2011/8/23 Doug Ewell <[email protected]>: > srivas sinnathurai <sisrivas at blueyonder dot co dot uk> wrote: > >> If same codes within PUA becomes standard for different purposes, > > They aren't "standard." Two different private agreements could assign > different characters to the same PUA code points. > >> how to get both working using same font? > > You can't.
I do agree. >> How to instruct text docs, what font if different fonts are used? > > There's no standard way to specify even one font or private agreement in > plain text, let alone how to switch between them within the same > document. This is not an intended use of the PUA. There exists such standard in the context of plain-text rendering, because of font fallback mechanisms (in Windows with Uniscribe, such fallback mechanism is not tunable per user preferences, as the list of alternative fonts that are tried is fixed by the implementation of Uniscribe; but anyway it still exists), which implies that multiple fonts will be scanned with an order of preference; font fallback is involved each time a character is not mapped on the selected font but may be mapped in another font. Such mechanism is exactly similar to the explicit fallback mechanism in CSS (where one provides an ordered comma-separated list of font-family names), but that also extends this list of fonts automatically using the default font fallback mechanisms used for plain-text rendering. In other words, even if you can't instruct a plain-text to use glyphs from one font or from another for the same code point (PUA here), such possibility still exists in rich-text rendering, because all glyphs can become selectable as variants (including the variants listed in the same font for the same glyph, in standardized OpenType features, provided that the rich-text application implements such glyph-selection mechanism). PUAs are effectively not meant to supply the PUA agreement. This has to be provided elsewhere, but a font can perfectly transport this agreement (for the font as a whole which is separately selectable, just like its designed glyph variants are individually selectable by some typographic feature tables, as well as by index, for example several swash variants of the same letter with more or less decorations). If you can use font fallbacks, then you can render the same text containing distinct PUAs designed for distinct PUA agreements (and this demonstrates the utility of the conscript registry, which allows cooperation between authors of separate agreements, that have accepted to encode their PUA characters with non-conflicting PUA code point assignments). -- Philippe.
