ref: 25a9bd9be066c7ff31bfb345cf9c76c5e408ca04
parent: bb435da213cf6221fdc060adb75cac8a8468ac12
author: Werner Lemberg <[email protected]>
date: Sun May 24 05:21:31 EDT 2015
Clean up and improve description of MS hinting engine versions.
--- a/include/ftttdrv.h
+++ b/include/ftttdrv.h
@@ -67,13 +67,13 @@
* 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_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.
+ *
* {- * 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
+ * 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
* }
*
- * Since FreeType doesn't provide all capabilities of DWrite ClearType,
- * using version~38 seems justified.
+ * 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