[HarfBuzz] Some beginner questions about using Harfbuzz API

[Nikolay Sivov]

On 30.11.2015 20:33, Deepak Jois wrote:
> I am trying to build a Lua interface to Harfbuzz that I can hopefully
> use with LuaTeX some day. I have got something working after taking
> inspiration from the harfbuzz-tutorial code[2], and the Lua
> bindings[3] in Simon Cozen’s SILE package. What I have works for the
> basic cases, but I had a few questions about some API functions that I
> wanted to clarify here.
>
> For reference, here[1] is the code I am referring to. My questions are:
>
> * Why are there two different ways to create font-like objects? There
> is hb_face_create/hb_font_create (used in SILE) and hb_ft_font_create
> (used in harfbuzz-tutorial). Any guidelienes on what one should use?

It depends on what you want, and prototypes are self explanatory - 
hb_face_create() uses blob, hb_ft_font_create() uses FT_Face, so it's 
meant to be used together with freetype library.
>
> * Are there any helper methods to create font blobs from files?
> Currently the code just slurps the file contents (in Lua) and then
> passes it on as a char * to the C code.

hb_blob_create() ? I don't think there's any api that will read a file 
for you, so you have to read it yourself.

>
> * I don’t fully understand the concept of font indexes. I guess it’s
> something meant to disambiguate fonts in font-collections. However,
> what about the case when a single file contains a single font? I am
> using 0 as the font-index for those cases. Is that fine?

Exactly, it's only meaningful for collections. 0 is what should be use 
in other cases.

>
> * What does the call hb_ot_font_set_funcs(hb_font) do?

...

>
> * I suppose the way to get glyph names is to use
> hb_font_get_glyph_name(…). However that does not work in my code,
> although hb-shape seems to report the glyph names correctly. All I get
> are empty strings. Any clues as to what I might be doing wrong.

It depends on backend, from the looks of it, names are only returned for 
freetype-based font objects.

>
> *  Why does the harfbuzz-tutorial code divide the values by 64 before
> printing it out or sending it to the Cairo drawing API. I am guessing
> it has something to do with DPI values of output display devices but I
> don’t fully understand it. Is that an arbitrary choice, or the most
> commonly used value?

I don't know about this specific tutorial, but it sounds like fixed 
point value conversion.

>
> Thanks
> Deepak
>
> [1]: https://github.com/deepakjois/luatex-harfbuzz-shaper/blob/master/luaharfbuzz.c#L19
>
> [2]: https://github.com/behdad/harfbuzz-tutorial
>
> [3]: https://github.com/simoncozens/sile/blob/master/src/justenoughharfbuzz.c
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