forked from minhngoc25a/freetype2
329 lines
15 KiB
C
329 lines
15 KiB
C
/***************************************************************************/
|
|
/* */
|
|
/* 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_ _Backwards_ _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 backwards 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 backwards
|
|
* 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 */
|