[HarfBuzz] Units of members of hb_glyph_position_t

[Behdad Esfahbod]

On Tue, May 28, 2019 at 11:44 AM Eli Zaretskii <eliz at gnu.org> wrote:

> > 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
> > hb_face_get_upem().
>
> 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?
>

You pick what value you want to represent one pixel as.  Say, you choose
1024.  Then if you want to render at "16px" font size, you set scale to
16*1024.  That's all.

-- 
behdad
http://behdad.org/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/harfbuzz/attachments/20190528/6656d580/attachment.html>