The Tor Browser: modules/freetype2/include/ftcffdrv.h@b8a032363ba2 (annotated)

modules/freetype2/include/ftcffdrv.h@b8a032363ba2 (annotated)

modules/freetype2/include/ftcffdrv.h

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /***************************************************************************/
 /*                                                                         */
 /*  ftcffdrv.h                                                             */
 /*                                                                         */
 /*    FreeType API for controlling the CFF driver (specification only).    */
 /*                                                                         */
 /*  Copyright 2013 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 __FTCFFDRV_H__
 #define __FTCFFDRV_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:
    *   cff_driver
    *
    * @title:
    *   The CFF driver
    *
    * @abstract:
    *   Controlling the CFF driver module.
    *
    * @description:
    *   While FreeType's CFF driver doesn't expose API functions by itself,
    *   it is possible to control its behaviour with @FT_Property_Set and
    *   @FT_Property_Get.  The list below gives the available properties
    *   together with the necessary macros and structures.
    *
    *   The CFF driver's module name is `cff'.
    *
    *   *Hinting* *and* *antialiasing* *principles* *of* *the* *new* *engine*
    *
    *   The rasterizer is positioning horizontal features (e.g., ascender
    *   height & x-height, or crossbars) on the pixel grid and minimizing the
    *   amount of antialiasing applied to them, while placing vertical
    *   features (vertical stems) on the pixel grid without hinting, thus
    *   representing the stem position and weight accurately.  Sometimes the
    *   vertical stems may be only partially black.  In this context,
    *   `antialiasing' means that stems are not positioned exactly on pixel
    *   borders, causing a fuzzy appearance.
    *
    *   There are two principles behind this approach.
    *
    *   1) No hinting in the horizontal direction: Unlike `superhinted'
    *   TrueType, which changes glyph widths to accommodate regular
    *   inter-glyph spacing, Adobe's approach is `faithful to the design' in
    *   representing both the glyph width and the inter-glyph spacing
    *   designed for the font.  This makes the screen display as close as it
    *   can be to the result one would get with infinite resolution, while
    *   preserving what is considered the key characteristics of each glyph.
    *   Note that the distances between unhinted and grid-fitted positions at
    *   small sizes are comparable to kerning values and thus would be
    *   noticeable (and distracting) while reading if hinting were applied.
    *
    *   One of the reasons to not hint horizontally is antialiasing for LCD
    *   screens: The pixel geometry of modern displays supplies three
    *   vertical sub-pixels as the eye moves horizontally across each visible
    *   pixel.  On devices where we can be certain this characteristic is
    *   present a rasterizer can take advantage of the sub-pixels to add
    *   increments of weight.  In Western writing systems this turns out to
    *   be the more critical direction anyway; the weights and spacing of
    *   vertical stems (see above) are central to Armenian, Cyrillic, Greek,
    *   and Latin type designs.  Even when the rasterizer uses greyscale
    *   antialiasing instead of color (a necessary compromise when one
    *   doesn't know the screen characteristics), the unhinted vertical
    *   features preserve the design's weight and spacing much better than
    *   aliased type would.
    *
    *   2) Aligment in the vertical direction: Weights and spacing along the
    *   y~axis are less critical; what is much more important is the visual
    *   alignment of related features (like cap-height and x-height).  The
    *   sense of alignment for these is enhanced by the sharpness of grid-fit
    *   edges, while the cruder vertical resolution (full pixels instead of
    *   1/3 pixels) is less of a problem.
    *
    *   On the technical side, horizontal alignment zones for ascender,
    *   x-height, and other important height values (traditionally called
    *   `blue zones') as defined in the font are positioned independently,
    *   each being rounded to the nearest pixel edge, taking care of
    *   overshoot suppression at small sizes, stem darkening, and scaling.
    *
    *   Hstems (this is, hint values defined in the font to help align
    *   horizontal features) that fall within a blue zone are said to be
    *   `captured' and are aligned to that zone.  Uncaptured stems are moved
    *   in one of four ways, top edge up or down, bottom edge up or down.
    *   Unless there are conflicting hstems, the smallest movement is taken
    *   to minimize distortion.
    */
   /**************************************************************************
    *
    * @property:
    *   hinting-engine
    *
    * @description:
    *   Thanks to Adobe, which contributed a new hinting (and parsing)
    *   engine, an application can select between `freetype' and `adobe' if
    *   compiled with CFF_CONFIG_OPTION_OLD_ENGINE.  If this configuration
    *   macro isn't defined, `hinting-engine' does nothing.
    *
    *   The default engine is `freetype' if CFF_CONFIG_OPTION_OLD_ENGINE is
    *   defined, and `adobe' otherwise.
    *
    *   The following example code demonstrates how to select Adobe's hinting
    *   engine (omitting the error handling).
    *
    *   {
    *     FT_Library  library;
    *     FT_UInt     hinting_engine = FT_CFF_HINTING_ADOBE;
    *
    *
    *     FT_Init_FreeType( &library );
    *
    *     FT_Property_Set( library, "cff",
    *                               "hinting-engine", &hinting_engine );
    *   }
    *
    * @note:
    *   This property can be used with @FT_Property_Get also.
    *
    */
   /**************************************************************************
    *
    * @enum:
    *   FT_CFF_HINTING_XXX
    *
    * @description:
    *   A list of constants used for the @hinting-engine property to select
    *   the hinting engine for CFF fonts.
    *
    * @values:
    *   FT_CFF_HINTING_FREETYPE ::
    *     Use the old FreeType hinting engine.
    *
    *   FT_CFF_HINTING_ADOBE ::
    *     Use the hinting engine contributed by Adobe.
    *
    */
 #define FT_CFF_HINTING_FREETYPE  0
 #define FT_CFF_HINTING_ADOBE     1
   /**************************************************************************
    *
    * @property:
    *   no-stem-darkening
    *
    * @description:
    *   By default, the Adobe CFF engine darkens stems at smaller sizes,
    *   regardless of hinting, to enhance contrast.  This feature requires
    *   a rendering system with proper gamma correction.  Setting this
    *   property, stem darkening gets switched off.
    *
    *   Note that stem darkening is never applied if @FT_LOAD_NO_SCALE is set.
    *
    *   {
    *     FT_Library  library;
    *     FT_Bool     no_stem_darkening = TRUE;
    *
    *
    *     FT_Init_FreeType( &library );
    *
    *     FT_Property_Set( library, "cff",
    *                               "no-stem-darkening", &no_stem_darkening );
    *   }
    *
    * @note:
    *   This property can be used with @FT_Property_Get also.
    *
    */
   /**************************************************************************
    *
    * @property:
    *   darkening-parameters
    *
    * @description:
    *   By default, the Adobe CFF engine darkens stems as follows (if the
    *   `no-stem-darkening' property isn't set):
    *
    *   {
    *     stem width <= 0.5px:   darkening amount = 0.4px
    *     stem width  = 1px:     darkening amount = 0.275px
    *     stem width  = 1.667px: darkening amount = 0.275px
    *     stem width >= 2.333px: darkening amount = 0px
    *   }
    *
    *   and piecewise linear in-between.  Using the `darkening-parameters'
    *   property, these four control points can be changed, as the following
    *   example demonstrates.
    *
    *   {
    *     FT_Library  library;
    *     FT_Int      darken_params[8] = {  500, 300,   // x1, y1
    *                                      1000, 200,   // x2, y2
    *                                      1500, 100,   // x3, y3
    *                                      2000,   0 }; // x4, y4
    *
    *
    *     FT_Init_FreeType( &library );
    *
    *     FT_Property_Set( library, "cff",
    *                               "darkening-parameters", darken_params );
    *   }
    *
    *   The x~values give the stem width, and the y~values the darkening
    *   amount.  The unit is 1000th of pixels.  All coordinate values must be
    *   positive; the x~values must be monotonically increasing; the
    *   y~values must be monotonically decreasing and smaller than or
    *   equal to 500 (corresponding to half a pixel); the slope of each
    *   linear piece must be shallower than -1 (e.g., -.4).
    *
    * @note:
    *   This property can be used with @FT_Property_Get also.
    *
    */
  /* */
 FT_END_HEADER
 #endif /* __FTCFFDRV_H__ */
 /* END */

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modules/freetype2/include/ftcffdrv.h@b8a032363ba2 (annotated)

modules/freetype2/include/ftcffdrv.h