[cairo] Reference counting of cairo free type face

[Julian Viereck]

> Cairo maintains a holdover cache of the MRU scaled fonts (outside of any
> context) which accounts for the extra references. The
> cairo_font_face_finish() / cairo_scaled_font_finish() interface has been
> proposed to decouple those extra references in a user-controllable way.
> However, in order to hook into the destroy notification for when the
> font is released you want to use cairo_scaled_font_set_user_data().

Thanks a lot for this input Chris.

Is there an upper limit for the fonts being cached? If so, I wouldn't 
bother to have some constant memory overhead for font faces that are no 
longer needed.

Best,

Julian

On Wed Nov 14 10:01:47 2012, Chris Wilson wrote:
> On Tue, 13 Nov 2012 22:17:00 +0100, Julian Viereck <julian.viereck at googlemail.com> wrote:
>> Hi there,
>>
>> I try to add font support to the node-canvas [1] library, which uses
>> cairo for rendering. I use the freetype2 library to load a font face and
>> then use it in cairo. The font rendering is working, but I got stuck on
>> memory management of the cairo font face. In particular, the cairo font
>> face's reference counter is not reset to 1 at the point I expect it to be.
>>
>> Here's what I'm doing. I concentrate on the essential parts only. Let
>> "ref(cr_face)" be "cairo_font_face_get_reference_count(cr_face)" to get
>> the internal reference counter for the cairo font face.
>>
>>       // Create a free type font face from a font file.
>>       FT_New_Face(library, *filePath, faceIdx, &ft_face);
>>       // Create a cairo font face.
>>       cr_face = cairo_ft_font_face_create_for_ft_face(ft_face, 0);
>>
>>       // At this point ref(cr_face) == 1.
>>
>>       // Set the cairo font face on a cairo rendering context.
>>       cairo_set_font_face(ctx, cr_face);
>>
>>       // At this point ref(cr_face) == 2. I guess that's okay, as the
>> font is requird for rendering.
>>
>>       cairo_text_path(ctx, str);
>>
>>       // At this point ref(cr_face) == 4.
>>
>>       cairo_destroy(ctx);
>>
>>       // At this point ref(cr_face) == 3.
>>
>> Shouldn't the reference count after the "cairo_destory" be 1 again and
>> the cairo font face deleted? Do I need to perform some manually cleanup
>> first?
>
> Cairo maintains a holdover cache of the MRU scaled fonts (outside of any
> context) which accounts for the extra references. The
> cairo_font_face_finish() / cairo_scaled_font_finish() interface has been
> proposed to decouple those extra references in a user-controllable way.
> However, in order to hook into the destroy notification for when the
> font is released you want to use cairo_scaled_font_set_user_data().
> -Chris
>