Here's a portion of the description of the 'pref' feature tag in the OpenType tag registry:
Function: Substitutes the pre-base form of a consonant. In some scripts of south or southeast Asia, such as Khmer, the conjoined form of certain consonants is always denoted as a pre-base form. In the case of some scripts of south India, variations in writing conventions exist such that a conjoined Ra consonant may be written as a pre-base form, or a below-base or post-base form. Fonts may be designed to support one or another convention. If a font is designed to support a writing convention in which conjoined Ra is a pre-base form, the Pre-Base Forms feature would be used. Example: In the Khmer script, the consonant Ra has a pre-base subscript form called Coeng Ra. When the sequence of Coeng followed by Ra occurs, its pre-base form is substituted. Peter -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Saturday, July 23, 2011 11:35 PM To: Peter Constable; [email protected]; [email protected]; Philippe Verdy Subject: RE: Prepending vowel exception in Lontara/Buginese script ? From: Peter Constable <[email protected]> > Van is mistaken in his understanding of OpenType Layout. There is no > mechanism to describe re-ordering in OpenType Layout tables in a font. > That must be handled by the OTL client software. > > Peter Well crapola. So that reordering is completely controlled by Unisribe, Peter? Then what is the pre-base forms/substitution OT features for? I had always thought that vowel reordering was their purpose. -Van
