/***************************************************************************/ | |
/* */ | |
/* 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[cff] | |
* darkening-parameters[cff] | |
* | |
*/ | |
/************************************************************************** | |
* | |
* @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[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. | |
* | |
*/ | |
/************************************************************************** | |
* | |
* @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. | |
* | |
*/ | |
/* */ | |
FT_END_HEADER | |
#endif /* __FTCFFDRV_H__ */ | |
/* END */ |