Re: Pango: Accessing x-height/mean-line in Pango layout

[Olivier Fourdan <ofourdan redhat com>]

Hi Behdad,

Behdad Esfahbod said the following on 12/18/2012 06:19 PM:
> On 12-12-18 04:32 AM, Olivier Fourdan wrote:
> > [...]
> > While this would mostly work, it's not perfect as the positioning is done
> > based on the logical rect, which may vary depending on the text, ie "Papa" and
> > "papa" would not have the same height. Beside, to improve the positioning,
> > it's been requested that the label is placed at the middle of the lowercase
> > characters.
> That's not correct.  "Papa" and "papa" have the same logical height.  That's
> the whole difference between logical and ink extents.

Yes, you're right, of course, but what I meant is that "Papa" and 
"papa" do not have the same height /visually/ so using the logical 
rect does not give the expected result.

Using the ink rect is not suitable either as "Papa", "Mama", "papa" 
and "mama" would all have different heights.

> > Looking at typefaces definitions here [1] and [2], the positioning should be
> > ideally made based in between the baseline and the mean-line (baseline -
> > x-height).
> >
> > I wonder, how can I access the x-height (or better the meanline) using the
> > Pango API? Is there a simple API for this?
> Not directly.  If you are using pangofc, you can obtain the FT_Face from the
> PangoFont, then get the (I think) "OS/2" table of the font, and get the
> x-height from in there.

Humm, that sounds quite complicated and platform specific, no?

So I took another approach to the problem.

Assuming Pango rightfully places the strikethrough, I retrieved the 
position of the strikethrough with something like:

    context = pango_layout_get_context (layout);
    language = pango_language_get_default ();
    desc = pango_layout_get_font_description (layout);
    metrics = pango_context_get_metrics (context, desc, language);

    baseline = pango_layout_get_baseline(layout);
    strikethrough =  pango_font_metrics_get_strikethrough_position 
(metrics);
    thickness =  pango_font_metrics_get_underline_thickness (metrics);

    return PANGO_PIXELS(baseline - strikethrough - thickness / 2);

And that gives (what I believe is) an acceptable result, at least from 
my point of view.

Cheers,
Olivier