Re: [ft-devel] A question about freetype and harfbuzz

[Behdad Esfahbod]

On Mon, 2007-05-28 at 23:48 +0200, Werner LEMBERG wrote:
> > [...] I believe that any OpenType Layout engine should do its own
> > validation,
> 
> Why?  What is bad about letting FreeType doing that?  Afterwards you
> can omit any error handling...

For one thing, FreeType is not necessarily available.  We are removing
FreeType dependency from HarfBuzz, and Qt wants to use HarfBuzz on
Windows too.

The other reason is that, for example, HarfBuzz is forgiving about some
problems.  A nonexistent lookup index for example is automatically
ignored.  Or a subtable that is not ever referenced can have an
(invalid) offset of zero and it will still work.  These kind of
exceptions were added to make fonts that already worked with Windows
work with Pango too.  My plan for the rewrite I'm doing is, whenever I
see a problem in a table, instead of rejecting the font, or having to
check for validity all the time, just modify the data in place to make
it valid.  That will partically defeat the idea of using mmap()ed fonts,
but it's not a big deal.

>     Werner
-- 
behdad
http://behdad.org/

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