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1 /***************************************************************************/ | |
2 /* */ | |
3 /* ftcffdrv.h */ | |
4 /* */ | |
5 /* FreeType API for controlling the CFF driver (specification only). */ | |
6 /* */ | |
7 /* Copyright 2013, 2014 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 */ | |
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