[cairo] NO_HINTING problems with custom FT_Face

Ian Britten britten at caris.com
Wed Oct 14 07:57:20 PDT 2009

Hi all,
   [ Cairo v1.8.8, FreeType v2.3.9 ]
I was hoping that someone could help me get to the root of a problem
I'm having when using my custom FreeType font driver via Cairo.  I
realize it's not truly a Cairo problem per-se, but I know that there
are some people here knowledgeable about this stuff, and I'm almost at
wits end ... :(

- When I create+draw my text upright and use load_flags=0, the text
   draws as expected.  However, when I use load_flags=FT_LOAD_NO_HINTING,
   my text no longer draws.

- I have a custom FT fontdriver, for accessing our custom font format
   seamlessly as a FT_Face.
- I have a non-Cairo rendering system, in which the driver works as
   needed (Admittedly, in very limited/controlled workflows).
- I'm trying to render this text through Cairo, which is putting
   untested expectations on my driver, and which I'm trying to fix.
- Although I obviously can't reproduce this in a sample (due to the
   custom font driver), the Cairo code is pretty generic.  I simply
   get my FT font, set it in the context, set the colour/size/etc, and
   do a show_glyphs().  The same code works with TT fonts though.

I'm reasonably certain the problem is with my font driver, specifically
the FT_Slot_LoadFunc callback(*) not handling the given load_flags
parameter correctly.  However, I'm not sure what Cairo is expecting
of my driver, since Cairo sets some of those flags when loading
glyphs, and the FT documentation of those flags is a bit sparse.
(*) Called by FT_Load_Glyph(), called by _cairo_ft_scaled_glyph_init()

In the case that works (I set load_flags=0), my driver received a
In the case that doesn't work (load_flags=FT_LOAD_NO_HINTING), my
driver received a load_flags=514

I suspect that the problem is that my driver doesn't do anything when
FT_LOAD_IGNORE_TRANSFORM is absent, and Cairo is expecting it to have
done _something_ to various fields...  I'm just not sure what, and in
what units/format (eg: double? 26.6? 16.16?).  I've been flailing
around some, but with no success yet.

So, is anyone familiar with this realm?  (And, is anyone still reading
this? :P )  Can anyone offer any description or suggestion of what I
need to do?
Or, if I'm completely off-base, does anyone have any other ideas why
my text doesn't draw when I turn off hinting?

Many thanks for any help!

More information about the cairo mailing list