[MPEG-OTSPEC] Co-existence of glyph rendering source types

[Dave Crossland]

I'm not sure "political reasons" is a good principle. The political reasons
have a technical basis, right?

On Fri, Sep 11, 2020, 7:23 PM Adam Twardoch (Lists) <list.adam at twardoch.com>
wrote:

> Ps. In my world, OFF2 would bring down the number of flavors to just 4:
> glyf (TT, optionally also cubic), CFF2 (for political reasons), SVG and
> sbix.
>
> Drop EBDT, CBDT and CFF.
>
> On Sat, 12 Sep 2020 at 00:43, Adam Twardoch (Lists) <
> list.adam at twardoch.com> wrote:
>
>> OpenType currently has provisions for multiple sources for glyph
>> rendering:
>>
>> 1. Outlines
>>
>> - glyf (optionally with fvar)
>> - CFF
>> - CFF2 (with or without variations)
>> - SVG
>>
>> 2. Bitmaps
>>
>> - EBDT
>> - CBDT (uses PNG)
>> - sbix (also uses PNG)
>>
>> In the spec, the relationship between these seven flavors is extremely
>> convoluted, and sometimes contradictory.
>>
>> https://docs.microsoft.com/en-us/typography/opentype/spec/cbdt says
>> nothing about whether some other glyph rendering source (e.g. glyf) must or
>> may be present in the font or not.
>>
>> https://docs.microsoft.com/en-us/typography/opentype/spec/sbix says that
>> the font "may also" contain glyf or CFF, but makes no mention of SVG, CBDT
>> or CFF2.
>>
>> https://docs.microsoft.com/en-us/typography/opentype/spec/svg says »For
>> every SVG glyph description, there must be a corresponding TrueType, CFF or
>> CFF2 glyph description in the font.« No mention of sbix or CBDT.
>>
>> https://docs.microsoft.com/en-us/typography/opentype/spec/otff says » An
>> OpenType font file contains data, in table format, that comprises either a
>> TrueType or a Compact Font Format (CFF) outline font. « By this, one might
>> read that am sbix-only SFNT or a CBDT-only SFNT or an EBDT-only SFNT or
>> perhaps even a CFF2-only SFNT is not an OpenType font.
>>
>> The same page says » OpenType fonts that contain TrueType outlines should
>> use the value of 0x00010000 for the sfntVersion. OpenType fonts containing
>> CFF data (version 1 or 2) should use 0x4F54544F ('OTTO', when
>> re-interpreted as a Tag) for sfntVersion.« Here CFF is used as “1 or 2”,
>> while in the SVG spec, CFF is named separately from CFF2 so it must mean
>> "1". Again, no guidance for sbix-only, CBDT-only or EBDT-only fonts.
>>
>> Same document: » Required Tables. Whether TrueType or CFF outlines are
>> used in an OpenType font« Again, no word on non-outline fonts (sbix-only,
>> CBDT-only, EBDT-only).
>>
>> Same document: » OpenType fonts may also contain bitmaps of glyphs, in
>> addition to outlines.« But the sbix table says outlines are optional.
>> Contradiction?
>>
>> In the same document 'maxp' is listed as required in the same way as
>> 'head' is but
>> https://docs.microsoft.com/en-us/typography/opentype/spec/maxp says »
>> This table establishes the memory requirements for this font. Fonts with
>> CFF data must use Version 0.5 of this table, specifying only the numGlyphs
>> field. Fonts with TrueType outlines must use Version 1.0 of this table,
>> where all data is required.« It’s unclear if “CFF” is inclusive of CFF2
>> here or not, and it’s unclear what role the table has with bitmap-only
>> fonts.
>>
>> Same on
>> https://docs.microsoft.com/en-us/typography/opentype/spec/hmtx — the
>> description goes into detail about glyf, CFF & CFF2 but makes no reference
>> to bitmap-only fonts.
>>
>> Therefore, the spec does not provide sufficient info about which tables
>> should or must be in an outline-less sbix-based, CBDT-based or EBDT-based
>> font.
>>
>> https://docs.microsoft.com/en-us/typography/opentype/spec/ebdt and
>> https://docs.microsoft.com/en-us/typography/opentype/spec/eblc make no
>> mention of their relationship to the outline formats, or to other bitmap
>> formats.
>>
>>
>> https://docs.microsoft.com/en-us/typography/opentype/spec/recom#embedded-bitmaps
>> explicitly allowes EBDT-only fonts and says that all required tables except
>> glyf and loca should be there. Does that still include maxp & hmtx?
>>
>> Since recommendations allow EBDT-only, one could think that CBDT-only is
>> also allowed, because somewhere it says that CBDT uses similar concepts to
>> EBDT. But maybe someone else might think differently.
>>
>> sbix and CBDT also make no mention of their relationship with each other,
>> and I think sbix/CBDT are not coordinated either.
>>
>> There is a reluctance on some parties’ part to support SVG in fonts, but
>> I can easily see how either CBDT or sbix or both could be used for SVG the
>> same way as EBDT was used for TT outlines — as a hand-tuned representation
>> for small sizes. An engine might prefer the fast PNG rendering for smaller
>> sizes and switch to slower SVG for larger sizes, or if it doesn't have an
>> SVG renderer, it still might render upscaled PNGs from sbix/CBDT (as Apple
>> used to do with sbix-only, and as Google still does with CBDT-only).
>>
>> In OFF2, we might do away with EBDT/CBDT entirely. EBDT is no better than
>> sbix, in fact it's worse because it has a max PPM of 127 (!). sbix can be
>> used for all kinds of bitmaps, and its PNGs can be monochrome or grayscale.
>>
>> I don't see why OT fonts that have SVG also MUST have glyf/CFF(2), while
>> sbix does not have to have glyf/CFF(2), and EBDT/CBDT also, in practice,
>> doesn't have to have outlines.
>>
>> If an sbix-only font (without glyf/CFF(2)) is valid (at least some part
>> of the spec suggests it is), then a font with sbix and SVG but without glyf
>> or CFF(2) also should be valid, bit isn't.
>>
>> Finally, I don't think it's clear whether a CFF2 table and a CFF table
>> can or cannot co-exist in the same font.
>>
>> https://docs.microsoft.com/en-us/typography/opentype/spec/recom says
>> »Mixing Outline Formats
>> Both Microsoft and Adobe recommend against mixing outline formats within
>> a single font. Choose the format that meets your feature requirements.«
>>
>> But of course...
>> https://docs.microsoft.com/en-us/typography/opentype/spec/otff lists
>> “SVG Outlines” as the 3rd kind of outlines next to “TrueType Outlines” and
>> “CFF Outlines”, and the SVG table spec says you *must* mix SVG Outlines
>> with either TT or CFF outlines, so obviously that recommendation is, uh,
>> err... 😁
>>
>> I hope we can clarify these relationships.
>>
>> A.
>>
>>
>>
>>
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>>
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