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Re: [ft] Off by one error in freetype (or in programmer?)

From: Werner LEMBERG
Subject: Re: [ft] Off by one error in freetype (or in programmer?)
Date: Fri, 06 Apr 2018 08:40:03 +0200 (CEST)

>> Nice, but completely irrelevant.  For the conversion of font
>> outlines to pixmaps or graymaps, pixels *are* rectangles, both by
>> definition and construction.
> That cannot be true.


> The edges of the rectangles would give rise to aliasing, aka
> stair-stepping.
> <>

Of course there will be stair-stepping!  Just have a look into the
additional documentation files that accompany the old TrueType
specification (both Apple and MS versions).  You'll see rectangles
everywhere talking about B/W rasterization.

The definition in TrueType whether a B/W pixel gets switched on or off
is very simple: If the pixel center is within or on the outline
boundary, it gets displayed.  [I'm ignoring dropout handling for the
sake of clarity.]

A graphics system might apply filters to display such bitmaps on
screens, but this is not covered by the TT specification.

The situation is different for ClearType since it does take care of
the physical reality of screens.  However, this doesn't change the
fact that glyph outlines are shifted by (-0.5,-0.5) before applying
TrueType bytecode instructions.


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