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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 1/256 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 1/256 units.
FT_LCD_FILTER_LEGACY |
FT_LCD_FILTER_LEGACY1 | The legacy libXft color filter is no longer supported and ignored.

#### since

2.3.0

## 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

## 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 1/256 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

## 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

## 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