On Mon, Jul 11, 2011 at 1:48 AM, Werner LEMBERG
<address@hidden> wrote:
> > I've been working on a bug for webkit related to an odd behavior
> > with the Ahem font in the Qt framework. I tracked it down to Qt
> > either receiving incorrect descent/ascent values from
> > FT_Size_Metrics or using the Freetype library incorrectly.
>
> > When using FT_Set_Char_Size(face, width, height, h_res, v_res),
> > should face->size->metrics have the proper values for all font
> > types afterwards or is it necessary for Qt to handle knowing
> > truetype fonts use ((TT_Size)face->size)->root.metrics instead?
BTW, right after a call to FT_Set_Char_Size, `face->size->metrics' and
`(TT_Size)face->size)->root.metrics' point to the same object... You
probably mean something different?
My mistake, I meant ((TT_Size)face->size)->metrics has different values than face->size->metrics. The way it is currently, the only way to get to the tt_size_reset values are to cast to TT_Size. The metrics there are correct, the root.metrics are the values before executing tt_size_reset.
> > If face->size->metrics is supposed to have proper values after
> > calling FT_Set_Char_Size, then there is a bug in either
> > ttdriver.c or ttobjs.c.
> >
> > ttdriver.c line 248:
> > if ( FT_IS_SCALABLE( size->face ) )
> > error = tt_size_reset( ttsize );
> > will need to copy ttsize->metrics over to ttsize->root.metrics
> >
> > or
> >
> > ttobjs.c line 1057:
> > metrics = &size->metrics;
> > will need to be
> > metrics = &size->root.metrics;
> >
> > are some potential fixes.
This is a difficult issue. The question is what `proper values'
actually mean. From the documentation for FT_Size_Metrics:
Note that due to glyph hinting, these values might not be exact for
certain fonts. Thus they must be treated as unreliable with an
error margin of at least one pixel!
Normally, FreeType does grid-fitting of metrics (cf. the stuff guarded
by the GRID_FIT_METRICS macro in ftobjs.c's
ft_recompute_scaled_metrics) using FLOOR and CEIL if hinting is
requested for, regardless of the font format. For TrueType fonts,
this gets replaced internally with ROUND if the font asks for it (in
tt_size_reset). This explains the one-pixel difference you experience
sometimes.
Mhmm. Perhaps the rounding code in `tt_size_reset' should be executed
only if GRID_FIT_METRICS is undefined? The metrics values actually
*are* already rounded to integers...
Chia-I, can you comment also?
Werner