[HarfBuzz] How to get hb_face_t and hb_font_t without Freetype?

[Jonathan Kew]

On 11/05/2019 20:38, Eli Zaretskii wrote:
>> Cc: Behdad Esfahbod <behdad at behdad.org>,
>>   "harfbuzz at lists.freedesktop.org" <harfbuzz at lists.freedesktop.org>
>> From: Jonathan Kew <jfkthame at gmail.com>
>> Date: Sat, 11 May 2019 20:11:17 +0100
>>
>>> Yes. The font file.  Maybe describe what you are trying to do?
>>>
>>
>> If you've got access to the font as a file or as a single buffer in
>> memory, then wrapping the entire thing as a blob and handing it to
>> hb_face_create will be simplest.
> 
> Would wrapping in a blob the buffer returned by GetFontData be enough?

If you use GetFontData to get the complete font as a single buffer (i.e. 
pass zero for the dwTable parameter), yes.

Alternatively, you could use hb_face_create_for_tables, with a 
reference_table_func that uses GetFontData to read individual tables 
when harfbuzz asks for them.

> 
>> In a case where you don't necessarily have easy access to the complete
>> font file, but have platform APIs that you can use to retrieve specific
>> font tables (like IDWriteFontFace::TryGetFontTable on Windows, or
>> CGFontCopyTableForTag on macOS), that's where you might prefer to use
>> hb_face_create_for_tables (like Firefox does). This expects you to
>> provide a reference_table_func that will return a blob containing the
>> data of any given font table (identified by its 32-bit OpenType table tag).
> 
> So there should be a function for each of the OpenType table tag, each
> function returning a pointer to the table's data?

There's a single reference_table_func that is passed a table tag and 
returns a blob for the requested table. How that reference_table_func is 
implemented is up to you, depending on what the platform provides; in 
practice, most platforms offer a single function that similarly accepts 
a table tag and returns that table.

HTH,

JK