Re: [ft] Freetype Scan Converter

[Robin Watts]

In message <address@hidden>
          Werner LEMBERG <address@hidden> wrote:
> > Apologies if this is an FAQ, but is there some documentation for
> > the algorithm anywhere?
> Unfortunately, no.  This is David's work, and I have to take my hat
> off to him :-)

Would David be available to comment on it then?

> I would be glad if you could contribute something, extending
> FreeType's `raster.txt' file with an appropriate section.

I'd have a go, except at the moment I feel my understanding is such
that I'd possibly only lead people astray.

> libart is available from
> 
>   http://git.gnome.org/browse/libart_lgpl/
> 
> and documentation for it seems to be here:
> 
>   http://www.gnome.org/~mathieu/libart/libart.html

I'd found libart, but not the html documention, so thanks for that.
As far as I can tell, from a quick scan through, the docs there say
purely that each pixel gets a delta value, which describes the change
in the percentage coverage from pixel to pixel - all sensible enough,
and clearly enabling you to get exactly the values you need to
antialias with.

I can't see any reference to how those values are calculated though.

> > I understand that each cell stores a "cover" and an "area" value
> > for a pixel, but the exact definition of the values stored there
> > is eluding me.  [...]
> The above page seems to nicely document it; perhaps not completely,
> but there is definitely more than we have.

Sadly (according to the documentation at least) the values held by
libART bear no resemblance (that I can see) to the cover and area
values held by freetype.

Even if the libART page did say exactly how its values were
calculated that wouldn't immediately help with understanding
freetypes code as they don't appear to be obviously related to the
values that freetype carries around.

There is a note in freetype saying that it's implementation, while
inspired by libART, is quite different. Hence while I will try to
find some time to dig through libARTs code and understand it, I fear
it won't immediately help.

If David was available to give a quick explaination that could save
me a lot of time, and I could then maybe have a go at writing
something.

Thanks,

Robin
-- 
Robin Watts,             Email: <mailto:address@hidden>
Warm Silence Software,   WWW:   <http://www.wss.co.uk/>
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