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...
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