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Diffstat (limited to 'include/freetype/ftlcdfil.h')
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diff --git a/include/freetype/ftlcdfil.h b/include/freetype/ftlcdfil.h new file mode 100644 index 0000000..a9dd3ea --- /dev/null +++ b/include/freetype/ftlcdfil.h @@ -0,0 +1,292 @@ +/***************************************************************************/ +/* */ +/* ftlcdfil.h */ +/* */ +/* FreeType API for color filtering of subpixel bitmap glyphs */ +/* (specification). */ +/* */ +/* Copyright 2006-2015 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 __FT_LCD_FILTER_H__ +#define __FT_LCD_FILTER_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. + * + * 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. + * + * 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. + * + * 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. + * + * 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. + * + * The filter affects glyph bitmaps rendered through @FT_Render_Glyph, + * @FT_Outline_Get_Bitmap, @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 larger or taller than the dimensions of the corresponding + * outline with regards to the pixel grid. For example, for + * @FT_RENDER_MODE_LCD, the filter adds up to 3~pixels to the left, and + * up to 3~pixels to the right. + * + * The bitmap offset values are adjusted correctly, so clients shouldn't + * need to modify their layout and glyph positioning code when enabling + * the filter. + * + * @since: + * 2.3.0 + */ + FT_EXPORT( FT_Error ) + FT_Library_SetLcdFilter( FT_Library library, + FT_LcdFilter filter ); + + + /************************************************************************** + * + * @func: + * FT_Library_SetLcdFilterWeights + * + * @description: + * Use this function to override the filter weights selected by + * @FT_Library_SetLcdFilter. By default, FreeType uses the quintuple + * (0x00, 0x55, 0x56, 0x55, 0x00) for FT_LCD_FILTER_LIGHT, and (0x08, + * 0x4D, 0x56, 0x4D, 0x08) for FT_LCD_FILTER_DEFAULT. + * + * @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. + * + * This function must be called after @FT_Library_SetLcdFilter to have + * any effect. + * + * @since: + * 2.4.0 + */ + FT_EXPORT( FT_Error ) + FT_Library_SetLcdFilterWeights( FT_Library library, + unsigned char *weights ); + + /* */ + + +FT_END_HEADER + +#endif /* __FT_LCD_FILTER_H__ */ + + +/* END */ |