Re: Adding tis620.2533-0 into Thai Pango engine

[Theppitak Karoonboonyanan <thep links nectec or th>]

Dear K.Chookij,

IMO, the code point assignments of both Windows and Solaris extensions are
not different in the shaper engine's point of view. The difference is only
the U+F71B glyph shape, not the different code tables as in the case of
tis620-0, tis620-1 and tis620-2.

So, I don't think tis620-3 is necessary here.

However, let me guess that you mean to choose to handle tis620-3 with
Wtt2.0 cell clustering engine. (Please excuse me if I'm wrong.)  But I
think Wtt2.0 can be applied to other tis620 encodings as well.

It should be nice if there is a way for users to choose their preferred
shaping engine. Otherwise, just choose the best one for them.

-Theppitak.

On Wed, 1 Nov 2000, Chookij Vanatham wrote:

> 
> K.Theppitak,
> 
> Regarding the issue of adding tis620.2533-0 in Thai pango engine,
> I've been thinking if actually, I would say that even Solaris uses Thai
> fonts used in WindowsThai extension but, actually, we modified the glyph
> at U+F71B which is used for bad sequence indicator.
> 
> Then, may be, we should have tis620-3 for Thai Solaris extension.
> 
> Let me know what you think and if you agree, please let me know
> how this convention on encoding field on XLFD will be maintained
> and well-known to anybody as the standard.
> 
> Thanks,
> 
> Chookij V.
> 
> 
> ] 
> ] ] 
> ] ] According to our experience, there are three different practices of Thai
> ] ] fonts for rendering :
> ] ] 
> ] ] 1. Plain tis620 : combining characters are placed at the safe positions to
> ] ]    prevent collapsion. There are two practices of this kind :
> ] ]    - negative-offset-zero-width diacritics (this makes the fonts apply to
> ] ]      many applications, such as Netscape, which support Western fonts
> ] ]      without knowing they are rendering Thai fonts)
> ] ]    - real monospace fonts (used in mule/emacs; this requires the
> ] ]      applications to combine characters into cells)
> ] ] 
> ] ] 2. MacThai extension : an extended tis620 code set, by using codes in the
> ] ]    range 0x80-0x9f and in some free slots to keep the prepositioned
> ] ]    combining characters. This needs a shaping algorithm to produce 
> ] ]    elegant rendering.
> ] ] 
> ] ] 3. WindowsThai extension : similar to MacThai extension, but used in
> ] ]    Windows Thai Editions.
> ] ] 
> ] ] The last two code sets are mapped to their own private area of Unicode and
> ] ] cannot be used together.
> ] ] 
> ] ] So, we are now discussing about a convention on the encoding field on XLFD
> ] ] to distinguish the three code sets :-
> ] ] 
> ] ]   -tis620-0   for plain tis620
> ] ]   -tis620-1   for MacThai extension
> ] ]   -tis620-2   for WindowsThai extension
> ] ] 
> ] ] Note that the years in the registry field are omitted, because tis620.2529
> ] ] and tis620.2533 do not differ in content. Both can be referred to as
> ] ] tis620 without confusion.
> ] ] 
> ] ] However, Mr Chookij, could you please describe how Wtt2.0 is implemented
> ] ] in Solaris,
> ] In Solaris, we have Wtt2.0 rule for the display. Wtt2.0 provides the table
> ] to check whether the current Thai character can be combined with the previous
> ] character. If it can't be combined like those invalid Thai sequences such as,
> ] U+0E48 + U+0E34 --> this is invalid sequence, then U+0E34 will be displayed
> ] as its own cell.
> ] 
> 

____________________________________________________________________
Theppitak Karoonboonyanan
Software and Language Engineering Laboratory, NECTEC
http://www.links.nectec.or.th/~thep/  mailto:theppitak nectec or th