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 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 */ 112 113 114 /************************************************************************** 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 /* */ 247 248 FT_END_HEADER 249 250 251 #endif /* __FTCFFDRV_H__ */ 252 253 254 /* END */ 255
