From: John Hudson <>
Date: Mon, 22 Aug 2011 10:10:18 -0700

Shriramana Sharma wrote:

> I was just noting
> that the glyph tables themselves don't *use* the actual codepoints of
> the characters getting ligated (while they *refer* to them).

Characters are mapped to glyph IDs in the font cmap tables.

Glyph IDs are mapped to other glyph IDs (one-to-one, one-to-many,
many-to-one, or one-to-one-of-many) in the layout GSUB table.

> No! See Behdad's post -- it is clearly said that the lookup will still
> be in logical order (1001, 1012) -> (1540) and not in visual order as
> you say.

I think there may be some confusion in this discussion over what
constitutes 'visual order'. I try to avoid the term because it is
difficult for right-to-left readers to accustom themselves to thinking
of visual order as anything other than right-to-left. I prefer the term
'reading order' or 'resolved order', i.e. resolved bidi and script
shaping order, which may have involved integrated reordering (reordering
within the glyph processing) as in the case of Indic scripts.

> Nope -- they are placed in the lookup table in *logical* order. IIUC the
> entire sequence of glyphs is only reordered from RTL at the very end.
> Peter or Behdad, can you corroborate this?

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.


Tiro Typeworks
Gulf Islands, BC
The criminologist's definition of 'public order
crimes' comes perilously close to the historian's
description of 'working-class leisure-time activity.'
  - Sidney Harring, _Policing a Class Society_
Received on Mon Aug 22 2011 - 12:12:55 CDT

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