metric for block coverage

[Richard Wordingham via Unicode]

On Sun, 18 Feb 2018 13:05:29 +0100
Adam Borowski via Unicode <unicode at unicode.org> wrote:

> On Sun, Feb 18, 2018 at 02:14:46AM -0800, James Kass wrote:

> > You probably already know that basic script coverage information is
> > stored internally in OpenType fonts in the OS/2 table.
> > 
> > https://docs.microsoft.com/en-us/typography/opentype/spec/os2  
> 
> It's only a single bit without a meaning beyond "range is considered
> functional".  No "basic coverage" vs "good coverage" vs "full
> coverage".

It's worse than that when a script uses characters primarily
associated with another script.  For example, to have any confidence
that my Tai Tham font will be used for U+0E4A THAI CHARACTER MAI
TRI or U+0E4B THAI CHARACTER MAI CHATTAWA placed on U+1A4B TAI THAM
LETTER A, I have to set the Thai bit, even though I only have four Thai
characters in my font.  (The other two are punctuation.)

Richard.