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 */ 263
