ftlcdfil.h

Go to the documentation of this file.

/****************************************************************************
 *
 * 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
   *   subpixels 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
   *   equally.
   *
   *   A good 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 is then 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 rendering, the
   *   dimensions of LCD bitmaps can be either wider or taller than the
   *   dimensions of the corresponding 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 */