lib/shared/include/ftcffdrv.h

   1 /***************************************************************************/
   2 /*                                                                         */
   3 /*  ftcffdrv.h                                                             */
   4 /*                                                                         */
   5 /*    FreeType API for controlling the CFF driver (specification only).    */
   6 /*                                                                         */
   7 /*  Copyright 2013-2015 by                                                 */
   8 /*  David Turner, Robert Wilhelm, and Werner Lemberg.                      */
   9 /*                                                                         */
  10 /*  This file is part of the FreeType project, and may only be used,       */
  11 /*  modified, and distributed under the terms of the FreeType project      */
  12 /*  license, LICENSE.TXT.  By continuing to use, modify, or distribute     */
  13 /*  this file you indicate that you have read the license and              */
  14 /*  understand and accept it fully.                                        */
  15 /*                                                                         */
  16 /***************************************************************************/
  17
  18
  19 #ifndef __FTCFFDRV_H__
  20 #define __FTCFFDRV_H__
  21
  22 #include <ft2build.h>
  23 #include FT_FREETYPE_H
  24
  25 #ifdef FREETYPE_H
  26 #error "freetype.h of FreeType 1 has been loaded!"
  27 #error "Please fix the directory search order for header files"
  28 #error "so that freetype.h of FreeType 2 is found first."
  29 #endif
  30
  31
  32 FT_BEGIN_HEADER
  33
  34
  35   /**************************************************************************
  36    *
  37    * @section:
  38    *   cff_driver
  39    *
  40    * @title:
  41    *   The CFF driver
  42    *
  43    * @abstract:
  44    *   Controlling the CFF driver module.
  45    *
  46    * @description:
  47    *   While FreeType's CFF driver doesn't expose API functions by itself,
  48    *   it is possible to control its behaviour with @FT_Property_Set and
  49    *   @FT_Property_Get.  The list below gives the available properties
  50    *   together with the necessary macros and structures.
  51    *
  52    *   The CFF driver's module name is `cff'.
  53    *
  54    *   *Hinting* *and* *antialiasing* *principles* *of* *the* *new* *engine*
  55    *
  56    *   The rasterizer is positioning horizontal features (e.g., ascender
  57    *   height & x-height, or crossbars) on the pixel grid and minimizing the
  58    *   amount of antialiasing applied to them, while placing vertical
  59    *   features (vertical stems) on the pixel grid without hinting, thus
  60    *   representing the stem position and weight accurately.  Sometimes the
  61    *   vertical stems may be only partially black.  In this context,
  62    *   `antialiasing' means that stems are not positioned exactly on pixel
  63    *   borders, causing a fuzzy appearance.
  64    *
  65    *   There are two principles behind this approach.
  66    *
  67    *   1) No hinting in the horizontal direction: Unlike `superhinted'
  68    *   TrueType, which changes glyph widths to accommodate regular
  69    *   inter-glyph spacing, Adobe's approach is `faithful to the design' in
  70    *   representing both the glyph width and the inter-glyph spacing
  71    *   designed for the font.  This makes the screen display as close as it
  72    *   can be to the result one would get with infinite resolution, while
  73    *   preserving what is considered the key characteristics of each glyph.
  74    *   Note that the distances between unhinted and grid-fitted positions at
  75    *   small sizes are comparable to kerning values and thus would be
  76    *   noticeable (and distracting) while reading if hinting were applied.
  77    *
  78    *   One of the reasons to not hint horizontally is antialiasing for LCD
  79    *   screens: The pixel geometry of modern displays supplies three
  80    *   vertical sub-pixels as the eye moves horizontally across each visible
  81    *   pixel.  On devices where we can be certain this characteristic is
  82    *   present a rasterizer can take advantage of the sub-pixels to add
  83    *   increments of weight.  In Western writing systems this turns out to
  84    *   be the more critical direction anyway; the weights and spacing of
  85    *   vertical stems (see above) are central to Armenian, Cyrillic, Greek,
  86    *   and Latin type designs.  Even when the rasterizer uses greyscale
  87    *   antialiasing instead of color (a necessary compromise when one
  88    *   doesn't know the screen characteristics), the unhinted vertical
  89    *   features preserve the design's weight and spacing much better than
  90    *   aliased type would.
  91    *
  92    *   2) Aligment in the vertical direction: Weights and spacing along the
  93    *   y~axis are less critical; what is much more important is the visual
  94    *   alignment of related features (like cap-height and x-height).  The
  95    *   sense of alignment for these is enhanced by the sharpness of grid-fit
  96    *   edges, while the cruder vertical resolution (full pixels instead of
  97    *   1/3 pixels) is less of a problem.
  98    *
  99    *   On the technical side, horizontal alignment zones for ascender,
 100    *   x-height, and other important height values (traditionally called
 101    *   `blue zones') as defined in the font are positioned independently,
 102    *   each being rounded to the nearest pixel edge, taking care of
 103    *   overshoot suppression at small sizes, stem darkening, and scaling.
 104    *
 105    *   Hstems (this is, hint values defined in the font to help align
 106    *   horizontal features) that fall within a blue zone are said to be
 107    *   `captured' and are aligned to that zone.  Uncaptured stems are moved
 108    *   in one of four ways, top edge up or down, bottom edge up or down.
 109    *   Unless there are conflicting hstems, the smallest movement is taken
 110    *   to minimize distortion.
 111    *
 112    * @order:
 113    *   hinting-engine
 114    *   no-stem-darkening
 115    *   darkening-parameters
 116    *
 117    */
 118
 119
 120   /**************************************************************************
 121    *
 122    * @property:
 123    *   hinting-engine
 124    *
 125    * @description:
 126    *   Thanks to Adobe, which contributed a new hinting (and parsing)
 127    *   engine, an application can select between `freetype' and `adobe' if
 128    *   compiled with CFF_CONFIG_OPTION_OLD_ENGINE.  If this configuration
 129    *   macro isn't defined, `hinting-engine' does nothing.
 130    *
 131    *   The default engine is `freetype' if CFF_CONFIG_OPTION_OLD_ENGINE is
 132    *   defined, and `adobe' otherwise.
 133    *
 134    *   The following example code demonstrates how to select Adobe's hinting
 135    *   engine (omitting the error handling).
 136    *
 137    *   {
 138    *     FT_Library  library;
 139    *     FT_UInt     hinting_engine = FT_CFF_HINTING_ADOBE;
 140    *
 141    *
 142    *     FT_Init_FreeType( &library );
 143    *
 144    *     FT_Property_Set( library, "cff",
 145    *                               "hinting-engine", &hinting_engine );
 146    *   }
 147    *
 148    * @note:
 149    *   This property can be used with @FT_Property_Get also.
 150    *
 151    */
 152
 153
 154   /**************************************************************************
 155    *
 156    * @enum:
 157    *   FT_CFF_HINTING_XXX
 158    *
 159    * @description:
 160    *   A list of constants used for the @hinting-engine property to select
 161    *   the hinting engine for CFF fonts.
 162    *
 163    * @values:
 164    *   FT_CFF_HINTING_FREETYPE ::
 165    *     Use the old FreeType hinting engine.
 166    *
 167    *   FT_CFF_HINTING_ADOBE ::
 168    *     Use the hinting engine contributed by Adobe.
 169    *
 170    */
 171 #define FT_CFF_HINTING_FREETYPE  0
 172 #define FT_CFF_HINTING_ADOBE     1
 173
 174
 175   /**************************************************************************
 176    *
 177    * @property:
 178    *   no-stem-darkening
 179    *
 180    * @description:
 181    *   By default, the Adobe CFF engine darkens stems at smaller sizes,
 182    *   regardless of hinting, to enhance contrast.  This feature requires
 183    *   a rendering system with proper gamma correction.  Setting this
 184    *   property, stem darkening gets switched off.
 185    *
 186    *   Note that stem darkening is never applied if @FT_LOAD_NO_SCALE is set.
 187    *
 188    *   {
 189    *     FT_Library  library;
 190    *     FT_Bool     no_stem_darkening = TRUE;
 191    *
 192    *
 193    *     FT_Init_FreeType( &library );
 194    *
 195    *     FT_Property_Set( library, "cff",
 196    *                               "no-stem-darkening", &no_stem_darkening );
 197    *   }
 198    *
 199    * @note:
 200    *   This property can be used with @FT_Property_Get also.
 201    *
 202    */
 203
 204
 205   /**************************************************************************
 206    *
 207    * @property:
 208    *   darkening-parameters
 209    *
 210    * @description:
 211    *   By default, the Adobe CFF engine darkens stems as follows (if the
 212    *   `no-stem-darkening' property isn't set):
 213    *
 214    *   {
 215    *     stem width <= 0.5px:   darkening amount = 0.4px
 216    *     stem width  = 1px:     darkening amount = 0.275px
 217    *     stem width  = 1.667px: darkening amount = 0.275px
 218    *     stem width >= 2.333px: darkening amount = 0px
 219    *   }
 220    *
 221    *   and piecewise linear in-between.  At configuration time, these four
 222    *   control points can be set with the macro
 223    *   `CFF_CONFIG_OPTION_DARKENING_PARAMETERS'.  At runtime, the control
 224    *   points can be changed using the `darkening-parameters' property, as
 225    *   the following example demonstrates.
 226    *
 227    *   {
 228    *     FT_Library  library;
 229    *     FT_Int      darken_params[8] = {  500, 300,   // x1, y1
 230    *                                      1000, 200,   // x2, y2
 231    *                                      1500, 100,   // x3, y3
 232    *                                      2000,   0 }; // x4, y4
 233    *
 234    *
 235    *     FT_Init_FreeType( &library );
 236    *
 237    *     FT_Property_Set( library, "cff",
 238    *                               "darkening-parameters", darken_params );
 239    *   }
 240    *
 241    *   The x~values give the stem width, and the y~values the darkening
 242    *   amount.  The unit is 1000th of pixels.  All coordinate values must be
 243    *   positive; the x~values must be monotonically increasing; the
 244    *   y~values must be monotonically decreasing and smaller than or
 245    *   equal to 500 (corresponding to half a pixel); the slope of each
 246    *   linear piece must be shallower than -1 (e.g., -.4).
 247    *
 248    * @note:
 249    *   This property can be used with @FT_Property_Get also.
 250    *
 251    */
 252
 253   /* */
 254
 255
 256 FT_END_HEADER
 257
 258
 259 #endif /* __FTCFFDRV_H__ */
 260
 261
 262 /* END */