diff options
Diffstat (limited to 'modules/freetype2/src/truetype/ttinterp.h')
| -rw-r--r-- | modules/freetype2/src/truetype/ttinterp.h | 62 |
1 files changed, 39 insertions, 23 deletions
diff --git a/modules/freetype2/src/truetype/ttinterp.h b/modules/freetype2/src/truetype/ttinterp.h index 33a9b122f..2966439ea 100644 --- a/modules/freetype2/src/truetype/ttinterp.h +++ b/modules/freetype2/src/truetype/ttinterp.h @@ -4,7 +4,7 @@ /* */ /* TrueType bytecode interpreter (specification). */ /* */ -/* Copyright 1996-2016 by */ +/* Copyright 1996-2018 by */ /* David Turner, Robert Wilhelm, and Werner Lemberg. */ /* */ /* This file is part of the FreeType project, and may only be used, */ @@ -253,23 +253,38 @@ FT_BEGIN_HEADER #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). + * FreeType supports ClearType-like hinting of TrueType fonts through + * the version 40 interpreter. This is achieved through several hacks + * in the base (v35) interpreter, as detailed below. * - * 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. + * ClearType is an umbrella term for several rendering techniques + * employed by Microsoft's various GUI and rendering toolkit + * implementations, most importantly: subpixel rendering for using the + * RGB subpixels of LCDs to approximately triple the perceived + * resolution on the x-axis and subpixel hinting for positioning stems + * on subpixel borders. TrueType programming is explicit, i.e., fonts + * must be programmed to take advantage of ClearType's possibilities. + * + * When ClearType was introduced, it seemed unlikely that all fonts + * would be reprogrammed, so Microsoft decided to implement a backward + * compatibility mode. It employs several simple to complicated + * assumptions and tricks, many of them font-dependent, that modify the + * interpretation of the bytecode contained in these fonts to retrofit + * them into a ClearType-y look. The quality of the results varies. + * Most (web)fonts that were released since then have come to rely on + * these hacks to render correctly, even some of Microsoft's flagship + * fonts (e.g., Calibri, Cambria, Segoe UI). + * + * FreeType's minimal subpixel hinting code (interpreter version 40) + * employs a small list of font-agnostic hacks loosely based on the + * public information available on Microsoft's compatibility mode[2]. + * The focus is on modern (web)fonts rather than legacy fonts that were + * made for monochrome rendering. It will not match ClearType rendering + * exactly. Unlike the `Infinality' code (interpreter version 38) that + * came before, it will not try to toggle hacks for specific fonts for + * performance and complexity reasons. It will fall back to version 35 + * behavior for tricky fonts[1] or when monochrome rendering is + * requested. * * Major hacks * @@ -316,12 +331,12 @@ FT_BEGIN_HEADER * very specific patterns (`superhinting') for pre-ClearType-displays, * the worse the results. * - * Microsoft defines a way to turn off backwards compatibility and + * Microsoft defines a way to turn off backward 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 + * interpreter assumes backward compatibility by default, which can be * turned off the same way by executing the following in the control * program (cf. `Ins_INSTCTRL'). * @@ -331,7 +346,7 @@ FT_BEGIN_HEADER * [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'. + * backward compatibility turned off). Cf. `TT_RunIns'. * * [2] Proposed by Microsoft's Greg Hitchcock in * https://www.microsoft.com/typography/cleartype/truetypecleartype.aspx @@ -347,7 +362,8 @@ FT_BEGIN_HEADER * */ - /* Using v40 implies subpixel hinting. Used to detect interpreter */ + /* Using v40 implies subpixel hinting, unless FT_RENDER_MODE_MONO has been + * requested. Used to detect interpreter */ /* version switches. `_lean' to differentiate from the Infinality */ /* `subpixel_hinting', which is managed differently. */ FT_Bool subpixel_hinting_lean; @@ -357,10 +373,10 @@ FT_BEGIN_HEADER /* is managed differently. */ FT_Bool vertical_lcd_lean; - /* Default to backwards compatibility mode in v40 interpreter. If */ + /* Default to backward 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; + FT_Bool backward_compatibility; /* Useful for detecting and denying post-IUP trickery that is usually */ /* used to fix pixel patterns (`superhinting'). */ |