ref: 9e422b67c8f1a2e06cb934c2867e0536729793f4
dir: /include/freetype/ftlcdfil.h/
/****************************************************************************
*
* ftlcdfil.h
*
* FreeType API for color filtering of subpixel bitmap glyphs
* (specification).
*
* Copyright (C) 2006-2020 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 1/256th 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 1/256th 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 1/256th 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 */