lib/3rdparty/freetype/include/ftlcdfil.h

   1 /***************************************************************************/
   2 /*                                                                         */
   3 /*  ftlcdfil.h                                                             */
   4 /*                                                                         */
   5 /*    FreeType API for color filtering of subpixel bitmap glyphs           */
   6 /*    (specification).                                                     */
   7 /*                                                                         */
   8 /*  Copyright 2006-2008, 2010, 2013 by                                     */
   9 /*  David Turner, Robert Wilhelm, and Werner Lemberg.                      */
  10 /*                                                                         */
  11 /*  This file is part of the FreeType project, and may only be used,       */
  12 /*  modified, and distributed under the terms of the FreeType project      */
  13 /*  license, LICENSE.TXT.  By continuing to use, modify, or distribute     */
  14 /*  this file you indicate that you have read the license and              */
  15 /*  understand and accept it fully.                                        */
  16 /*                                                                         */
  17 /***************************************************************************/
  18
  19
  20 #ifndef __FT_LCD_FILTER_H__
  21 #define __FT_LCD_FILTER_H__
  22
  23 #include <ft2build.h>
  24 #include FT_FREETYPE_H
  25
  26 #ifdef FREETYPE_H
  27 #error "freetype.h of FreeType 1 has been loaded!"
  28 #error "Please fix the directory search order for header files"
  29 #error "so that freetype.h of FreeType 2 is found first."
  30 #endif
  31
  32
  33 FT_BEGIN_HEADER
  34
  35   /***************************************************************************
  36    *
  37    * @section:
  38    *   lcd_filtering
  39    *
  40    * @title:
  41    *   LCD Filtering
  42    *
  43    * @abstract:
  44    *   Reduce color fringes of LCD-optimized bitmaps.
  45    *
  46    * @description:
  47    *   The @FT_Library_SetLcdFilter API can be used to specify a low-pass
  48    *   filter, which is then applied to LCD-optimized bitmaps generated
  49    *   through @FT_Render_Glyph.  This is useful to reduce color fringes
  50    *   that would occur with unfiltered rendering.
  51    *
  52    *   Note that no filter is active by default, and that this function is
  53    *   *not* implemented in default builds of the library.  You need to
  54    *   #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file
  55    *   in order to activate it.
  56    *
  57    *   FreeType generates alpha coverage maps, which are linear by nature.
  58    *   For instance, the value 0x80 in bitmap representation means that
  59    *   (within numerical precision) 0x80/0xff fraction of that pixel is
  60    *   covered by the glyph's outline.  The blending function for placing
  61    *   text over a background is
  62    *
  63    *   {
  64    *     dst = alpha * src + (1 - alpha) * dst    ,
  65    *   }
  66    *
  67    *   which is known as OVER.  However, when calculating the output of the
  68    *   OVER operator, the source colors should first be transformed to a
  69    *   linear color space, then alpha blended in that space, and transformed
  70    *   back to the output color space.
  71    *
  72    *   When linear light blending is used, the default FIR5 filtering
  73    *   weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as
  74    *   they have been designed for black on white rendering while lacking
  75    *   gamma correction.  To preserve color neutrality, weights for a FIR5
  76    *   filter should be chosen according to two free parameters `a' and `c',
  77    *   and the FIR weights should be
  78    *
  79    *   {
  80    *     [a - c, a + c, 2 * a, a + c, a - c]    .
  81    *   }
  82    *
  83    *   This formula generates equal weights for all the color primaries
  84    *   across the filter kernel, which makes it colorless.  One suggested
  85    *   set of weights is
  86    *
  87    *   {
  88    *     [0x10, 0x50, 0x60, 0x50, 0x10]    ,
  89    *   }
  90    *
  91    *   where `a' has value 0x30 and `b' value 0x20.  The weights in filter
  92    *   may have a sum larger than 0x100, which increases coloration slightly
  93    *   but also improves contrast.
  94    */
  95
  96
  97   /****************************************************************************
  98    *
  99    * @enum:
 100    *   FT_LcdFilter
 101    *
 102    * @description:
 103    *   A list of values to identify various types of LCD filters.
 104    *
 105    * @values:
 106    *   FT_LCD_FILTER_NONE ::
 107    *     Do not perform filtering.  When used with subpixel rendering, this
 108    *     results in sometimes severe color fringes.
 109    *
 110    *   FT_LCD_FILTER_DEFAULT ::
 111    *     The default filter reduces color fringes considerably, at the cost
 112    *     of a slight blurriness in the output.
 113    *
 114    *   FT_LCD_FILTER_LIGHT ::
 115    *     The light filter is a variant that produces less blurriness at the
 116    *     cost of slightly more color fringes than the default one.  It might
 117    *     be better, depending on taste, your monitor, or your personal vision.
 118    *
 119    *   FT_LCD_FILTER_LEGACY ::
 120    *     This filter corresponds to the original libXft color filter.  It
 121    *     provides high contrast output but can exhibit really bad color
 122    *     fringes if glyphs are not extremely well hinted to the pixel grid.
 123    *     In other words, it only works well if the TrueType bytecode
 124    *     interpreter is enabled *and* high-quality hinted fonts are used.
 125    *
 126    *     This filter is only provided for comparison purposes, and might be
 127    *     disabled or stay unsupported in the future.
 128    *
 129    * @since:
 130    *   2.3.0
 131    */
 132   typedef enum  FT_LcdFilter_
 133   {
 134     FT_LCD_FILTER_NONE    = 0,
 135     FT_LCD_FILTER_DEFAULT = 1,
 136     FT_LCD_FILTER_LIGHT   = 2,
 137     FT_LCD_FILTER_LEGACY  = 16,
 138
 139     FT_LCD_FILTER_MAX   /* do not remove */
 140
 141   } FT_LcdFilter;
 142
 143
 144   /**************************************************************************
 145    *
 146    * @func:
 147    *   FT_Library_SetLcdFilter
 148    *
 149    * @description:
 150    *   This function is used to apply color filtering to LCD decimated
 151    *   bitmaps, like the ones used when calling @FT_Render_Glyph with
 152    *   @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V.
 153    *
 154    * @input:
 155    *   library ::
 156    *     A handle to the target library instance.
 157    *
 158    *   filter ::
 159    *     The filter type.
 160    *
 161    *     You can use @FT_LCD_FILTER_NONE here to disable this feature, or
 162    *     @FT_LCD_FILTER_DEFAULT to use a default filter that should work
 163    *     well on most LCD screens.
 164    *
 165    * @return:
 166    *   FreeType error code.  0~means success.
 167    *
 168    * @note:
 169    *   This feature is always disabled by default.  Clients must make an
 170    *   explicit call to this function with a `filter' value other than
 171    *   @FT_LCD_FILTER_NONE in order to enable it.
 172    *
 173    *   Due to *PATENTS* covering subpixel rendering, this function doesn't
 174    *   do anything except returning `FT_Err_Unimplemented_Feature' if the
 175    *   configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
 176    *   defined in your build of the library, which should correspond to all
 177    *   default builds of FreeType.
 178    *
 179    *   The filter affects glyph bitmaps rendered through @FT_Render_Glyph,
 180    *   @FT_Outline_Get_Bitmap, @FT_Load_Glyph, and @FT_Load_Char.
 181    *
 182    *   It does _not_ affect the output of @FT_Outline_Render and
 183    *   @FT_Outline_Get_Bitmap.
 184    *
 185    *   If this feature is activated, the dimensions of LCD glyph bitmaps are
 186    *   either larger or taller than the dimensions of the corresponding
 187    *   outline with regards to the pixel grid.  For example, for
 188    *   @FT_RENDER_MODE_LCD, the filter adds up to 3~pixels to the left, and
 189    *   up to 3~pixels to the right.
 190    *
 191    *   The bitmap offset values are adjusted correctly, so clients shouldn't
 192    *   need to modify their layout and glyph positioning code when enabling
 193    *   the filter.
 194    *
 195    * @since:
 196    *   2.3.0
 197    */
 198   FT_EXPORT( FT_Error )
 199   FT_Library_SetLcdFilter( FT_Library    library,
 200                            FT_LcdFilter  filter );
 201
 202
 203   /**************************************************************************
 204    *
 205    * @func:
 206    *   FT_Library_SetLcdFilterWeights
 207    *
 208    * @description:
 209    *   Use this function to override the filter weights selected by
 210    *   @FT_Library_SetLcdFilter.  By default, FreeType uses the quintuple
 211    *   (0x00, 0x55, 0x56, 0x55, 0x00) for FT_LCD_FILTER_LIGHT, and (0x10,
 212    *   0x40, 0x70, 0x40, 0x10) for FT_LCD_FILTER_DEFAULT and
 213    *   FT_LCD_FILTER_LEGACY.
 214    *
 215    * @input:
 216    *   library ::
 217    *     A handle to the target library instance.
 218    *
 219    *   weights ::
 220    *     A pointer to an array; the function copies the first five bytes and
 221    *     uses them to specify the filter weights.
 222    *
 223    * @return:
 224    *   FreeType error code.  0~means success.
 225    *
 226    * @note:
 227    *   Due to *PATENTS* covering subpixel rendering, this function doesn't
 228    *   do anything except returning `FT_Err_Unimplemented_Feature' if the
 229    *   configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
 230    *   defined in your build of the library, which should correspond to all
 231    *   default builds of FreeType.
 232    *
 233    *   This function must be called after @FT_Library_SetLcdFilter to have
 234    *   any effect.
 235    *
 236    * @since:
 237    *   2.4.0
 238    */
 239   FT_EXPORT( FT_Error )
 240   FT_Library_SetLcdFilterWeights( FT_Library      library,
 241                                   unsigned char  *weights );
 242
 243   /* */
 244
 245
 246 FT_END_HEADER
 247
 248 #endif /* __FT_LCD_FILTER_H__ */
 249
 250
 251 /* END */