[HarfBuzz] Units of members of hb_glyph_position_t
eliz at gnu.org
Tue May 28 15:44:50 UTC 2019
> From: Behdad Esfahbod <behdad at behdad.org>
> Date: Mon, 27 May 2019 21:21:10 -0400
> Cc: "harfbuzz at lists.freedesktop.org" <harfbuzz at lists.freedesktop.org>
> control those units (if they are under the client program's control).
> They are controlled mainly using hb_font_set_scale().
> In particular, I get a huge value of x_advance for the letter U+05EA
> HEBREW LETTER TAV when it is followed by U+05BB HEBREW POINT QUBUTS.
> The value of x_advance I get is 1229, which is too large even after
> dividing by 64 (which, btw, I still am not sure is TRT in my case,
> FreeType works in 26.6 fixed-point, ie. 64 units per 1.0. That's where the 64 value comes from. And you
> don't see it in your code because hb_ft_font_create* sets that on hb_font for you.
> In your Windows code, you should call hb_font_set_scale(). I believe right now you are *not* calling, and you
> get values in the face's UPEM. That's the default scale for fonts. You can get the face UPEM using
OK, I figured out how to scale the units from UPEM to pixels for a
given font size, and now I see reasonable results after such scaling.
However, I think something is still amiss, because I still don't
understand how to determine the values with which to call
hb_font_set_scale. Say I call it with an integer value N, what will
that produce in terms of values of hb_glyph_position_t? Will the
values there be in the 0..N range, where N means the full height of
the em box? If so, how would I then convert those values to pixels --
this conversion will need the font size as well, right? And if so, I
might well leave the values in UPEM units, and convert them to pixels
by hand. I feel that I'm still missing something, since you said "you
should call hb_font_set_scale". So presumably if I call that
function, conversion to pixels will somehow become easier?
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