1 /**************************************************************************** 2 * 3 * ftlcdfil.h 4 * 5 * FreeType API for color filtering of subpixel bitmap glyphs 6 * (specification). 7 * 8 * Copyright (C) 2006-2019 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_rendering 40 * 41 * @title: 42 * Subpixel Rendering 43 * 44 * @abstract: 45 * API to control subpixel rendering. 46 * 47 * @description: 48 * FreeType provides two alternative subpixel rendering technologies. 49 * Should you define `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` in your 50 * `ftoption.h` file, this enables patented ClearType-style rendering. 51 * Otherwise, Harmony LCD rendering is enabled. These technologies are 52 * controlled differently and API described below, although always 53 * available, performs its function when appropriate method is enabled 54 * and does nothing otherwise. 55 * 56 * ClearType-style LCD rendering exploits the color-striped structure of 57 * LCD pixels, increasing the available resolution in the direction of 58 * the stripe (usually horizontal RGB) by a factor of~3. Using the 59 * subpixels coverages unfiltered can create severe color fringes 60 * especially when rendering thin features. Indeed, to produce 61 * black-on-white text, the nearby color subpixels must be dimmed 62 * equally. 63 * 64 * A good 5-tap FIR filter should be applied to subpixel coverages 65 * regardless of pixel boundaries and should have these properties: 66 * 67 * 1. It should be symmetrical, like {~a, b, c, b, a~}, to avoid 68 * any shifts in appearance. 69 * 70 * 2. It should be color-balanced, meaning a~+ b~=~c, to reduce color 71 * fringes by distributing the computed coverage for one subpixel to 72 * all subpixels equally. 73 * 74 * 3. It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain 75 * overall brightness. 76 * 77 * Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is sharper but is less 78 * forgiving of non-ideal gamma curves of a screen (and viewing angles), 79 * beveled filters are fuzzier but more tolerant. 80 * 81 * Use the @FT_Library_SetLcdFilter or @FT_Library_SetLcdFilterWeights 82 * API to specify a low-pass filter, which is then applied to 83 * subpixel-rendered bitmaps generated through @FT_Render_Glyph. 84 * 85 * Harmony LCD rendering is suitable to panels with any regular subpixel 86 * structure, not just monitors with 3 color striped subpixels, as long 87 * as the color subpixels have fixed positions relative to the pixel 88 * center. In this case, each color channel is then rendered separately 89 * after shifting the outline opposite to the subpixel shift so that the 90 * coverage maps are aligned. This method is immune to color fringes 91 * because the shifts do not change integral coverage. 92 * 93 * The subpixel geometry must be specified by xy-coordinates for each 94 * subpixel. By convention they may come in the RGB order: {{-1/3, 0}, 95 * {0, 0}, {1/3, 0}} for standard RGB striped panel or {{-1/6, 1/4}, 96 * {-1/6, -1/4}, {1/3, 0}} for a certain PenTile panel. 97 * 98 * Use the @FT_Library_SetLcdGeometry API to specify subpixel positions. 99 * If one follows the RGB order convention, the same order applies to the 100 * resulting @FT_PIXEL_MODE_LCD and @FT_PIXEL_MODE_LCD_V bitmaps. Note, 101 * however, that the coordinate frame for the latter must be rotated 102 * clockwise. Harmony with default LCD geometry is equivalent to 103 * ClearType with light filter. 104 * 105 * As a result of ClearType filtering or Harmony rendering, the 106 * dimensions of LCD bitmaps can be either wider or taller than the 107 * dimensions of the corresponding outline with regard to the pixel grid. 108 * For example, for @FT_RENDER_MODE_LCD, the filter adds 2~subpixels to 109 * the left, and 2~subpixels to the right. The bitmap offset values are 110 * adjusted accordingly, so clients shouldn't need to modify their layout 111 * and glyph positioning code when enabling the filter. 112 * 113 * The ClearType and Harmony rendering is applicable to glyph bitmaps 114 * rendered through @FT_Render_Glyph, @FT_Load_Glyph, @FT_Load_Char, and 115 * @FT_Glyph_To_Bitmap, when @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V 116 * is specified. This API does not control @FT_Outline_Render and 117 * @FT_Outline_Get_Bitmap. 118 * 119 * The described algorithms can completely remove color artefacts when 120 * combined with gamma-corrected alpha blending in linear space. Each of 121 * the 3~alpha values (subpixels) must by independently used to blend one 122 * color channel. That is, red alpha blends the red channel of the text 123 * color with the red channel of the background pixel. 124 */ 125 126 127 /************************************************************************** 128 * 129 * @enum: 130 * FT_LcdFilter 131 * 132 * @description: 133 * A list of values to identify various types of LCD filters. 134 * 135 * @values: 136 * FT_LCD_FILTER_NONE :: 137 * Do not perform filtering. When used with subpixel rendering, this 138 * results in sometimes severe color fringes. 139 * 140 * FT_LCD_FILTER_DEFAULT :: 141 * This is a beveled, normalized, and color-balanced five-tap filter 142 * with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256th units. 143 * 144 * FT_LCD_FILTER_LIGHT :: 145 * this is a boxy, normalized, and color-balanced three-tap filter with 146 * weights of [0x00 0x55 0x56 0x55 0x00] in 1/256th units. 147 * 148 * FT_LCD_FILTER_LEGACY :: 149 * FT_LCD_FILTER_LEGACY1 :: 150 * This filter corresponds to the original libXft color filter. It 151 * provides high contrast output but can exhibit really bad color 152 * fringes if glyphs are not extremely well hinted to the pixel grid. 153 * This filter is only provided for comparison purposes, and might be 154 * disabled or stay unsupported in the future. The second value is 155 * provided for compatibility with FontConfig, which historically used 156 * different enumeration, sometimes incorrectly forwarded to FreeType. 157 * 158 * @since: 159 * 2.3.0 (`FT_LCD_FILTER_LEGACY1` since 2.6.2) 160 */ 161 typedef enum FT_LcdFilter_ 162 { 163 FT_LCD_FILTER_NONE = 0, 164 FT_LCD_FILTER_DEFAULT = 1, 165 FT_LCD_FILTER_LIGHT = 2, 166 FT_LCD_FILTER_LEGACY1 = 3, 167 FT_LCD_FILTER_LEGACY = 16, 168 169 FT_LCD_FILTER_MAX /* do not remove */ 170 171 } FT_LcdFilter; 172 173 174 /************************************************************************** 175 * 176 * @function: 177 * FT_Library_SetLcdFilter 178 * 179 * @description: 180 * This function is used to apply color filtering to LCD decimated 181 * bitmaps, like the ones used when calling @FT_Render_Glyph with 182 * @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V. 183 * 184 * @input: 185 * library :: 186 * A handle to the target library instance. 187 * 188 * filter :: 189 * The filter type. 190 * 191 * You can use @FT_LCD_FILTER_NONE here to disable this feature, or 192 * @FT_LCD_FILTER_DEFAULT to use a default filter that should work well 193 * on most LCD screens. 194 * 195 * @return: 196 * FreeType error code. 0~means success. 197 * 198 * @note: 199 * This feature is always disabled by default. Clients must make an 200 * explicit call to this function with a `filter` value other than 201 * @FT_LCD_FILTER_NONE in order to enable it. 202 * 203 * Due to **PATENTS** covering subpixel rendering, this function doesn't 204 * do anything except returning `FT_Err_Unimplemented_Feature` if the 205 * configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is not 206 * defined in your build of the library, which should correspond to all 207 * default builds of FreeType. 208 * 209 * @since: 210 * 2.3.0 211 */ 212 FT_EXPORT( FT_Error ) 213 FT_Library_SetLcdFilter( FT_Library library, 214 FT_LcdFilter filter ); 215 216 217 /************************************************************************** 218 * 219 * @function: 220 * FT_Library_SetLcdFilterWeights 221 * 222 * @description: 223 * This function can be used to enable LCD filter with custom weights, 224 * instead of using presets in @FT_Library_SetLcdFilter. 225 * 226 * @input: 227 * library :: 228 * A handle to the target library instance. 229 * 230 * weights :: 231 * A pointer to an array; the function copies the first five bytes and 232 * uses them to specify the filter weights in 1/256th units. 233 * 234 * @return: 235 * FreeType error code. 0~means success. 236 * 237 * @note: 238 * Due to **PATENTS** covering subpixel rendering, this function doesn't 239 * do anything except returning `FT_Err_Unimplemented_Feature` if the 240 * configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is not 241 * defined in your build of the library, which should correspond to all 242 * default builds of FreeType. 243 * 244 * LCD filter weights can also be set per face using @FT_Face_Properties 245 * with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS. 246 * 247 * @since: 248 * 2.4.0 249 */ 250 FT_EXPORT( FT_Error ) 251 FT_Library_SetLcdFilterWeights( FT_Library library, 252 unsigned char *weights ); 253 254 255 /************************************************************************** 256 * 257 * @type: 258 * FT_LcdFiveTapFilter 259 * 260 * @description: 261 * A typedef for passing the five LCD filter weights to 262 * @FT_Face_Properties within an @FT_Parameter structure. 263 * 264 * @since: 265 * 2.8 266 * 267 */ 268 #define FT_LCD_FILTER_FIVE_TAPS 5 269 270 typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS]; 271 272 273 /************************************************************************** 274 * 275 * @function: 276 * FT_Library_SetLcdGeometry 277 * 278 * @description: 279 * This function can be used to modify default positions of color 280 * subpixels, which controls Harmony LCD rendering. 281 * 282 * @input: 283 * library :: 284 * A handle to the target library instance. 285 * 286 * sub :: 287 * A pointer to an array of 3 vectors in 26.6 fractional pixel format; 288 * the function modifies the default values, see the note below. 289 * 290 * @return: 291 * FreeType error code. 0~means success. 292 * 293 * @note: 294 * Subpixel geometry examples: 295 * 296 * - {{-21, 0}, {0, 0}, {21, 0}} is the default, corresponding to 3 color 297 * stripes shifted by a third of a pixel. This could be an RGB panel. 298 * 299 * - {{21, 0}, {0, 0}, {-21, 0}} looks the same as the default but can 300 * specify a BGR panel instead, while keeping the bitmap in the same 301 * RGB888 format. 302 * 303 * - {{0, 21}, {0, 0}, {0, -21}} is the vertical RGB, but the bitmap 304 * stays RGB888 as a result. 305 * 306 * - {{-11, 16}, {-11, -16}, {22, 0}} is a certain PenTile arrangement. 307 * 308 * This function does nothing and returns `FT_Err_Unimplemented_Feature` 309 * in the context of ClearType-style subpixel rendering when 310 * `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is defined in your build of the 311 * library. 312 * 313 * @since: 314 * 2.10.0 315 */ 316 FT_EXPORT( FT_Error ) 317 FT_Library_SetLcdGeometry( FT_Library library, 318 FT_Vector sub[3] ); 319 320 /* */ 321 322 323 FT_END_HEADER 324 325 #endif /* FTLCDFIL_H_ */ 326 327 328 /* END */ 329