ref: 2e7bb5e825880301e762f41fd0efa2aa18a4421f
dir: /include/freetype/ftttdrv.h/
/***************************************************************************/
/* */
/* ftttdrv.h */
/* */
/* FreeType API for controlling the TrueType driver */
/* (specification only). */
/* */
/* Copyright 2013-2017 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 FTTTDRV_H_
#define FTTTDRV_H_
#include <ft2build.h>
#include FT_FREETYPE_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:
* tt_driver
*
* @title:
* The TrueType driver
*
* @abstract:
* Controlling the TrueType driver module.
*
* @description:
* While FreeType's TrueType driver doesn't expose API functions by
* itself, it is possible to control its behaviour with @FT_Property_Set
* and @FT_Property_Get. The following lists the available properties
* together with the necessary macros and structures.
*
* The TrueType driver's module name is `truetype'.
*
* We start with a list of definitions, kindly provided by Greg
* Hitchcock.
*
* _Bi-Level_ _Rendering_
*
* Monochromatic rendering, exclusively used in the early days of
* TrueType by both Apple and Microsoft. Microsoft's GDI interface
* supported hinting of the right-side bearing point, such that the
* advance width could be non-linear. Most often this was done to
* achieve some level of glyph symmetry. To enable reasonable
* performance (e.g., not having to run hinting on all glyphs just to
* get the widths) there was a bit in the head table indicating if the
* side bearing was hinted, and additional tables, `hdmx' and `LTSH', to
* cache hinting widths across multiple sizes and device aspect ratios.
*
* _Font_ _Smoothing_
*
* Microsoft's GDI implementation of anti-aliasing. Not traditional
* anti-aliasing as the outlines were hinted before the sampling. The
* widths matched the bi-level rendering.
*
* _ClearType_ _Rendering_
*
* Technique that uses physical subpixels to improve rendering on LCD
* (and other) displays. Because of the higher resolution, many methods
* of improving symmetry in glyphs through hinting the right-side
* bearing were no longer necessary. This lead to what GDI calls
* `natural widths' ClearType, see
* http://www.beatstamm.com/typography/RTRCh4.htm#Sec21. Since hinting
* has extra resolution, most non-linearity went away, but it is still
* possible for hints to change the advance widths in this mode.
*
* _ClearType_ _Compatible_ _Widths_
*
* One of the earliest challenges with ClearType was allowing the
* implementation in GDI to be selected without requiring all UI and
* documents to reflow. To address this, a compatible method of
* rendering ClearType was added where the font hints are executed once
* to determine the width in bi-level rendering, and then re-run in
* ClearType, with the difference in widths being absorbed in the font
* hints for ClearType (mostly in the white space of hints); see
* http://www.beatstamm.com/typography/RTRCh4.htm#Sec20. Somewhat by
* definition, compatible width ClearType allows for non-linear widths,
* but only when the bi-level version has non-linear widths.
*
* _ClearType_ _Subpixel_ _Positioning_
*
* One of the nice benefits of ClearType is the ability to more crisply
* display fractional widths; unfortunately, the GDI model of integer
* bitmaps did not support this. However, the WPF and Direct Write
* frameworks do support fractional widths. DWrite calls this `natural
* mode', not to be confused with GDI's `natural widths'. Subpixel
* positioning, in the current implementation of Direct Write,
* unfortunately does not support hinted advance widths, see
* http://www.beatstamm.com/typography/RTRCh4.htm#Sec22. Note that the
* TrueType interpreter fully allows the advance width to be adjusted in
* this mode, just the DWrite client will ignore those changes.
*
* _ClearType_ _Backward_ _Compatibility_
*
* This is a set of exceptions made in the TrueType interpreter to
* minimize hinting techniques that were problematic with the extra
* resolution of ClearType; see
* http://www.beatstamm.com/typography/RTRCh4.htm#Sec1 and
* http://www.microsoft.com/typography/cleartype/truetypecleartype.aspx.
* This technique is not to be confused with ClearType compatible
* widths. ClearType backward compatibility has no direct impact on
* changing advance widths, but there might be an indirect impact on
* disabling some deltas. This could be worked around in backward
* compatibility mode.
*
* _Native_ _ClearType_ _Mode_
*
* (Not to be confused with `natural widths'.) This mode removes all
* the exceptions in the TrueType interpreter when running with
* ClearType. Any issues on widths would still apply, though.
*
*/
/**************************************************************************
*
* @property:
* interpreter-version
*
* @description:
* Currently, three versions are available, two representing the
* bytecode interpreter with subpixel hinting support (old `Infinality'
* code and new stripped-down and higher performance `minimal' code) and
* one without, respectively. The default is subpixel support if
* TT_CONFIG_OPTION_SUBPIXEL_HINTING is defined, and no subpixel support
* otherwise (since it isn't available then).
*
* If subpixel hinting is on, many TrueType bytecode instructions behave
* differently compared to B/W or grayscale rendering (except if `native
* ClearType' is selected by the font). Microsoft's main idea is to
* render at a much increased horizontal resolution, then sampling down
* the created output to subpixel precision. However, many older fonts
* are not suited to this and must be specially taken care of by
* applying (hardcoded) tweaks in Microsoft's interpreter.
*
* Details on subpixel hinting and some of the necessary tweaks can be
* found in Greg Hitchcock's whitepaper at
* `http://www.microsoft.com/typography/cleartype/truetypecleartype.aspx'.
* Note that FreeType currently doesn't really `subpixel hint' (6x1, 6x2,
* or 6x5 supersampling) like discussed in the paper. Depending on the
* chosen interpreter, it simply ignores instructions on vertical stems
* to arrive at very similar results.
*
* The following example code demonstrates how to deactivate subpixel
* hinting (omitting the error handling).
*
* {
* FT_Library library;
* FT_Face face;
* FT_UInt interpreter_version = TT_INTERPRETER_VERSION_35;
*
*
* FT_Init_FreeType( &library );
*
* FT_Property_Set( library, "truetype",
* "interpreter-version",
* &interpreter_version );
* }
*
* @note:
* This property can be used with @FT_Property_Get also.
*
* This property can be set via the `FREETYPE_PROPERTIES' environment
* variable (using values `35', `38', or `40').
*/
/**************************************************************************
*
* @enum:
* TT_INTERPRETER_VERSION_XXX
*
* @description:
* A list of constants used for the @interpreter-version property to
* select the hinting engine for Truetype fonts.
*
* The numeric value in the constant names represents the version
* number as returned by the `GETINFO' bytecode instruction.
*
* @values:
* TT_INTERPRETER_VERSION_35 ::
* Version~35 corresponds to MS rasterizer v.1.7 as used e.g. in
* Windows~98; only grayscale and B/W rasterizing is supported.
*
* TT_INTERPRETER_VERSION_38 ::
* Version~38 corresponds to MS rasterizer v.1.9; it is roughly
* equivalent to the hinting provided by DirectWrite ClearType (as can
* be found, for example, in the Internet Explorer~9 running on
* Windows~7). It is used in FreeType to select the `Infinality'
* subpixel hinting code. The code may be removed in a future
* version.
*
* TT_INTERPRETER_VERSION_40 ::
* Version~40 corresponds to MS rasterizer v.2.1; it is roughly
* equivalent to the hinting provided by DirectWrite ClearType (as can
* be found, for example, in Microsoft's Edge Browser on Windows~10).
* It is used in FreeType to select the `minimal' subpixel hinting
* code, a stripped-down and higher performance version of the
* `Infinality' code.
*
* @note:
* This property controls the behaviour of the bytecode interpreter
* and thus how outlines get hinted. It does *not* control how glyph
* get rasterized! In particular, it does not control subpixel color
* filtering.
*
* If FreeType has not been compiled with the configuration option
* TT_CONFIG_OPTION_SUBPIXEL_HINTING, selecting version~38 or~40 causes
* an `FT_Err_Unimplemented_Feature' error.
*
* Depending on the graphics framework, Microsoft uses different
* bytecode and rendering engines. As a consequence, the version
* numbers returned by a call to the `GETINFO' bytecode instruction are
* more convoluted than desired.
*
* Here are two tables that try to shed some light on the possible
* values for the MS rasterizer engine, together with the additional
* features introduced by it.
*
* {
* GETINFO framework version feature
* -------------------------------------------------------------------
* 3 GDI (Win 3.1), v1.0 16-bit, first version
* TrueImage
* 33 GDI (Win NT 3.1), v1.5 32-bit
* HP Laserjet
* 34 GDI (Win 95) v1.6 font smoothing,
* new SCANTYPE opcode
* 35 GDI (Win 98/2000) v1.7 (UN)SCALED_COMPONENT_OFFSET
* bits in composite glyphs
* 36 MGDI (Win CE 2) v1.6+ classic ClearType
* 37 GDI (XP and later), v1.8 ClearType
* GDI+ old (before Vista)
* 38 GDI+ old (Vista, Win 7), v1.9 subpixel ClearType,
* WPF Y-direction ClearType,
* additional error checking
* 39 DWrite (before Win 8) v2.0 subpixel ClearType flags
* in GETINFO opcode,
* bug fixes
* 40 GDI+ (after Win 7), v2.1 Y-direction ClearType flag
* DWrite (Win 8) in GETINFO opcode,
* Gray ClearType
* }
*
* The `version' field gives a rough orientation only, since some
* applications provided certain features much earlier (as an example,
* Microsoft Reader used subpixel and Y-direction ClearType already in
* Windows 2000). Similarly, updates to a given framework might include
* improved hinting support.
*
* {
* version sampling rendering comment
* x y x y
* --------------------------------------------------------------
* v1.0 normal normal B/W B/W bi-level
* v1.6 high high gray gray grayscale
* v1.8 high normal color-filter B/W (GDI) ClearType
* v1.9 high high color-filter gray Color ClearType
* v2.1 high normal gray B/W Gray ClearType
* v2.1 high high gray gray Gray ClearType
* }
*
* Color and Gray ClearType are the two available variants of
* `Y-direction ClearType', meaning grayscale rasterization along the
* Y-direction; the name used in the TrueType specification for this
* feature is `symmetric smoothing'. `Classic ClearType' is the
* original algorithm used before introducing a modified version in
* Win~XP. Another name for v1.6's grayscale rendering is `font
* smoothing', and `Color ClearType' is sometimes also called `DWrite
* ClearType'. To differentiate between today's Color ClearType and the
* earlier ClearType variant with B/W rendering along the vertical axis,
* the latter is sometimes called `GDI ClearType'.
*
* `Normal' and `high' sampling describe the (virtual) resolution to
* access the rasterized outline after the hinting process. `Normal'
* means 1 sample per grid line (i.e., B/W). In the current Microsoft
* implementation, `high' means an extra virtual resolution of 16x16 (or
* 16x1) grid lines per pixel for bytecode instructions like `MIRP'.
* After hinting, these 16 grid lines are mapped to 6x5 (or 6x1) grid
* lines for color filtering if Color ClearType is activated.
*
* Note that `Gray ClearType' is essentially the same as v1.6's
* grayscale rendering. However, the GETINFO instruction handles it
* differently: v1.6 returns bit~12 (hinting for grayscale), while v2.1
* returns bits~13 (hinting for ClearType), 18 (symmetrical smoothing),
* and~19 (Gray ClearType). Also, this mode respects bits 2 and~3 for
* the version~1 gasp table exclusively (like Color ClearType), while
* v1.6 only respects the values of version~0 (bits 0 and~1).
*
* Keep in mind that the features of the above interpreter versions
* might not map exactly to FreeType features or behavior because it is
* a fundamentally different library with different internals.
*
*/
#define TT_INTERPRETER_VERSION_35 35
#define TT_INTERPRETER_VERSION_38 38
#define TT_INTERPRETER_VERSION_40 40
/* */
FT_END_HEADER
#endif /* FTTTDRV_H_ */
/* END */