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freetype/include/ftlcdfil.h

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/***************************************************************************/
/* */
/* ftlcdfil.h */
/* */
/* FreeType API for color filtering of subpixel bitmap glyphs */
/* (specification). */
/* */
/* Copyright 2006-2008, 2010, 2013 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 __FT_LCD_FILTER_H__
#define __FT_LCD_FILTER_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 LCD-optimized bitmaps.
*
* @description:
* The @FT_Library_SetLcdFilter API can be used to specify a low-pass
* filter, which is then applied to LCD-optimized bitmaps generated
* through @FT_Render_Glyph. This is useful to reduce color fringes
* that would occur with unfiltered rendering.
*
* 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.
*
* FreeType generates alpha coverage maps, which are linear by nature.
* For instance, the value 0x80 in bitmap representation means that
* (within numerical precision) 0x80/0xff fraction of that pixel is
* covered by the glyph's outline. The blending function for placing
* text over a background is
*
* {
* dst = alpha * src + (1 - alpha) * dst ,
* }
*
* which is known as OVER. However, when calculating the output of the
* OVER operator, the source colors should first be transformed to a
* linear color space, then alpha blended in that space, and transformed
* back to the output color space.
*
* When linear light blending is used, the default FIR5 filtering
* weights (as given by FT_LCD_FILTER_DEFAULT) are no longer optimal, as
* they have been designed for black on white rendering while lacking
* gamma correction. To preserve color neutrality, weights for a FIR5
* filter should be chosen according to two free parameters `a' and `c',
* and the FIR weights should be
*
* {
* [a - c, a + c, 2 * a, a + c, a - c] .
* }
*
* This formula generates equal weights for all the color primaries
* across the filter kernel, which makes it colorless. One suggested
* set of weights is
*
* {
* [0x10, 0x50, 0x60, 0x50, 0x10] ,
* }
*
* where `a' has value 0x30 and `b' value 0x20. The weights in filter
* may have a sum larger than 0x100, which increases coloration slightly
* but also improves contrast.
*/
/****************************************************************************
*
* @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.
*
* FT_LCD_FILTER_LIGHT ::
* The light filter is a variant that produces less blurriness at the
* cost of slightly more color fringes than the default one. It might
* be better, depending on taste, your monitor, or your personal vision.
*
* 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.
*
* @since:
* 2.3.0
*/
typedef enum FT_LcdFilter_
{
FT_LCD_FILTER_NONE = 0,
FT_LCD_FILTER_DEFAULT = 1,
FT_LCD_FILTER_LIGHT = 2,
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.
*
* The filter affects glyph bitmaps rendered through @FT_Render_Glyph,
* @FT_Outline_Get_Bitmap, @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 larger or taller than the dimensions of the corresponding
* outline with regards to the pixel grid. For example, for
* @FT_RENDER_MODE_LCD, the filter adds up to 3~pixels to the left, and
* up to 3~pixels to the right.
*
* The bitmap offset values are adjusted correctly, so clients shouldn't
* need to modify their layout and glyph positioning code when enabling
* the filter.
*
* @since:
* 2.3.0
*/
FT_EXPORT( FT_Error )
FT_Library_SetLcdFilter( FT_Library library,
FT_LcdFilter filter );
/**************************************************************************
*
* @func:
* FT_Library_SetLcdFilterWeights
*
* @description:
* Use this function to override the filter weights selected by
* @FT_Library_SetLcdFilter. By default, FreeType uses the quintuple
* (0x00, 0x55, 0x56, 0x55, 0x00) for FT_LCD_FILTER_LIGHT, and (0x10,
* 0x40, 0x70, 0x40, 0x10) for FT_LCD_FILTER_DEFAULT and
* FT_LCD_FILTER_LEGACY.
*
* @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.
*
* This function must be called after @FT_Library_SetLcdFilter to have
* any effect.
*
* @since:
* 2.4.0
*/
FT_EXPORT( FT_Error )
FT_Library_SetLcdFilterWeights( FT_Library library,
unsigned char *weights );
/* */
FT_END_HEADER
#endif /* __FT_LCD_FILTER_H__ */
/* END */
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