On 3/1/14 06:12, James Clark wrote:
On Mon, Dec 30, 2013 at 7:26 PM, Jonathan Kew <[email protected] <mailto:[email protected]>> wrote:There's overlap here with the process of font-matching (choosing the font(s) to be used for a given text sequence), which is clearly out of scope for harfbuzz. If a given Unicode character is not supported (exactly, or via a *canonical* [de]composition) by a given font, there are several possible outcomes: just render the font's .notdef glyph; render some synthetic representation of the codepoint (hexbox); render a compatibility-equivalent character/sequence, if such exists; choose a different font. I agree with this model. There's a general problem of what you might call "fallback": what to do if the requested font does not have a glyph for the requested character. This makes me realize that I don't understand the big picture of how this fallback process interacts with harfbuzz. In order to do fallback, you need to do character to glyph mapping.
Not necessarily. You need to know the character repertoire supported by the font, but you may not need to actually map to glyphs. In Firefox, for instance, font fallback is done based on a per-font *bit* map of supported Unicode codepoints. So at the font fallback stage, we know whether the character is present, but do not map it to a glyph.
If the application is expected to do fallback before calling harfbuzz, why does harfbuzz expect chars rather than glyphs as input to the shaping process?
One good reason, at least, would be that shaping requires harfbuzz to have access to Unicode character properties, and it is not necessarily possible to derive these from glyph IDs.
I would have expected there to be some application callback that harfbuzz would call when there is a need to do fallback; the application would use this to tell harfbuzz how to handle this situation for this particular character. Some of these harfbuzz could handle by itself, others would require cooperation between the application and harfbuzz.
This doesn't fit into the harfbuzz model, where the "unit of work" is a buffer that is shaped with one specified font according to the rules of one script and language system.
Harfbuzz does not attempt to manage larger blocks of text that may involve multiple directions, scripts, languages or fonts. All this belongs (IMO) to a higher level of the software stack (such as Pango), which will require a richer data model than HB's buffer, and is responsible for font fallback as well as for dividing the text into direction/script/language/font runs, each of which can be separately shaped by harfbuzz.
Using compatibility decompositions to provide a fallback rendering would be one of the options such a higher-level component might use. Pushing this down into HB itself seems quite problematic to me, given the huge variety of types of compatibility decompositions, some of which require some kind of additional styling to avoid corrupting the intended meaning of the data.
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