tor-browser

ftlcdfil.h (11744B)

---

/****************************************************************************
*
* ftlcdfil.h
*
*   FreeType API for color filtering of subpixel bitmap glyphs
*   (specification).
*
* Copyright (C) 2006-2025 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 */