG
> table be removed.  Ie. an encoder is encouraged to keep an a DSIG
> table with zero signatures.  That's a valid table that is only eight
> bytes.  (00 00 00 01 00
> 00 00 00).

Interesting.  So rather than removing the table all together (as the latest draft currently recommends, http://www.w3.org/TR/WOFF2/), just removing all of the signatures from the table if present.

That sound great!