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

Sat Sep 12 02:42:54 CEST 2020

On Sat, 12 Sep 2020 at 01:45, Renzhi Li <Renzhi.Li at microsoft.com> wrote:

>
>    - Format is mostly like TT but allowing "shape fragments" for data
>       sharing. A shape fragment could be a contour, or a *part* of a
>       contour, like a serif shape.
>       - No composite glyphs and components, shape sharing is completely
>       transparent.
>
>
Shape fragments are already sort of implemented in some font editors (e.g.
FontLab 7) and also you can view CFF(2) subroutines as such shape
fragments. I can easily see how they can be beneficial, especially in
variable scenarios.

There may be a piece of contour that not only looks the same in many glyphs
but it also varies in the same way in all the glyphs. Associating the
variations with it and the re-using the whole part may be good for size.

On the other hand, with VFs, there may also be “false friends” —
sub-contours that look the same in a particular master but vary
differently, at least in some portion of the design space.

I think it’d be beneficial to check the actual size benefits in a good
sample of quality VFs for these situations, before drafting that proposal.

CFF subroutine compression has been around for a long time, so it could be
used as a metric. In real life, what's the current size benefit of
subroutinized vs. non-subroutinized?

A.

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> *From:* mpeg-otspec <mpeg-otspec-bounces at lists.aau.at> on behalf of Dave
> Crossland <dcrossland at google.com>
>
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> *Sent:* Friday, September 11, 2020 16:34
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> *To:* Adam Twardoch (Lists) <list.adam at twardoch.com>
>
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> *Cc:* mpeg-otspec <mpeg-otspec at lists.aau.at>
>
>
>
> *Subject:* [EXTERNAL] Re: [MPEG-OTSPEC] Co-existence of glyph rendering
> source types
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> I'm not sure "political reasons" is a good principle. The political
> reasons have a technical basis, right?
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> On Fri, Sep 11, 2020, 7:23 PM Adam Twardoch (Lists) <
> list.adam at twardoch.com> wrote:
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> 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.
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> Drop EBDT, CBDT and CFF.
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> On Sat, 12 Sep 2020 at 00:43, Adam Twardoch (Lists) <
> list.adam at twardoch.com> wrote:
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> OpenType currently has provisions for multiple sources for glyph
> rendering:
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> 1. Outlines
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> - glyf (optionally with fvar)
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> - CFF
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> - CFF2 (with or without variations)
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> - SVG
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> 2. Bitmaps
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> - EBDT
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> - CBDT (uses PNG)
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> - sbix (also uses PNG)
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> In the spec, the relationship between these seven flavors is extremely
> convoluted, and sometimes contradictory.
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> https://docs.microsoft.com/en-us/typography/opentype/spec/cbdt
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Fcbdt&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659402348&sdata=2tVhDUno4%2BBq9k9urcb2BB7tOO58cW4Mnm32PQJ95ZQ%3D&reserved=0>
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> says nothing about whether some other glyph rendering source (e.g. glyf)
> must or may be present in the font or not.
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> https://docs.microsoft.com/en-us/typography/opentype/spec/sbix
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Fsbix&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659412342&sdata=cvWOLEkXBDom9oxwml8h4YHTFbT%2BJEjrJK4VzpHrKg4%3D&reserved=0>
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> says that the font "may also" contain glyf or CFF, but makes no mention of
> SVG, CBDT or CFF2.
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> https://docs.microsoft.com/en-us/typography/opentype/spec/svg
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Fsvg&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659412342&sdata=Ijuy%2FRnfMOLVW96Tsauigv6ZxpB8pUu%2FWTZPDc%2B67hY%3D&reserved=0>
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> 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.
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> https://docs.microsoft.com/en-us/typography/opentype/spec/otff
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Fotff&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659422339&sdata=W%2Fcc%2B8V6UQjb7YLaZbjJMrWlEDocifHSlygiIdBzcLo%3D&reserved=0>
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> 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
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> SFNT is not an OpenType font.
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> 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.«
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> 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.
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> 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).
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> Same document: » OpenType fonts may also contain bitmaps of glyphs, in
> addition to outlines.« But the sbix table says outlines are optional.
> Contradiction?
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> In the same document 'maxp' is listed as required in the same way as
> 'head' is but
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> https://docs.microsoft.com/en-us/typography/opentype/spec/maxp
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Fmaxp&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659422339&sdata=W9pmetXwIIbhnrKtYNywMS1AMyevWBZJUtd%2F7AK%2FeYI%3D&reserved=0>
> 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
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> 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.
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> Same on
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> https://docs.microsoft.com/en-us/typography/opentype/spec/hmtx
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Fhmtx&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659432331&sdata=qRAiffd%2F1bptT4j32rGGJtu6u8K1oj5JPt%2FcgQ4p%2Brc%3D&reserved=0>
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> — the description goes into detail about glyf, CFF & CFF2 but makes no
> reference to bitmap-only fonts.
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> 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.
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> https://docs.microsoft.com/en-us/typography/opentype/spec/ebdt
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Febdt&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659432331&sdata=oIS2HpcMj8ha2TFMu6scwQw8PAM8NKYypngmOQ9H9%2BI%3D&reserved=0>
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> and
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> https://docs.microsoft.com/en-us/typography/opentype/spec/eblc
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Feblc&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659442328&sdata=DTf%2B9BnRYY%2BwAWJZtzD6xJoQkT1zk8m2ttWuwdkfsCg%3D&reserved=0>
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> make no mention of their relationship to the outline formats, or to other
> bitmap formats.
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> https://docs.microsoft.com/en-us/typography/opentype/spec/recom#embedded-bitmaps
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Frecom%23embedded-bitmaps&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659442328&sdata=q7VjBF2rFCpJ9kj9%2ByQJ440NsQcpm1D%2FI50%2F%2FQ0b9NM%3D&reserved=0>
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> explicitly allowes EBDT-only fonts and says that all required tables
> except glyf and loca should be there. Does that still include maxp & hmtx?
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> 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.
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> sbix and CBDT also make no mention of their relationship with each other,
> and I think sbix/CBDT are not coordinated either.
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> 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
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> with CBDT-only).
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> 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.
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> 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.
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> 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.
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> 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.
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> https://docs.microsoft.com/en-us/typography/opentype/spec/recom
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Frecom&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659452319&sdata=%2BxYr7nAngUjPQJ0PaEs2b5A6NDe4warYAMuSh7tlSME%3D&reserved=0>
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> says »Mixing Outline Formats
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> Both Microsoft and Adobe recommend against mixing outline formats within a
> single font. Choose the format that meets your feature requirements.«
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> But of course...
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> https://docs.microsoft.com/en-us/typography/opentype/spec/otff
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdocs.microsoft.com%2Fen-us%2Ftypography%2Fopentype%2Fspec%2Fotff&data=02%7C01%7Crenzhi.li%40microsoft.com%7C032984bc5e5d40f1179d08d856ab375e%7C72f988bf86f141af91ab2d7cd011db47%7C0%7C0%7C637354640659452319&sdata=wMuVyEbVt7qcxqFXGi4adiGHNgYmgg8fa0JUTb7Apkw%3D&reserved=0>
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> 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... 😁
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> I hope we can clarify these relationships.
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> A.
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