ref: 0ad326236607df0802dc72c4f13b3f35484b7672
dir: /include/freetype/ftcffdrv.h/
/***************************************************************************/
/* */
/* ftcffdrv.h */
/* */
/* FreeType API for controlling the CFF driver (specification only). */
/* */
/* Copyright 2013-2017 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) Alignment 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.
*
* @order:
* hinting-engine[cff]
* no-stem-darkening[cff]
* darkening-parameters[cff]
* random-seed
*
*/
/**************************************************************************
*
* @property:
* hinting-engine[cff]
*
* @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.
*
* This property can be set via the `FREETYPE_PROPERTIES' environment
* variable (using values `adobe' or `freetype').
*/
/**************************************************************************
*
* @enum:
* FT_CFF_HINTING_XXX
*
* @description:
* A list of constants used for the @hinting-engine[cff] 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[cff]
*
* @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.
*
* This property can be set via the `FREETYPE_PROPERTIES' environment
* variable (using values 1 and 0 for `on' and `off', respectively).
* It can also be set per face using @FT_Face_Properties with
* @FT_PARAM_TAG_STEM_DARKENING.
*
*/
/**************************************************************************
*
* @property:
* darkening-parameters[cff]
*
* @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. At configuration time, these four
* control points can be set with the macro
* `CFF_CONFIG_OPTION_DARKENING_PARAMETERS'. At runtime, the control
* points can be changed using the `darkening-parameters' property, 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.
*
* This property can be set via the `FREETYPE_PROPERTIES' environment
* variable, using eight comma-separated integers without spaces. Here
* the above example, using `\' to break the line for readability.
*
* {
* FREETYPE_PROPERTIES=\
* cff:darkening-parameters=500,300,1000,200,1500,100,2000,0
* }
*/
/**************************************************************************
*
* @property:
* random-seed
*
* @description:
* By default, the seed value for the CFF `random' operator is set to a
* random value. However, mainly for debugging purposes, it is often
* necessary to use a known value as a seed so that the pseudo-random
* number sequences generated by `random' are repeatable.
*
* The `random-seed' property does that. Its argument is a signed 32bit
* integer; if the value is zero or negative, the seed given by the
* `intitialRandomSeed' private DICT operator in a CFF file gets used
* (or a default value if there is no such operator). If the value is
* positive, use it instead of `initialRandomSeed', which is
* consequently ignored.
*
* @note:
* This property can be set via the `FREETYPE_PROPERTIES' environment
* variable. It can also be set per face using @FT_Face_Properties with
* @FT_PARAM_TAG_RANDOM_SEED.
*
*/
/**************************************************************************
*
* @constant:
* FT_PARAM_TAG_RANDOM_SEED
*
* @description:
* An @FT_Parameter tag to be used with @FT_Face_Properties. The
* corresponding 32bit signed integer argument overrides the CFF
* module's random seed value with a face-specific one; see
* @random-seed.
*
*/
#define FT_PARAM_TAG_RANDOM_SEED \
FT_MAKE_TAG( 's', 'e', 'e', 'd' )
/* */
FT_END_HEADER
#endif /* FTCFFDRV_H_ */
/* END */