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

   1 /***************************************************************************/
   2 /*                                                                         */
   3 /*  ftlcdfil.h                                                             */
   4 /*                                                                         */
   5 /*    FreeType API for color filtering of subpixel bitmap glyphs           */
   6 /*    (specification).                                                     */
   7 /*                                                                         */
   8 /*  Copyright 2006-2015 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 subpixel-rendered bitmaps.
  45    *
  46    * @description:
  47    *   Subpixel rendering exploits the color-striped structure of LCD
  48    *   pixels, increasing the available resolution in the direction of the
  49    *   stripe (usually horizontal RGB) by a factor of~3.  Since these
  50    *   subpixels are color pixels, using them unfiltered creates severe
  51    *   color fringes.  Use the @FT_Library_SetLcdFilter API to specify a
  52    *   low-pass filter, which is then applied to subpixel-rendered bitmaps
  53    *   generated through @FT_Render_Glyph.  The filter sacrifices some of
  54    *   the higher resolution to reduce color fringes, making the glyph image
  55    *   slightly blurrier.  Positional improvements will remain.
  56    *
  57    *   Note that no filter is active by default, and that this function is
  58    *   *not* implemented in default builds of the library.  You need to
  59    *   #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file
  60    *   in order to activate it.
  61    *
  62    *   A filter should have two properties:
  63    *
  64    *   1) It should be normalized, meaning the sum of the 5~components
  65    *      should be 256 (0x100).  It is possible to go above or under this
  66    *      target sum, however: going under means tossing out contrast, going
  67    *      over means invoking clamping and thereby non-linearities that
  68    *      increase contrast somewhat at the expense of greater distortion
  69    *      and color-fringing.  Contrast is better enhanced through stem
  70    *      darkening.
  71    *
  72    *   2) It should be color-balanced, meaning a filter `{~a, b, c, b, a~}'
  73    *      where a~+ b~=~c.  It distributes the computed coverage for one
  74    *      subpixel to all subpixels equally, sacrificing some won resolution
  75    *      but drastically reducing color-fringing.  Positioning improvements
  76    *      remain!  Note that color-fringing can only really be minimized
  77    *      when using a color-balanced filter and alpha-blending the glyph
  78    *      onto a surface in linear space; see @FT_Render_Glyph.
  79    *
  80    *   Regarding the form, a filter can be a `boxy' filter or a `beveled'
  81    *   filter.  Boxy filters are sharper but are less forgiving of non-ideal
  82    *   gamma curves of a screen (viewing angles!), beveled filters are
  83    *   fuzzier but more tolerant.
  84    *
  85    *   Examples:
  86    *
  87    *   - [0x10 0x40 0x70 0x40 0x10] is beveled and neither balanced nor
  88    *     normalized.
  89    *
  90    *   - [0x1A 0x33 0x4D 0x33 0x1A] is beveled and balanced but not
  91    *     normalized.
  92    *
  93    *   - [0x19 0x33 0x66 0x4c 0x19] is beveled and normalized but not
  94    *     balanced.
  95    *
  96    *   - [0x00 0x4c 0x66 0x4c 0x00] is boxily beveled and normalized but not
  97    *     balanced.
  98    *
  99    *   - [0x00 0x55 0x56 0x55 0x00] is boxy, normalized, and almost
 100    *     balanced.
 101    *
 102    *   - [0x08 0x4D 0x56 0x4D 0x08] is beveled, normalized and, almost
 103    *     balanced.
 104    *
 105    *   It is important to understand that linear alpha blending and gamma
 106    *   correction is critical for correctly rendering glyphs onto surfaces
 107    *   without artifacts and even more critical when subpixel rendering is
 108    *   involved.
 109    *
 110    *   Each of the 3~alpha values (subpixels) is independently used to blend
 111    *   one color channel.  That is, red alpha blends the red channel of the
 112    *   text color with the red channel of the background pixel.  The
 113    *   distribution of density values by the color-balanced filter assumes
 114    *   alpha blending is done in linear space; only then color artifacts
 115    *   cancel out.
 116    */
 117
 118
 119   /****************************************************************************
 120    *
 121    * @enum:
 122    *   FT_LcdFilter
 123    *
 124    * @description:
 125    *   A list of values to identify various types of LCD filters.
 126    *
 127    * @values:
 128    *   FT_LCD_FILTER_NONE ::
 129    *     Do not perform filtering.  When used with subpixel rendering, this
 130    *     results in sometimes severe color fringes.
 131    *
 132    *   FT_LCD_FILTER_DEFAULT ::
 133    *     The default filter reduces color fringes considerably, at the cost
 134    *     of a slight blurriness in the output.
 135    *
 136    *     It is a beveled, normalized, and color-balanced five-tap filter
 137    *     that is more forgiving to screens with non-ideal gamma curves and
 138    *     viewing angles.  Note that while color-fringing is reduced, it can
 139    *     only be minimized by using linear alpha blending and gamma
 140    *     correction to render glyphs onto surfaces.
 141    *
 142    *   FT_LCD_FILTER_LIGHT ::
 143    *     The light filter is a variant that is sharper at the cost of
 144    *     slightly more color fringes than the default one.
 145    *
 146    *     It is a boxy, normalized, and color-balanced three-tap filter that
 147    *     is less forgiving to screens with non-ideal gamma curves and
 148    *     viewing angles.  This filter works best when the rendering system
 149    *     uses linear alpha blending and gamma correction to render glyphs
 150    *     onto surfaces.
 151    *
 152    *   FT_LCD_FILTER_LEGACY ::
 153    *     This filter corresponds to the original libXft color filter.  It
 154    *     provides high contrast output but can exhibit really bad color
 155    *     fringes if glyphs are not extremely well hinted to the pixel grid.
 156    *     In other words, it only works well if the TrueType bytecode
 157    *     interpreter is enabled *and* high-quality hinted fonts are used.
 158    *
 159    *     This filter is only provided for comparison purposes, and might be
 160    *     disabled or stay unsupported in the future.
 161    *
 162    *   FT_LCD_FILTER_LEGACY1 ::
 163    *     For historical reasons, the FontConfig library returns a different
 164    *     enumeration value for legacy LCD filtering.  To make code work that
 165    *     (incorrectly) forwards FontConfig's enumeration value to
 166    *     @FT_Library_SetLcdFilter without proper mapping, it is thus easiest
 167    *     to have another enumeration value, which is completely equal to
 168    *     `FT_LCD_FILTER_LEGACY'.
 169    *
 170    * @since:
 171    *   2.3.0 (`FT_LCD_FILTER_LEGACY1' since 2.6.2)
 172    */
 173   typedef enum  FT_LcdFilter_
 174   {
 175     FT_LCD_FILTER_NONE    = 0,
 176     FT_LCD_FILTER_DEFAULT = 1,
 177     FT_LCD_FILTER_LIGHT   = 2,
 178     FT_LCD_FILTER_LEGACY1 = 3,
 179     FT_LCD_FILTER_LEGACY  = 16,
 180
 181     FT_LCD_FILTER_MAX   /* do not remove */
 182
 183   } FT_LcdFilter;
 184
 185
 186   /**************************************************************************
 187    *
 188    * @func:
 189    *   FT_Library_SetLcdFilter
 190    *
 191    * @description:
 192    *   This function is used to apply color filtering to LCD decimated
 193    *   bitmaps, like the ones used when calling @FT_Render_Glyph with
 194    *   @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V.
 195    *
 196    * @input:
 197    *   library ::
 198    *     A handle to the target library instance.
 199    *
 200    *   filter ::
 201    *     The filter type.
 202    *
 203    *     You can use @FT_LCD_FILTER_NONE here to disable this feature, or
 204    *     @FT_LCD_FILTER_DEFAULT to use a default filter that should work
 205    *     well on most LCD screens.
 206    *
 207    * @return:
 208    *   FreeType error code.  0~means success.
 209    *
 210    * @note:
 211    *   This feature is always disabled by default.  Clients must make an
 212    *   explicit call to this function with a `filter' value other than
 213    *   @FT_LCD_FILTER_NONE in order to enable it.
 214    *
 215    *   Due to *PATENTS* covering subpixel rendering, this function doesn't
 216    *   do anything except returning `FT_Err_Unimplemented_Feature' if the
 217    *   configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
 218    *   defined in your build of the library, which should correspond to all
 219    *   default builds of FreeType.
 220    *
 221    *   The filter affects glyph bitmaps rendered through @FT_Render_Glyph,
 222    *   @FT_Outline_Get_Bitmap, @FT_Load_Glyph, and @FT_Load_Char.
 223    *
 224    *   It does _not_ affect the output of @FT_Outline_Render and
 225    *   @FT_Outline_Get_Bitmap.
 226    *
 227    *   If this feature is activated, the dimensions of LCD glyph bitmaps are
 228    *   either larger or taller than the dimensions of the corresponding
 229    *   outline with regards to the pixel grid.  For example, for
 230    *   @FT_RENDER_MODE_LCD, the filter adds up to 3~pixels to the left, and
 231    *   up to 3~pixels to the right.
 232    *
 233    *   The bitmap offset values are adjusted correctly, so clients shouldn't
 234    *   need to modify their layout and glyph positioning code when enabling
 235    *   the filter.
 236    *
 237    * @since:
 238    *   2.3.0
 239    */
 240   FT_EXPORT( FT_Error )
 241   FT_Library_SetLcdFilter( FT_Library    library,
 242                            FT_LcdFilter  filter );
 243
 244
 245   /**************************************************************************
 246    *
 247    * @func:
 248    *   FT_Library_SetLcdFilterWeights
 249    *
 250    * @description:
 251    *   Use this function to override the filter weights selected by
 252    *   @FT_Library_SetLcdFilter.  By default, FreeType uses the quintuple
 253    *   (0x00, 0x55, 0x56, 0x55, 0x00) for FT_LCD_FILTER_LIGHT, and (0x10,
 254    *   0x40, 0x70, 0x40, 0x10) for FT_LCD_FILTER_DEFAULT and
 255    *   FT_LCD_FILTER_LEGACY.
 256    *
 257    * @input:
 258    *   library ::
 259    *     A handle to the target library instance.
 260    *
 261    *   weights ::
 262    *     A pointer to an array; the function copies the first five bytes and
 263    *     uses them to specify the filter weights.
 264    *
 265    * @return:
 266    *   FreeType error code.  0~means success.
 267    *
 268    * @note:
 269    *   Due to *PATENTS* covering subpixel rendering, this function doesn't
 270    *   do anything except returning `FT_Err_Unimplemented_Feature' if the
 271    *   configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
 272    *   defined in your build of the library, which should correspond to all
 273    *   default builds of FreeType.
 274    *
 275    *   This function must be called after @FT_Library_SetLcdFilter to have
 276    *   any effect.
 277    *
 278    * @since:
 279    *   2.4.0
 280    */
 281   FT_EXPORT( FT_Error )
 282   FT_Library_SetLcdFilterWeights( FT_Library      library,
 283                                   unsigned char  *weights );
 284
 285   /* */
 286
 287
 288 FT_END_HEADER
 289
 290 #endif /* __FT_LCD_FILTER_H__ */
 291
 292
 293 /* END */