ref: c1abd6aa4715db225a71ecbe006fd7dbedf82fcf
dir: /include/freetype/ftlcdfil.h/
/***************************************************************************/
/* */
/* ftlcdfil.h */
/* */
/* FreeType API for color filtering of subpixel bitmap glyphs */
/* (specification). */
/* */
/* Copyright 2006-2017 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 FTLCDFIL_H_
#define FTLCDFIL_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:
* lcd_filtering
*
* @title:
* LCD Filtering
*
* @abstract:
* Reduce color fringes of subpixel-rendered bitmaps.
*
* @description:
* Subpixel rendering exploits the color-striped structure of LCD
* pixels, increasing the available resolution in the direction of the
* stripe (usually horizontal RGB) by a factor of~3. Since these
* subpixels are color pixels, using them unfiltered creates severe
* color fringes. Use the @FT_Library_SetLcdFilter API to specify a
* low-pass filter, which is then applied to subpixel-rendered bitmaps
* generated through @FT_Render_Glyph. The filter sacrifices some of
* the higher resolution to reduce color fringes, making the glyph image
* slightly blurrier. Positional improvements will remain.
*
* Note that no filter is active by default, and that this function is
* *not* implemented in default builds of the library. You need to
* #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your `ftoption.h' file
* in order to activate it and explicitly call @FT_Library_SetLcdFilter
* to enable it.
*
* A filter should have two properties:
*
* 1) It should be normalized, meaning the sum of the 5~components
* should be 256 (0x100). It is possible to go above or under this
* target sum, however: going under means tossing out contrast, going
* over means invoking clamping and thereby non-linearities that
* increase contrast somewhat at the expense of greater distortion
* and color-fringing. Contrast is better enhanced through stem
* darkening.
*
* 2) It should be color-balanced, meaning a filter `{~a, b, c, b, a~}'
* where a~+ b~=~c. It distributes the computed coverage for one
* subpixel to all subpixels equally, sacrificing some won resolution
* but drastically reducing color-fringing. Positioning improvements
* remain! Note that color-fringing can only really be minimized
* when using a color-balanced filter and alpha-blending the glyph
* onto a surface in linear space; see @FT_Render_Glyph.
*
* Regarding the form, a filter can be a `boxy' filter or a `beveled'
* filter. Boxy filters are sharper but are less forgiving of non-ideal
* gamma curves of a screen (viewing angles!), beveled filters are
* fuzzier but more tolerant.
*
* Examples:
*
* - [0x10 0x40 0x70 0x40 0x10] is beveled and neither balanced nor
* normalized.
*
* - [0x1A 0x33 0x4D 0x33 0x1A] is beveled and balanced but not
* normalized.
*
* - [0x19 0x33 0x66 0x4c 0x19] is beveled and normalized but not
* balanced.
*
* - [0x00 0x4c 0x66 0x4c 0x00] is boxily beveled and normalized but not
* balanced.
*
* - [0x00 0x55 0x56 0x55 0x00] is boxy, normalized, and almost
* balanced.
*
* - [0x08 0x4D 0x56 0x4D 0x08] is beveled, normalized and, almost
* balanced.
*
* The filter affects glyph bitmaps rendered through @FT_Render_Glyph,
* @FT_Load_Glyph, and @FT_Load_Char. It does _not_ affect the output
* of @FT_Outline_Render and @FT_Outline_Get_Bitmap.
*
* If this feature is activated, the dimensions of LCD glyph bitmaps are
* either wider or taller than the dimensions of the corresponding
* outline with regard to the pixel grid. For example, for
* @FT_RENDER_MODE_LCD, the filter adds 3~subpixels to the left, and
* 3~subpixels to the right. The bitmap offset values are adjusted
* accordingly, so clients shouldn't need to modify their layout and
* glyph positioning code when enabling the filter.
*
* It is important to understand that linear alpha blending and gamma
* correction is critical for correctly rendering glyphs onto surfaces
* without artifacts and even more critical when subpixel rendering is
* involved.
*
* Each of the 3~alpha values (subpixels) is independently used to blend
* one color channel. That is, red alpha blends the red channel of the
* text color with the red channel of the background pixel. The
* distribution of density values by the color-balanced filter assumes
* alpha blending is done in linear space; only then color artifacts
* cancel out.
*/
/****************************************************************************
*
* @enum:
* FT_LcdFilter
*
* @description:
* A list of values to identify various types of LCD filters.
*
* @values:
* FT_LCD_FILTER_NONE ::
* Do not perform filtering. When used with subpixel rendering, this
* results in sometimes severe color fringes.
*
* FT_LCD_FILTER_DEFAULT ::
* The default filter reduces color fringes considerably, at the cost
* of a slight blurriness in the output.
*
* It is a beveled, normalized, and color-balanced five-tap filter
* that is more forgiving to screens with non-ideal gamma curves and
* viewing angles. Note that while color-fringing is reduced, it can
* only be minimized by using linear alpha blending and gamma
* correction to render glyphs onto surfaces. The default filter
* weights are [0x08 0x4D 0x56 0x4D 0x08].
*
* FT_LCD_FILTER_LIGHT ::
* The light filter is a variant that is sharper at the cost of
* slightly more color fringes than the default one.
*
* It is a boxy, normalized, and color-balanced three-tap filter that
* is less forgiving to screens with non-ideal gamma curves and
* viewing angles. This filter works best when the rendering system
* uses linear alpha blending and gamma correction to render glyphs
* onto surfaces. The light filter weights are
* [0x00 0x55 0x56 0x55 0x00].
*
* FT_LCD_FILTER_LEGACY ::
* This filter corresponds to the original libXft color filter. It
* provides high contrast output but can exhibit really bad color
* fringes if glyphs are not extremely well hinted to the pixel grid.
* In other words, it only works well if the TrueType bytecode
* interpreter is enabled *and* high-quality hinted fonts are used.
*
* This filter is only provided for comparison purposes, and might be
* disabled or stay unsupported in the future.
*
* FT_LCD_FILTER_LEGACY1 ::
* For historical reasons, the FontConfig library returns a different
* enumeration value for legacy LCD filtering. To make code work that
* (incorrectly) forwards FontConfig's enumeration value to
* @FT_Library_SetLcdFilter without proper mapping, it is thus easiest
* to have another enumeration value, which is completely equal to
* `FT_LCD_FILTER_LEGACY'.
*
* @since:
* 2.3.0 (`FT_LCD_FILTER_LEGACY1' since 2.6.2)
*/
typedef enum FT_LcdFilter_
{
FT_LCD_FILTER_NONE = 0,
FT_LCD_FILTER_DEFAULT = 1,
FT_LCD_FILTER_LIGHT = 2,
FT_LCD_FILTER_LEGACY1 = 3,
FT_LCD_FILTER_LEGACY = 16,
FT_LCD_FILTER_MAX /* do not remove */
} FT_LcdFilter;
/**************************************************************************
*
* @func:
* FT_Library_SetLcdFilter
*
* @description:
* This function is used to apply color filtering to LCD decimated
* bitmaps, like the ones used when calling @FT_Render_Glyph with
* @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V.
*
* @input:
* library ::
* A handle to the target library instance.
*
* filter ::
* The filter type.
*
* You can use @FT_LCD_FILTER_NONE here to disable this feature, or
* @FT_LCD_FILTER_DEFAULT to use a default filter that should work
* well on most LCD screens.
*
* @return:
* FreeType error code. 0~means success.
*
* @note:
* This feature is always disabled by default. Clients must make an
* explicit call to this function with a `filter' value other than
* @FT_LCD_FILTER_NONE in order to enable it.
*
* Due to *PATENTS* covering subpixel rendering, this function doesn't
* do anything except returning `FT_Err_Unimplemented_Feature' if the
* configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
* defined in your build of the library, which should correspond to all
* default builds of FreeType.
*
* @since:
* 2.3.0
*/
FT_EXPORT( FT_Error )
FT_Library_SetLcdFilter( FT_Library library,
FT_LcdFilter filter );
/**************************************************************************
*
* @func:
* FT_Library_SetLcdFilterWeights
*
* @description:
* This function can be used to enable LCD filter with custom weights,
* instead of using presets in @FT_Library_SetLcdFilter.
*
* @input:
* library ::
* A handle to the target library instance.
*
* weights ::
* A pointer to an array; the function copies the first five bytes and
* uses them to specify the filter weights.
*
* @return:
* FreeType error code. 0~means success.
*
* @note:
* Due to *PATENTS* covering subpixel rendering, this function doesn't
* do anything except returning `FT_Err_Unimplemented_Feature' if the
* configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
* defined in your build of the library, which should correspond to all
* default builds of FreeType.
*
* LCD filter weights can also be set per face using @FT_Face_Properties
* with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS.
*
* @since:
* 2.4.0
*/
FT_EXPORT( FT_Error )
FT_Library_SetLcdFilterWeights( FT_Library library,
unsigned char *weights );
/*
* @constant:
* FT_PARAM_TAG_LCD_FILTER_WEIGHTS
*
* @description:
* An @FT_Parameter tag to be used with @FT_Face_Properties. The
* corresponding argument specifies the five LCD filter weights for a
* given face (if using @FT_LOAD_TARGET_LCD, for example), overriding
* the global default values or the values set up with
* @FT_Library_Set_LcdFilterWeights.
*
*/
#define FT_PARAM_TAG_LCD_FILTER_WEIGHTS \
FT_MAKE_TAG( 'l', 'c', 'd', 'f' )
/*
* @type:
* FT_LcdFiveTapFilter
*
* @description:
* A typedef for passing the five LCD filter weights to
* @FT_Face_Properties within an @FT_Parameter structure.
*
*/
#define FT_LCD_FILTER_FIVE_TAPS 5
typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS];
/* */
FT_END_HEADER
#endif /* FTLCDFIL_H_ */
/* END */