[MPEG-OTSPEC] Questions about variable composites

Skef Iterum skef at skef.org
Wed Aug 23 21:48:09 CEST 2023

I'm not surprised at this -- that seems like the right decision for the 
general case. Some sort of flag (or flags) to influence that might make 
sense for COLR 3.

However, that's a separate issue from whether hinting could be made to 
work with variable compositing, where one would at least want the 
option, and more specifically whether hinting could work with both glyf 
and CFF2 if the compositing info were in a distinct table. I think:

 1. It's clear that hinting could work the same way with glyf whether
    the data were in the glyf table or separate from it, as long as
    there were flags specifics to that case.
 2. One can define analogous flags for CFF2. There does seem to be a
    viable compositing model for PostScript-style hinting.


On 8/23/23 06:47, Hin-Tak Leung wrote:
> TL;DR.. I just like to point out that, having looked at google's COLR 
> code in skia/chrome recently, I believe I have noticed that google 
> folks have disabled hinting for color fonts in skia/chrome. I don't 
> know of the reason (you can check skia/chrome commit log yourself), 
> but this probably make sense for two reasons: color fonts tend to be 
> used at much larger sizes, for which hinting matters less; 2nd reason, 
> it is a whole lot of complication if you have to think about sub-pixel 
> rendering and color fringes/artefacts from interaction of colour fonts 
> with sub-pixel rendering...
> In other words, hinting of color fonts is either not needed, or too 
> complicated to implement:-).
> On Tuesday, 22 August 2023 at 08:08:03 BST, Skef Iterum 
> <skef at skef.org> wrote:
> Since the TypeCon meeting last week I've been preoccupied with a 
> number of questions about the variable composite proposal.  I hope 
> these are the good, potentially productive kind of question rather 
> than the crabby, lets-just-not-do-this type, but they aren't small. 
> I'm wondering if things might be significantly better with some 
> significant changes.
> I've boiled these thoughts into two interrelated multi-part questions, 
> which I have added as issues in the boring-expansion-spec GitHub 
> repository and will reproduce here. The linked issues seem like good 
> contexts for subsequent discussion.
>         Should variable composites be in the glyf table, and why?
>         (#103
>         <https://github.com/harfbuzz/boring-expansion-spec/issues/103>)
> I think I understand how we got to the current proposal. Roughly:
>  1. The variable composites specification extends the current glyf
>     composites mechanism.
>  2. Leaving variable composites in the glyf table saves some bytes, in
>     that the offsets can remain in loca and you share the Tuple
>     Variation Store offsets with gvar.
> However:
>  1. Maybe the overall variable composites system shouldn't be so
>     directly derived from the glyf mechanism (see the other question).
>  2. Everything proposed would seem to apply just as well to pulling
>     outlines out of a CFF2 table.
>  3. We already have a model for how to do this in an external table,
>     that being COLR.
> Right now, a system that understands COLR starts by looking in that 
> table for an entry. If it finds one, it pulls path data from either 
> glyf or CFF(2). If it doesn't, it falls back to glyf or CFF(2).  All 
> of this happens "below"/subsequent to shaping:
> (shaping) -> COLR -> (glyf | CFF(2))
> It seems like what "variable compositing" amounts to is an additional, 
> simplified shaping step. Call it "intra-glyph shaping", which occurs here:
> (inter-glyph shaping) -> COLR -> (intra-glyph shaping) -> (glyf | CFF2)
> The only reason the system doesn't already look like this is that the 
> compositing data is stored in the glyf table.
> Set aside the question of other potential changes and just consider 
> the current proposal: If one wanted to have this mechanism for CFF2 
> also, would it be substantially different? If it had to live inside 
> the CFF2 table it would be formatted differently (with blends instead 
> of a separate tuple variation store, perhaps using floats instead of 
> fixed-point values of different scales, etc.) But would the meaning of 
> the parameters be any different? Would other parameters be needed, or 
> redundant, in the CFF2 case?  I don't see how, or why.
> So suppose the system worked this way instead:
>  1. Variable composite data is in its own table, call it "vcmp". It
>     has some top-level mechanism for mapping data to GIDs analogous to
>     that of COLR. The per-glyph tuple variation stores could be at an
>     offset within the data.
>  2. For the sake of argument, leave the per-glyph format exactly like
>     it is now, except for an additional `hint flags` field in the
>     component record (and minus the stuff needed to play nice in the
>     glyf table, like `numberOfContours`).
>  3. Prohibit the use of the existing `glyf` composite mechanism when
>     using this separate table.
>  4. Specify that when there is path data for a GID in the (glyf |
>     CFF(2)) table, and that GID also has a composite entry, the path
>     data is added with no transformation to the composite data. (This
>     was asked for toward the end of the TypeCon meeting.)
>  5. Specify that when there is hinting data for a GID in the (glyf |
>     CFF(2)) table, (TrueType instructions or CFF stems) and that GID
>     also has a composite entry, the relationship of the additional
>     hinting data to the component hinting data is determined by the
>     hint flags.
> The main thing to work out with this system would be the details of 
> the hint flags, but those problems are analogous for the two path data 
> sources.  Maybe you need different flags for glyf and for CFF2 — which 
> could overlap, because one assumes mixing sources is off the table — 
> but in each case the only thing to be worked out is how to reconcile 
> the hinting data. (We know this because we already have COLR, so we 
> already have implementations that grab data from the bottom-level 
> tables, alter the points according to affine transformations, and 
> render the results.)
> This change would have these cons:
>  1. A modest increase in size, due redundant loca/gvar/vcmp offset
>     entries and duplication across the tuple variation stores (header,
>     regions).
>  2. ?
> And these pros:
>  1. Assuming someone does the work of specifying the hinting behavior
>     for CFF2, the system would work just as well with CFF2 and glyf.
>     This reduces pressure on glyf format changes. CFF2 already goes
>     above 64k glyphs, already supports cubics, and can already
>     losslessly represent quadratics as cubics (at the cost of using
>     floating point values in the conversion, when that precision is
>     needed).
>  2. If the composite system needs to do other things, its internal
>     structure doesn't need to be so closely tied to the older glyf
>     composite mechanism.
> Note: Although I can't make any promises, I've thought through some of 
> what one would need to say about CFF2 hinting and variable components. 
> It does seem like there could be a viable model here where overall 
> hinting quality could approach that of the current system. 
> ("Decompositing" to CFF (or CFF2) would involve some hinting 
> compromises, but that's already true for CFF2 to CFF because of overlap.)
>         Variable Compositing is analogous to shaping. So what about
>         substitution? (#104
>         <https://github.com/harfbuzz/boring-expansion-spec/issues/104>)
> I noted in the other question that "variable compositing" seems to 
> amount to an additional, simplified shaping step. However, as 
> specified the system only includes an analog of positioning, and lacks 
> an analog of substitution.
> Let's consider a specific case.
> Suppose that you are working in a model that has three conceptual 
> layers: atoms, molecules, and glyphs. Perhaps these are exposed by a 
> font editor.
> For a given molecule, the designer decides she wants the outline of 
> one atom to change within one sub-region of design space, and a 
> different atom to change within a slightly different sub-region of 
> design space. The molecule is used in 25 different glyphs. With the 
> existing proposal it seems like there are two options:
>  1. Force the designer to play tricks with the masters so that all
>     versions of the atom interpolate, and then position the masters in
>     design space right next to each other for quick interpolations.
>     This increases the burden on the designer.
>  2. Allow the designer to specify different, non-interpolable versions
>     of an atom in different subspaces of design space, and sort things
>     out in the compiled font.
> In our example, it seems like the only option for 2 with the current 
> proposal would be to use GSUB's `rvrn` or something similar. Given 
> that the molecule has four versions (for each permutation of default 
> and altered atom), you would need 100 GIDs to handle the 25 glyphs. 
> You would also need to either duplicate the composite data for the 
> other, always-present atoms across the four molecules, or add another 
> "base molecule" layer into the hierarchy to collect that data together 
> to avoid duplication.
> Now, of course, in *some* cases you'll need to do something like this 
> anyway: mainly when swapping an atom affects the metrics of the 
> ultimate glyph. But such cases seem like the exception rather than the 
> rule.
> So:
>  1. Should there be some more targeted way of supporting this sort of
>     case in a variable composite model?
>  2. Does this suggest that the model should draw a little bit more
>     from GSUB/GPOS and perhaps be less closely tied to the older glyf
>     model? (For example, you might need distinct positioning data for
>     the different atoms that can be substituted into a molecule,
>     perhaps loosely analogous to distinct contextual positioning GPOS
>     rules that could apply after a substitution.)
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