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Subpixel Rendering

Synopsis

FreeType provides two alternative subpixel rendering technologies. Should you define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your ftoption.h file, this enables 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 subpixel coverages unfiltered can create severe color fringes especially when rendering thin features. Indeed, to produce black-on-white text, the nearby color subpixels must be dimmed evenly. Therefore, an equalizing 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.

Boxy 3-tap filter {0, ⅓, ⅓, ⅓, 0} is sharper but is less forgiving of non-ideal gamma curves of a screen (and 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 can be 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: {{-⅓, 0}, {0, 0}, {⅓, 0}} for standard RGB striped panel or {{-⅙, ¼}, {-⅙, -¼}, {⅓, 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 shifts, the resulting dimensions of LCD bitmaps can be slightly wider or taller than the dimensions the original 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.

The ClearType and Harmony rendering is applicable to glyph bitmaps rendered through FT_Render_Glyph, FT_Load_Glyph, FT_Load_Char, and FT_Glyph_To_Bitmap, when FT_RENDER_MODE_LCD or FT_RENDER_MODE_LCD_V is specified. This API does not control FT_Outline_Render and FT_Outline_Get_Bitmap.

The described algorithms can completely remove color artefacts when combined with gamma-corrected alpha blending in linear space. 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.

FT_LcdFilter

Defined in FT_LCD_FILTER_H (freetype/ftlcdfil.h).

  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;

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)

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FT_Library_SetLcdFilter

Defined in FT_LCD_FILTER_H (freetype/ftlcdfil.h).

  FT_EXPORT( FT_Error )
  FT_Library_SetLcdFilter( FT_Library    library,
                           FT_LcdFilter  filter );

This function is used to change filter applied 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

Since 2.10.3 the LCD filtering is enabled with FT_LCD_FILTER_DEFAULT. It is no longer necessary to call this function explicitly except to choose a different filter or disable filtering altogether with FT_LCD_FILTER_NONE.

This function does nothing but returns FT_Err_Unimplemented_Feature if the configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not defined in your build of the library.

since

2.3.0

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FT_Library_SetLcdFilterWeights

Defined in FT_LCD_FILTER_H (freetype/ftlcdfil.h).

  FT_EXPORT( FT_Error )
  FT_Library_SetLcdFilterWeights( FT_Library      library,
                                  unsigned char  *weights );

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

This function does nothing but returns FT_Err_Unimplemented_Feature if the configuration macro FT_CONFIG_OPTION_SUBPIXEL_RENDERING is not defined in your build of the library.

LCD filter weights can also be set per face using FT_Face_Properties with FT_PARAM_TAG_LCD_FILTER_WEIGHTS.

since

2.4.0

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FT_LcdFiveTapFilter

Defined in FT_LCD_FILTER_H (freetype/ftlcdfil.h).

#define FT_LCD_FILTER_FIVE_TAPS  5

  typedef FT_Byte  FT_LcdFiveTapFilter[FT_LCD_FILTER_FIVE_TAPS];

A typedef for passing the five LCD filter weights to FT_Face_Properties within an FT_Parameter structure.

since

2.8

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FT_Library_SetLcdGeometry

Defined in FT_LCD_FILTER_H (freetype/ftlcdfil.h).

  FT_EXPORT( FT_Error )
  FT_Library_SetLcdGeometry( FT_Library  library,
                             FT_Vector   sub[3] );

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 to 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

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