Re: i18n-ISSUE-401: Clarify initial letter requirements and alignment points

Richard Ishida wrote:

> It currently says "in Devanagari the hanging baseline may be preferred".

> This wording implies that in other cases the hanging baseline may not be
> preferred. If so, and if this refers to drop intial letters, what are
> the alternatives, and what are the font metrics or other considerations
> involved in alignment?

On the font metrics side, I would argue that we don't really have the 
data point we need, or that the data is mis-specified.

I think the notion of 'hanging baseline' should be completely discarded 
or, at least, given some very different name. What we're talking about, 
in scripts such as Devanagari and Bengali, is head-line alignment, which 
is something independent of the baseline position. The baseline, in font 
metrics terms, is the y-direction 0 distance of the internal font grid, 
or the global adjustment of the same applied to a font or to a 
particular script subset of a font to align *with a different script* as 
specified in the font BASE table [1]. The characteristic case of BASE 
table baseline adjustment is lowering of CJK ideographic characters when 
used in the context of Latin text. The BASE table specification also 
gives examples of a 'hanging baseline' setting for Devanagari script, 
but I've yet to see this implemented in either fonts or software, and I 
believe it is based on a mistaken notion.

What is needed for head-line alignment of different sizes or fonts of 
Indian scripts such as Devanagari, including -- or even especially -- 
the case of drop initial letters, is a head-line alignment value, 
specified in font units, *expressed as a distance from the baseline*. 
You really need these two pieces of data: the baseline and the head-line 
alignment, and from those you can calculate the scaling of an initial 
letter such that the head-line of the initial aligns with that of the 
first line of text and the baseline of the initial letter aligns with 
that of the secondary line of text (determined by how many lines the 
initial covers). [This corresponds to use of the cap-height metric in 
scaling and aligning Latin drop caps.]

For a variety of reasons, the BASE table is implemented in relatively 
few fonts (only Adobe seem to have consistently supported it, and only, 
to my knowledge, for CJK ideographic baseline adjustment). This means 
that even if the 'hanging baseline' value of the BASE table were 
interpreted in browsers and other software as the head-line alignment 
value and not confused with baseline alignment, this data point wouldn't 
be available in the fonts.

JH


1. http://www.microsoft.com/typography/otspec/base.htm


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Received on Friday, 16 January 2015 19:09:21 UTC