Re: RTL PUA?

On 22 August 2011 22:40, John Hudson <john_at_tiro.ca> wrote:

>
>
>
>>
> Glyph ID inputs for OTL processing are according to reading/resolved order.
> This is typically the same as logical order, but the term logical order
> really applies to character strings, not glyph strings, which are much more
> maleable. The order of input strings in GSUB lookups or contexts is
> dependent not only on the underlying character order, but also on the
> results of previous GSUB lookups. So while, unlike AAT and Graphite,
> OpenType Layout doesn't explicitly provide for glyph re-ordering, some kinds
> of glyph reordering are possible using sequences of contextual lookups to
> duplicate a glyph in a second location in the string and then remove the
> first instance. We use this in some Devanagari fonts to enable subsequent
> ligation of short ikar variants to the left of a consonant base with reph
> marks to the right of that base.
> र्क्मि
>
> JH
>
>
> Open Font Format GSUB tables contain glyphs in visual (or reading order)
and not in logical order. The logical order of characters is transformed to
a visual order of characters before the font layer. Thus for Devanagari, the
I matra is moved left and the reph is moved right. It is this visual
syllable of characters that is transformed to a visual syllable of glyphIds
using the cmap table in the Open font.
Consider Devanagari <RKMI>. The logical order of characters is <Ra Halant Ka
Halant Ma IMatra>. This logical order is transformed to the visual order of
characters <IMatra Ka Halant Ma Ra Halant>, then to the visual order of
GIds <IMatraGId KaGId HalantGId MaGId RaGId HalantGId>. GSub tables will
transform <KaGId HalantGId> -> <HalfKaGId>.
<RaGId HalantGId> will be transformed to <rephGId>. So finally, the visual
order of GIds will be <IMatraGId HalfKaGId MaGId rephGId>. This can be
further beautified but is already in a readable form. It will appear as
र्क्मि
So no reordering is needed in the Open font for general case.

For special cases, (example <RDMI> , logical <Ra Halant Da Halant Ma
IMatra>), reordering in the font is needed. Here, <DaGId HalantGId MaGId>
does not reduce to <HalfDAGId MaGid> in most Hindi fonts, but remains as
<DaGId HalantGId MaGId> . The visual order of Gids for the syllable is
  <IMatraGId DaGId HalantGId MaGId RaGId HalantGId>.
It is converted to <IMatraGId DaGId HalantGId MaGId rephGId> by the font.
Now the font has to reorder the IMatrGId and the rephGId over the retained
HalantGId. The correct glyph sequence would be
<DaGId rephGId HalantGId IMatraGId MaGId >
र्द,मि

(Using Gmail I have created RDMI. The halant is shown by the comma. How does
one create Halant?)

Comparing <RKMI> and <RDMI> we can see that font level reordering of
glyphIds is needed only for <RKMI>.

vinod kumar

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