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

   1 /***************************************************************************/
   2 /*                                                                         */
   3 /*  ftttdrv.h                                                              */
   4 /*                                                                         */
   5 /*    FreeType API for controlling the TrueType driver                     */
   6 /*    (specification only).                                                */
   7 /*                                                                         */
   8 /*  Copyright 2013-2015 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 __FTTTDRV_H__
  21 #define __FTTTDRV_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    *
  38    * @section:
  39    *   tt_driver
  40    *
  41    * @title:
  42    *   The TrueType driver
  43    *
  44    * @abstract:
  45    *   Controlling the TrueType driver module.
  46    *
  47    * @description:
  48    *   While FreeType's TrueType driver doesn't expose API functions by
  49    *   itself, it is possible to control its behaviour with @FT_Property_Set
  50    *   and @FT_Property_Get.  The following lists the available properties
  51    *   together with the necessary macros and structures.
  52    *
  53    *   The TrueType driver's module name is `truetype'.
  54    *
  55    *   We start with a list of definitions, kindly provided by Greg
  56    *   Hitchcock.
  57    *
  58    *   _Bi-Level_ _Rendering_
  59    *
  60    *   Monochromatic rendering, exclusively used in the early days of
  61    *   TrueType by both Apple and Microsoft.  Microsoft's GDI interface
  62    *   supported hinting of the right-side bearing point, such that the
  63    *   advance width could be non-linear.  Most often this was done to
  64    *   achieve some level of glyph symmetry.  To enable reasonable
  65    *   performance (e.g., not having to run hinting on all glyphs just to
  66    *   get the widths) there was a bit in the head table indicating if the
  67    *   side bearing was hinted, and additional tables, `hdmx' and `LTSH', to
  68    *   cache hinting widths across multiple sizes and device aspect ratios.
  69    *
  70    *   _Font_ _Smoothing_
  71    *
  72    *   Microsoft's GDI implementation of anti-aliasing.  Not traditional
  73    *   anti-aliasing as the outlines were hinted before the sampling.  The
  74    *   widths matched the bi-level rendering.
  75    *
  76    *   _ClearType_ _Rendering_
  77    *
  78    *   Technique that uses physical subpixels to improve rendering on LCD
  79    *   (and other) displays.  Because of the higher resolution, many methods
  80    *   of improving symmetry in glyphs through hinting the right-side
  81    *   bearing were no longer necessary.  This lead to what GDI calls
  82    *   `natural widths' ClearType, see
  83    *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec21.  Since hinting
  84    *   has extra resolution, most non-linearity went away, but it is still
  85    *   possible for hints to change the advance widths in this mode.
  86    *
  87    *   _ClearType_ _Compatible_ _Widths_
  88    *
  89    *   One of the earliest challenges with ClearType was allowing the
  90    *   implementation in GDI to be selected without requiring all UI and
  91    *   documents to reflow.  To address this, a compatible method of
  92    *   rendering ClearType was added where the font hints are executed once
  93    *   to determine the width in bi-level rendering, and then re-run in
  94    *   ClearType, with the difference in widths being absorbed in the font
  95    *   hints for ClearType (mostly in the white space of hints); see
  96    *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec20.  Somewhat by
  97    *   definition, compatible width ClearType allows for non-linear widths,
  98    *   but only when the bi-level version has non-linear widths.
  99    *
 100    *   _ClearType_ _Subpixel_ _Positioning_
 101    *
 102    *   One of the nice benefits of ClearType is the ability to more crisply
 103    *   display fractional widths; unfortunately, the GDI model of integer
 104    *   bitmaps did not support this.  However, the WPF and Direct Write
 105    *   frameworks do support fractional widths.  DWrite calls this `natural
 106    *   mode', not to be confused with GDI's `natural widths'.  Subpixel
 107    *   positioning, in the current implementation of Direct Write,
 108    *   unfortunately does not support hinted advance widths, see
 109    *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec22.  Note that the
 110    *   TrueType interpreter fully allows the advance width to be adjusted in
 111    *   this mode, just the DWrite client will ignore those changes.
 112    *
 113    *   _ClearType_ _Backwards_ _Compatibility_
 114    *
 115    *   This is a set of exceptions made in the TrueType interpreter to
 116    *   minimize hinting techniques that were problematic with the extra
 117    *   resolution of ClearType; see
 118    *   http://www.beatstamm.com/typography/RTRCh4.htm#Sec1 and
 119    *   http://www.microsoft.com/typography/cleartype/truetypecleartype.aspx.
 120    *   This technique is not to be confused with ClearType compatible
 121    *   widths.  ClearType backwards compatibility has no direct impact on
 122    *   changing advance widths, but there might be an indirect impact on
 123    *   disabling some deltas.  This could be worked around in backwards
 124    *   compatibility mode.
 125    *
 126    *   _Native_ _ClearType_ _Mode_
 127    *
 128    *   (Not to be confused with `natural widths'.)  This mode removes all
 129    *   the exceptions in the TrueType interpreter when running with
 130    *   ClearType.  Any issues on widths would still apply, though.
 131    *
 132    */
 133
 134
 135   /**************************************************************************
 136    *
 137    * @property:
 138    *   interpreter-version
 139    *
 140    * @description:
 141    *   Currently, two versions are available, representing the bytecode
 142    *   interpreter with and without subpixel hinting support,
 143    *   respectively.  The default is subpixel support if
 144    *   TT_CONFIG_OPTION_SUBPIXEL_HINTING is defined, and no subpixel
 145    *   support otherwise (since it isn't available then).
 146    *
 147    *   If subpixel hinting is on, many TrueType bytecode instructions behave
 148    *   differently compared to B/W or grayscale rendering (except if `native
 149    *   ClearType' is selected by the font).  The main idea is to render at a
 150    *   much increased horizontal resolution, then sampling down the created
 151    *   output to subpixel precision.  However, many older fonts are not
 152    *   suited to this and must be specially taken care of by applying
 153    *   (hardcoded) font-specific tweaks.
 154    *
 155    *   Details on subpixel hinting and some of the necessary tweaks can be
 156    *   found in Greg Hitchcock's whitepaper at
 157    *   `http://www.microsoft.com/typography/cleartype/truetypecleartype.aspx'.
 158    *
 159    *   The following example code demonstrates how to activate subpixel
 160    *   hinting (omitting the error handling).
 161    *
 162    *   {
 163    *     FT_Library  library;
 164    *     FT_Face     face;
 165    *     FT_UInt     interpreter_version = TT_INTERPRETER_VERSION_38;
 166    *
 167    *
 168    *     FT_Init_FreeType( &library );
 169    *
 170    *     FT_Property_Set( library, "truetype",
 171    *                               "interpreter-version",
 172    *                               &interpreter_version );
 173    *   }
 174    *
 175    * @note:
 176    *   This property can be used with @FT_Property_Get also.
 177    *
 178    */
 179
 180
 181   /**************************************************************************
 182    *
 183    * @enum:
 184    *   TT_INTERPRETER_VERSION_XXX
 185    *
 186    * @description:
 187    *   A list of constants used for the @interpreter-version property to
 188    *   select the hinting engine for Truetype fonts.
 189    *
 190    *   The numeric value in the constant names represents the version
 191    *   number as returned by the `GETINFO' bytecode instruction.
 192    *
 193    * @values:
 194    *   TT_INTERPRETER_VERSION_35 ::
 195    *     Version~35 corresponds to MS rasterizer v.1.7 as used e.g. in
 196    *     Windows~98; only grayscale and B/W rasterizing is supported.
 197    *
 198    *   TT_INTERPRETER_VERSION_38 ::
 199    *     Version~38 corresponds to MS rasterizer v.1.9; it is roughly
 200    *     equivalent to the hinting provided by DirectWrite ClearType (as
 201    *     can be found, for example, in the Internet Explorer~9 running on
 202    *     Windows~7).
 203    *
 204    * @note:
 205    *   This property controls the behaviour of the bytecode interpreter
 206    *   and thus how outlines get hinted.  It does *not* control how glyph
 207    *   get rasterized!  In particular, it does not control subpixel color
 208    *   filtering.
 209    *
 210    *   If FreeType has not been compiled with configuration option
 211    *   FT_CONFIG_OPTION_SUBPIXEL_HINTING, selecting version~38 causes an
 212    *   `FT_Err_Unimplemented_Feature' error.
 213    *
 214    *   Depending on the graphics framework, Microsoft uses different
 215    *   bytecode and rendering engines.  As a consequence, the version
 216    *   numbers returned by a call to the `GETINFO' bytecode instruction are
 217    *   more convoluted than desired.
 218    *
 219    *   Here are two tables that try to shed some light on the possible
 220    *   values for the MS rasterizer engine, together with the additional
 221    *   features introduced by it.
 222    *
 223    *   {
 224    *     GETINFO framework               version feature
 225    *     -------------------------------------------------------------------
 226    *         3   GDI (Win 3.1),            v1.0  16-bit, first version
 227    *             TrueImage
 228    *        33   GDI (Win NT 3.1),         v1.5  32-bit
 229    *             HP Laserjet
 230    *        34   GDI (Win 95)              v1.6  font smoothing,
 231    *                                             new SCANTYPE opcode
 232    *        35   GDI (Win 98/2000)         v1.7  (UN)SCALED_COMPONENT_OFFSET
 233    *                                               bits in composite glyphs
 234    *        36   MGDI (Win CE 2)           v1.6+ classic ClearType
 235    *        37   GDI (XP and later),       v1.8  ClearType
 236    *             GDI+ old (before Vista)
 237    *        38   GDI+ old (Vista, Win 7),  v1.9  subpixel ClearType,
 238    *             WPF                             Y-direction ClearType,
 239    *                                             additional error checking
 240    *        39   DWrite (before Win 8)     v2.0  subpixel ClearType flags
 241    *                                               in GETINFO opcode,
 242    *                                             bug fixes
 243    *        40   GDI+ (after Win 7),       v2.1  Y-direction ClearType flag
 244    *             DWrite (Win 8)                    in GETINFO opcode,
 245    *                                             Gray ClearType
 246    *   }
 247    *
 248    *   The `version' field gives a rough orientation only, since some
 249    *   applications provided certain features much earlier (as an example,
 250    *   Microsoft Reader used subpixel and Y-direction ClearType already in
 251    *   Windows 2000).  Similarly, updates to a given framework might include
 252    *   improved hinting support.
 253    *
 254    *   {
 255    *      version   sampling          rendering        comment
 256    *               x        y       x           y
 257    *     --------------------------------------------------------------
 258    *       v1.0   normal  normal  B/W           B/W    bi-level
 259    *       v1.6   high    high    gray          gray   grayscale
 260    *       v1.8   high    normal  color-filter  B/W    (GDI) ClearType
 261    *       v1.9   high    high    color-filter  gray   Color ClearType
 262    *       v2.1   high    normal  gray          B/W    Gray ClearType
 263    *       v2.1   high    high    gray          gray   Gray ClearType
 264    *   }
 265    *
 266    *   Color and Gray ClearType are the two available variants of
 267    *   `Y-direction ClearType', meaning grayscale rasterization along the
 268    *   Y-direction; the name used in the TrueType specification for this
 269    *   feature is `symmetric smoothing'.  `Classic ClearType' is the
 270    *   original algorithm used before introducing a modified version in
 271    *   Win~XP.  Another name for v1.6's grayscale rendering is `font
 272    *   smoothing', and `Color ClearType' is sometimes also called `DWrite
 273    *   ClearType'.  To differentiate between today's Color ClearType and the
 274    *   earlier ClearType variant with B/W rendering along the vertical axis,
 275    *   the latter is sometimes called `GDI ClearType'.
 276    *
 277    *   `Normal' and `high' sampling describe the (virtual) resolution to
 278    *   access the rasterized outline after the hinting process.  `Normal'
 279    *   means 1 sample per grid line (i.e., B/W).  In the current Microsoft
 280    *   implementation, `high' means an extra virtual resolution of 16x16 (or
 281    *   16x1) grid lines per pixel for bytecode instructions like `MIRP'.
 282    *   After hinting, these 16 grid lines are mapped to 6x5 (or 6x1) grid
 283    *   lines for color filtering if Color ClearType is activated.
 284    *
 285    *   Note that `Gray ClearType' is essentially the same as v1.6's
 286    *   grayscale rendering.  However, the GETINFO instruction handles it
 287    *   differently: v1.6 returns bit~12 (hinting for grayscale), while v2.1
 288    *   returns bits~13 (hinting for ClearType), 18 (symmetrical smoothing),
 289    *   and~19 (Gray ClearType).  Also, this mode respects bits 2 and~3 for
 290    *   the version~1 gasp table exclusively (like Color ClearType), while
 291    *   v1.6 only respects the values of version~0 (bits 0 and~1).
 292    *
 293    *   FreeType doesn't provide all capabilities of the most recent
 294    *   ClearType incarnation, thus we identify our subpixel support as
 295    *   version~38.
 296    *
 297    */
 298 #define TT_INTERPRETER_VERSION_35  35
 299 #define TT_INTERPRETER_VERSION_38  38
 300
 301  /* */
 302
 303
 304 FT_END_HEADER
 305
 306
 307 #endif /* __FTTTDRV_H__ */
 308
 309
 310 /* END */