324 lines
12 KiB
C
324 lines
12 KiB
C
/****************************************************************************
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*
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* ftlcdfil.h
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*
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* FreeType API for color filtering of subpixel bitmap glyphs
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* (specification).
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*
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* Copyright (C) 2006-2022 by
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* David Turner, Robert Wilhelm, and Werner Lemberg.
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*
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* This file is part of the FreeType project, and may only be used,
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* modified, and distributed under the terms of the FreeType project
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* license, LICENSE.TXT. By continuing to use, modify, or distribute
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* this file you indicate that you have read the license and
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* understand and accept it fully.
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*
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*/
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#ifndef FTLCDFIL_H_
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#define FTLCDFIL_H_
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#include <freetype/freetype.h>
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#include <freetype/ftparams.h>
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#ifdef FREETYPE_H
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#error "freetype.h of FreeType 1 has been loaded!"
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#error "Please fix the directory search order for header files"
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#error "so that freetype.h of FreeType 2 is found first."
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#endif
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FT_BEGIN_HEADER
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/**************************************************************************
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*
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* @section:
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* lcd_rendering
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*
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* @title:
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* Subpixel Rendering
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*
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* @abstract:
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* API to control subpixel rendering.
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*
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* @description:
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* FreeType provides two alternative subpixel rendering technologies.
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* Should you define `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` in your
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* `ftoption.h` file, this enables ClearType-style rendering.
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* Otherwise, Harmony LCD rendering is enabled. These technologies are
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* controlled differently and API described below, although always
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* available, performs its function when appropriate method is enabled
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* and does nothing otherwise.
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*
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* ClearType-style LCD rendering exploits the color-striped structure of
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* LCD pixels, increasing the available resolution in the direction of
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* the stripe (usually horizontal RGB) by a factor of~3. Using the
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* subpixel coverages unfiltered can create severe color fringes
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* especially when rendering thin features. Indeed, to produce
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* black-on-white text, the nearby color subpixels must be dimmed
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* evenly. Therefore, an equalizing 5-tap FIR filter should be applied
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* to subpixel coverages regardless of pixel boundaries and should have
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* these properties:
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*
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* 1. It should be symmetrical, like {~a, b, c, b, a~}, to avoid
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* any shifts in appearance.
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*
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* 2. It should be color-balanced, meaning a~+ b~=~c, to reduce color
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* fringes by distributing the computed coverage for one subpixel to
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* all subpixels equally.
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*
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* 3. It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain
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* overall brightness.
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*
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* Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is sharper but is less
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* forgiving of non-ideal gamma curves of a screen (and viewing angles),
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* beveled filters are fuzzier but more tolerant.
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*
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* Use the @FT_Library_SetLcdFilter or @FT_Library_SetLcdFilterWeights
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* API to specify a low-pass filter, which is then applied to
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* subpixel-rendered bitmaps generated through @FT_Render_Glyph.
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*
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* Harmony LCD rendering is suitable to panels with any regular subpixel
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* structure, not just monitors with 3 color striped subpixels, as long
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* as the color subpixels have fixed positions relative to the pixel
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* center. In this case, each color channel can be rendered separately
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* after shifting the outline opposite to the subpixel shift so that the
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* coverage maps are aligned. This method is immune to color fringes
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* because the shifts do not change integral coverage.
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*
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* The subpixel geometry must be specified by xy-coordinates for each
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* subpixel. By convention they may come in the RGB order: {{-1/3, 0},
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* {0, 0}, {1/3, 0}} for standard RGB striped panel or {{-1/6, 1/4},
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* {-1/6, -1/4}, {1/3, 0}} for a certain PenTile panel.
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*
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* Use the @FT_Library_SetLcdGeometry API to specify subpixel positions.
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* If one follows the RGB order convention, the same order applies to the
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* resulting @FT_PIXEL_MODE_LCD and @FT_PIXEL_MODE_LCD_V bitmaps. Note,
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* however, that the coordinate frame for the latter must be rotated
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* clockwise. Harmony with default LCD geometry is equivalent to
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* ClearType with light filter.
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*
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* As a result of ClearType filtering or Harmony shifts, the resulting
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* dimensions of LCD bitmaps can be slightly wider or taller than the
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* dimensions the original outline with regard to the pixel grid.
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* For example, for @FT_RENDER_MODE_LCD, the filter adds 2~subpixels to
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* the left, and 2~subpixels to the right. The bitmap offset values are
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* adjusted accordingly, so clients shouldn't need to modify their layout
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* and glyph positioning code when enabling the filter.
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*
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* The ClearType and Harmony rendering is applicable to glyph bitmaps
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* rendered through @FT_Render_Glyph, @FT_Load_Glyph, @FT_Load_Char, and
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* @FT_Glyph_To_Bitmap, when @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V
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* is specified. This API does not control @FT_Outline_Render and
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* @FT_Outline_Get_Bitmap.
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*
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* The described algorithms can completely remove color artefacts when
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* combined with gamma-corrected alpha blending in linear space. Each of
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* the 3~alpha values (subpixels) must by independently used to blend one
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* color channel. That is, red alpha blends the red channel of the text
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* color with the red channel of the background pixel.
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*/
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/**************************************************************************
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*
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* @enum:
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* FT_LcdFilter
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*
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* @description:
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* A list of values to identify various types of LCD filters.
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*
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* @values:
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* FT_LCD_FILTER_NONE ::
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* Do not perform filtering. When used with subpixel rendering, this
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* results in sometimes severe color fringes.
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*
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* FT_LCD_FILTER_DEFAULT ::
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* This is a beveled, normalized, and color-balanced five-tap filter
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* with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256th units.
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*
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* FT_LCD_FILTER_LIGHT ::
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* this is a boxy, normalized, and color-balanced three-tap filter with
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* weights of [0x00 0x55 0x56 0x55 0x00] in 1/256th units.
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*
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* FT_LCD_FILTER_LEGACY ::
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* FT_LCD_FILTER_LEGACY1 ::
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* This filter corresponds to the original libXft color filter. It
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* provides high contrast output but can exhibit really bad color
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* fringes if glyphs are not extremely well hinted to the pixel grid.
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* This filter is only provided for comparison purposes, and might be
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* disabled or stay unsupported in the future. The second value is
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* provided for compatibility with FontConfig, which historically used
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* different enumeration, sometimes incorrectly forwarded to FreeType.
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*
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* @since:
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* 2.3.0 (`FT_LCD_FILTER_LEGACY1` since 2.6.2)
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*/
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typedef enum FT_LcdFilter_
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{
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FT_LCD_FILTER_NONE = 0,
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FT_LCD_FILTER_DEFAULT = 1,
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FT_LCD_FILTER_LIGHT = 2,
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FT_LCD_FILTER_LEGACY1 = 3,
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FT_LCD_FILTER_LEGACY = 16,
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FT_LCD_FILTER_MAX /* do not remove */
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} FT_LcdFilter;
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/**************************************************************************
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*
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* @function:
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* FT_Library_SetLcdFilter
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*
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* @description:
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* This function is used to change filter applied to LCD decimated
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* bitmaps, like the ones used when calling @FT_Render_Glyph with
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* @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V.
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*
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* @input:
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* library ::
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* A handle to the target library instance.
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*
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* filter ::
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* The filter type.
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*
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* You can use @FT_LCD_FILTER_NONE here to disable this feature, or
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* @FT_LCD_FILTER_DEFAULT to use a default filter that should work well
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* on most LCD screens.
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*
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* @return:
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* FreeType error code. 0~means success.
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*
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* @note:
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* Since 2.10.3 the LCD filtering is enabled with @FT_LCD_FILTER_DEFAULT.
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* It is no longer necessary to call this function explicitly except
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* to choose a different filter or disable filtering altogether with
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* @FT_LCD_FILTER_NONE.
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*
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* This function does nothing but returns `FT_Err_Unimplemented_Feature`
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* if the configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is
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* not defined in your build of the library.
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*
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* @since:
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* 2.3.0
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*/
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FT_EXPORT( FT_Error )
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FT_Library_SetLcdFilter( FT_Library library,
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FT_LcdFilter filter );
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/**************************************************************************
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*
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* @function:
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* FT_Library_SetLcdFilterWeights
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*
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* @description:
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* This function can be used to enable LCD filter with custom weights,
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* instead of using presets in @FT_Library_SetLcdFilter.
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*
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* @input:
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* library ::
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* A handle to the target library instance.
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*
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* weights ::
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* A pointer to an array; the function copies the first five bytes and
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* uses them to specify the filter weights in 1/256th units.
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*
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* @return:
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* FreeType error code. 0~means success.
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*
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* @note:
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* This function does nothing but returns `FT_Err_Unimplemented_Feature`
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* if the configuration macro `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is
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* not defined in your build of the library.
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*
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* LCD filter weights can also be set per face using @FT_Face_Properties
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* with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS.
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*
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* @since:
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* 2.4.0
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*/
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FT_EXPORT( FT_Error )
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FT_Library_SetLcdFilterWeights( FT_Library library,
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unsigned char *weights );
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/**************************************************************************
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*
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* @type:
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* FT_LcdFiveTapFilter
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*
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* @description:
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* A typedef for passing the five LCD filter weights to
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* @FT_Face_Properties within an @FT_Parameter structure.
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*
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* @since:
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* 2.8
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*
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*/
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#define FT_LCD_FILTER_FIVE_TAPS 5
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typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS];
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/**************************************************************************
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*
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* @function:
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* FT_Library_SetLcdGeometry
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*
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* @description:
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* This function can be used to modify default positions of color
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* subpixels, which controls Harmony LCD rendering.
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*
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* @input:
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* library ::
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* A handle to the target library instance.
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*
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* sub ::
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* A pointer to an array of 3 vectors in 26.6 fractional pixel format;
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* the function modifies the default values, see the note below.
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*
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* @return:
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* FreeType error code. 0~means success.
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*
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* @note:
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* Subpixel geometry examples:
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*
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* - {{-21, 0}, {0, 0}, {21, 0}} is the default, corresponding to 3 color
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* stripes shifted by a third of a pixel. This could be an RGB panel.
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*
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* - {{21, 0}, {0, 0}, {-21, 0}} looks the same as the default but can
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* specify a BGR panel instead, while keeping the bitmap in the same
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* RGB888 format.
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*
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* - {{0, 21}, {0, 0}, {0, -21}} is the vertical RGB, but the bitmap
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* stays RGB888 as a result.
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*
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* - {{-11, 16}, {-11, -16}, {22, 0}} is a certain PenTile arrangement.
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*
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* This function does nothing and returns `FT_Err_Unimplemented_Feature`
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* in the context of ClearType-style subpixel rendering when
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* `FT_CONFIG_OPTION_SUBPIXEL_RENDERING` is defined in your build of the
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* library.
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*
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* @since:
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* 2.10.0
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*/
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FT_EXPORT( FT_Error )
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FT_Library_SetLcdGeometry( FT_Library library,
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FT_Vector sub[3] );
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/* */
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FT_END_HEADER
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#endif /* FTLCDFIL_H_ */
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/* END */
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