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Diffstat (limited to 'include/ftlcdfil.h')
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diff --git a/include/ftlcdfil.h b/include/ftlcdfil.h new file mode 100644 index 0000000..39206f0 --- /dev/null +++ b/include/ftlcdfil.h @@ -0,0 +1,251 @@ +/***************************************************************************/ +/* */ +/* ftlcdfil.h */ +/* */ +/* FreeType API for color filtering of subpixel bitmap glyphs */ +/* (specification). */ +/* */ +/* Copyright 2006-2008, 2010, 2013 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 LCD-optimized bitmaps. + * + * @description: + * The @FT_Library_SetLcdFilter API can be used to specify a low-pass + * filter, which is then applied to LCD-optimized bitmaps generated + * through @FT_Render_Glyph. This is useful to reduce color fringes + * that would occur with unfiltered rendering. + * + * 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. + * + * FreeType generates alpha coverage maps, which are linear by nature. + * For instance, the value 0x80 in bitmap representation means that + * (within numerical precision) 0x80/0xff fraction of that pixel is + * covered by the glyph's outline. The blending function for placing + * text over a background is + * + * { + * dst = alpha * src + (1 - alpha) * dst , + * } + * + * which is known as OVER. However, when calculating the output of the + * OVER operator, the source colors should first be transformed to a + * linear color space, then alpha blended in that space, and transformed + * back to the output color space. + * + * When linear light blending is used, the default FIR5 filtering + * weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as + * they have been designed for black on white rendering while lacking + * gamma correction. To preserve color neutrality, weights for a FIR5 + * filter should be chosen according to two free parameters `a' and `c', + * and the FIR weights should be + * + * { + * [a - c, a + c, 2 * a, a + c, a - c] . + * } + * + * This formula generates equal weights for all the color primaries + * across the filter kernel, which makes it colorless. One suggested + * set of weights is + * + * { + * [0x10, 0x50, 0x60, 0x50, 0x10] , + * } + * + * where `a' has value 0x30 and `b' value 0x20. The weights in filter + * may have a sum larger than 0x100, which increases coloration slightly + * but also improves contrast. + */ + + + /**************************************************************************** + * + * @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. + * + * FT_LCD_FILTER_LIGHT :: + * The light filter is a variant that produces less blurriness at the + * cost of slightly more color fringes than the default one. It might + * be better, depending on taste, your monitor, or your personal vision. + * + * 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. + * + * @since: + * 2.3.0 + */ + typedef enum FT_LcdFilter_ + { + FT_LCD_FILTER_NONE = 0, + FT_LCD_FILTER_DEFAULT = 1, + FT_LCD_FILTER_LIGHT = 2, + 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 (0x10, + * 0x40, 0x70, 0x40, 0x10) for FT_LCD_FILTER_DEFAULT and + * FT_LCD_FILTER_LEGACY. + * + * @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 */ |