Re: [ft-devel] Two patches for FT

[Werner LEMBERG]

> > With the current CVS of fontforge (earlier versions won't work
> > correctly) I've debugged the instructions of the `8' glyph.  It is
> > a rounding problem, and it seems to be a very subtle one.  The
> > last DELTAP1 instruction in the hinting code moves point 17 0.375
> > units up (at 11 ppem).  This causes a shape change as shown in the
> > attached images.  As can be seen, the spline almost hits the pixel
> > center which decides whether the pixel is on or off.  Within
> > Windows, the pixel center is *below* the curve, so it is off.
> > Within FreeType, the pixel center is *above* (or on) the curve, so
> > it is on.  David?
>
> If I understand correctly what you're saying, the rendering
> difference would come from the way Windows and FreeType tesselate
> the bezier arc.

I don't think so.  I rather believe that the hinted point coordinates
differ slightly.  Is it possible on a Windows box to get the hinted
outline coordinates of glyph `8' in tahoma.ttf (version 3.15 from
Windows XP) at 11ppem?  Perhaps using VOLT?  This would answer the
question, and it would be of *great* help to identify the problem in
the FreeType code.  BTW, a similar problem appears with glyph `6'.

> Could someone give me the coordinates of the three control points.
> It should be easy to check wether the pixel center should be below
> or above the curve with some computational tricks.  The graphics
> seems to show that the arc should be below the pixel center, but I
> hope it's wrong.

The arc is definitely below the pixel center, if you zoom in with
FontForge.

> Another possibility would be that the point position before the
> DELTAP1 isn't correct, which would represent a different challenge
> to debug.

Exactly.  Rogier, what does your TT interpreter return for this
particular glyph?


    Werner