[HarfBuzz] HarfBuzz glyph offsets

[Jonathan Blow]

Well, there appears to have been no kerning problem, I only needed to add
HB's offset to FT's bearing and everything comes out right.

I was thinking of HB as a drop-in layer on top of FT as regards glyph
metrics, so this may just have been an error in my mental model. Though I
am not sure how one is supposed to form the proper mental model in an
efficient way... ... ...

On Wed, Dec 23, 2015 at 2:37 PM, Jonathan Blow <jon at number-none.com> wrote:

>
>>
>> FreeType does not give you any offsets. Unless you mean the side
>> bearings, but they are very different things and shouldn’t be used for
>> glyph placement at all. You are getting 0 offsets because either the
>> font does not do any special positioning here (not uncommon for old
>> Chinese fonts), or something is wrong in the way the text is shaped.
>>
>
> ​Okay, this is confusing. Maybe this has to do with different cultural
> assumptions or different ideas about what the software looks like, but
> bearings are just offsets to me. When I look at this diagram from the
> FreeType documentation:
>
> http://www.freetype.org/freetype2/docs/tutorial/step2.html
>
> It says that wherever the cursor is, add bearingX as an offset and draw
> the thing in the box. And in fact this is how I draw things when they show
> up in the proper places. So I am not sure what you mean here; my guess is
> that you are thinking of there being a little bit of a software stack
> living below you that itself adds bearingX to draw the thing in the box, so
> that it would be incorrect for you to do so?
>
> So then my interpretation is that the 'offsets' from HB are additional and
> are not meant to include bearings, and are only for kerning or for like
> things, and I should actually add both offsets together I guess.
>
>
>> For Arabic, it looks like you are applying the kerning on the wrong
>> direction, see how the ا in لطراز is moving closer to the ز instead of
>> the ر as I’d expect it.
>>
>
> ​That's quite possible, I will investigate. My old code, pre-HB, would
> switch to drawing glyphs right-to-left for RTL languages, but upon starting
> to use HB I was pretty confused because it looks to me like HB is always
> giving me glyphs in left-to-right order, even for RTL languages (if there
> is a setting that changes this, I have no idea). So at some point I just
> simplified all the rendering code to always be LTR, though it is possible I
> made a mistake or else for some reason kerning is RTL while everything else
> is LTR which would be really weird??
>
> Oh wait ... I remember something about kerning in otf/ttf always having to
> do with the logical order of glyphs and not the display order; does HB keep
> this convention? I had interpreted offsets as being screen positions, not
> deltas in some abstract kerning space, so that could be the source of the
> issue.
>  ​
>
>
>>
>> If you are not setting the correct text script, then you are unlikely to
>> get correct output. You need to analyse the text and pass the correct
>> script to HarfBuzz (resolving characters with common and inhere tied
>> script properties, etc).
>>
>
> ​In this case we have a fixed set of languages (currently 15) that we are
> just swapping in and out, so I can set this stuff on a per-language basis,
> so I guess for each one I need to know:
>
> * what to pass to hb_buffer_set_script​
>
> ​* what to pass to hb_buffer_set_language (I am not sure how this latter
> affects results in different ways than the former)
>
> ... Anything else?​
>
>
>
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