[mpeg-OTspec] New cmap format

[John Hudson]

On 03/04/12 7:25 PM, Martin Hosken wrote:

> Rather than burdening every font shaping engine and rendering engine already in existence with a table format that people are liable to ignore for a long while yet, how about using a tool to autogenerate all your compound glyph references for you. A tool like ttfbuilder could do that after font generation. Alternatively one could write a tool to autogenerate the necessary GSUB rules to break apart the ligatures.

I already have such development tools, and it still seems to me a lot of 
extra stuff to manage, as well as resulting in larger and more complex 
fonts than would be necessary if one didn't need to include in them 
glyphs that may never, in fact, be displayed.

> I agree, shaping engines should do the right thing here and decompose where a composed cmap entry is missing. In fact they should also compose decompose sequences where a composed cmap entry exists. So the issue cuts both ways. Another reason why just having a decomposing cmap isn't necessarily going to solve the problem for everyone.

Most engines already perform composing operations for precomposed 
Unicodes, because there was a time when the processing gain of 
performing composition at the character level seemed significant. And 
may still be significant for some people, which is why I am not 
proposing to change anything that already exists or force anyone to stop 
making fonts in the way they already make them. I'm suggesting a 
mechanism that would allow us to make fonts in new ways that are smaller 
and simpler.

I should clarify that these are two separate matters:

1. Engines should be able to map precomposed Unicodes in text to 
decomposed Unicodes in a font, if these are the only ones available, 
rather than displaying a .notdef glyph.

2. In addition, there may be a use for a mapping mechanism from any 
single Unicode (not only those with canonical decompositions) to a 
sequence of multiple glyphs that can display the character, without the 
intermediary step of mapping a default glyph for that Unicode.


JH



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