255 lines
9.7 KiB
C
255 lines
9.7 KiB
C
/***************************************************************************/
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/* */
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/* ftcffdrv.h */
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/* */
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/* FreeType API for controlling the CFF driver (specification only). */
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/* */
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/* Copyright 2013 by */
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/* David Turner, Robert Wilhelm, and Werner Lemberg. */
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/* */
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/* This file is part of the FreeType project, and may only be used, */
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/* modified, and distributed under the terms of the FreeType project */
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/* license, LICENSE.TXT. By continuing to use, modify, or distribute */
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/* this file you indicate that you have read the license and */
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/* understand and accept it fully. */
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/* */
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/***************************************************************************/
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#ifndef __FTCFFDRV_H__
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#define __FTCFFDRV_H__
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#include <ft2build.h>
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#include FT_FREETYPE_H
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#ifdef FREETYPE_H
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#error "freetype.h of FreeType 1 has been loaded!"
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#error "Please fix the directory search order for header files"
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#error "so that freetype.h of FreeType 2 is found first."
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#endif
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FT_BEGIN_HEADER
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/**************************************************************************
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*
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* @section:
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* cff_driver
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*
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* @title:
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* The CFF driver
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*
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* @abstract:
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* Controlling the CFF driver module.
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*
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* @description:
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* While FreeType's CFF driver doesn't expose API functions by itself,
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* it is possible to control its behaviour with @FT_Property_Set and
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* @FT_Property_Get. The list below gives the available properties
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* together with the necessary macros and structures.
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*
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* The CFF driver's module name is `cff'.
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*
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* *Hinting* *and* *antialiasing* *principles* *of* *the* *new* *engine*
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*
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* The rasterizer is positioning horizontal features (e.g., ascender
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* height & x-height, or crossbars) on the pixel grid and minimizing the
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* amount of antialiasing applied to them, while placing vertical
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* features (vertical stems) on the pixel grid without hinting, thus
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* representing the stem position and weight accurately. Sometimes the
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* vertical stems may be only partially black. In this context,
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* `antialiasing' means that stems are not positioned exactly on pixel
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* borders, causing a fuzzy appearance.
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*
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* There are two principles behind this approach.
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*
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* 1) No hinting in the horizontal direction: Unlike `superhinted'
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* TrueType, which changes glyph widths to accommodate regular
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* inter-glyph spacing, Adobe's approach is `faithful to the design' in
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* representing both the glyph width and the inter-glyph spacing
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* designed for the font. This makes the screen display as close as it
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* can be to the result one would get with infinite resolution, while
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* preserving what is considered the key characteristics of each glyph.
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* Note that the distances between unhinted and grid-fitted positions at
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* small sizes are comparable to kerning values and thus would be
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* noticeable (and distracting) while reading if hinting were applied.
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*
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* One of the reasons to not hint horizontally is antialiasing for LCD
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* screens: The pixel geometry of modern displays supplies three
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* vertical sub-pixels as the eye moves horizontally across each visible
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* pixel. On devices where we can be certain this characteristic is
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* present a rasterizer can take advantage of the sub-pixels to add
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* increments of weight. In Western writing systems this turns out to
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* be the more critical direction anyway; the weights and spacing of
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* vertical stems (see above) are central to Armenian, Cyrillic, Greek,
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* and Latin type designs. Even when the rasterizer uses greyscale
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* antialiasing instead of color (a necessary compromise when one
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* doesn't know the screen characteristics), the unhinted vertical
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* features preserve the design's weight and spacing much better than
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* aliased type would.
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*
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* 2) Aligment in the vertical direction: Weights and spacing along the
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* y~axis are less critical; what is much more important is the visual
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* alignment of related features (like cap-height and x-height). The
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* sense of alignment for these is enhanced by the sharpness of grid-fit
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* edges, while the cruder vertical resolution (full pixels instead of
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* 1/3 pixels) is less of a problem.
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*
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* On the technical side, horizontal alignment zones for ascender,
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* x-height, and other important height values (traditionally called
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* `blue zones') as defined in the font are positioned independently,
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* each being rounded to the nearest pixel edge, taking care of
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* overshoot suppression at small sizes, stem darkening, and scaling.
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*
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* Hstems (this is, hint values defined in the font to help align
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* horizontal features) that fall within a blue zone are said to be
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* `captured' and are aligned to that zone. Uncaptured stems are moved
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* in one of four ways, top edge up or down, bottom edge up or down.
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* Unless there are conflicting hstems, the smallest movement is taken
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* to minimize distortion.
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*/
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/**************************************************************************
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*
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* @property:
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* hinting-engine
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*
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* @description:
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* Thanks to Adobe, which contributed a new hinting (and parsing)
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* engine, an application can select between `freetype' and `adobe' if
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* compiled with CFF_CONFIG_OPTION_OLD_ENGINE. If this configuration
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* macro isn't defined, `hinting-engine' does nothing.
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*
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* The default engine is `freetype' if CFF_CONFIG_OPTION_OLD_ENGINE is
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* defined, and `adobe' otherwise.
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*
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* The following example code demonstrates how to select Adobe's hinting
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* engine (omitting the error handling).
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*
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* {
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* FT_Library library;
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* FT_UInt hinting_engine = FT_CFF_HINTING_ADOBE;
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*
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*
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* FT_Init_FreeType( &library );
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*
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* FT_Property_Set( library, "cff",
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* "hinting-engine", &hinting_engine );
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* }
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*
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* @note:
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* This property can be used with @FT_Property_Get also.
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*
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*/
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/**************************************************************************
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*
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* @enum:
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* FT_CFF_HINTING_XXX
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*
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* @description:
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* A list of constants used for the @hinting-engine property to select
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* the hinting engine for CFF fonts.
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*
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* @values:
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* FT_CFF_HINTING_FREETYPE ::
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* Use the old FreeType hinting engine.
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*
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* FT_CFF_HINTING_ADOBE ::
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* Use the hinting engine contributed by Adobe.
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*
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*/
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#define FT_CFF_HINTING_FREETYPE 0
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#define FT_CFF_HINTING_ADOBE 1
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/**************************************************************************
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*
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* @property:
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* no-stem-darkening
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*
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* @description:
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* By default, the Adobe CFF engine darkens stems at smaller sizes,
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* regardless of hinting, to enhance contrast. This feature requires
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* a rendering system with proper gamma correction. Setting this
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* property, stem darkening gets switched off.
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*
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* Note that stem darkening is never applied if @FT_LOAD_NO_SCALE is set.
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*
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* {
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* FT_Library library;
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* FT_Bool no_stem_darkening = TRUE;
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*
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*
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* FT_Init_FreeType( &library );
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*
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* FT_Property_Set( library, "cff",
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* "no-stem-darkening", &no_stem_darkening );
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* }
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*
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* @note:
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* This property can be used with @FT_Property_Get also.
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*
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*/
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/**************************************************************************
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*
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* @property:
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* darkening-parameters
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*
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* @description:
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* By default, the Adobe CFF engine darkens stems as follows (if the
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* `no-stem-darkening' property isn't set):
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*
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* {
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* stem width <= 0.5px: darkening amount = 0.4px
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* stem width = 1px: darkening amount = 0.275px
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* stem width = 1.667px: darkening amount = 0.275px
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* stem width >= 2.333px: darkening amount = 0px
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* }
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*
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* and piecewise linear in-between. Using the `darkening-parameters'
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* property, these four control points can be changed, as the following
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* example demonstrates.
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*
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* {
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* FT_Library library;
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* FT_Int darken_params[8] = { 500, 300, // x1, y1
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* 1000, 200, // x2, y2
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* 1500, 100, // x3, y3
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* 2000, 0 }; // x4, y4
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*
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*
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* FT_Init_FreeType( &library );
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*
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* FT_Property_Set( library, "cff",
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* "darkening-parameters", darken_params );
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* }
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*
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* The x~values give the stem width, and the y~values the darkening
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* amount. The unit is 1000th of pixels. All coordinate values must be
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* positive; the x~values must be monotonically increasing; the
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* y~values must be monotonically decreasing and smaller than or
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* equal to 500 (corresponding to half a pixel); the slope of each
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* linear piece must be shallower than -1 (e.g., -.4).
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*
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* @note:
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* This property can be used with @FT_Property_Get also.
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*
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*/
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/* */
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FT_END_HEADER
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#endif /* __FTCFFDRV_H__ */
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/* END */
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