1 /**************************************************************************** 2 * 3 * ftlcdfil.h 4 * 5 * FreeType API for color filtering of subpixel bitmap glyphs 6 * (specification). 7 * 8 * Copyright (C) 2006-2020 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 <freetype/freetype.h> 24 #include <freetype/ftparams.h> 25 26 #ifdef FREETYPE_H 27 #error "freetype.h of FreeType 1 has been loaded!" 28 #error "Please fix the directory search order for header files" 29 #error "so that freetype.h of FreeType 2 is found first." 30 #endif 31 32 33 FT_BEGIN_HEADER 34 35 /************************************************************************** 36 * 37 * @section: 38 * lcd_rendering 39 * 40 * @title: 41 * Subpixel Rendering 42 * 43 * @abstract: 44 * API to control subpixel rendering. 45 * 46 * @description: 47 * FreeType provides two alternative subpixel rendering technologies. 48 * Should you define `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` in your 49 * `ftoption.h` file, this enables ClearType-style rendering. 50 * Otherwise, Harmony LCD rendering is enabled. These technologies are 51 * controlled differently and API described below, although always 52 * available, performs its function when appropriate method is enabled 53 * and does nothing otherwise. 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. Using the 58 * subpixels coverages unfiltered can create severe color fringes 59 * especially when rendering thin features. Indeed, to produce 60 * black-on-white text, the nearby color subpixels must be dimmed 61 * equally. 62 * 63 * A good 5-tap FIR filter should be applied to subpixel coverages 64 * regardless of pixel boundaries and should have these properties: 65 * 66 * 1. It should be symmetrical, like {~a, b, c, b, a~}, to avoid 67 * any shifts in appearance. 68 * 69 * 2. It should be color-balanced, meaning a~+ b~=~c, to reduce color 70 * fringes by distributing the computed coverage for one subpixel to 71 * all subpixels equally. 72 * 73 * 3. It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain 74 * overall brightness. 75 * 76 * Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is sharper but is less 77 * forgiving of non-ideal gamma curves of a screen (and viewing angles), 78 * beveled filters are fuzzier but more tolerant. 79 * 80 * Use the @FT_Library_SetLcdFilter or @FT_Library_SetLcdFilterWeights 81 * API to specify a low-pass filter, which is then applied to 82 * subpixel-rendered bitmaps generated through @FT_Render_Glyph. 83 * 84 * Harmony LCD rendering is suitable to panels with any regular subpixel 85 * structure, not just monitors with 3 color striped subpixels, as long 86 * as the color subpixels have fixed positions relative to the pixel 87 * center. In this case, each color channel is then rendered separately 88 * after shifting the outline opposite to the subpixel shift so that the 89 * coverage maps are aligned. This method is immune to color fringes 90 * because the shifts do not change integral coverage. 91 * 92 * The subpixel geometry must be specified by xy-coordinates for each 93 * subpixel. By convention they may come in the RGB order: {{-1/3, 0}, 94 * {0, 0}, {1/3, 0}} for standard RGB striped panel or {{-1/6, 1/4}, 95 * {-1/6, -1/4}, {1/3, 0}} for a certain PenTile panel. 96 * 97 * Use the @FT_Library_SetLcdGeometry API to specify subpixel positions. 98 * If one follows the RGB order convention, the same order applies to the 99 * resulting @FT_PIXEL_MODE_LCD and @FT_PIXEL_MODE_LCD_V bitmaps. Note, 100 * however, that the coordinate frame for the latter must be rotated 101 * clockwise. Harmony with default LCD geometry is equivalent to 102 * ClearType with light filter. 103 * 104 * As a result of ClearType filtering or Harmony rendering, the 105 * dimensions of LCD bitmaps can be either wider or taller than the 106 * dimensions of the corresponding outline with regard to the pixel grid. 107 * For example, for @FT_RENDER_MODE_LCD, the filter adds 2~subpixels to 108 * the left, and 2~subpixels to the right. The bitmap offset values are 109 * adjusted accordingly, so clients shouldn't need to modify their layout 110 * and glyph positioning code when enabling the filter. 111 * 112 * The ClearType and Harmony rendering is applicable to glyph bitmaps 113 * rendered through @FT_Render_Glyph, @FT_Load_Glyph, @FT_Load_Char, and 114 * @FT_Glyph_To_Bitmap, when @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V 115 * is specified. This API does not control @FT_Outline_Render and 116 * @FT_Outline_Get_Bitmap. 117 * 118 * The described algorithms can completely remove color artefacts when 119 * combined with gamma-corrected alpha blending in linear space. Each of 120 * the 3~alpha values (subpixels) must by independently used to blend one 121 * color channel. That is, red alpha blends the red channel of the text 122 * color with the red channel of the background pixel. 123 */ 124 125 126 /************************************************************************** 127 * 128 * @enum: 129 * FT_LcdFilter 130 * 131 * @description: 132 * A list of values to identify various types of LCD filters. 133 * 134 * @values: 135 * FT_LCD_FILTER_NONE :: 136 * Do not perform filtering. When used with subpixel rendering, this 137 * results in sometimes severe color fringes. 138 * 139 * FT_LCD_FILTER_DEFAULT :: 140 * This is a beveled, normalized, and color-balanced five-tap filter 141 * with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256th units. 142 * 143 * FT_LCD_FILTER_LIGHT :: 144 * this is a boxy, normalized, and color-balanced three-tap filter with 145 * weights of [0x00 0x55 0x56 0x55 0x00] in 1/256th units. 146 * 147 * FT_LCD_FILTER_LEGACY :: 148 * FT_LCD_FILTER_LEGACY1 :: 149 * This filter corresponds to the original libXft color filter. It 150 * provides high contrast output but can exhibit really bad color 151 * fringes if glyphs are not extremely well hinted to the pixel grid. 152 * This filter is only provided for comparison purposes, and might be 153 * disabled or stay unsupported in the future. The second value is 154 * provided for compatibility with FontConfig, which historically used 155 * different enumeration, sometimes incorrectly forwarded to FreeType. 156 * 157 * @since: 158 * 2.3.0 (`FT_LCD_FILTER_LEGACY1` since 2.6.2) 159 */ 160 typedef enum FT_LcdFilter_ 161 { 162 FT_LCD_FILTER_NONE = 0, 163 FT_LCD_FILTER_DEFAULT = 1, 164 FT_LCD_FILTER_LIGHT = 2, 165 FT_LCD_FILTER_LEGACY1 = 3, 166 FT_LCD_FILTER_LEGACY = 16, 167 168 FT_LCD_FILTER_MAX /* do not remove */ 169 170 } FT_LcdFilter; 171 172 173 /************************************************************************** 174 * 175 * @function: 176 * FT_Library_SetLcdFilter 177 * 178 * @description: 179 * This function is used to change filter applied to LCD decimated 180 * bitmaps, like the ones used when calling @FT_Render_Glyph with 181 * @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V. 182 * 183 * @input: 184 * library :: 185 * A handle to the target library instance. 186 * 187 * filter :: 188 * The filter type. 189 * 190 * You can use @FT_LCD_FILTER_NONE here to disable this feature, or 191 * @FT_LCD_FILTER_DEFAULT to use a default filter that should work well 192 * on most LCD screens. 193 * 194 * @return: 195 * FreeType error code. 0~means success. 196 * 197 * @note: 198 * Since 2.10.3 the LCD filtering is enabled with @FT_LCD_FILTER_DEFAULT. 199 * It is no longer necessary to call this function explicitly except 200 * to choose a different filter or disable filtering altogether with 201 * @FT_LCD_FILTER_NONE. 202 * 203 * This function does nothing but returns `FT_Err_Unimplemented_Feature` 204 * if the configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is 205 * not defined in your build of the library. 206 * 207 * @since: 208 * 2.3.0 209 */ 210 FT_EXPORT( FT_Error ) 211 FT_Library_SetLcdFilter( FT_Library library, 212 FT_LcdFilter filter ); 213 214 215 /************************************************************************** 216 * 217 * @function: 218 * FT_Library_SetLcdFilterWeights 219 * 220 * @description: 221 * This function can be used to enable LCD filter with custom weights, 222 * instead of using presets in @FT_Library_SetLcdFilter. 223 * 224 * @input: 225 * library :: 226 * A handle to the target library instance. 227 * 228 * weights :: 229 * A pointer to an array; the function copies the first five bytes and 230 * uses them to specify the filter weights in 1/256th units. 231 * 232 * @return: 233 * FreeType error code. 0~means success. 234 * 235 * @note: 236 * This function does nothing but returns `FT_Err_Unimplemented_Feature` 237 * if the configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is 238 * not defined in your build of the library. 239 * 240 * LCD filter weights can also be set per face using @FT_Face_Properties 241 * with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS. 242 * 243 * @since: 244 * 2.4.0 245 */ 246 FT_EXPORT( FT_Error ) 247 FT_Library_SetLcdFilterWeights( FT_Library library, 248 unsigned char *weights ); 249 250 251 /************************************************************************** 252 * 253 * @type: 254 * FT_LcdFiveTapFilter 255 * 256 * @description: 257 * A typedef for passing the five LCD filter weights to 258 * @FT_Face_Properties within an @FT_Parameter structure. 259 * 260 * @since: 261 * 2.8 262 * 263 */ 264 #define FT_LCD_FILTER_FIVE_TAPS 5 265 266 typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS]; 267 268 269 /************************************************************************** 270 * 271 * @function: 272 * FT_Library_SetLcdGeometry 273 * 274 * @description: 275 * This function can be used to modify default positions of color 276 * subpixels, which controls Harmony LCD rendering. 277 * 278 * @input: 279 * library :: 280 * A handle to the target library instance. 281 * 282 * sub :: 283 * A pointer to an array of 3 vectors in 26.6 fractional pixel format; 284 * the function modifies the default values, see the note below. 285 * 286 * @return: 287 * FreeType error code. 0~means success. 288 * 289 * @note: 290 * Subpixel geometry examples: 291 * 292 * - {{-21, 0}, {0, 0}, {21, 0}} is the default, corresponding to 3 color 293 * stripes shifted by a third of a pixel. This could be an RGB panel. 294 * 295 * - {{21, 0}, {0, 0}, {-21, 0}} looks the same as the default but can 296 * specify a BGR panel instead, while keeping the bitmap in the same 297 * RGB888 format. 298 * 299 * - {{0, 21}, {0, 0}, {0, -21}} is the vertical RGB, but the bitmap 300 * stays RGB888 as a result. 301 * 302 * - {{-11, 16}, {-11, -16}, {22, 0}} is a certain PenTile arrangement. 303 * 304 * This function does nothing and returns `FT_Err_Unimplemented_Feature` 305 * in the context of ClearType-style subpixel rendering when 306 * `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is defined in your build of the 307 * library. 308 * 309 * @since: 310 * 2.10.0 311 */ 312 FT_EXPORT( FT_Error ) 313 FT_Library_SetLcdGeometry( FT_Library library, 314 FT_Vector sub[3] ); 315 316 /* */ 317 318 319 FT_END_HEADER 320 321 #endif /* FTLCDFIL_H_ */ 322 323 324 /* END */ 325