opentype layout(GPOS table)

[li bo <libo imc gmail com>]

Hi all,

Now I'm trying to implement indic language by using opentype font. And I know less about opentype, and just know that the opentype layout(GSUB and GPOS) helps shape and combine indic characters.
I have read opentype specification, but i'm not very clear about it.
I want to ask that how GPOS table works, Does it generate the absolute coordinate for indic characters or the relative coordinate(offset to previous glyph)?
Thanks,
libo