Re: [ilreq] How are underlines positioned in Indic text?

A few thoughts on this:

1. Ideally, underline position and thickness should be defined at the 
font level, with CSS only providing fallback default behaviour. There 
are, however, some issues with this:

a) In OpenType fonts, values for underlinePosition and 
underlineThickness are stored in the 
'[post](https://www.microsoft.com/typography/otspec/post.htm)' table. 
There is one pair of values for the whole font, regardless of how many
 different scripts are supported, and currently no way to specify 
different values for different scripts or language (writing) systems.

b) font tools tend to provide a default setting for these two values, 
and a lot of font makers appear not to alter the default setting, with
 the result that the data may not produce best results.

I think both of these issues could be addressed by defining 
localisable underline thickness and position values, probably as an 
extension of the existing but little-used 'BASE' table. This would not
 only provide layout software with appropriate values for different 
scripts and writing systems, but the newly defined data structure may 
well encourage font developers (and font tool makers) to pay more 
attention to this aspect of font production. This could also assist 
CSS in determining whether the font data is reliable.

[These comments also apply to the strikethrough height and thickness 
values, currently implemented in the 'OS/2' table, and similarly 
under-localised.]

2. A lot of writing systems have conjunct forms, below marks, and/or 
vertical stacks that cut through the height of an underline. In 
addition to determining the appropriate height and thickness of 
underlines, it is also desirable to determine the preferred behaviour 
of underlining with descending glyphs, which might also differ 
depending on writing system.

This is complicated by the fact that the depth of a given sequence of 
characters in a complex script is font-dependent, and cannot be 
determined or even reliably guessed at from the character string.

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Received on Monday, 21 November 2016 20:12:04 UTC