BidiMirrored property and ancient scripts (Was Re: Plain text custom fraction input)
richard.wordingham at ntlworld.com
Fri Jul 24 14:29:58 CDT 2015
On Fri, 24 Jul 2015 09:28:05 -0700
Ken Whistler <kenwhistler at att.net> wrote:
> First there is a general issue of general mirroring of body text for
> some ancient scripts, which in paleographic contexts often followed
> conventions (no longer seen, except in rare edge cases) of having the
> direction of glyph orientation switch depending on line orientation.
Direction switching is commonplace in didactic text for Ancient Egyptian
in modern texts. Right-to-left text is also natural when showing how
to normalise hieratic or demotic to hieroglyphs.
> It is a dextroverse versus
> sinistroverse layout issue, as nearly all of this kind of epigraphic
> text does not occur in *bi*directional contexts at all -- but rather
> in text where everything goes one direction.
Remember that parentheses in pure Arabic or Hebrew text without numbers
are also mirrored. The same would apply for N'ko, where numbers are
Please remind us of the purpose of RLO and LRO. Are you suggesting
that their use may be 'out of scope' in some contexts?
Recall Bidi rule L4:
"A character is depicted by a mirrored glyph if and
only if (a) the resolved directionality of that character is R, and (b)
the Bidi_Mirrored property value of that character is Yes.
The Bidi_Mirrored property is defined by Section 4.7, Bidi Mirrored of
[Unicode]; the property values are specified in [UCD]. This rule can be
overridden in certain cases; see HL6."
The higher-level protocols are beyond the control of a supplier of
plain text. It is not good that they may be kept secret from the
user displaying the text, as would often be the case defined by a
protocol that says that the font automatically selected defines the
mirroring or not.
> Note that even in scripts that have this
> behavior paleographically, the occurrence of RTL versus LTR versions
> may differ statistically over time and eventually die out in favor
> of one direction or the other. See Old Italic. For that matter,
> see ancient Greek, which had RTL, LTR, and boustrophedon, but
> which eventually settled on strictly LTR layout.
The question is about controlling mirroring when the 'abnormal'
direction (largely as defined by the UCD) is used, not whether it is
More information about the Unicode