Re: [ft-devel] Severe rendering bug

[Tobias Ringström]

On 01/02/2012 10:40 PM, Werner LEMBERG wrote:
> > Looking at the Freetype TrueType interpreter implementation, the
> > projection vector is in pixel space, meaning that the 300%
> > horizontal stretch that I use in my example has been applied.
> >
> > The problem is that in Ins_IP, the projections are calculated using
> > orus coordinate vectors, which are in em-space (NOT stretched).
> >
> > If I change from using orus to org, the instruction behaves as I
> > would expect it to from reading the TrueType specification, and
> > there are no artefacts.
>
> Can you prepare a patch, please?  I currently don't have the time to
> really delve into this issue, but maybe `orus' is correct, but should
> be scaled properly...

The difference between org and orus is that org has rounding errors 
caused by the scaling to 26.6 fixed point pixel coordinates, so scaling 
orus seems to be more correct, although I struggle to see how it could 
matter in practice.  But since the rest of the code really likes to use 
orus, it's best to keep using it in IP as well I suppose.

The attached patch fixes the bug, and does not change the case where 
x_scale and y_scale are equal, so it should be quite safe and not 
introduce any new bugs.

Please review this patch and consider it for inclusion in the next 
release of Freetype.

/Tobias

freetype-scaled-IP.patch
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