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| 1 /***************************************************************************/ | |
| 2 /* */ | |
| 3 /* ftlcdfil.h */ | |
| 4 /* */ | |
| 5 /* FreeType API for color filtering of subpixel bitmap glyphs */ | |
| 6 /* (specification). */ | |
| 7 /* */ | |
| 8 /* Copyright 2006-2008, 2010, 2013 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 __FT_LCD_FILTER_H__ | |
| 21 #define __FT_LCD_FILTER_H__ | |
| 22 | |
| 23 #include <ft2build.h> | |
| 24 #include FT_FREETYPE_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_filtering | |
| 39 * | |
| 40 * @title: | |
| 41 * LCD Filtering | |
| 42 * | |
| 43 * @abstract: | |
| 44 * Reduce color fringes of LCD-optimized bitmaps. | |
| 45 * | |
| 46 * @description: | |
| 47 * The @FT_Library_SetLcdFilter API can be used to specify a low-pass | |
| 48 * filter, which is then applied to LCD-optimized bitmaps generated | |
| 49 * through @FT_Render_Glyph. This is useful to reduce color fringes | |
| 50 * that would occur with unfiltered rendering. | |
| 51 * | |
| 52 * Note that no filter is active by default, and that this function is | |
| 53 * *not* implemented in default builds of the library. You need to | |
| 54 * #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file | |
| 55 * in order to activate it. | |
| 56 * | |
| 57 * FreeType generates alpha coverage maps, which are linear by nature. | |
| 58 * For instance, the value 0x80 in bitmap representation means that | |
| 59 * (within numerical precision) 0x80/0xff fraction of that pixel is | |
| 60 * covered by the glyph's outline. The blending function for placing | |
| 61 * text over a background is | |
| 62 * | |
| 63 * { | |
| 64 * dst = alpha * src + (1 - alpha) * dst , | |
| 65 * } | |
| 66 * | |
| 67 * which is known as OVER. However, when calculating the output of the | |
| 68 * OVER operator, the source colors should first be transformed to a | |
| 69 * linear color space, then alpha blended in that space, and transformed | |
| 70 * back to the output color space. | |
| 71 * | |
| 72 * When linear light blending is used, the default FIR5 filtering | |
| 73 * weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as | |
| 74 * they have been designed for black on white rendering while lacking | |
| 75 * gamma correction. To preserve color neutrality, weights for a FIR5 | |
| 76 * filter should be chosen according to two free parameters `a' and `c', | |
| 77 * and the FIR weights should be | |
| 78 * | |
| 79 * { | |
| 80 * [a - c, a + c, 2 * a, a + c, a - c] . | |
| 81 * } | |
| 82 * | |
| 83 * This formula generates equal weights for all the color primaries | |
| 84 * across the filter kernel, which makes it colorless. One suggested | |
| 85 * set of weights is | |
| 86 * | |
| 87 * { | |
| 88 * [0x10, 0x50, 0x60, 0x50, 0x10] , | |
| 89 * } | |
| 90 * | |
| 91 * where `a' has value 0x30 and `b' value 0x20. The weights in filter | |
| 92 * may have a sum larger than 0x100, which increases coloration slightly | |
| 93 * but also improves contrast. | |
| 94 */ | |
| 95 | |
| 96 | |
| 97 /**************************************************************************** | |
| 98 * | |
| 99 * @enum: | |
| 100 * FT_LcdFilter | |
| 101 * | |
| 102 * @description: | |
| 103 * A list of values to identify various types of LCD filters. | |
| 104 * | |
| 105 * @values: | |
| 106 * FT_LCD_FILTER_NONE :: | |
| 107 * Do not perform filtering. When used with subpixel rendering, this | |
| 108 * results in sometimes severe color fringes. | |
| 109 * | |
| 110 * FT_LCD_FILTER_DEFAULT :: | |
| 111 * The default filter reduces color fringes considerably, at the cost | |
| 112 * of a slight blurriness in the output. | |
| 113 * | |
| 114 * FT_LCD_FILTER_LIGHT :: | |
| 115 * The light filter is a variant that produces less blurriness at the | |
| 116 * cost of slightly more color fringes than the default one. It might | |
| 117 * be better, depending on taste, your monitor, or your personal vision. | |
| 118 * | |
| 119 * FT_LCD_FILTER_LEGACY :: | |
| 120 * This filter corresponds to the original libXft color filter. It | |
| 121 * provides high contrast output but can exhibit really bad color | |
| 122 * fringes if glyphs are not extremely well hinted to the pixel grid. | |
| 123 * In other words, it only works well if the TrueType bytecode | |
| 124 * interpreter is enabled *and* high-quality hinted fonts are used. | |
| 125 * | |
| 126 * This filter is only provided for comparison purposes, and might be | |
| 127 * disabled or stay unsupported in the future. | |
| 128 * | |
| 129 * @since: | |
| 130 * 2.3.0 | |
| 131 */ | |
| 132 typedef enum FT_LcdFilter_ | |
| 133 { | |
| 134 FT_LCD_FILTER_NONE = 0, | |
| 135 FT_LCD_FILTER_DEFAULT = 1, | |
| 136 FT_LCD_FILTER_LIGHT = 2, | |
| 137 FT_LCD_FILTER_LEGACY = 16, | |
| 138 | |
| 139 FT_LCD_FILTER_MAX /* do not remove */ | |
| 140 | |
| 141 } FT_LcdFilter; | |
| 142 | |
| 143 | |
| 144 /************************************************************************** | |
| 145 * | |
| 146 * @func: | |
| 147 * FT_Library_SetLcdFilter | |
| 148 * | |
| 149 * @description: | |
| 150 * This function is used to apply color filtering to LCD decimated | |
| 151 * bitmaps, like the ones used when calling @FT_Render_Glyph with | |
| 152 * @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V. | |
| 153 * | |
| 154 * @input: | |
| 155 * library :: | |
| 156 * A handle to the target library instance. | |
| 157 * | |
| 158 * filter :: | |
| 159 * The filter type. | |
| 160 * | |
| 161 * You can use @FT_LCD_FILTER_NONE here to disable this feature, or | |
| 162 * @FT_LCD_FILTER_DEFAULT to use a default filter that should work | |
| 163 * well on most LCD screens. | |
| 164 * | |
| 165 * @return: | |
| 166 * FreeType error code. 0~means success. | |
| 167 * | |
| 168 * @note: | |
| 169 * This feature is always disabled by default. Clients must make an | |
| 170 * explicit call to this function with a `filter' value other than | |
| 171 * @FT_LCD_FILTER_NONE in order to enable it. | |
| 172 * | |
| 173 * Due to *PATENTS* covering subpixel rendering, this function doesn't | |
| 174 * do anything except returning `FT_Err_Unimplemented_Feature' if the | |
| 175 * configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not | |
| 176 * defined in your build of the library, which should correspond to all | |
| 177 * default builds of FreeType. | |
| 178 * | |
| 179 * The filter affects glyph bitmaps rendered through @FT_Render_Glyph, | |
| 180 * @FT_Outline_Get_Bitmap, @FT_Load_Glyph, and @FT_Load_Char. | |
| 181 * | |
| 182 * It does _not_ affect the output of @FT_Outline_Render and | |
| 183 * @FT_Outline_Get_Bitmap. | |
| 184 * | |
| 185 * If this feature is activated, the dimensions of LCD glyph bitmaps are | |
| 186 * either larger or taller than the dimensions of the corresponding | |
| 187 * outline with regards to the pixel grid. For example, for | |
| 188 * @FT_RENDER_MODE_LCD, the filter adds up to 3~pixels to the left, and | |
| 189 * up to 3~pixels to the right. | |
| 190 * | |
| 191 * The bitmap offset values are adjusted correctly, so clients shouldn't | |
| 192 * need to modify their layout and glyph positioning code when enabling | |
| 193 * the filter. | |
| 194 * | |
| 195 * @since: | |
| 196 * 2.3.0 | |
| 197 */ | |
| 198 FT_EXPORT( FT_Error ) | |
| 199 FT_Library_SetLcdFilter( FT_Library library, | |
| 200 FT_LcdFilter filter ); | |
| 201 | |
| 202 | |
| 203 /************************************************************************** | |
| 204 * | |
| 205 * @func: | |
| 206 * FT_Library_SetLcdFilterWeights | |
| 207 * | |
| 208 * @description: | |
| 209 * Use this function to override the filter weights selected by | |
| 210 * @FT_Library_SetLcdFilter. By default, FreeType uses the quintuple | |
| 211 * (0x00, 0x55, 0x56, 0x55, 0x00) for FT_LCD_FILTER_LIGHT, and (0x10, | |
| 212 * 0x40, 0x70, 0x40, 0x10) for FT_LCD_FILTER_DEFAULT and | |
| 213 * FT_LCD_FILTER_LEGACY. | |
| 214 * | |
| 215 * @input: | |
| 216 * library :: | |
| 217 * A handle to the target library instance. | |
| 218 * | |
| 219 * weights :: | |
| 220 * A pointer to an array; the function copies the first five bytes and | |
| 221 * uses them to specify the filter weights. | |
| 222 * | |
| 223 * @return: | |
| 224 * FreeType error code. 0~means success. | |
| 225 * | |
| 226 * @note: | |
| 227 * Due to *PATENTS* covering subpixel rendering, this function doesn't | |
| 228 * do anything except returning `FT_Err_Unimplemented_Feature' if the | |
| 229 * configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not | |
| 230 * defined in your build of the library, which should correspond to all | |
| 231 * default builds of FreeType. | |
| 232 * | |
| 233 * This function must be called after @FT_Library_SetLcdFilter to have | |
| 234 * any effect. | |
| 235 * | |
| 236 * @since: | |
| 237 * 2.4.0 | |
| 238 */ | |
| 239 FT_EXPORT( FT_Error ) | |
| 240 FT_Library_SetLcdFilterWeights( FT_Library library, | |
| 241 unsigned char *weights ); | |
| 242 | |
| 243 /* */ | |
| 244 | |
| 245 | |
| 246 FT_END_HEADER | |
| 247 | |
| 248 #endif /* __FT_LCD_FILTER_H__ */ | |
| 249 | |
| 250 | |
| 251 /* END */ | |
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Chromium Code Reviews
Issue 1416993005:
Merge to XFA: Update bundled freetype to 2.6.1 (Closed)
Base URL: https://pdfium.googlesource.com/pdfium.git@xfa