Better document various TrueType hinting modes.

Text based on private communication with Greg Hitchcock and added to the
repository with his permission.

include/freetype/ftttdrv.h

@@ -52,6 +52,83 @@ FT_BEGIN_HEADER
    *
    *   The TrueType driver's module name is `truetype'.
    *
+   *   We start with a list of definitions, kindly provided by Greg
+   *   Hitchcock.
+   *
+   *   _Bi-Level_ _Rendering_
+   *
+   *   Monochromatic rendering, exclusively used in the early days of
+   *   TrueType by both Apple and Microsoft.  Microsoft's GDI interface
+   *   supported hinting of the right-side bearing point, such that the
+   *   advance width could be non-linear.  Most often this was done to
+   *   achieve some level of glyph symmetry.  To enable reasonable
+   *   performance (e.g., not having to run hinting on all glyphs just to
+   *   get the widths) there was a bit in the head table indicating if the
+   *   side bearing was hinted, and additional tables, `hdmx' and `LTSH', to
+   *   cache hinting widths across multiple sizes and device aspect ratios.
+   *
+   *   _Font_ _Smoothing_
+   *
+   *   Microsoft's GDI implementation of anti-aliasing.  Not traditional
+   *   anti-aliasing as the outlines were hinted before the sampling.  The
+   *   widths matched the bi-level rendering.
+   *
+   *   _ClearType_ _Rendering_
+   *
+   *   Technique that uses physical subpixels to improve rendering on LCD
+   *   (and other) displays.  Because of the higher resolution, many methods
+   *   of improving symmetry in glyphs through hinting the right-side
+   *   bearing were no longer necessary.  This lead to what GDI calls
+   *   `natural widths' ClearType, see
+   *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec21.  Since hinting
+   *   has extra resolution, most non-linearity went away, but it is still
+   *   possible for hints to change the advance widths in this mode.
+   *
+   *   _ClearType_ _Compatible_ _Widths_
+   *
+   *   One of the earliest challenges with ClearType was allowing the
+   *   implementation in GDI to be selected without requiring all UI and
+   *   documents to reflow.  To address this, a compatible method of
+   *   rendering ClearType was added where the font hints are executed once
+   *   to determine the width in bi-level rendering, and then re-run in
+   *   ClearType, with the difference in widths being absorbed in the font
+   *   hints for ClearType (mostly in the white space of hints); see
+   *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec20.  Somewhat by
+   *   definition, compatible width ClearType allows for non-linear widths,
+   *   but only when the bi-level version has non-linear widths.
+   *
+   *   _ClearType_ _Subpixel_ _Positioning_
+   *
+   *   One of the nice benefits of ClearType is the ability to more crisply
+   *   display fractional widths; unfortunately, the GDI model of integer
+   *   bitmaps did not support this.  However, the WPF and Direct Write
+   *   frameworks do support fractional widths.  DWrite calls this `natural
+   *   mode', not to be confused with GDI's `natural widths'.  Subpixel
+   *   positioning, in the current implementation of Direct Write,
+   *   unfortunately does not support hinted advance widths, see
+   *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec22.  Note that the
+   *   TrueType interpreter fully allows the advance width to be adjusted in
+   *   this mode, just the DWrite client will ignore those changes.
+   *
+   *   _ClearType_ _Backwards_ _Compatibility_
+   *
+   *   This is a set of exceptions made in the TrueType interpreter to
+   *   minimize hinting techniques that were problematic with the extra
+   *   resolution of ClearType; see
+   *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec1 and
+   *   http://www.microsoft.com/typography/cleartype/truetypecleartype.aspx.
+   *   This technique is not to be confused with ClearType compatible
+   *   widths.  ClearType backwards compatibility has no direct impact on
+   *   changing advance widths, but there might be an indirect impact on
+   *   disabling some deltas.  This could be worked around in backwards
+   *   compatibility mode.
+   *
+   *   _Native_ _ClearType_ _Mode_
+   *
+   *   (Not to be confused with `natural widths'.)  This mode removes all
+   *   the exceptions in the TrueType interpreter when running with
+   *   ClearType.  Any issues on widths would still apply, though.
+   *
    */