[cairo] font thrashing

[David Reveman]

On Fri, 2004-04-30 at 16:15 -0400, John Ellson wrote:
> There needs to be some caching of font lookups, probably here in 
> cairo_gstate.c:
> 
> 
>     cairo_status_t
>     _cairo_gstate_select_font (cairo_gstate_t       *gstate,
>                                const char           *family,
>                                cairo_font_slant_t   slant,
>                                cairo_font_weight_t  weight)
>     {
>         if (gstate->font != NULL)
>             cairo_font_destroy (gstate->font);
>             
>                                                                                
> 
>         gstate->font = _cairo_font_create (family, slant, weight);
>     
>                                                                                
> 
>         return CAIRO_STATUS_SUCCESS;
>     }
> 
> 
> 
> If the same font request is repeated (from the SVG for example) then
> the previous font resolution (and its glyph caches) should be reused.
> 
> Probably there should be a most-recently-used cache of 10 or so fonts.
> 
> We noticed severe performance problems today when rendering an svg file 
> that
> respecified the same font multiple times.   David Reveman's glyph cache 
> patch
> didn't help because the cache was destroyed each time the font was 
> respecified.
> 
> 
> I wrote the generic font and glyph caching code in Tom Boutell's gd, so
> I could take a shot at coding this, unless anyone else wants to claim it?

The font and its glyph cache will not be destroyed if you take a
reference to the font.

example:

caior_select_font (cr, "Times",..
font1 = cairo_current_font (cr);
cairo_font_reference (font1);

caior_select_font (cr, "Sans",..
font2 = cairo_current_font (cr);
cairo_font_reference (font2);
...

font1 and font2 can then be set current with cairo_set_font and the
glyph cache will not be destroyed until you call cairo_font_destroy.

We can of course include this type of font caching in cairo but I think
it's better that we leave it to the application. 

I cache fonts like this in Waimea WM and it works great. Adding this
kind of font cache to libsvg-cairo should be easy.

-David