@ -36,28 +36,29 @@ FT_BEGIN_HEADER
/***************************************************************************
*
* @ section :
* lcd_filt ering
* lcd_rend ering
*
* @ title :
* LCD Filt ering
* Subpixel Rend ering
*
* @ abstract :
* Remove color fringes of subpixel - rendered bitmaps .
* API to control subpixel rendering .
*
* @ description :
* FreeType provides two alternative subpixel rendering technologies .
* 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 .
* ` ftoption . h ' , this enables patented ClearType - style rendering .
* Otherwise , Harmony LCD rendering is enabled . These technologies are
* controlled differently and API described below , although always
* available , performs its function when appropriate method is enabled
* and does nothing otherwise .
*
* 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 component smust be dimmed equally .
* black - on - white text , the nearby color subpixel smust be dimmed equally .
*
* A good 5 - tap FIR filter should be applied to subpixel coverages
* regardless of pixel boundaries and should have these properties :
@ -72,31 +73,54 @@ FT_BEGIN_HEADER
* 3 ) It should be normalized , meaning 2 a ~ + 2 b ~ + 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
* The distribution of density values by the color - balanced filter assumes
* alpha blending is done in linear space ; only then color artifacts
* cancel out . 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 .
*
* Use the @ FT_Library_SetLcdFilter or @ FT_Library_SetLcdFilterWeights
* API to specify a low - pass filter , which is then applied to
* subpixel - rendered bitmaps generated through @ FT_Render_Glyph .
*
* Harmony LCD rendering is suitable to panels with any regular subpixel
* structure , not just monitors with 3 color striped subpixels , as long as
* the color subpixels have fixed positions relative to the pixel center .
* In this case , each color channel is then rendered separately after
* shifting the outline opposite to the subpixel shift so that the
* coverage maps are aligned . This method is immune to color fringes
* because the shifts do not change integral coverage .
*
* The subpixel geometry must be specified by xy - coordinates for each
* subpixel . By convention they may come in the RGB order :
*
* { { - 1 / 3 , 0 } , { 0 , 0 } , { 1 / 3 , 0 } } for hRGB striped panel ( default ) ,
* { { 1 / 3 , 0 } , { 0 , 0 } , { - 1 / 3 , 0 } } for hBGR striped panel ,
* { { 0 , 1 / 3 } , { 0 , 0 } , { 0 , - 1 / 3 } } for vRGB striped panel ,
* { { - 1 / 6 , 1 / 4 } , { - 1 / 6 , - 1 / 4 } , { 1 / 3 , 0 } } for a certain PenTile panel .
*
* Use the @ FT_Library_SetLcdGeometry API to specify subpixel positions .
* If one follows the RGB order convention , the same order applies
* to the resulting @ FT_PIXEL_MODE_LCD and @ FT_PIXEL_MODE_LCD_V bitmaps .
* Note , however , that the coordinate frame for the latter must be rotated
* clockwise . Harmony with default LCD geometry is equivalent to
* ClearType with light filter .
*
* As a result of ClearType filtering or Harmony rendering , the dimensions
* of LCD bitmaps can be 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 .
* The ClearType and Harmony rendering is applicable to glyph bitmaps
* rendered through @ FT_Render_Glyph , @ FT_Load_Glyph , and @ FT_Load_Char .
* @ FT_RENDER_MODE_LCD or @ FT_RENDER_MODE_LCD_V and these technologies do
* not apply to @ FT_Outline_Render and @ FT_Outline_Get_Bitmap directly .
* Each of the 3 ~ alpha values ( subpixels ) must by 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 .
*/
@ -247,6 +271,53 @@ FT_BEGIN_HEADER
typedef FT_Byte FT_LcdFiveTapFilter [ FT_LCD_FILTER_FIVE_TAPS ] ;
/**************************************************************************
*
* @ func :
* FT_Library_SetLcdGeometry
*
* @ description :
* This function can be used to modify default positions of color
* subpixels , which controls Harmony LCD rendering .
*
* @ input :
* library : :
* A handle to the target library instance .
*
* sub : :
* A pointer to an array of 3 vectors in 26.6 fractional pixel format ;
* the function modifies the default values , see the note below .
*
* @ return :
* FreeType error code . 0 ~ means success .
*
* @ note :
* Subpixel geometry examples :
*
* - { { - 21 , 0 } , { 0 , 0 } , { 21 , 0 } } is the default , corresponding 3 color
* stripes shifted by a third of a pixel . This could be an RGB panel .
*
* - { { 21 , 0 } , { 0 , 0 } , { - 21 , 0 } } looks the same as the default but
* can specify a BGR panel instead , while keeping the bitmap in the same
* RGB888 format .
*
* - { { 0 , 21 } , { 0 , 0 } , { 0 , - 21 } } is the vertical RGB , but the bitmap
* stays RGB888 as a result .
*
* - { { - 11 , 16 } , { - 11 , - 16 } , { 22 , 0 } } is a certain PenTile arrangement .
*
* This function does nothing and returns ` FT_Err_Unimplemented_Feature '
* in the context of ClearType - style subpixel rendering when
* FT_CONFIG_OPTION_SUBPIXEL_RENDERING is defined in your build of the
* library .
*
* @ since :
* 2.10 .0
*/
FT_EXPORT ( FT_Error )
FT_Library_SetLcdGeometry ( FT_Library library ,
FT_Vector sub [ 3 ] ) ;
/* */
Alexei Podtelezhnikov
7 years ago