Commit 5deb92fd authored by Eli Zaretskii's avatar Eli Zaretskii
Browse files

doc/lispref/display.texi (Bidirectional Display): New section.

parent be18c5a5
2011-07-23 Eli Zaretskii <eliz@gnu.org>
* display.texi (Bidirectional Display): New section.
2011-07-16 Lars Magne Ingebrigtsen <larsi@gnus.org>
Tim Cross <theophilusx@gmail.com> (tiny change)
Glenn Morris <rgm@gnu.org>
......
......@@ -34,6 +34,8 @@ that Emacs presents to the user.
* Display Tables:: How to specify other conventions.
* Beeping:: Audible signal to the user.
* Window Systems:: Which window system is being used.
* Bidirectional Display:: Display of bidirectional scripts, such as
Arabic and Farsi.
@end menu
@node Refresh Screen
......@@ -5966,3 +5968,131 @@ This hook is used for internal purposes: setting up communication with
the window system, and creating the initial window. Users should not
interfere with it.
@end defvar
@node Bidirectional Display
@section Bidirectional Display
@cindex bidirectional display
@cindex right-to-left text
Emacs can display text written in scripts, such as Arabic, Farsi,
and Hebrew, whose natural ordering of horizontal text for display is
from right to left. However, digits and Latin text embedded in these
scripts are still displayed left to right. It is also not uncommon to
have small portions of text in Arabic or Hebrew embedded in otherwise
Latin document, e.g., as comments and strings in a program source
file. Likewise, small portions of Latin text can be embedded in an
Arabic or Farsi document. For these reasons, text that uses these
scripts is actually @dfn{bidirectional}: a mixture of runs of
left-to-right and right-to-left characters.
This section describes the facilities and options provided by Emacs
for editing and displaying bidirectional text.
@cindex logical order
@cindex visual order
@cindex unicode bidirectional algorithm
Emacs stores right-to-left and bidirectional text in the so-called
@dfn{logical} (or @dfn{reading}) order: the buffer or string position
of the first character you read precedes that of the next character.
Reordering of bidirectional text into the @dfn{visual} order happens
at display time. As result, character positions no longer increase
monotonically with their positions on display. Emacs implements the
Unicode Bidirectional Algorithm (a.k.a.@: @acronym{UBA}) described in
the Unicode Standard Annex #9, for reordering of bidirectional text
for display. Reordering of bidirectional text for display in Emacs is
a ``Full bidirectionality'' class implementation of the @acronym{UBA}.
@defvar bidi-display-reordering
The buffer-local variable @code{bidi-display-reordering} controls
whether text in the buffer is reordered for display. If its value is
non-@code{nil}, Emacs reorders characters that have right-to-left
directionality when they are displayed. The default value is
@code{nil}. Text in overlay strings (@pxref{Overlay
Properties,,before-string}), display strings (@pxref{Overlay
Properties,,display}), and @code{display} text properties
(@pxref{Display Property}) is also reordered if the buffer whose text
includes these strings is reordered for display. Turning off
@code{bidi-display-reordering} for a buffer turns off reordering of
all the overlay and display strings in that buffer.
Reordering of strings that are unrelated to any buffer, such as text
displayed on the mode line (@pxref{Mode Line Format}) or header line
(@pxref{Header Lines}), is controlled by the default value of
@code{bidi-display-reordering}.
@end defvar
@cindex unibyte buffers, and bidi reordering
Emacs does not reorder text in unibyte buffers, even if
@code{bidi-display-reordering} is non-@code{nil} in such a buffer.
This is because unibyte buffers contain raw bytes, not characters, and
thus don't have bidirectional properties defined for them which are
required for correct reordering. Therefore, to test whether text in a
buffer will be reordered for display, it is not enough to test the
value of @code{bidi-display-reordering} alone. The correct test is
this:
@example
(if (and enable-multibyte-characters
bidi-display-reordering)
;; Buffer is being reordered for display
)
@end example
In contrast to unibyte buffers, unibyte display and overlay strings
@emph{are} reordered, if their parent buffer is reordered. This is
because plain-@sc{ascii} strings are stored by Emacs as unibyte
strings. If a unibyte display or overlay string includes
non-@sc{ascii} characters, these characters are assumed to have
left-to-right direction.
@cindex display properties, and bidi reordering of text
Text covered by @code{display} text properties, by overlays with
@code{display} properties whose value is a string, and by any other
properties that replace buffer text, is treated as a single unit when
it is reordered for display. That is, the entire chunk of text
covered by these properties is reordered together. Moreover, the
bidirectional properties of the characters in this chunk of text are
ignored, and Emacs reorders them as if they were replaced with a
single character @code{u+FFFC}, known as the @dfn{Object Replacement
Character}. This means that placing a display property over a portion
of text may change the way that the surrounding text is reordered for
display. To prevent this unexpected effect, always place such
properties on text whose directionality is identical with text that
surrounds it.
@cindex base direction of a paragraph
Each paragraph of bidirectional text can have its own @dfn{base
direction}, either right-to-left or left-to-right. Text in
left-to-right paragraphs is displayed beginning at the left margin of
the window and is truncated or continued when it reaches the right
margin. By contrast, display of text in right-to-left paragraphs
begins at the right margin and is continued or truncated at the left
margin.
@defvar bidi-paragraph-direction
Emacs determines the base direction of each paragraph dynamically,
based on the text at the beginning of the paragraph. The precise
method of determining the base direction is specified by the
@acronym{UBA}; in a nutshell, the first character in a paragraph that
has an explicit directionality determines the base direction of the
paragraph. However, sometimes a buffer may need to force a certain
base direction for its paragraphs. For example, a buffer that visits
a source code of a program should force all its paragraphs to be
displayed left to right. The variable
@code{bidi-paragraph-direction}, if non-@code{nil}, disables the
dynamic determination of the base direction, and instead forces all
paragraphs in the buffer to have the direction specified by its
buffer-local value. The value can be either @code{right-to-left} or
@code{left-to-right}. Any other value is interpreted as @code{nil}.
@end defvar
@defun current-bidi-paragraph-direction &optional buffer
This function returns the paragraph direction at point in the named
@var{buffer}. The returned value is a symbol, either
@code{left-to-right} or @code{right-to-left}. If @var{buffer} is
omitted or @code{nil}, it defaults to the current buffer. If the
buffer-local value of the variable @code{bidi-paragraph-direction} is
non-@code{nil}, the returned value will be identical to that value;
otherwise, the returned value reflects the paragraph direction
determined dynamically by Emacs.
@end defun
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