Richard Wordingham wrote:
On Mon, 17 Jun 2013 08:47:10 +0530
Suresh P<[email protected]> wrote:
Behdad Esfahbod wrote:
On 13-06-13 07:57 AM, Suresh P wrote:
Behdad Esfahbod wrote:
On 13-06-11 07:29 AM, Suresh P wrote:
I want to know whether it is possible to prevent pref and blwf
rules being applied using contextual rules
But I find the ligation(for pref and blwf) always happen
irrespective of the rules in the contextual lookup.
Okay, as per the spec.
Show us the font and malfunctioning sequence.
There is no malfunctioning as such. But is looks weird when one see a
pre-base RA in front of a base YA ( and a below base LA subjoining
YA ) because it is never written like this. Instead, in actual use,
ligation (and reordering) is avoided and written with explicit
virama. A test case is attached herewith and it applies to any font
which uses pre-base RA and below base LA. It is better left to the
font developer to decide the contexts.
I'll offer my understanding of the situation. If my explanation
doesn't help you, it may be necessary to examine the actual <GSUB>
table of your font to explain what is happening, and other tables will
provide context for it.
So what you want in exactly one lookup for the <blwf> feature:
In context <ya virama la> do nothing
else in context <virama la> ligate virama la.
It seems neither blwf ligature lookup for <VIRAMA LA> nor pref lookup
for <VIRAMA RA> honour the contextual lookups. :( The calt feature I
used for YA VIRAMA LA with fontforge was:
//
lookup caltContextualAlternatesinMalay {
lookupflag 0;
ignore sub [\b_ya \m_U \b_la]'
} caltContextualAlternatesinMalay;
feature calt {
script mlm2;
language dflt ;
lookup caltContextualAlternatesinMalay;
} calt;
//
Also tried [\b_ya] [\m_U]' [\b_la].
(The second, normal condition will invoke a ligature lookup.)
That should do what you want; are you having trouble expressing that
in the lookup definition language you are using?
The same trick should work with the <pref> feature:
In context <ya virama ra> do nothing
else in context <virama ra> ligate virama ra.
Again, the second, normal condition will invoke a ligature lookup.
This ligature lookup should *not* be listed under the <pref> feature.
The OpenType specification at
http://www.microsoft.com/typography/OpenTypeDev/malayalam/intro.htm
says:
"Reorder pre-base reordering consonants: If a pre-base reordering
consonant is found, reorder it according to the following rules:
1. Only reorder a glyph produced by substitution during application
of the <pref> feature. (Note that a font may shape a Ra consonant
with the <pref> feature generally but block it in certain
contexts.)
..."
This is exactly the logic you want.
yes
RA will be classified as a
pre-base reordering consonant, but not reordered in the context <ya
virama ra> because there will be no substitution resulting from
application of the <pref> feature. I don't know if this rule applies
to the old Malayalam rendering engine - you may need to ensure that you
have the script tag <mlm2> rather than <mlym> in the <GSUB> table.
The font contains script tags for both versions and I think harfbuzz
defaults to mlm2.
Richard.
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