Re: [ft-devel] Severe rendering bug

[Werner LEMBERG]

>> This is a bug in the hinting instructions of DejaVu for this glyph.
>> The setup of the projection vector for the affected IP instructions
>> is bad.  I suggest that you make a bug report.
> 
> I'm certainly not an expert, but I think you might be wrong, and
> that this actually is a bug in the freetype hinter, specifically in
> the IP instruction in the face of a non-square font size and
> non-orthogonal projection vectors.

Good catch!  This is indeed a possibility.

> I just investigated this today with no prior knowledge of the
> hinting language, so I can of course be horribly wrong.  The
> pictures you attached look like output from Fontforge which uses the
> Freetype hinting engine, so it would of course have the same bugs.

Correct.

> The TrueType Viewer Tool on the other hand has its own hinting
> interpreter, and it does not have any of the problems that I
> reported.

Interesting!  I've tried to make the source compile on GNU/Linux
(again), but unfortunately it uses non-standard (old) C++ and is based
on Qt3.  Recent g++ compilers refuse to compile it, and porting to Qt4
is non-trivial.  Using Qt3 libraries I managed to compile it, but it
segfaults, and I wasn't able to fix it :-(

BUT: Right now I've tried the Windows binary, and it works on my
GNU/Linux box :-)

> 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...

> It would be great if someone could verify my reasoning, dispute it,
> or just spread some light in general.

It would be great if someone could try the Windows hinter (with
ClearType disabled)...

> Also, and explanation of what orus means would be very welcome
> bonus. :-)

>From include/internal/tttypes.h:

  FT_Vector*  org;         /* original point coordinates  */
  FT_Vector*  cur;         /* current point coordinates   */
  FT_Vector*  orus;        /* original (unscaled) point coordinates */


    Werner