Re: ISO 10646-1 Level 2

From: Theppitak Karoonboonyanan (
Date: Tue Jun 22 1999 - 13:12:53 EDT

On Tue, 22 Jun 1999, Markus Kuhn wrote:

> Is it for ISO 10646-1 Level 2 scripts sufficient, if only one single
> combining character is allowed to follow a normal character or is there
> some reasonable very small upper limit on the number of combining
> characters per base character?

The upper limit for the number of combining characters in normal Thai text
is 2. Please also note that Thai text is displayed in 4 levels: base line,
below, above, and top. This is important for the design of font to use the
overstriking method.

> It might even be feasible to allow more than one combining character, as
> long as they do not overlap, for example if there is a mark both above
> and below the base character and the order of the combining characters
> does not matter, as long as we do not have to reposition combining
> characters and as long as the upper limit of combining characters per
> character is small (say 2).

Yes, up to 2 combining characters can follow a base character in Thai.
The characters below and above base line never occur at the same time in
general. But the ones at the top level are always possible to appear.

> Does ISO 10646-1 Level 2 support with simple overstriking and one
> combining character per base character sound like a sensible approach to
> get support for scripts like Thai into xterm?

For Thai: overstriking - yes; combining characters <= 2.

Theppitak Karoonboonyanan
Software and Language Engineering Laboratory, NECTEC

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