Issues with docs for cv01-cv99 Feature Parameters table

[cmyr]

Description

The docs for this table describes several fields that are supposed to be name IDs, but are explicitly allowed to be 'NULL'.

• conceptually, NULL is not meaningful for a non-offset type.
• Does NULL mean 0x0000? If so, this is ambiguous, because 0 is a valid Name ID, and is likely to be mishandled by clients.
• If we want to represent "value not present" for a Name ID, a better choice would be 0xFFFF: this is already used by convention in various other places. For instance in the 'fvar' docs, we see: "Values of 6 and 0xFFFF can be used; otherwise, values must be greater than 255 and less than 32768. If the value is 0xFFFF, then the value is ignored, and no PostScript name equivalent is provided for the instance."

concretely, I would suggest that the docs be updated to remove use of the word 'NULL', and to explicitly suggest a value of 0xFFFF to represent "no name is provided". I also think it good to describe the convention of using 0xFFFF for an empty value in the Name ID docs.

Page URL

https://learn.microsoft.com/en-us/typography/opentype/spec/features_ae

Content source URL

https://github.com/MicrosoftDocs/typography/blob/live/typographydocs/opentype/spec/features_ae.md