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Clean up and improve description of MS hinting engine versions.

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2.6.5
Werner Lemberg 10 years ago
parent bb435da213
commit 25a9bd9be0
1 changed files with 84 additions and 21 deletions
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  1. 105
      include/ftttdrv.h

105
include/ftttdrv.h
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@ -67,13 +67,13 @@ FT_BEGIN_HEADER
* 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. 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.
* 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
@ -135,24 +135,87 @@ FT_BEGIN_HEADER
* `FT_Err_Unimplemented_Feature' error.
*
* Depending on the graphics framework, Microsoft uses different
* bytecode engines. As a consequence, the version numbers returned by
* a call to the `GETINFO[1]' bytecode instruction are more convoluted
* than desired.
* bytecode and rendering engines. As a consequence, the version
* numbers returned by a call to the `GETINFO[1]' 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.
*
* {
* framework Windows version result of GETINFO[1]
* ----------------------------------------------------
* GDI before XP 35
* GDI XP and later 37
* GDI+ old before Vista 37
* GDI+ old Vista, 7 38
* GDI+ after 7 40
* DWrite before 8 39
* DWrite 8 and later 40
* 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
* }
*
* Since FreeType doesn't provide all capabilities of DWrite ClearType,
* using version~38 seems justified.
* 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

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