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Diffstat (limited to 'include/ftcffdrv.h')
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diff --git a/include/ftcffdrv.h b/include/ftcffdrv.h new file mode 100644 index 0000000..e4d039d --- /dev/null +++ b/include/ftcffdrv.h @@ -0,0 +1,254 @@ +/***************************************************************************/ +/* */ +/* 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 */ |