ftcffdrv.h revision ec0bab5697bb31ba980810145f62e3799946ec60 
1/***************************************************************************/
2/*                                                                         */
3/*  ftcffdrv.h                                                             */
4/*                                                                         */
5/*    FreeType API for controlling the CFF driver (specification only).    */
6/*                                                                         */
7/*  Copyright 2013 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
32FT_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
113
114  /**************************************************************************
115   *
116   * @property:
117   *   hinting-engine
118   *
119   * @description:
120   *   Thanks to Adobe, which contributed a new hinting (and parsing)
121   *   engine, an application can select between `freetype' and `adobe' if
122   *   compiled with CFF_CONFIG_OPTION_OLD_ENGINE.  If this configuration
123   *   macro isn't defined, `hinting-engine' does nothing.
124   *
125   *   The default engine is `freetype' if CFF_CONFIG_OPTION_OLD_ENGINE is
126   *   defined, and `adobe' otherwise.
127   *
128   *   The following example code demonstrates how to select Adobe's hinting
129   *   engine (omitting the error handling).
130   *
131   *   {
132   *     FT_Library  library;
133   *     FT_UInt     hinting_engine = FT_CFF_HINTING_ADOBE;
134   *
135   *
136   *     FT_Init_FreeType( &library );
137   *
138   *     FT_Property_Set( library, "cff",
139   *                               "hinting-engine", &hinting_engine );
140   *   }
141   *
142   * @note:
143   *   This property can be used with @FT_Property_Get also.
144   *
145   */
146
147
148  /**************************************************************************
149   *
150   * @enum:
151   *   FT_CFF_HINTING_XXX
152   *
153   * @description:
154   *   A list of constants used for the @hinting-engine property to select
155   *   the hinting engine for CFF fonts.
156   *
157   * @values:
158   *   FT_CFF_HINTING_FREETYPE ::
159   *     Use the old FreeType hinting engine.
160   *
161   *   FT_CFF_HINTING_ADOBE ::
162   *     Use the hinting engine contributed by Adobe.
163   *
164   */
165#define FT_CFF_HINTING_FREETYPE  0
166#define FT_CFF_HINTING_ADOBE     1
167
168
169  /**************************************************************************
170   *
171   * @property:
172   *   no-stem-darkening
173   *
174   * @description:
175   *   By default, the Adobe CFF engine darkens stems at smaller sizes,
176   *   regardless of hinting, to enhance contrast.  This feature requires
177   *   a rendering system with proper gamma correction.  Setting this
178   *   property, stem darkening gets switched off.
179   *
180   *   Note that stem darkening is never applied if @FT_LOAD_NO_SCALE is set.
181   *
182   *   {
183   *     FT_Library  library;
184   *     FT_Bool     no_stem_darkening = TRUE;
185   *
186   *
187   *     FT_Init_FreeType( &library );
188   *
189   *     FT_Property_Set( library, "cff",
190   *                               "no-stem-darkening", &no_stem_darkening );
191   *   }
192   *
193   * @note:
194   *   This property can be used with @FT_Property_Get also.
195   *
196   */
197
198
199  /**************************************************************************
200   *
201   * @property:
202   *   darkening-parameters
203   *
204   * @description:
205   *   By default, the Adobe CFF engine darkens stems as follows (if the
206   *   `no-stem-darkening' property isn't set):
207   *
208   *   {
209   *     stem width <= 0.5px:   darkening amount = 0.4px
210   *     stem width  = 1px:     darkening amount = 0.275px
211   *     stem width  = 1.667px: darkening amount = 0.275px
212   *     stem width >= 2.333px: darkening amount = 0px
213   *   }
214   *
215   *   and piecewise linear in-between.  Using the `darkening-parameters'
216   *   property, these four control points can be changed, as the following
217   *   example demonstrates.
218   *
219   *   {
220   *     FT_Library  library;
221   *     FT_Int      darken_params[8] = {  500, 300,   // x1, y1
222   *                                      1000, 200,   // x2, y2
223   *                                      1500, 100,   // x3, y3
224   *                                      2000,   0 }; // x4, y4
225   *
226   *
227   *     FT_Init_FreeType( &library );
228   *
229   *     FT_Property_Set( library, "cff",
230   *                               "darkening-parameters", darken_params );
231   *   }
232   *
233   *   The x~values give the stem width, and the y~values the darkening
234   *   amount.  The unit is 1000th of pixels.  All coordinate values must be
235   *   positive; the x~values must be monotonically increasing; the
236   *   y~values must be monotonically decreasing and smaller than or
237   *   equal to 500 (corresponding to half a pixel); the slope of each
238   *   linear piece must be shallower than -1 (e.g., -.4).
239   *
240   * @note:
241   *   This property can be used with @FT_Property_Get also.
242   *
243   */
244
245
246 /* */
247
248FT_END_HEADER
249
250
251#endif /* __FTCFFDRV_H__ */
252
253
254/* END */
255