Re: Pango Fc Shaper gives incorrect geometry information

[Nicola Fontana <ntd entidi it>]

Il Thu, 04 Sep 2014 16:34:29 +0100 Simon Cozens <simon simon-cozens org> scrisse:

> ...
> For instance, the code below on OS X (using the pango-basic-coretext module) 
> returns the correct answer, 7.070312. On Linux, it reports the width of a 12pt 
> Gentium "z" as 7.000000pts. The width of an "m" is 12.851562pt on OS X, but 
> 12.000000pts on Linux.

A bit late but... for the posterity.

I had this very same issue recently. I think this is not a bug: my
guess is this is the expected behavior of the hinting algorithm. I
found some rounding code in pango triggered by it, e.g.:

https://git.gnome.org/browse/pango/tree/pango/pango-layout.c?h=1.40.12#n4688

When the device unit is pixel it is not a big deal, but if you use
points (AFAIK the default device unit on a cairo PDF surface) the
result is quite gross.

> ...
> #include <pango/pangocairo.h>
>
> int main() {
>    PangoFontMap * fm = pango_cairo_font_map_get_default ();
>    PangoContext * pango = pango_font_map_create_context(fm);
>    PangoAttrList * pal = pango_attr_list_new();
>    PangoAttribute * pa1 = pango_attr_family_new("Gentium");
>    PangoAttribute * pa2 = pango_attr_size_new(PANGO_SCALE * 12);
>    PangoItem* charitem;
>    PangoGlyphString* pgs = pango_glyph_string_new();
>    PangoGlyphInfo g;
>
>    pango_attr_list_insert(pal, pa1);
>    pango_attr_list_insert(pal, pa2);

Disabling the hint metrics just before pango_itemize() returns the
expected result:

    cairo_font_options_t *options = cairo_font_options_create();
    cairo_font_options_set_hint_metrics(options, CAIRO_HINT_METRICS_OFF);
    pango_cairo_context_set_font_options(pango, options);
    cairo_font_options_destroy(options);

>    charitem = pango_itemize(pango, "m", 0, 1, pal, NULL)->data;
>    pango_shape("m", 1, &(charitem->analysis), pgs);
>    g = (pgs->glyphs)[0];
>    printf("%f\n",g.geometry.width/(double)PANGO_SCALE);
> }

Ciao.
-- 
Nicola