This is a proposal on how to extend the lookup LookupFlags to support more flags than currently can fit into the unassigned flags.
The lookuptable is extended to support multiple flags in anticipation of extra flag needs, and a single flag is added which has meaning for Attachment lookups :
LookupTable:
| Type | Name | Description |
|---|---|---|
| uint16 | LookupType | Different enumerations for GSUB and GPOS |
| uint16 | LookupFlag | Lookup qualifiers |
| uint16 | SubTableCount | Number of subtables in this lookup |
| Offset | Subtable[SubTableCount] | Array of offset to Subtables from beginning of Lookup table |
| uint16 | MarkFilteringSet | Only present if UserMarkFilteringSet of LookupFlag is set |
| uint16 | ExtraFlag | More lookup qualifiers, only present if ExtraFlags of LookupFlag is set |
LookupFlag bit enumeration:
| Type | Name | Description |
|---|---|---|
| 0x0001 | RightToLeft | Only used for cursive attachment |
| 0x0002 | IgnoreBaseGlyphs | If set, skips over base glyphs |
| 0x0004 | IgnoreLigatures | If set, skips over ligatures |
| 0x0008 | IgnoreMarks | If set, skips over all combining marks |
| 0x0010 | UserMarkFilteringSet | If set, use mark filtering |
| 0x0060 | Reserved | Set to zero |
| 0x0080 | ExtraFlags | If set, the ExtraFlag field is included and used |
| 0xFF00 | MarkAttachmentType | If not zero, skips over all marks of attachmant type different from specified. |
ExtraFlag bit enumeration:
| Type | Name | Description |
|---|---|---|
| 0x7FFF | Reserved | Set to zero |
| 0x8000 | Reserved | Set to zero. Reserved for future flags extension |
Do we want to extend the ExtraFlags to 32-bits and save a level of future chaining?
We are already expecting 4 bits to be used in future proposals.