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Diffstat (limited to 'modules/freetype2/include/freetype/ftlcdfil.h')
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diff --git a/modules/freetype2/include/freetype/ftlcdfil.h b/modules/freetype2/include/freetype/ftlcdfil.h new file mode 100644 index 0000000000..d3723e16f6 --- /dev/null +++ b/modules/freetype2/include/freetype/ftlcdfil.h @@ -0,0 +1,323 @@ +/**************************************************************************** + * + * ftlcdfil.h + * + * FreeType API for color filtering of subpixel bitmap glyphs + * (specification). + * + * Copyright (C) 2006-2023 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 <freetype/freetype.h> +#include <freetype/ftparams.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_rendering + * + * @title: + * Subpixel Rendering + * + * @abstract: + * API to control subpixel rendering. + * + * @description: + * FreeType provides two alternative subpixel rendering technologies. + * Should you define `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` in your + * `ftoption.h` file, this enables ClearType-style rendering. + * Otherwise, Harmony LCD rendering is enabled. These technologies are + * controlled differently and API described below, although always + * available, performs its function when appropriate method is enabled + * and does nothing otherwise. + * + * ClearType-style LCD 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. Using the + * subpixel coverages unfiltered can create severe color fringes + * especially when rendering thin features. Indeed, to produce + * black-on-white text, the nearby color subpixels must be dimmed + * evenly. Therefore, an equalizing 5-tap FIR filter should be applied + * to subpixel coverages regardless of pixel boundaries and should have + * these properties: + * + * 1. It should be symmetrical, like {~a, b, c, b, a~}, to avoid + * any shifts in appearance. + * + * 2. It should be color-balanced, meaning a~+ b~=~c, to reduce color + * fringes by distributing the computed coverage for one subpixel to + * all subpixels equally. + * + * 3. It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain + * overall brightness. + * + * Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is sharper but is less + * forgiving of non-ideal gamma curves of a screen (and viewing angles), + * beveled filters are fuzzier but more tolerant. + * + * Use the @FT_Library_SetLcdFilter or @FT_Library_SetLcdFilterWeights + * API to specify a low-pass filter, which is then applied to + * subpixel-rendered bitmaps generated through @FT_Render_Glyph. + * + * Harmony LCD rendering is suitable to panels with any regular subpixel + * structure, not just monitors with 3 color striped subpixels, as long + * as the color subpixels have fixed positions relative to the pixel + * center. In this case, each color channel can be rendered separately + * after shifting the outline opposite to the subpixel shift so that the + * coverage maps are aligned. This method is immune to color fringes + * because the shifts do not change integral coverage. + * + * The subpixel geometry must be specified by xy-coordinates for each + * subpixel. By convention they may come in the RGB order: {{-1/3, 0}, + * {0, 0}, {1/3, 0}} for standard RGB striped panel or {{-1/6, 1/4}, + * {-1/6, -1/4}, {1/3, 0}} for a certain PenTile panel. + * + * Use the @FT_Library_SetLcdGeometry API to specify subpixel positions. + * If one follows the RGB order convention, the same order applies to the + * resulting @FT_PIXEL_MODE_LCD and @FT_PIXEL_MODE_LCD_V bitmaps. Note, + * however, that the coordinate frame for the latter must be rotated + * clockwise. Harmony with default LCD geometry is equivalent to + * ClearType with light filter. + * + * As a result of ClearType filtering or Harmony shifts, the resulting + * dimensions of LCD bitmaps can be slightly wider or taller than the + * dimensions the original outline with regard to the pixel grid. + * For example, for @FT_RENDER_MODE_LCD, the filter adds 2~subpixels to + * the left, and 2~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. + * + * The ClearType and Harmony rendering is applicable to glyph bitmaps + * rendered through @FT_Render_Glyph, @FT_Load_Glyph, @FT_Load_Char, and + * @FT_Glyph_To_Bitmap, when @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V + * is specified. This API does not control @FT_Outline_Render and + * @FT_Outline_Get_Bitmap. + * + * The described algorithms can completely remove color artefacts when + * combined with gamma-corrected alpha blending in linear space. Each of + * the 3~alpha values (subpixels) must by 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. + */ + + + /************************************************************************** + * + * @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 :: + * This is a beveled, normalized, and color-balanced five-tap filter + * with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256 units. + * + * FT_LCD_FILTER_LIGHT :: + * this is a boxy, normalized, and color-balanced three-tap filter with + * weights of [0x00 0x55 0x56 0x55 0x00] in 1/256 units. + * + * FT_LCD_FILTER_LEGACY :: + * FT_LCD_FILTER_LEGACY1 :: + * 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. + * This filter is only provided for comparison purposes, and might be + * disabled or stay unsupported in the future. The second value is + * provided for compatibility with FontConfig, which historically used + * different enumeration, sometimes incorrectly forwarded to FreeType. + * + * @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; + + + /************************************************************************** + * + * @function: + * FT_Library_SetLcdFilter + * + * @description: + * This function is used to change filter applied 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: + * Since 2.10.3 the LCD filtering is enabled with @FT_LCD_FILTER_DEFAULT. + * It is no longer necessary to call this function explicitly except + * to choose a different filter or disable filtering altogether with + * @FT_LCD_FILTER_NONE. + * + * This function does nothing but returns `FT_Err_Unimplemented_Feature` + * if the configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is + * not defined in your build of the library. + * + * @since: + * 2.3.0 + */ + FT_EXPORT( FT_Error ) + FT_Library_SetLcdFilter( FT_Library library, + FT_LcdFilter filter ); + + + /************************************************************************** + * + * @function: + * 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 in 1/256 units. + * + * @return: + * FreeType error code. 0~means success. + * + * @note: + * This function does nothing but returns `FT_Err_Unimplemented_Feature` + * if the configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is + * not defined in your build of the library. + * + * 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 ); + + + /************************************************************************** + * + * @type: + * FT_LcdFiveTapFilter + * + * @description: + * A typedef for passing the five LCD filter weights to + * @FT_Face_Properties within an @FT_Parameter structure. + * + * @since: + * 2.8 + * + */ +#define FT_LCD_FILTER_FIVE_TAPS 5 + + typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS]; + + + /************************************************************************** + * + * @function: + * FT_Library_SetLcdGeometry + * + * @description: + * This function can be used to modify default positions of color + * subpixels, which controls Harmony LCD rendering. + * + * @input: + * library :: + * A handle to the target library instance. + * + * sub :: + * A pointer to an array of 3 vectors in 26.6 fractional pixel format; + * the function modifies the default values, see the note below. + * + * @return: + * FreeType error code. 0~means success. + * + * @note: + * Subpixel geometry examples: + * + * - {{-21, 0}, {0, 0}, {21, 0}} is the default, corresponding to 3 color + * stripes shifted by a third of a pixel. This could be an RGB panel. + * + * - {{21, 0}, {0, 0}, {-21, 0}} looks the same as the default but can + * specify a BGR panel instead, while keeping the bitmap in the same + * RGB888 format. + * + * - {{0, 21}, {0, 0}, {0, -21}} is the vertical RGB, but the bitmap + * stays RGB888 as a result. + * + * - {{-11, 16}, {-11, -16}, {22, 0}} is a certain PenTile arrangement. + * + * This function does nothing and returns `FT_Err_Unimplemented_Feature` + * in the context of ClearType-style subpixel rendering when + * `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is defined in your build of the + * library. + * + * @since: + * 2.10.0 + */ + FT_EXPORT( FT_Error ) + FT_Library_SetLcdGeometry( FT_Library library, + FT_Vector sub[3] ); + + /* */ + + +FT_END_HEADER + +#endif /* FTLCDFIL_H_ */ + + +/* END */ |