[HarfBuzz] hb_glyph_position_t::x_offset and scaled FreeType fonts

[Jamie Dale]

The scale applied to the hb_font seems to affect how badly this breaks
(it's currently less bad, but still broken).

It is possible that I was just never setting the hb_font scale correctly as
I was handling scaling internally from my user-data and ignoring the font
setting.

-Jamie.

On 8 May 2017 at 21:15, Jamie Dale <jamiedale88+harfbuzz at gmail.com> wrote:

> Something is definitely causing it to change when the font is scaled
> though:
>
>  Scale | x_offset
> ~0.666 |  2141
> ~0.832 |  340
> ~1.000 | -1461
> ~1.165 | -3262
> ~1.332 | -5063
>
> -Jamie.
>
> On 8 May 2017 at 20:45, Jamie Dale <jamiedale88+harfbuzz at gmail.com> wrote:
>
>> Hey all,
>>
>> I'm running HarfBuzz 1.2.4 with a modified hb_font implementation based
>> on the FreeType implementation in the vanilla version.
>>
>> One of the differences is that I pass through the FreeType scale as
>> user-data and apply it when getting the metrics from FreeType (via
>> FT_Set_Transform).
>>
>> We've been running with this for a while, and haven't noticed any issues
>> until today when we tried it with some Thai text and found that the
>> hb_glyph_position_t::x_offset was coming back wrong once the text was
>> scaled (weirdly this same issue doesn't seem to affect
>> hb_glyph_position_t::y_offset).
>>
>> When the scale is 1, everything is fine, but when the scale is <1 the
>> glyph is too far right (suggesting that the offset is too large), and when
>> the scale is >1 the glyph is too far left (suggesting that the offset is
>> too small).
>>
>> That would suggest to me that the value I'm getting back hasn't been
>> scaled appropriately for the actual FreeType glyph. Unfortunately I'm not
>> familiar enough with how HarfBuzz calculates that value to know where that
>> lack of scale is happening.
>>
>> I do notice that there's no FreeType override for the
>> set_glyph_h_origin_func from hb_font. Could that be the issue?
>>
>> The Thai text being used is ฝึ (U+0E1D, U+0E36) and the font is Tahoma
>> (available in Windows).
>>
>> Has anyone else attempted something like this?
>>
>> Thanks,
>> Jamie.
>>
>
>
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