[mpeg-OTspec] Proposed mandatory and optional features of the Composite Font format

[Mikhail Leonov]

Hi everyone,
I would like to provide an update on mandatory and optional elements in the composite font format.

First, some background. During phone conference meetings that preceded the formal creation of this working group, parties involved concluded that, due to the diversity of font handling platforms and programming interfaces in the industry, it was not practically feasible to agree on a single predefined set of composite font elements and properties that would express existing font selection models and approaches. Instead, it was recommended to give composite font producers and consumers flexibility to introduce optional properties that wouldn't necessarily be used or even understood by all conformant consumers. At the same time, semantics of such properties should be defined as clearly as possible, so that an implementation that chooses to suport them can do so in a way compatible with other consumers and producers.

In addition to such optional properties, there are core pieces of the composite font format that conforming implementations must interpret correctly to provide the minimum degree of interoperability.

The purpose of this email is to start creating a list of mandatory and optional features in the composite font format.

The format is assumed to be based on the latest composite font syntax proposal uploaded to this AHG discussion list by Ken Lunde. Since I don't recall us discussing the root element mandated by the XML specification, I propose introducing a root element called CompositeFont.

Here is a complete composite font file that prescribes the consumer to use font "Times New Roman" for all Unicode characters and all languages:

<?xml version="1.0" encoding="UTF-8"?>
<CompositeFont>
    <ComponentFont Target="Times New Roman"/>
</CompositeFont>

I propose that a conforming composite font consumer must:
1. Recognize and parse basic XML structure as outlined above, including the standard XML header and the root element CompositeFont.
2. Reject composite font files that violate the standard XML specification.
3. Recognize, parse, and interpret all possible valid values for the following elements and attributes (these elements and attributes are furthermore called mandatory):
    a) ComponentFont element and its Target attribute. The interpretation of the Target attribute value is allowed to vary between consumers depending on the target environment font grouping model. For example, a consumer may be using OpenType Preferred names, Win32 names, Postscript names, WWS names, or some other font selection model that may not even be OpenType based. However, the consumer must use a font model that supports at least one font format and assigns name values to font files that conform to supported font formats. In addition, the consumer must parse comma-separator characters that can be used to separate multiple font names inside one Target attribute value, omit leading and trailing whitespace characters from font name values, and honor the escape sequence that encodes the comma character itself if it appears as a part of the font name. I propose double comma ",," as an escape mechanism for such cases.
    b) Encoding element and its Target and Original attributes. These are described in Ken Lunde's email, and I won't repeat the semantics here. Similar to the Target attribute semantics, the interpretation of Unicode characters supported by a font may vary depending on the target environment font model. For example, a consumer may prefer one cmap table format to another. However, the consumer must use a font model that provides an ability to obtain Unicode coverage for all supported font formats.
    c) Language element and its Target attribute. I would like to change the Target attribute values from the original definition provided by Ken, which used ISO 639-2/T language codes, to a definition that uses IETF language tags, as specified by RFC 4646. The key difference is ability to differentiate between multiple character sets for the same language, for example Simplified Chinese ("zh-Hans") vs. Traditional Chinese ("zh-Hant"). The conforming implementation must properly match language tags. There is a couple of interesting corner cases here that I would like to discuss in more detail - exact rules for matching language tags that are related to each other, and handling of empty language tags and cases where the language infomation is not available to the font selection algorithm.
4. Reject composite font files that don't contain at least one ComponentFont elements.
5. Interpret composite font elements in the order they appear in the font file.

In addition to the mandatory elements and attributes described above, there is a large group of elements and attributes that are considered optional, such as scale, common baseline and height metrics, the display name of the composite font itself, font style coercion, digital signature enforcement, optical size, required font version, font checksum validation, and others.

Before drilling into exact syntax of such properties, I would like to discuss one particular issue. I believe some of the optional properties can be safely ignored by the composite font consumers without breaking the intent of the composite font producer, while other optional properties should cause the consumers unable to honor their semantics to skip containing elements entirely. An example of the former class is handling of Scale attribute of the ComponentFont entry on platforms that don't support font scaling. An example of the latter class is required font version range. If the consumer is unable to extract a version from the font, then it is unable to honor the producer intent, and a font entry should arguably be skipped altogether. I propose that the composite font format should contain provisions that enable consumers to tell whether encountering an unknown attribute should cause the element it belongs to to be ignored or not.

Please note that the proposals above are by no means final, and any suggestions, corrections and additions are very welcome.

Thanks in advance, and best regards,
Mikhail Leonov
Microsoft
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