[mpeg-OTspec] CFR ascender and descender

[John Hudson]

Vladimir wrote:

> I am not sure if using OS/2 sTypo... values would be a good choice
> because in the majority of fonts the actual values recorded in those
> fields can't really be trusted.

Also, the sTypoAscender and -Descender values are expected to be used in 
concert with the sTypoLinegap value, and without that third piece of 
information they are useless.

> In particular, I would suggest to at least consider the following strategy:
> - if the CFR <FontMetrics> element defines ascender and descender values
> - the implementation will use the values defined by a CFR author;

Perhaps we also need to look more closely at what it means to 'use' 
those metrics, in terms of the distinction that OS/2 metrics have at 
least tried to make between linespacing metrics and bounding box metrics 
(even if in practice the two have been conflated most of the time).

> - if the ascender and descender values are not explicitly defined, the
> implementation will use the first, highest-priority component font to
> determine the ascender and descender values according to the algorithm
> defined in the "Baseline to Baseline Distances" section of the OT/OFF
> "Recommendations" clause.

This seems like a step in the right direction. Will need to review those 
recommendations again, though.

I understand that Google did a lot of cross-browser testing of 
linespacing behaviour, and as a result came up with recommendations for 
their webfonts that differ from the recommendations we've worked with on 
Microsoft fonts for the past decade.

J.


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