Update freetype to latest version of ASOP.

include/freetype/ftlcdfil.h

 /***************************************************************************/
 /*                                                                         */
 /*  ftlcdfil.h                                                             */
 /*                                                                         */
 /*    FreeType API for color filtering of subpixel bitmap glyphs           */
 /*    (specification).                                                     */
 /*                                                                         */
 /*  Copyright 2006, 2007, 2008, 2010 by                                    */
 /*  David Turner, Robert Wilhelm, and Werner Lemberg.                      */
 /*                                                                         */
@@ skipping 35 matching lines @@
    * @description:
    *   The @FT_Library_SetLcdFilter API can be used to specify a low-pass
    *   filter which is then applied to LCD-optimized bitmaps generated
    *   through @FT_Render_Glyph.  This is useful to reduce color fringes
    *   which would occur with unfiltered rendering.
    *
    *   Note that no filter is active by default, and that this function is
    *   *not* implemented in default builds of the library.  You need to
    *   #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file
    *   in order to activate it.
+   *
+   *   FreeType generates alpha coverage maps, which are linear by nature.
+   *   For instance, the value 0x80 in bitmap representation means that
+   *   (within numerical precision) 0x80/0xff fraction of that pixel is
+   *   covered by the glyph's outline.  The blending function for placing
+   *   text over a background is
+   *
+   *   {
+   *     dst = alpha * src + (1 - alpha) * dst    ,
+   *   }
+   *
+   *   which is known as OVER.  However, when calculating the output of the
+   *   OVER operator, the source colors should first be transformed to a
+   *   linear color space, then alpha blended in that space, and transformed
+   *   back to the output color space.
+   *
+   *   When linear light blending is used, the default FIR5 filtering
+   *   weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as
+   *   they have been designed for black on white rendering while lacking
+   *   gamma correction.  To preserve color neutrality, weights for a FIR5
+   *   filter should be chosen according to two free parameters `a' and `c',
+   *   and the FIR weights should be
+   *
+   *   {
+   *     [a - c, a + c, 2 * a, a + c, a - c]    .
+   *   }
+   *
+   *   This formula generates equal weights for all the color primaries
+   *   across the filter kernel, which makes it colorless.  One suggested
+   *   set of weights is
+   *
+   *   {
+   *     [0x10, 0x50, 0x60, 0x50, 0x10]    ,
+   *   }
+   *
+   *   where `a' has value 0x30 and `b' value 0x20.  The weights in filter
+   *   may have a sum larger than 0x100, which increases coloration slightly
+   *   but also improves contrast.
    */
 
 
   /****************************************************************************
    *
    * @enum:
    *   FT_LcdFilter
    *
    * @description:
    *   A list of values to identify various types of LCD filters.
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   /* */
 
 
 FT_END_HEADER
 
 #endif /* __FT_LCD_FILTER_H__ */
 
 
 /* END */