Re: Action 138 "Table directory" (was RE: Telcon minutes, Wednesday, Sep. 10)

On Fri, Sep 12, 2014 at 5:40 PM, Levantovsky, Vladimir <
Vladimir.Levantovsky@monotype.com> wrote:

>  Both SFNT and WOFF 1.0 have the table directory entries sorted in
> alphabetical order which is **not** the case with WOFF2.
>
>
>
> Looking at the "Table Directory" spec - there is no mentioning there that
> the table order is different from the original font and specifies the exact
> order in which the table s appear in the compressed font stream (we do
> mention it in passing in the compressed font format section). Should we
> make it explicit and define it here as well to make sure that there is no
> ambiguity?
>

Perhaps you added the following to the specification since, to clarify?  It
looks good to me.  Thank you Vlad.

http://dev.w3.org/webfonts/WOFF2/spec/

4. Table directory format

"Contrary to the way how table directory entries are specified in the
original input font file (where table directory entires are sorted in
ascending alphabetical order), the WOFF2 table directory entires define the
physical order of tables in which they have been processed and encoded as
part of the compressed font data stream. It is a decoder responsibility to
sort and reorder the table directory when the font file is decompressed."
>
>
>
> Thank you,
>
> Vlad
>
>
>
>
>
> *From:* Levantovsky, Vladimir
> *Sent:* Thursday, September 11, 2014 6:29 PM
> *To:* Levantovsky, Vladimir; w3c-webfonts-wg (public-webfonts-wg@w3.org)
> *Subject:* Action 138 "Table directory" (was RE: Telcon minutes,
> Wednesday, Sep. 10)
>
>
>
> Folks,
>
>
>
> Some thoughts that I want to run by you:
>
> 1)      As we discussed during the call - adding any padding requirements
> to table directory entries would probably be silly. Each entry has one byte
> of flags followed (in the majority of cases) by a single, variable length
> UIntBase128 value. All known table tags will not be there (at least those
> that we know of today) and only two tables will have an extra field when
> the transformed length UIntBase128 value is present. So, for the majority
> of cases each table entry would likely be 2-5 bytes long, and require
> 4-byte alignment and padding of up to 3 extra bytes would eat away any and
> all benefits of using variable-length encoding for table length.
>
> 2)      If the above is true, than there is no way to know ahead of time
> where one entry ends and another one begins - we only know the number of
> entries so a decoder has to process everything sequentially to figure out
> where the table directory ends and a compressed font stream begins. The
> compressed font stream does begin on 4-byte boundary and does require
> padding (as we agreed yesterday) but it is the only one among the
> compressed data streams where its initial starting position in the file is
> not apriori known (one need to process the table directory to figure that
> out).
>
>
>
> Would it be useful and/or make sense to have a field defined that would
> allow a decoder to start reading the compressed font stream immediately,
> before the table directory is parsed? If so, the reserved field (which I am
> not sure why we needed it other than for data alignment issues) could be
> used to signal the size of the table directory in bytes. I realize that
> Uint16 isn't much but for table directory that is likely to never have more
> than double-digit number of entries up to 11 bytes long (in the worst case)
> - UInt16 would be more than sufficient.
>
>
>
> Comments?
>
>
>
> Thank you,
>
> Vlad
>
>
>
>
>
> *From:* Levantovsky, Vladimir [mailto:Vladimir.Levantovsky@monotype.com
> <Vladimir.Levantovsky@monotype.com>]
> *Sent:* Wednesday, September 10, 2014 5:14 PM
> *To:* w3c-webfonts-wg (public-webfonts-wg@w3.org)
> *Subject:* Telcon minutes, Wednesday, Sep. 10
>
>
>
>
>
>       [5]
> http://lists.w3.org/Archives/Public/public-webfonts-wg/2014Aug/0013.html
>
>
>
>    Vlad: should all table directory conformance statements carry
>
>    forward? Would be a good ideal to review, wouldn't want to
>
>    automatically carry everything forward
>
>    ... esp. as there are some that do not apply (e.g. padding
>
>    between tables)
>
>
>
>    <scribe> ACTION: vlad review conformance statements for WOFF
>
>    1.0 and transfer the applicable ones over [recorded in
>
>    [6]http://www.w3.org/2014/09/10-webfonts-minutes.html#action01]
>
>
>
>    <trackbot> Created ACTION-138 - Review conformance statements
>
>    for woff 1.0 and transfer the applicable ones over [on Vladimir
>
>    Levantovsky - due 2014-09-17].
>
>
>
>
>