Thanks for the info. I actually started working with the extracted cluster information right before you sent this message, although that was to minimise the shaping requests I need to make by extracting the info (where possible) from the shaped data for the entire run of text.
I'll admit that colour only was a bad example, but aside from also being able to change the font or font size, our rich-text can also contain completely user-defined widgets. This can make extracting out the style information... tricky, since I don't really know how it's being used (and may actually be part of a nested control, such as a button or hyperlink). Rich-text itself is actually a secondary concern right now, my primary concern is selection highlighting (which uses a similar mechanism, as text is broken into runs where it is selected, since selection can change the text colour). That said, selection isn't allowed to change the font used so I can more easily combine the selected and non-selected text into a single shape, however I'm still unsure how ligatures would be handled in that case. I'll use English for simplicity since I can actually read it. Imagine I have the text "Magnificent", where the "fi" has been combined into a ligature. If I were to select "Magnif", then in order to change the colour of that portion of the text, the ligature would have to be split. This doesn't present a readability issue for English, but would it present issues for other languages? -Jamie. On 6 October 2015 at 22:45, Khaled Hosny <khaledho...@eglug.org> wrote: > On Tue, Oct 06, 2015 at 08:08:00PM +0100, Jamie Dale wrote: > > I suspect that the first shape has used some ligatures, and the second > > shape was unable to do that due to being unable to combine the glyphs (I > > have previously seen this with the "fi" ligature in English). > > > > If both of these forms are considered acceptable, then I'm happy enough, > > Shaping parts of text separately is generally a bad idea as you lose any > OpenType interaction between these parts, so you only do it when it is > absolutely necessary (e.g. due to font change). Though your second image > is still barely legible, it loses all the contextual substitutions > specified in the font and gives a very suboptimal result, but it can > make the text illegible in many other cases, for example when shaping > "لا". I expect Indic scripts to suffer more legibility-wise. > > The proper way it to identify rich-text attributes that shouldn’t break > shaping (color, underline, overline, etc.) and apply them after shaping, > using cluster values to do the reverse glyph to character index mapping > (while at it, use HB_BUFFER_CLUSTER_LEVEL_MONOTONE_CHARACTERS so that > you get more finer cluster mapping). > > Regards, > Khaled >
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