1 /***************************************************************************/ 2 /* */ 3 /* ftlcdfil.h */ 4 /* */ 5 /* FreeType API for color filtering of subpixel bitmap glyphs */ 6 /* (specification). */ 7 /* */ 8 /* Copyright 2006-2018 by */ 9 /* David Turner, Robert Wilhelm, and Werner Lemberg. */ 10 /* */ 11 /* This file is part of the FreeType project, and may only be used, */ 12 /* modified, and distributed under the terms of the FreeType project */ 13 /* license, LICENSE.TXT. By continuing to use, modify, or distribute */ 14 /* this file you indicate that you have read the license and */ 15 /* understand and accept it fully. */ 16 /* */ 17 /***************************************************************************/ 18 19 20 #ifndef FTLCDFIL_H_ 21 #define FTLCDFIL_H_ 22 23 #include <ft2build.h> 24 #include FT_FREETYPE_H 25 #include FT_PARAMETER_TAGS_H 26 27 #ifdef FREETYPE_H 28 #error "freetype.h of FreeType 1 has been loaded!" 29 #error "Please fix the directory search order for header files" 30 #error "so that freetype.h of FreeType 2 is found first." 31 #endif 32 33 34 FT_BEGIN_HEADER 35 36 /*************************************************************************** 37 * 38 * @section: 39 * lcd_filtering 40 * 41 * @title: 42 * LCD Filtering 43 * 44 * @abstract: 45 * Reduce color fringes of subpixel-rendered bitmaps. 46 * 47 * @description: 48 * Should you #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your 49 * `ftoption.h', which enables patented ClearType-style rendering, 50 * the LCD-optimized glyph bitmaps should be filtered to reduce color 51 * fringes inherent to this technology. The default FreeType LCD 52 * rendering uses different technology, and API described below, 53 * although available, does nothing. 54 * 55 * ClearType-style LCD rendering exploits the color-striped structure of 56 * LCD pixels, increasing the available resolution in the direction of 57 * the stripe (usually horizontal RGB) by a factor of~3. Since these 58 * subpixels are color pixels, using them unfiltered creates severe 59 * color fringes. Use the @FT_Library_SetLcdFilter API to specify a 60 * low-pass filter, which is then applied to subpixel-rendered bitmaps 61 * generated through @FT_Render_Glyph. The filter sacrifices some of 62 * the higher resolution to reduce color fringes, making the glyph image 63 * slightly blurrier. Positional improvements will remain. 64 * 65 * A filter should have two properties: 66 * 67 * 1) It should be normalized, meaning the sum of the 5~components 68 * should be 256 (0x100). It is possible to go above or under this 69 * target sum, however: going under means tossing out contrast, going 70 * over means invoking clamping and thereby non-linearities that 71 * increase contrast somewhat at the expense of greater distortion 72 * and color-fringing. Contrast is better enhanced through stem 73 * darkening. 74 * 75 * 2) It should be color-balanced, meaning a filter `{~a, b, c, b, a~}' 76 * where a~+ b~=~c. It distributes the computed coverage for one 77 * subpixel to all subpixels equally, sacrificing some won resolution 78 * but drastically reducing color-fringing. Positioning improvements 79 * remain! Note that color-fringing can only really be minimized 80 * when using a color-balanced filter and alpha-blending the glyph 81 * onto a surface in linear space; see @FT_Render_Glyph. 82 * 83 * Regarding the form, a filter can be a `boxy' filter or a `beveled' 84 * filter. Boxy filters are sharper but are less forgiving of non-ideal 85 * gamma curves of a screen (viewing angles!), beveled filters are 86 * fuzzier but more tolerant. 87 * 88 * Examples: 89 * 90 * - [0x10 0x40 0x70 0x40 0x10] is beveled and neither balanced nor 91 * normalized. 92 * 93 * - [0x1A 0x33 0x4D 0x33 0x1A] is beveled and balanced but not 94 * normalized. 95 * 96 * - [0x19 0x33 0x66 0x4c 0x19] is beveled and normalized but not 97 * balanced. 98 * 99 * - [0x00 0x4c 0x66 0x4c 0x00] is boxily beveled and normalized but not 100 * balanced. 101 * 102 * - [0x00 0x55 0x56 0x55 0x00] is boxy, normalized, and almost 103 * balanced. 104 * 105 * - [0x08 0x4D 0x56 0x4D 0x08] is beveled, normalized and, almost 106 * balanced. 107 * 108 * The filter affects glyph bitmaps rendered through @FT_Render_Glyph, 109 * @FT_Load_Glyph, and @FT_Load_Char. It does _not_ affect the output 110 * of @FT_Outline_Render and @FT_Outline_Get_Bitmap. 111 * 112 * If this feature is activated, the dimensions of LCD glyph bitmaps are 113 * either wider or taller than the dimensions of the corresponding 114 * outline with regard to the pixel grid. For example, for 115 * @FT_RENDER_MODE_LCD, the filter adds 3~subpixels to the left, and 116 * 3~subpixels to the right. The bitmap offset values are adjusted 117 * accordingly, so clients shouldn't need to modify their layout and 118 * glyph positioning code when enabling the filter. 119 * 120 * It is important to understand that linear alpha blending and gamma 121 * correction is critical for correctly rendering glyphs onto surfaces 122 * without artifacts and even more critical when subpixel rendering is 123 * involved. 124 * 125 * Each of the 3~alpha values (subpixels) is independently used to blend 126 * one color channel. That is, red alpha blends the red channel of the 127 * text color with the red channel of the background pixel. The 128 * distribution of density values by the color-balanced filter assumes 129 * alpha blending is done in linear space; only then color artifacts 130 * cancel out. 131 */ 132 133 134 /**************************************************************************** 135 * 136 * @enum: 137 * FT_LcdFilter 138 * 139 * @description: 140 * A list of values to identify various types of LCD filters. 141 * 142 * @values: 143 * FT_LCD_FILTER_NONE :: 144 * Do not perform filtering. When used with subpixel rendering, this 145 * results in sometimes severe color fringes. 146 * 147 * FT_LCD_FILTER_DEFAULT :: 148 * The default filter reduces color fringes considerably, at the cost 149 * of a slight blurriness in the output. 150 * 151 * It is a beveled, normalized, and color-balanced five-tap filter 152 * that is more forgiving to screens with non-ideal gamma curves and 153 * viewing angles. Note that while color-fringing is reduced, it can 154 * only be minimized by using linear alpha blending and gamma 155 * correction to render glyphs onto surfaces. The default filter 156 * weights are [0x08 0x4D 0x56 0x4D 0x08]. 157 * 158 * FT_LCD_FILTER_LIGHT :: 159 * The light filter is a variant that is sharper at the cost of 160 * slightly more color fringes than the default one. 161 * 162 * It is a boxy, normalized, and color-balanced three-tap filter that 163 * is less forgiving to screens with non-ideal gamma curves and 164 * viewing angles. This filter works best when the rendering system 165 * uses linear alpha blending and gamma correction to render glyphs 166 * onto surfaces. The light filter weights are 167 * [0x00 0x55 0x56 0x55 0x00]. 168 * 169 * FT_LCD_FILTER_LEGACY :: 170 * This filter corresponds to the original libXft color filter. It 171 * provides high contrast output but can exhibit really bad color 172 * fringes if glyphs are not extremely well hinted to the pixel grid. 173 * In other words, it only works well if the TrueType bytecode 174 * interpreter is enabled *and* high-quality hinted fonts are used. 175 * 176 * This filter is only provided for comparison purposes, and might be 177 * disabled or stay unsupported in the future. 178 * 179 * FT_LCD_FILTER_LEGACY1 :: 180 * For historical reasons, the FontConfig library returns a different 181 * enumeration value for legacy LCD filtering. To make code work that 182 * (incorrectly) forwards FontConfig's enumeration value to 183 * @FT_Library_SetLcdFilter without proper mapping, it is thus easiest 184 * to have another enumeration value, which is completely equal to 185 * `FT_LCD_FILTER_LEGACY'. 186 * 187 * @since: 188 * 2.3.0 (`FT_LCD_FILTER_LEGACY1' since 2.6.2) 189 */ 190 typedef enum FT_LcdFilter_ 191 { 192 FT_LCD_FILTER_NONE = 0, 193 FT_LCD_FILTER_DEFAULT = 1, 194 FT_LCD_FILTER_LIGHT = 2, 195 FT_LCD_FILTER_LEGACY1 = 3, 196 FT_LCD_FILTER_LEGACY = 16, 197 198 FT_LCD_FILTER_MAX /* do not remove */ 199 200 } FT_LcdFilter; 201 202 203 /************************************************************************** 204 * 205 * @func: 206 * FT_Library_SetLcdFilter 207 * 208 * @description: 209 * This function is used to apply color filtering to LCD decimated 210 * bitmaps, like the ones used when calling @FT_Render_Glyph with 211 * @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V. 212 * 213 * @input: 214 * library :: 215 * A handle to the target library instance. 216 * 217 * filter :: 218 * The filter type. 219 * 220 * You can use @FT_LCD_FILTER_NONE here to disable this feature, or 221 * @FT_LCD_FILTER_DEFAULT to use a default filter that should work 222 * well on most LCD screens. 223 * 224 * @return: 225 * FreeType error code. 0~means success. 226 * 227 * @note: 228 * This feature is always disabled by default. Clients must make an 229 * explicit call to this function with a `filter' value other than 230 * @FT_LCD_FILTER_NONE in order to enable it. 231 * 232 * Due to *PATENTS* covering subpixel rendering, this function doesn't 233 * do anything except returning `FT_Err_Unimplemented_Feature' if the 234 * configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not 235 * defined in your build of the library, which should correspond to all 236 * default builds of FreeType. 237 * 238 * @since: 239 * 2.3.0 240 */ 241 FT_EXPORT( FT_Error ) 242 FT_Library_SetLcdFilter( FT_Library library, 243 FT_LcdFilter filter ); 244 245 246 /************************************************************************** 247 * 248 * @func: 249 * FT_Library_SetLcdFilterWeights 250 * 251 * @description: 252 * This function can be used to enable LCD filter with custom weights, 253 * instead of using presets in @FT_Library_SetLcdFilter. 254 * 255 * @input: 256 * library :: 257 * A handle to the target library instance. 258 * 259 * weights :: 260 * A pointer to an array; the function copies the first five bytes and 261 * uses them to specify the filter weights. 262 * 263 * @return: 264 * FreeType error code. 0~means success. 265 * 266 * @note: 267 * Due to *PATENTS* covering subpixel rendering, this function doesn't 268 * do anything except returning `FT_Err_Unimplemented_Feature' if the 269 * configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not 270 * defined in your build of the library, which should correspond to all 271 * default builds of FreeType. 272 * 273 * LCD filter weights can also be set per face using @FT_Face_Properties 274 * with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS. 275 * 276 * @since: 277 * 2.4.0 278 */ 279 FT_EXPORT( FT_Error ) 280 FT_Library_SetLcdFilterWeights( FT_Library library, 281 unsigned char *weights ); 282 283 284 /* 285 * @type: 286 * FT_LcdFiveTapFilter 287 * 288 * @description: 289 * A typedef for passing the five LCD filter weights to 290 * @FT_Face_Properties within an @FT_Parameter structure. 291 * 292 * @since: 293 * 2.8 294 * 295 */ 296 #define FT_LCD_FILTER_FIVE_TAPS 5 297 298 typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS]; 299 300 301 /* */ 302 303 304 FT_END_HEADER 305 306 #endif /* FTLCDFIL_H_ */ 307 308 309 /* END */ 310
