| /***************************************************************************/ |
| /* */ |
| /* 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 */ |
