Thanks for the explanation, Eli. The idea of making tags not influence the reordering of the surrounding text nor the base direction was one of the first use case I thought of when I heard about the Isolation characters the first time. I believe it would make editing BiDi HTML much easier. I'll file a feature request about the idea.
Regards, Dov On Sun, Apr 5, 2015 at 11:48 AM, Eli Zaretskii <e...@gnu.org> wrote: > > Date: Sun, 5 Apr 2015 11:20:41 +0300 > > From: Dov Grobgeld <dov.grobg...@gmail.com> > > Cc: "Nadav Har'El" <n...@math.technion.ac.il>, Dotan Cohen < > dotanco...@gmail.com>, > > linux-il <linux-il@cs.huji.ac.il> > > > > I admit that I have very little knowledge about how font selection and > > reordering logic works in emacs. > > Font selection has absolutely no bearing on this discussion. The > visual order is decided based on the characters' properties, not on > how they will be drawn on the glass. For example, they will be > reordered even if they have no glyphs in any available font, or if you > set up a display table to, say, display Hebrew letters as upper-case > Latin letters. > > As to reordering, it happens at display time. The mechanism that > generates data structures used to draw font glyphs on the screen > produces visual order from the logical order, so that those data > structures hold the text already reordered. > > > One way of carrying out reordering of <h1>שלום!</h1> would be if the > emacs > > major mode was be able to inject the FSI...PDI characters before the > text is > > passed to the paragraph direction determining logic. Is this possible? > > No. The logic that determines base paragraph direction is part of the > reordering code which produces visual order from logical order. That > code runs as part of redisplay, and examines only the text that is in > the buffer. So the FSI..PDI embeddings must be present in buffer text > to do what you want. > > If something like what you want is required without changing buffer > text, there could be special text properties on some portions of > buffer text, that would be interpreted by the display code. But that > isn't implemented, mainly because no one expressed any special > interest in that, let alone presented a worked-out set of requirements > for it. >
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