forked from minhngoc25a/freetype2
258 lines
9.4 KiB
C
258 lines
9.4 KiB
C
/***************************************************************************/
|
|
/* */
|
|
/* 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 */
|