[ft-devel] rounding bug in MS TT rasterizer

[Werner LEMBERG]

Sergej, Paul,


from time to time we from the FreeType team get reports that our
engine doesn't rasterize certain TrueType glyphs correctly.  The most
noticeable and disturbing case is glyph `W' in the font `arialuni.ttf'
(see attached image).  Careful inspection has shown that the very
problem here (and in other cases) is a rounding bug in the MS
rasterizer which the bytecode instructions of arialuni.ttf explicitly
circumvent.

Rogier van Dalen writes:

  I see the very same asymmetry on my rasterizer.  Point 25 is the
  problematic point.  I've calculated it all; Microsoft rounds the
  x-coordinate 895.455782 up to 896 (14.0) in an interpolate
  instruction.  It is then rounded to half a pixel, to 14.5 according
  to the MS interpreter, to 13.5 according to FreeType's and mine.  A
  delta instruction then moves it left to 13.5 (12.5).  To conclude,
  the MS rasterizer does faulty rounding here, and a delta instruction
  could have been spared if they had implemented it properly.

I'm quite sure that this rounding bug has historical roots and won't
be fixed -- many, many fonts would render worse.  This means that we
have to emulate the behaviour of the MS rasterizer to get `correct'
results.

Can you tell us more about this problem?  Which instructions are
affected?  What exactly is the reason for the incorrect rounding?

Since so many fonts rely on this `feature' it should be documented --
ideally in the TrueType specification.


    Werner


PS: I wonder whether Apple's rasterizer renders this glyph similar to
    MS Windows...