Composite Font Requirements (was RE: [mpeg-OTspec] AHG on Open Font Format

[karstenluecke]

I like that you reduce the Composite Font Format intention to the question, which issue is the format to address?

As to 2.
["what are the defining metrics (e.g. max ascender, descender, leading) of the composite font and how closely do the components of a composite need to adhere to these metrics?"]
I think there are two aspects:
(a) Metrics that define ideal/recommended/automatic line-to-line distance. 
(a.1) Two columns of different-script texts do not necessarily need the same line-to-line distance. E.g. Latin--Arabic or Latin--Chinese/Japanese may even suffer from it. I am not sure if a composite font needs to impose "global" values here.
(a.2) In case of text which includes single different-script words or phrases, the font that provides glyphs for the "primary" script text may determine the line-to-line distance, and the other script would follow. Here, "scale" factors as suggested in Mr Leonov's 4. may jump in.
(b) Metrics that define maximum dimensions (OS/2.usWinAscent/Descent) should not have any impact on line-to-line distance anyway. If a composite font would provide these, they should be taken from the font with largest dimensions. There is no need to keep these values identical with every future composite font update or addition of other fonts to the composite font.
But that would be an ideal world.

Perhaps one more question which I cannot find addressed in the posts:

9.
Do Unicode ranges (a) defined in a composite font refer to precomposed character-glyphs only or do they also (b) include characters not covered in the font/cmap as such but would result from Unicode composition rules + separate base/mark glyphs + ccmp/mark/mkmk?
(b) would require that composite-font-savvy layout engines must, rather than may, support layout tables.

Best wishes,
Karsten Luecke