* include/freetype/ftcffdrv.h: Improve documentation.

This is based on blog entries from David Lemon and Dave Arnold (both
from Adobe) with kind permission.  Dave also helped in
proof-reading.

ChangeLog

@@ -1,3 +1,10 @@
+2013-08-02  Werner Lemberg  <wl@gnu.org>
+
+	* include/freetype/ftcffdrv.h: Improve documentation.
+	This is based on blog entries from David Lemon and Dave Arnold (both
+	from Adobe) with kind permission.  Dave also helped in
+	proof-reading.
+
 2013-08-02  Werner Lemberg  <wl@gnu.org>
 
 	[autofit] Move declaration of scripts into separate file.

include/freetype/ftcffdrv.h

@@ -46,11 +46,68 @@ FT_BEGIN_HEADER
    * @description:
    *   While FreeType's CFF driver doesn't expose API functions by itself,
    *   it is possible to control its behaviour with @FT_Property_Set and
-   *   @FT_Property_Get.  The following lists the available properties
+   *   @FT_Property_Get.  The list below gives the available properties
    *   together with the necessary macros and structures.
    *
    *   The CFF driver's module name is `cff'.
    *
+   *   *Hinting* *and* *antialiasing* *principles* *of* *the* *new* *engine*
+   *
+   *   The rasterizer is positioning horizontal features (e.g., ascender
+   *   height & x-height, or crossbars) on the pixel grid and minimizing the
+   *   amount of antialiasing applied to them, while placing vertical
+   *   features (vertical stems) on the pixel grid without hinting, thus
+   *   representing the stem position and weight accurately.  Sometimes the
+   *   vertical stems may be only partially black.  In this context,
+   *   `antialiasing' means that stems are not positioned exactly on pixel
+   *   borders, causing a fuzzy appearance.
+   *
+   *   There are two principles behind this approach.
+   *
+   *   1) No hinting in the horizontal direction: Unlike `superhinted'
+   *   TrueType which changes glyph widths to accommodate regular
+   *   inter-glyph spacing, Adobe's approach is `faithful to the design' in
+   *   representing both the glyph width and the inter-glyph spacing
+   *   designed for the font.  This makes the screen display as close as it
+   *   can be to the result one would get with infinite resolution, while
+   *   preserving what is considered the key characteristics of each glyph.
+   *   Note that the distances between unhinted and grid-fitted positions at
+   *   small sizes are comparable to kerning values and thus would be
+   *   noticeable (and distracting) while reading if hinting were applied.
+   *
+   *   One of the reasons to not hint horizontally is antialiasing for LCD
+   *   screens: The pixel geometry of modern displays supplies three
+   *   vertical sub-pixels as the eye moves horizontally across each visible
+   *   pixel.  On devices where we can be certain this characteristic is
+   *   present a rasterizer can take advantage of the sub-pixels to add
+   *   increments of weight.  In Western writing systems this turns out to
+   *   be the more critical direction anyway; the weights and spacing of
+   *   vertical stems (see above) are central to Armenian, Cyrillic, Greek,
+   *   and Latin type designs.  Even when the rasterizer uses greyscale
+   *   antialiasing instead of color (a necessary compromise when one
+   *   doesn't know the screen characteristics), the unhinted vertical
+   *   features preserve the design's weight and spacing much better than
+   *   aliased type would.
+   *
+   *   2) Aligment in the vertical direction: Weights and spacing along the
+   *   y~axis are less critical; what is much more important is the visual
+   *   alignment of related features (like cap-height and x-height).  The
+   *   sense of alignment for these is enhanced by the sharpness of grid-fit
+   *   edges, while the cruder vertical resolution (full pixels instead of
+   *   1/3 pixels) is less of a problem.
+   *
+   *   On the technical side, horizontal alignment zones for ascender,
+   *   x-height, and other important height values (traditionally called
+   *   `blue zones') as defined in the font are positioned independently,
+   *   each being rounded to the nearest pixel edge, taking care of
+   *   overshoot suppression at small sizes, stem darkening, and scaling.
+   *
+   *   Hstems (this is, hint values defined in the font to help align
+   *   horizontal features) that fall within a blue zone are said to be
+   *   `captured' and are aligned to that zone.  Uncaptured stems are moved
+   *   in one of four ways, top edge up or down, bottom edge up or down.
+   *   Unless there are conflicting hstems, the smallest movement is taken
+   *   to minimize distortion.
    */