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kc3-lang/freetype/include/ftcffdrv.h
Werner Lemberg
- include/ftcffdrv.h
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/***************************************************************************/ /* */ /* ftcffdrv.h */ /* */ /* FreeType API for controlling the CFF driver (specification only). */ /* */ /* Copyright 2013-2015 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. * * @order: * hinting-engine * no-stem-darkening * darkening-parameters * */ /************************************************************************** * * @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. 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. * */ /* */ FT_END_HEADER #endif /* __FTCFFDRV_H__ */ /* END */