Thanks!
My angle is mostly based on my project of a text editor which is
designed for working on RTL-heavy typesetting source files; think
something similar to LaTeX, where you could have line of RTL text, in
the middle of it an invocation of a function with an English name, and
in its' parameter list you would have more RTL text. Something like
"TEXT \foo{ARG} MORE TEXT" (with the usual UAX #9 convention of having
upper case characters represent RTL and lower case represent LTR).
With the BiDi algorithm as-written the directionality of the text leaks
out to the weak and natural direction syntax tokens and messes them up.
I fully know about explicit format control character as the way to
counter that, and the editor can make it easier for users to enter the
correct ones. The patch I was already able to land into Qt 6.9 helps a
lot with this. However, the ease of use can be significantly improved,
because the editor has semantic information about the meaning of the
code and could resolve directionality of weak and neutral characters
based on that information. So the user doesn't has to fiddle with adding
and removing the control characters as they edit the source - this can
be quite compelling UX.
As the _only_ leverage Scribe gives to modify the visual appearance of
text without interfering with the QTextDocument internal buffers (and
more importantly - undo stack) is the extra character formats in
QTextLayout, I was exploring use of that avenue. Seeing that
LayoutDirection is documented as also a character format property led me
thinking that this could be an opening.
However I acknowledge this is probably very niche use case, so if this
designation was never intended there is no need to further push the point.
Best,
Igor
On 03/11/2024 12:37, Volker Hilsheimer wrote:
On 31 Oct 2024, at 14:51, Igor Khanin <i...@khanin.biz> wrote:
Greetings,
I hope this is the right place to post this. I'm looking for advice as well as
any historical context I can get.
The documentation states that the `QTextFormat::LayoutDirection` text format
property is both a paragraph and a character property. When set on a block
format, it indeed affects the block's base direction via
`QTextBlock::textDirection`. However when set on a character format, to the
best of my understanding it appears to have no effect whatsoever. I'd expect it
to have some sort of similar impact on the BiDi algorithm - perhaps overriding
the directional character type of the character to be L or R, or maybe making a
sequence of characters with the same property value behave as if they were an
isolation run, or some such. But that can't obviously happen currently, as
`QBidiAlgorithm` has no access to any formats and finishes its' work before
character formats are resolved anyway.
This appears to be the case since Scribe was first released back in Qt 4, or at
the very least predates the current Git history. So my question is why then the
documentation says that? Was there an intention to implement some behavior that
then never happened?
Furthermore - would it make sense for this property to start _actually_ doing
anything on individual characters? I have a hacky POC, but don't want to waste
my own and other's time if this is something that would be considered too niche
or disruptive.
Best,
Igor
Hi Igor,
It’s the right place to post this, but it would help to understand what you are
trying to achieve with an implementation of layout direction on as a character
format. My understanding of the subject doesn't go very deep (and I can’t say
if the documentation is wrong or simply misleading - perhaps it’s a read-only
attribute), but to change layout direction within a block, you’d usually insert
Unicode control characters into your text.
Volker
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