Re: WOFF2 bug

I'd prefer we update the spec to match the reference implementation for
this one.

Cheers, Rod S.

On Tue, Oct 7, 2014 at 8:48 AM, Levantovsky, Vladimir <
Vladimir.Levantovsky@monotype.com> wrote:

>  Folks,
>
>
>
> I am working on the spec to incorporate actions 143 and 147 (
> https://www.w3.org/Fonts/WG/track/actions/pendingreview), and I found one
> more discrepancy between the spec and the reference implementation.
>
>
>
> The definition of the bboxStreamSize (see
> http://dev.w3.org/webfonts/WOFF2/spec/#glyf_table_format) says that this
> field represents the size of the bbox stream. However, the reference
> implementation (see my previous email below) writes the combined length of
> both bboxBitmap and bboxStream in this field.
>
> Since the size of bboxBitmap[n] is fixed and depends only on numGlyphs,
> the only variable part here _*is*_ the bboxStream size. It may not seem
> to make sense to inflate the bboxStreamSize with the size of the bboxBitmap
> that can be calculated based on the numGlyphs value from another field;
> however since the two data chunks are now grouped together and written out
> sequentially – having their combined length recorded in the header doesn’t
> hurt either.
>
>
>
> Should I just make the needed changes in the field description for the
> sake of keeping the reference implementation intact?
>
>
>
> Thank you,
>
> Vlad
>
>
>
>
>
> *From:* Levantovsky, Vladimir [mailto:Vladimir.Levantovsky@monotype.com]
> *Sent:* Tuesday, September 02, 2014 1:52 PM
> *To:* w3c-webfonts-wg (public-webfonts-wg@w3.org)
> *Subject:* WOFF2 bug
>
>
>
> Hello WG,
>
>
>
> One of my colleagues brought up a question about the discrepancy between
> the draft WOFF2 spec vs. the reference implementation – I believe he is on
> to something real that was overlooked all along.
>
>
>
> The spec description of the TransformedGlyfHeader says that:
>
> “The transformed table consists of a header with the size of each of the
> substreams, followed by the substreams in sequence.”
>
>
>
> The very last field of the header (according to the spec) is bboxBitmap
> array, which [as one would guess] is to be followed by nContourStream,
> nPointsStream, flagStream, glyphStream, compositeStream, bboxStream and
> instructionStream.
>
> (Note to editors: the spec is very light on the stream descriptions
> leaving developers to guess what they and their sequence are, some
> additional language needs to be added to address this).
>
>
>
> However, looking at the reference implementation (
> https://code.google.com/p/font-compression-reference/source/browse/woff2/transform.cc):
>
>
>   void GetTransformedGlyfBytes(std::vector<uint8_t>* result) {
>
>     WriteLong(result, 0);  // version
>
>     WriteUShort(result, n_glyphs_);
>
>     WriteUShort(result, 0);  // index_format, will be set later
>
>     WriteLong(result, n_contour_stream_.size());
>
>     WriteLong(result, n_points_stream_.size());
>
>     WriteLong(result, flag_byte_stream_.size());
>
>     WriteLong(result, glyph_stream_.size());
>
>     WriteLong(result, composite_stream_.size());
>
>     WriteLong(result, bbox_bitmap_.size() + bbox_stream_.size());
>
>     WriteLong(result, instruction_stream_.size());
>
>     WriteBytes(result, n_contour_stream_);
>
>     WriteBytes(result, n_points_stream_);
>
>     WriteBytes(result, flag_byte_stream_);
>
>     WriteBytes(result, glyph_stream_);
>
>     WriteBytes(result, composite_stream_);
>
>     WriteBytes(result, bbox_bitmap_);
>
>     WriteBytes(result, bbox_stream_);
>
>     WriteBytes(result, instruction_stream_);
>
>   }
>
>
>
> where the bbox_bitmap is written out much later in the process - in
> between compositeStream and bboxStream.
>
>
>
> This definitely breaks compatibility between the code and the spec, but
> considering that the code has now been widely adopted by two publicly
> released browser version (Chrome and Opera) and that large number of fonts
> served over the web in WOFF2 format implemented the same stream sequence as
> reference implementation – I think it would be much less of a pain to
> simply edit the spec to reflect the current implementation status.
>
>
>
> Let’s discuss it during the telcon tomorrow.
>
>
>
> Thank you,
>
> Vlad
>
>
>
>
>