[HarfBuzz] Contextual shaping of Malayalam post(pre)/below base forms

[Behdad Esfahbod]

On 13-06-26 06:41 PM, Richard Wordingham wrote:
> On Wed, 26 Jun 2013 11:09:46 -0400
> Behdad Esfahbod <behdad at behdad.org> wrote:
> 
>> On 13-06-18 06:46 PM, Richard Wordingham wrote:
>>> lookup pref_st2
>>>     | y1 xx r3 | -- No sequence indices for this context!
>>>     | xx r3 |
>>
>> This wouldn't work.  You should match y1 in backtrack sequence, not
>> input sequence.  Otherwise this is what happens, if you have a
>> sequence y1 xx r3:
>>
>>   - HB tries applying the lookup at position zero, matches the first
>> rule, stops,
>>
>>   - It then advances position by one, trying to apply the rule at
>> position one, now the second rule matches and ligature is formed.
>>
>> For a rule to stop a later one from applying they should apply to the
>> same input sequence at the same position, ie, both should have "xx
>> r3" as their input, and the first one should have more context in
>> terms of backtrack / lookahead...  It can have a longer input
>> sequence too, but they should start the same.
> 
> Applying the advice above, I tried
> 
> lookup pref_st2
>    y1 | xx r3 |
>       | xx r3 |
>          0 pref_lkp1
> end lookup
> 
> It worked!  That is, <YA, VIRAMA, RA> was not affected by shaping, but
> was rendered as three glyphs, as intended.
> 
> However, I have two follow-up remarks, for the actual failure mechanism
> is not as described by Behdad.  However, it does seem prudent to start
> the input context at the virama.
> 
> Firstly, I don't understand the advancing rule.  Given an input
> sequence y1 xx r3 u2 and the contexts | y1 xx r3 | and | xx r3 |, what
> I had expected is:
> 
> A1) The shaper tries applying the look up at y1, matches the first rule,
> stops.
> 
> A2) In accordance with the paragraph, "Once the substitutions are
> complete, the client should move to the glyph position immediately
> following the matched input sequence and resume the lookup process from
> there", the lookup then advances to uu, finds no match, and does not
> form the ligature.
> 
> Am I misunderstanding the specification?  Does the specification comply
> with Uniscribe?  (I've not been having much success with my experiments
> with Uniscribe, so I can't yet answer such questions myself.)

You aren't misunderstanding the spec.  I think you are correct and this is a
HarfBuzz bug.  Can you send your previous font to me to test with Uniscribe?
Or test it yourself?  That said, I can't nail it down in the code why this is
not working as intended.

Humm.  Ok, now I know.  In the particular sequence y1 is the base, right?  If
yes, then it is NOT marked for the pref feature.  <pref> is applied to all
characters after the base.  I believe that's why your original design wasn't
working.  Still I'm interested to double check that Uniscribe does the same.


> I haven't yet worked out how HarfBuzz distinguishes a pre-base <VIRAMA,
> RA> ligature formed by the <pref> feature from a post-base <VIRAMA, RA>
> ligature formed by the <pstf> feature,

It doesn't.

-- 
behdad
http://behdad.org/