ref: 02842872deb202f36cc473f790e3a555adb2c987
dir: /include/freetype/ftlcdfil.h/
/***************************************************************************/
/* */
/* ftlcdfil.h */
/* */
/* FreeType API for color filtering of subpixel bitmap glyphs */
/* (specification). */
/* */
/* Copyright 2006-2018 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
#include FT_PARAMETER_TAGS_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:
* Remove color fringes of subpixel-rendered bitmaps.
*
* @description:
* Should you #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your
* `ftoption.h', which enables patented ClearType-style rendering,
* the LCD-optimized glyph bitmaps should be filtered to reduce color
* fringes inherent to this technology. The default FreeType LCD
* rendering uses different technology, and API described below,
* although available, does nothing.
*
* ClearType-style LCD 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. Using the
* subpixels coverages unfiltered can create severe color fringes
* especially when rendering thin features. Indeed, to produce
* black-on-white text, the color components must be dimmed equally.
*
* A good 5-tap FIR filter should be applied to subpixel coverages
* regardless of pixel boundaries and should have these properties:
*
* 1) It should be symmetrical, like {~a, b, c, b, a~}, to avoid
* any shifts in appearance.
*
* 2) It should be color-balanced, meaning a~+ b~=~c, to reduce color
* fringes by distributing the computed coverage for one subpixel to
* all subpixels equally.
*
* 3) It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain
* overall brightness.
*
* 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 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 2~subpixels to the left, and
* 2~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.
*
* Only filtering along with gamma-corrected alpha blending can completely
* remove color fringes. Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is
* sharper but is less forgiving of non-ideal gamma curves of a screen
* (viewing angles!), beveled filters are fuzzier but more tolerant.
*
* 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 ::
* This is a beveled, normalized, and color-balanced five-tap filter
* with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256th units.
*
* FT_LCD_FILTER_LIGHT ::
* this is a boxy, normalized, and color-balanced three-tap filter
* with weights of [0x00 0x55 0x56 0x55 0x00] in 1/256th units.
*
* FT_LCD_FILTER_LEGACY ::
* FT_LCD_FILTER_LEGACY1 ::
* 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.
* This filter is only provided for comparison purposes, and might be
* disabled or stay unsupported in the future. The second value is
* provided for compatibility with FontConfig, which historically
* used different enumeration, sometimes incorrectly forwarded to
* FreeType.
*
* @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 in 1/256th units.
*
* @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 );
/*
* @type:
* FT_LcdFiveTapFilter
*
* @description:
* A typedef for passing the five LCD filter weights to
* @FT_Face_Properties within an @FT_Parameter structure.
*
* @since:
* 2.8
*
*/
#define FT_LCD_FILTER_FIVE_TAPS 5
typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS];
/* */
FT_END_HEADER
#endif /* FTLCDFIL_H_ */
/* END */