259 lines
8.8 KiB
C
259 lines
8.8 KiB
C
/****************************************************************************
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*
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* ftlcdfil.h
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*
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* FreeType API for color filtering of subpixel bitmap glyphs
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* (specification).
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*
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* Copyright 2006-2018 by
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* David Turner, Robert Wilhelm, and Werner Lemberg.
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*
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* This file is part of the FreeType project, and may only be used,
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* modified, and distributed under the terms of the FreeType project
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* license, LICENSE.TXT. By continuing to use, modify, or distribute
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* this file you indicate that you have read the license and
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* understand and accept it fully.
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*
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*/
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#ifndef FTLCDFIL_H_
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#define FTLCDFIL_H_
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#include <ft2build.h>
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#include FT_FREETYPE_H
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#include FT_PARAMETER_TAGS_H
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#ifdef FREETYPE_H
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#error "freetype.h of FreeType 1 has been loaded!"
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#error "Please fix the directory search order for header files"
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#error "so that freetype.h of FreeType 2 is found first."
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#endif
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FT_BEGIN_HEADER
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/***************************************************************************
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*
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* @section:
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* lcd_filtering
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*
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* @title:
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* LCD Filtering
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*
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* @abstract:
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* Remove color fringes of subpixel-rendered bitmaps.
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*
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* @description:
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* Should you #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your
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* `ftoption.h', which enables patented ClearType-style rendering,
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* the LCD-optimized glyph bitmaps should be filtered to reduce color
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* fringes inherent to this technology. The default FreeType LCD
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* rendering uses different technology, and API described below,
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* although available, does nothing.
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*
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* ClearType-style LCD rendering exploits the color-striped structure of
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* LCD pixels, increasing the available resolution in the direction of
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* the stripe (usually horizontal RGB) by a factor of~3. Using the
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* subpixels coverages unfiltered can create severe color fringes
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* especially when rendering thin features. Indeed, to produce
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* black-on-white text, the color components must be dimmed equally.
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*
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* A good 5-tap FIR filter should be applied to subpixel coverages
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* regardless of pixel boundaries and should have these properties:
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*
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* 1) It should be symmetrical, like {~a, b, c, b, a~}, to avoid
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* any shifts in appearance.
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*
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* 2) It should be color-balanced, meaning a~+ b~=~c, to reduce color
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* fringes by distributing the computed coverage for one subpixel to
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* all subpixels equally.
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*
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* 3) It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain
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* overall brightness.
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*
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* Use the @FT_Library_SetLcdFilter API to specify a low-pass filter,
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* which is then applied to subpixel-rendered bitmaps generated through
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* @FT_Render_Glyph. The filter affects glyph bitmaps rendered through
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* @FT_Render_Glyph, @FT_Load_Glyph, and @FT_Load_Char. It does _not_
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* affect the output of @FT_Outline_Render and @FT_Outline_Get_Bitmap.
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*
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* If this feature is activated, the dimensions of LCD glyph bitmaps are
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* either wider or taller than the dimensions of the corresponding
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* outline with regard to the pixel grid. For example, for
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* @FT_RENDER_MODE_LCD, the filter adds 2~subpixels to the left, and
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* 2~subpixels to the right. The bitmap offset values are adjusted
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* accordingly, so clients shouldn't need to modify their layout and
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* glyph positioning code when enabling the filter.
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*
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* Only filtering along with gamma-corrected alpha blending can completely
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* remove color fringes. Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is
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* sharper but is less forgiving of non-ideal gamma curves of a screen
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* (viewing angles!), beveled filters are fuzzier but more tolerant.
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*
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* Each of the 3~alpha values (subpixels) is independently used to blend
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* one color channel. That is, red alpha blends the red channel of the
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* text color with the red channel of the background pixel. The
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* distribution of density values by the color-balanced filter assumes
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* alpha blending is done in linear space; only then color artifacts
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* cancel out.
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*/
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/****************************************************************************
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*
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* @enum:
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* FT_LcdFilter
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*
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* @description:
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* A list of values to identify various types of LCD filters.
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*
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* @values:
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* FT_LCD_FILTER_NONE ::
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* Do not perform filtering. When used with subpixel rendering, this
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* results in sometimes severe color fringes.
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*
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* FT_LCD_FILTER_DEFAULT ::
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* This is a beveled, normalized, and color-balanced five-tap filter
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* with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256th units.
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*
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* FT_LCD_FILTER_LIGHT ::
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* this is a boxy, normalized, and color-balanced three-tap filter
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* with weights of [0x00 0x55 0x56 0x55 0x00] in 1/256th units.
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*
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* FT_LCD_FILTER_LEGACY ::
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* FT_LCD_FILTER_LEGACY1 ::
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* This filter corresponds to the original libXft color filter. It
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* provides high contrast output but can exhibit really bad color
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* fringes if glyphs are not extremely well hinted to the pixel grid.
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* This filter is only provided for comparison purposes, and might be
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* disabled or stay unsupported in the future. The second value is
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* provided for compatibility with FontConfig, which historically
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* used different enumeration, sometimes incorrectly forwarded to
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* FreeType.
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*
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* @since:
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* 2.3.0 (`FT_LCD_FILTER_LEGACY1' since 2.6.2)
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*/
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typedef enum FT_LcdFilter_
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{
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FT_LCD_FILTER_NONE = 0,
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FT_LCD_FILTER_DEFAULT = 1,
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FT_LCD_FILTER_LIGHT = 2,
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FT_LCD_FILTER_LEGACY1 = 3,
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FT_LCD_FILTER_LEGACY = 16,
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FT_LCD_FILTER_MAX /* do not remove */
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} FT_LcdFilter;
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/**************************************************************************
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*
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* @func:
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* FT_Library_SetLcdFilter
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*
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* @description:
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* This function is used to apply color filtering to LCD decimated
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* bitmaps, like the ones used when calling @FT_Render_Glyph with
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* @FT_RENDER_MODE_LCD or @FT_RENDER_MODE_LCD_V.
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*
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* @input:
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* library ::
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* A handle to the target library instance.
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*
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* filter ::
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* The filter type.
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*
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* You can use @FT_LCD_FILTER_NONE here to disable this feature, or
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* @FT_LCD_FILTER_DEFAULT to use a default filter that should work
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* well on most LCD screens.
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*
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* @return:
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* FreeType error code. 0~means success.
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*
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* @note:
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* This feature is always disabled by default. Clients must make an
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* explicit call to this function with a `filter' value other than
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* @FT_LCD_FILTER_NONE in order to enable it.
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*
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* Due to *PATENTS* covering subpixel rendering, this function doesn't
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* do anything except returning `FT_Err_Unimplemented_Feature' if the
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* configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
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* defined in your build of the library, which should correspond to all
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* default builds of FreeType.
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*
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* @since:
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* 2.3.0
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*/
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FT_EXPORT( FT_Error )
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FT_Library_SetLcdFilter( FT_Library library,
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FT_LcdFilter filter );
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/**************************************************************************
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*
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* @func:
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* FT_Library_SetLcdFilterWeights
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*
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* @description:
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* This function can be used to enable LCD filter with custom weights,
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* instead of using presets in @FT_Library_SetLcdFilter.
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*
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* @input:
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* library ::
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* A handle to the target library instance.
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*
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* weights ::
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* A pointer to an array; the function copies the first five bytes and
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* uses them to specify the filter weights in 1/256th units.
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*
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* @return:
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* FreeType error code. 0~means success.
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*
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* @note:
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* Due to *PATENTS* covering subpixel rendering, this function doesn't
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* do anything except returning `FT_Err_Unimplemented_Feature' if the
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* configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not
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* defined in your build of the library, which should correspond to all
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* default builds of FreeType.
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*
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* LCD filter weights can also be set per face using @FT_Face_Properties
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* with @FT_PARAM_TAG_LCD_FILTER_WEIGHTS.
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*
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* @since:
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* 2.4.0
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*/
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FT_EXPORT( FT_Error )
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FT_Library_SetLcdFilterWeights( FT_Library library,
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unsigned char *weights );
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/**************************************************************************
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*
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* @type:
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* FT_LcdFiveTapFilter
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*
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* @description:
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* A typedef for passing the five LCD filter weights to
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* @FT_Face_Properties within an @FT_Parameter structure.
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*
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* @since:
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* 2.8
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*
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*/
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#define FT_LCD_FILTER_FIVE_TAPS 5
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typedef FT_Byte FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS];
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/* */
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FT_END_HEADER
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#endif /* FTLCDFIL_H_ */
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/* END */
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