Why do the Hebrew Alphabetic Presentation Forms Exist

[Richard Wordingham]

On Thu, 4 Jun 2020 16:29:50 +0000 (UTC)
abrahamgross--- via Unicode <unicode at unicode.org> wrote:

> What? I don't understand what you're saying here
> 
> 2020/06/04 午後0:17:21 Richard Wordingham via Unicode
> <unicode at unicode.org>:
> > However, what if a word has two lamedhs, and
> > only is to be bent?  Are mere word-processors now up to handling
> > that and processing the whole word as a whole, even though
> > different parts have different feature settings?

Enabling and disabling features changes the set of rules a renderer
uses to convert a sequence of characters to a sequence of coloured
glyphs with defined relative positions.  Even in simple scripts, they
can control, amongst many other things, the horizontal spacing of
letters, and even adjustments to white space, even handling things that
were handled on typewriters by rules such as two spaces after commas and
three spaces after full stops.  (I'm told these were the RAF rules.)  A
set of rules is easiest to implement if the rules are the same for the
whole of the string being shaped.  One solution is to chop a string up
into runs with the same rules to be applied.  However, the font then
loses control over how the two parts line up.

Now, if you have two lamedhs in a word, you may wish to bend one and
not bend the other.  Any adjustment between letters becomes difficult
if the letters are subject to different rules.  An obvious question
would be, "Which rules apply to their interaction?"  Much as I dislike
the idea of use variation sequences to control 'stylistic' effects, it
does avoid these problems.  It does come with the cost of increasing
the number of glyphs whose interaction must be considered, though
there are tricks to reduce the amount of thought required.

Richard.