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Diffstat (limited to 'include/freetype/ftttdrv.h')
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diff --git a/include/freetype/ftttdrv.h b/include/freetype/ftttdrv.h new file mode 100644 index 0000000..dc0081a --- /dev/null +++ b/include/freetype/ftttdrv.h @@ -0,0 +1,310 @@ +/***************************************************************************/ +/* */ +/* ftttdrv.h */ +/* */ +/* FreeType API for controlling the TrueType driver */ +/* (specification only). */ +/* */ +/* Copyright 2013-2015 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, two versions are available, representing the bytecode + * interpreter with and without subpixel hinting support, + * 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). The 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) font-specific tweaks. + * + * 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'. + * + * The following example code demonstrates how to activate subpixel + * hinting (omitting the error handling). + * + * { + * FT_Library library; + * FT_Face face; + * FT_UInt interpreter_version = TT_INTERPRETER_VERSION_38; + * + * + * FT_Init_FreeType( &library ); + * + * FT_Property_Set( library, "truetype", + * "interpreter-version", + * &interpreter_version ); + * } + * + * @note: + * This property can be used with @FT_Property_Get also. + * + */ + + + /************************************************************************** + * + * @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). + * + * @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 configuration option + * FT_CONFIG_OPTION_SUBPIXEL_HINTING, selecting version~38 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). + * + * FreeType doesn't provide all capabilities of the most recent + * ClearType incarnation, thus we identify our subpixel support as + * version~38. + * + */ +#define TT_INTERPRETER_VERSION_35 35 +#define TT_INTERPRETER_VERSION_38 38 + + /* */ + + +FT_END_HEADER + + +#endif /* __FTTTDRV_H__ */ + + +/* END */ |