[HarfBuzz] Why mark glyphs are skipped when MarkBasePos matching?

[Shusaku KIMURA]

Thank you for quick and kindly response.

> One could have two lookups, one ignoring marks above, and one ignoring
> marks below, but that requires GDEF to categorise marks as marks above
> or as marks below.

Do you mean Mark Attachment Class Definition Table in GDEF? Certainly,
it would be possible to be solved by Mark Attachment Class Definition
Table and two lookup tables of different markAttachmentType.

I agree this solution is more complex than HarfBuzz's solution
(ignoring all marks), but I believe this solution is the way of
conforming to the OpenType Layout specification. Therefore I am not
sure whether the HarfBuzz's solution is really necessary.

I suppose that there was actual font data which expectes such
behavior, and the implementation was needed. Is this correct?

> A major complaint about OpenType is
> that the syntax of font files is defined, but not their semantics.

I am not familiar with whole of the OpenType specification, but I
studied OpenType Layout specification, and I agree with you for
example about BASE table. But GSUB and GPOS table are relatively clear
about the semantics including this topics, I think.

Please understand that I do not intend to criticize HarfBuzz
behavior. I am sorry when you are offended. I just want to know the
position of OpenType Layout specification for HarfBuzz implementation.

For HarfBuzz, is OpenType (Layout) specification not always absolute?

Thank you,

-- 
Shusaku KIMURA
skimura0 at gmail.com