diff options
Diffstat (limited to 'src/truetype/ttinterp.h')
| -rw-r--r-- | src/truetype/ttinterp.h | 147 |
1 files changed, 137 insertions, 10 deletions
diff --git a/src/truetype/ttinterp.h b/src/truetype/ttinterp.h index 32706d0..53f0944 100644 --- a/src/truetype/ttinterp.h +++ b/src/truetype/ttinterp.h @@ -4,7 +4,7 @@ /* */ /* TrueType bytecode interpreter (specification). */ /* */ -/* Copyright 1996-2015 by */ +/* Copyright 1996-2016 by */ /* David Turner, Robert Wilhelm, and Werner Lemberg. */ /* */ /* This file is part of the FreeType project, and may only be used, */ @@ -16,8 +16,8 @@ /***************************************************************************/ -#ifndef __TTINTERP_H__ -#define __TTINTERP_H__ +#ifndef TTINTERP_H_ +#define TTINTERP_H_ #include <ft2build.h> #include "ttobjs.h" @@ -99,7 +99,7 @@ FT_BEGIN_HEADER } TT_CallRec, *TT_CallStack; -#ifdef TT_CONFIG_OPTION_SUBPIXEL_HINTING +#ifdef TT_SUPPORT_SUBPIXEL_HINTING_INFINALITY /*************************************************************************/ /* */ @@ -138,7 +138,7 @@ FT_BEGIN_HEADER } SPH_Font_Class; -#endif /* TT_CONFIG_OPTION_SUBPIXEL_HINTING */ +#endif /* TT_SUPPORT_SUBPIXEL_HINTING_INFINALITY */ /*************************************************************************/ @@ -170,6 +170,7 @@ FT_BEGIN_HEADER pts, twilight; + FT_Long pointSize; /* in 26.6 format */ FT_Size_Metrics metrics; TT_Size_Metrics tt_metrics; /* size metrics */ @@ -247,9 +248,135 @@ FT_BEGIN_HEADER TT_Set_CVT_Func func_write_cvt; /* write a cvt entry (in pixels) */ TT_Set_CVT_Func func_move_cvt; /* incr a cvt entry (in pixels) */ - FT_Bool grayscale; /* are we hinting for grayscale? */ - -#ifdef TT_CONFIG_OPTION_SUBPIXEL_HINTING + FT_Bool grayscale; /* bi-level hinting and */ + /* grayscale rendering */ + +#ifdef TT_SUPPORT_SUBPIXEL_HINTING_MINIMAL + /* + * Modern TrueType fonts are usually rendered through Microsoft's + * collection of rendering techniques called ClearType (e.g., subpixel + * rendering and subpixel hinting). When ClearType was introduced, most + * fonts were not ready. Microsoft decided to implement a backwards + * compatibility mode that employed several simple to complicated + * assumptions and tricks that modified the interpretation of the + * bytecode contained in these fonts to make them look ClearType-y + * somehow. Most (web)fonts that were released since then have come to + * rely on these hacks to render correctly, even some of Microsoft's + * flagship ClearType fonts (Calibri, Cambria, Segoe UI). + * + * The minimal subpixel hinting code (interpreter version 40) employs a + * small list of font-agnostic hacks to bludgeon non-native-ClearType + * fonts (except tricky ones[1]) into submission. It will not try to + * toggle hacks for specific fonts for performance and complexity + * reasons. The focus is on modern (web)fonts rather than legacy fonts + * that were made for black-and-white rendering. + * + * Major hacks + * + * - Any point movement on the x axis is ignored (cf. `Direct_Move' and + * `Direct_Move_X'). This has the smallest code footprint and single + * biggest effect. The ClearType way to increase resolution is + * supersampling the x axis, the FreeType way is ignoring instructions + * on the x axis, which gives the same result in the majority of + * cases. + * + * - Points are not moved post-IUP (neither on the x nor on the y axis), + * except the x component of diagonal moves post-IUP (cf. + * `Direct_Move', `Direct_Move_Y', `Move_Zp2_Point'). Post-IUP + * changes are commonly used to `fix' pixel patterns which has little + * use outside monochrome rendering. + * + * - SHPIX and DELTAP don't execute unless moving a composite on the + * y axis or moving a previously y touched point. SHPIX additionally + * denies movement on the x axis (cf. `Ins_SHPIX' and `Ins_DELTAP'). + * Both instructions are commonly used to `fix' pixel patterns for + * monochrome or Windows's GDI rendering but make little sense for + * FreeType rendering. Both can distort the outline. See [2] for + * details. + * + * - The hdmx table and modifications to phantom points are ignored. + * Bearings and advance widths remain unchanged (except rounding them + * outside the interpreter!), cf. `compute_glyph_metrics' and + * `TT_Hint_Glyph'. Letting non-native-ClearType fonts modify spacing + * might mess up spacing. + * + * Minor hacks + * + * - FLIPRGON, FLIPRGOFF, and FLIPPT don't execute post-IUP. This + * prevents dents in e.g. Arial-Regular's `D' and `G' glyphs at + * various sizes. + * + * (Post-IUP is the state after both IUP[x] and IUP[y] have been + * executed.) + * + * The best results are achieved for fonts that were from the outset + * designed with ClearType in mind, meaning they leave the x axis mostly + * alone and don't mess with the `final' outline to produce more + * pleasing pixel patterns. The harder the designer tried to produce + * very specific patterns (`superhinting') for pre-ClearType-displays, + * the worse the results. + * + * Microsoft defines a way to turn off backwards compatibility and + * interpret instructions as before (called `native ClearType')[2][3]. + * The font designer then regains full control and is responsible for + * making the font work correctly with ClearType without any + * hand-holding by the interpreter or rasterizer[4]. The v40 + * interpreter assumes backwards compatibility by default, which can be + * turned off the same way by executing the following in the control + * program (cf. `Ins_INSTCTRL'). + * + * #PUSH 4,3 + * INSTCTRL[] + * + * [1] Tricky fonts as FreeType defines them rely on the bytecode + * interpreter to display correctly. Hacks can interfere with them, + * so they get treated like native ClearType fonts (v40 with + * backwards compatibility turned off). Cf. `TT_RunIns'. + * + * [2] Proposed by Microsoft's Greg Hitchcock in + * https://www.microsoft.com/typography/cleartype/truetypecleartype.aspx + * + * [3] Beat Stamm describes it in more detail: + * http://www.beatstamm.com/typography/RTRCh4.htm#Sec12 + * + * [4] The list of `native ClearType' fonts is small at the time of this + * writing; I found the following on a Windows 10 Update 1511 + * installation: Constantia, Corbel, Sitka, Malgun Gothic, Microsoft + * JhengHei (Bold and UI Bold), Microsoft YaHei (Bold and UI Bold), + * SimSun, NSimSun, and Yu Gothic. + * + */ + + /* Using v40 implies subpixel hinting. Used to detect interpreter */ + /* version switches. `_lean' to differentiate from the Infinality */ + /* `subpixel_hinting', which is managed differently. */ + FT_Bool subpixel_hinting_lean; + + /* Long side of a LCD subpixel is vertical (e.g., screen is rotated). */ + /* `_lean' to differentiate from the Infinality `vertical_lcd', which */ + /* is managed differently. */ + FT_Bool vertical_lcd_lean; + + /* Default to backwards compatibility mode in v40 interpreter. If */ + /* this is false, it implies the interpreter is in v35 or in native */ + /* ClearType mode. */ + FT_Bool backwards_compatibility; + + /* Useful for detecting and denying post-IUP trickery that is usually */ + /* used to fix pixel patterns (`superhinting'). */ + FT_Bool iupx_called; + FT_Bool iupy_called; + + /* ClearType hinting and grayscale rendering, as used by Universal */ + /* Windows Platform apps (Windows 8 and above). Like the standard */ + /* colorful ClearType mode, it utilizes a vastly increased virtual */ + /* resolution on the x axis. Different from bi-level hinting and */ + /* grayscale rendering, the old mode from Win9x days that roughly */ + /* adheres to the physical pixel grid on both axes. */ + FT_Bool grayscale_cleartype; +#endif /* TT_SUPPORT_SUBPIXEL_HINTING_MINIMAL */ + +#ifdef TT_SUPPORT_SUBPIXEL_HINTING_INFINALITY TT_Round_Func func_round_sphn; /* subpixel rounding function */ FT_Bool subpixel_hinting; /* Using subpixel hinting? */ @@ -279,7 +406,7 @@ FT_BEGIN_HEADER FT_ULong sph_in_func_flags; /* flags to indicate if in */ /* special functions */ -#endif /* TT_CONFIG_OPTION_SUBPIXEL_HINTING */ +#endif /* TT_SUPPORT_SUBPIXEL_HINTING_INFINALITY */ } TT_ExecContextRec; @@ -382,7 +509,7 @@ FT_BEGIN_HEADER FT_END_HEADER -#endif /* __TTINTERP_H__ */ +#endif /* TTINTERP_H_ */ /* END */ |