Edit File: ftlcdfil.h
/***************************************************************************/ /* */ /* ftlcdfil.h */ /* */ /* FreeType API for color filtering of subpixel bitmap glyphs */ /* (specification). */ /* */ /* Copyright 2006-2017 by */ /* David Turner, Robert Wilhelm, and Werner Lemberg. */ /* */ /* This file is part of the FreeType project, and may only be used, */ /* modified, and distributed under the terms of the FreeType project */ /* license, LICENSE.TXT. By continuing to use, modify, or distribute */ /* this file you indicate that you have read the license and */ /* understand and accept it fully. */ /* */ /***************************************************************************/ #ifndef FTLCDFIL_H_ #define FTLCDFIL_H_ #include <ft2build.h> #include FT_FREETYPE_H #ifdef FREETYPE_H #error "freetype.h of FreeType 1 has been loaded!" #error "Please fix the directory search order for header files" #error "so that freetype.h of FreeType 2 is found first." #endif FT_BEGIN_HEADER /*************************************************************************** * * @section: * lcd_filtering * * @title: * LCD Filtering * * @abstract: * Reduce color fringes of subpixel-rendered bitmaps. * * @description: * Subpixel rendering exploits the color-striped structure of LCD * pixels, increasing the available resolution in the direction of the * stripe (usually horizontal RGB) by a factor of~3. Since these * subpixels are color pixels, using them unfiltered creates severe * color fringes. Use the @FT_Library_SetLcdFilter API to specify a * low-pass filter, which is then applied to subpixel-rendered bitmaps * generated through @FT_Render_Glyph. The filter sacrifices some of * the higher resolution to reduce color fringes, making the glyph image * slightly blurrier. Positional improvements will remain. * * Note that no filter is active by default, and that this function is * *not* implemented in default builds of the library. You need to * #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file * in order to activate it and explicitly call @FT_Library_SetLcdFilter * to enable it. * * A filter should have two properties: * * 1) It should be normalized, meaning the sum of the 5~components * should be 256 (0x100). It is possible to go above or under this * target sum, however: going under means tossing out contrast, going * over means invoking clamping and thereby non-linearities that * increase contrast somewhat at the expense of greater distortion * and color-fringing. Contrast is better enhanced through stem * darkening. * * 2) It should be color-balanced, meaning a filter `{~a, b, c, b, a~}' * where a~+ b~=~c. It distributes the computed coverage for one * subpixel to all subpixels equally, sacrificing some won resolution * but drastically reducing color-fringing. Positioning improvements * remain! Note that color-fringing can only really be minimized * when using a color-balanced filter and alpha-blending the glyph * onto a surface in linear space; see @FT_Render_Glyph. * * Regarding the form, a filter can be a `boxy' filter or a `beveled' * filter. Boxy filters are sharper but are less forgiving of non-ideal * gamma curves of a screen (viewing angles!), beveled filters are * fuzzier but more tolerant. * * Examples: * * - [0x10 0x40 0x70 0x40 0x10] is beveled and neither balanced nor * normalized. * * - [0x1A 0x33 0x4D 0x33 0x1A] is beveled and balanced but not * normalized. * * - [0x19 0x33 0x66 0x4c 0x19] is beveled and normalized but not * balanced. * * - [0x00 0x4c 0x66 0x4c 0x00] is boxily beveled and normalized but not * balanced. * * - [0x00 0x55 0x56 0x55 0x00] is boxy, normalized, and almost * balanced. * * - [0x08 0x4D 0x56 0x4D 0x08] is beveled, normalized and, almost * balanced. * * The filter affects glyph bitmaps rendered through @FT_Render_Glyph, * @FT_Load_Glyph, and @FT_Load_Char. It does _not_ affect the output * of @FT_Outline_Render and @FT_Outline_Get_Bitmap. * * If this feature is activated, the dimensions of LCD glyph bitmaps are * either wider or taller than the dimensions of the corresponding * outline with regard to the pixel grid. For example, for * @FT_RENDER_MODE_LCD, the filter adds 3~subpixels to the left, and * 3~subpixels to the right. The bitmap offset values are adjusted * accordingly, so clients shouldn't need to modify their layout and * glyph positioning code when enabling the filter. * * It is important to understand that linear alpha blending and gamma * correction is critical for correctly rendering glyphs onto surfaces * without artifacts and even more critical when subpixel rendering is * involved. * * Each of the 3~alpha values (subpixels) is independently used to blend * one color channel. That is, red alpha blends the red channel of the * text color with the red channel of the background pixel. The * distribution of density values by the color-balanced filter assumes * alpha blending is done in linear space; only then color artifacts * cancel out. */ /**************************************************************************** * * @enum: * FT_LcdFilter * * @description: * A list of values to identify various types of LCD filters. * * @values: * FT_LCD_FILTER_NONE :: * Do not perform filtering. When used with subpixel rendering, this * results in sometimes severe color fringes. * * FT_LCD_FILTER_DEFAULT :: * The default filter reduces color fringes considerably, at the cost * of a slight blurriness in the output. * * It is a beveled, normalized, and color-balanced five-tap filter * that is more forgiving to screens with non-ideal gamma curves and * viewing angles. Note that while color-fringing is reduced, it can * only be minimized by using linear alpha blending and gamma * correction to render glyphs onto surfaces. The default filter * weights are [0x08 0x4D 0x56 0x4D 0x08]. * * FT_LCD_FILTER_LIGHT :: * The light filter is a variant that is sharper at the cost of * slightly more color fringes than the default one. * * It is a boxy, normalized, and color-balanced three-tap filter that * is less forgiving to screens with non-ideal gamma curves and * viewing angles. This filter works best when the rendering system * uses linear alpha blending and gamma correction to render glyphs * onto surfaces. The light filter weights are * [0x00 0x55 0x56 0x55 0x00]. * * FT_LCD_FILTER_LEGACY :: * This filter corresponds to the original libXft color filter. It * provides high contrast output but can exhibit really bad color * fringes if glyphs are not extremely well hinted to the pixel grid. * In other words, it only works well if the TrueType bytecode * interpreter is enabled *and* high-quality hinted fonts are used. * * This filter is only provided for comparison purposes, and might be * disabled or stay unsupported in the future. * * FT_LCD_FILTER_LEGACY1 :: * For historical reasons, the FontConfig library returns a different * enumeration value for legacy LCD filtering. To make code work that * (incorrectly) forwards FontConfig's enumeration value to * @FT_Library_SetLcdFilter without proper mapping, it is thus easiest * to have another enumeration value, which is completely equal to * `FT_LCD_FILTER_LEGACY'. * * @since: * 2.3.0 (`FT_LCD_FILTER_LEGACY1' since 2.6.2) */ typedef enum FT_LcdFilter_ { FT_LCD_FILTER_NONE = 0, FT_LCD_FILTER_DEFAULT = 1, FT_LCD_FILTER_LIGHT = 2, FT_LCD_FILTER_LEGACY1 = 3, FT_LCD_FILTER_LEGACY = 16, FT_LCD_FILTER_MAX /* do not remove */ } FT_LcdFilter; /************************************************************************** * * @func: * FT_Library_SetLcdFilter * * @description: * This function is used to apply color filtering to LCD decimated * bitmaps, like the ones used when calling @FT_Render_Glyph with * @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V. * * @input: * library :: * A handle to the target library instance. * * filter :: * The filter type. * * You can use @FT_LCD_FILTER_NONE here to disable this feature, or * @FT_LCD_FILTER_DEFAULT to use a default filter that should work * well on most LCD screens. * * @return: * FreeType error code. 0~means success. * * @note: * This feature is always disabled by default. Clients must make an * explicit call to this function with a `filter' value other than * @FT_LCD_FILTER_NONE in order to enable it. * * Due to *PATENTS* covering subpixel rendering, this function doesn't * do anything except returning `FT_Err_Unimplemented_Feature' if the * configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not * defined in your build of the library, which should correspond to all * default builds of FreeType. * * @since: * 2.3.0 */ FT_EXPORT( FT_Error ) FT_Library_SetLcdFilter( FT_Library library, FT_LcdFilter filter ); /************************************************************************** * * @func: * FT_Library_SetLcdFilterWeights * * @description: * This function can be used to enable LCD filter with custom weights, * instead of using presets in @FT_Library_SetLcdFilter. * * @input: * library :: * A handle to the target library instance. * * weights :: * A pointer to an array; the function copies the first five bytes and * uses them to specify the filter weights. * * @return: * FreeType error code. 0~means success. * * @note: * Due to *PATENTS* covering subpixel rendering, this function doesn't * do anything except returning `FT_Err_Unimplemented_Feature' if the * configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not * defined in your build of the library, which should correspond to all * default builds of FreeType. * * LCD filter weights can also be set per face using @FT_Face_Properties * with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS. * * @since: * 2.4.0 */ FT_EXPORT( FT_Error ) FT_Library_SetLcdFilterWeights( FT_Library library, unsigned char *weights ); /************************************************************************** * * @constant: * FT_PARAM_TAG_LCD_FILTER_WEIGHTS * * @description: * An @FT_Parameter tag to be used with @FT_Face_Properties. The * corresponding argument specifies the five LCD filter weights for a * given face (if using @FT_LOAD_TARGET_LCD, for example), overriding * the global default values or the values set up with * @FT_Library_SetLcdFilterWeights. * */ #define FT_PARAM_TAG_LCD_FILTER_WEIGHTS \ FT_MAKE_TAG( 'l', 'c', 'd', 'f' ) /* * @type: * FT_LcdFiveTapFilter * * @description: * A typedef for passing the five LCD filter weights to * @FT_Face_Properties within an @FT_Parameter structure. * */ #define FT_LCD_FILTER_FIVE_TAPS 5 typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS]; /* */ FT_END_HEADER #endif /* FTLCDFIL_H_ */ /* END */