From: William_J_G Overington (wjgo_10009@btinternet.com)
Date: Mon Dec 07 2009 - 03:59:27 CST
On Friday 4 December 2009, Eric Muller <emuller@adobe.com> wrote:
> This is another example of my mantra that what matters is
> the combination of the layout engine and the font. If you
> were to try this font with the flash.text.engine in Flash
> Player 10, you would get what you expect: "f ZWJ i"
> ligates regardless of the styling (i.e. even if the styling
> asks for no ligatures; the ZWJ takes precedence over
> styling), "f ZWNJ i" does not ligate regardless of the
> styling, and "f i" ligates or not depending on the styling.
> While it certainly takes some support in the font (having a
> ligature, and exposing it somehow), layout engines are in
> the business of interpreting the characters and driving the
> font appropriately. What matters is the aggregate.
>
> Eric.
The combination of layout engine and the font is indeed important.
Even if an OpenType font has a glyph for the ligature and the font also includes glyph substitution rules for both the "f ZWJ i" case and the "f i" case, the ligature glyph will not be displayed unless the application program is OpenType-aware.
Thus it is often useful also to make the glyph available by a direct mapping. The fi ligature is something of a special case, as the precomposed ligature has a regular Unicode mapping: most Latin ligatures do not have a regular Unicode mapping; yet such precomposed ligature glyphs can be mapped into the Private Use Area if so desired. Whilst in a perfect situation where all of the applications that would ever be used with the font would be OpenType-aware there would be no need for such a Private Use Area mapping of a ligature glyph, at the present time there are various applications that can use characters from plane 0 of Unicode, yet are not OpenType-aware and therefore cannot handle glyph substitution.
Whilst recognising that using Private Use Area mappings can sometimes be less than a perfect solution, nevertheless in some situations the use of a Private Use Area mapping can be useful. For example, suppose that a graphic file for adding to the web is being produced and the application that produces the graphic file is not OpenType-aware; in such a situation the use of a Private Use Area mapping can allow the graphic to be produced without any interoperability problems of using a Private Use Area mapping.
At least one major font manufacturer includes Private Use Area mappings for ligatures and also for alternates in at least some of its OpenType fonts.
Something which I like to do in my own fonts is to include a glyph for U+200D the Zero Width Joiner. This means that when using a non-OpenType-aware application program that the specific request for a ligature is clear.
William Overington
7 December 2009
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