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ftcffdrv.h
1/***************************************************************************/
2/* */
3/* ftcffdrv.h */
4/* */
5/* FreeType API for controlling the CFF driver (specification only). */
6/* */
7/* Copyright 2013-2016 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 * @order:
113 * hinting-engine
114 * no-stem-darkening[cff]
115 * darkening-parameters[cff]
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[cff]
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[cff]
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
256FT_END_HEADER
257
258
259#endif /* FTCFFDRV_H_ */
260
261
262/* END */
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