bug#29078: 25.2; font issue with FreeType 2.8; should not use the rounded ascender and descender

[Werner LEMBERG]

>> For TrueType fonts, FreeType 2.8 uses different rounding rules for
>> values in the FT_Size_Metrics structure, which is apparently used
>> by Emacs (since there are visible changes). In particular, one can
>> now have ascender + descender > height. The consequences are:
>>
>> 1. Tables look a bit ugly.
>>
>> 2. Windows (with the same number of text lines) are unnecessarily
>>    higher than before.
>>
>> 3. This can break existing configurations, where Emacs would be
>>    opened at start up, with an expected window size.
>>
>> I could notice changes at least with the default Mono font, which
>> appears to be DejaVu Sans Mono.
>>
>> Upstream now recommends to use the values from the FT_Face
>> structure and scale them manually:
> 
> I don't have FreeType2.8 to test, but you're saying we need to do this?
> 
> diff --git a/src/ftfont.c b/src/ftfont.c
> index 35f5923376..d16bf09a1e 100644
> --- a/src/ftfont.c
> +++ b/src/ftfont.c
> @@ -1153,18 +1153,9 @@ ftfont_open2 (struct frame *f,
>    upEM = ft_face->units_per_EM;
>    scalable = (INTEGERP (AREF (entity, FONT_AVGWIDTH_INDEX))
>             && XINT (AREF (entity, FONT_AVGWIDTH_INDEX)) == 0);
> -  if (scalable)
> -    {
> -      font->ascent = ft_face->ascender * size / upEM + 0.5;
> -      font->descent = - ft_face->descender * size / upEM + 0.5;
> -      font->height = ft_face->height * size / upEM + 0.5;
> -    }
> -  else
> -    {
> -      font->ascent = ft_face->size->metrics.ascender >> 6;
> -      font->descent = - ft_face->size->metrics.descender >> 6;
> -      font->height = ft_face->size->metrics.height >> 6;
> -    }
> +  font->ascent = ft_face->ascender * size / upEM + 0.5;
> +  font->descent = - ft_face->descender * size / upEM + 0.5;
> +  font->height = ft_face->height * size / upEM + 0.5;
>    if (INTEGERP (AREF (entity, FONT_SPACING_INDEX)))
>      spacing = XINT (AREF (entity, FONT_SPACING_INDEX));
>    else

No.  The `scalable' branch must make a distinction between TrueType
and non-TrueType fonts if the font gets fully hinted (i.e., if the
TrueType bytecode gets interpreted), something like

  if (scalable)
    {
      if (use_truetype_bytecode_hinting(font))
        {
          /* use TrueType rules for rounding */
          font->ascent = ROUND(ft_face->ascender * size / upEM)
          font->descent = ROUND(-ft_face->descender * size / upEM);
          font->height = font->ascent + font->descent;
        }
      else
        {
          font->ascent = CEIL(ft_face->ascender * size / upEM);
          font->descent = FLOOR(-ft_face->descender * size / upEM);
          font->height = ROUND(ft_face->height * size / upEM);
        }
    }
  ...


    Werner