FreeType-2.6.5 API Reference

LCD Filtering

Synopsis

FT_LcdFilterFT_Library_SetLcdFilterWeights
FT_Library_SetLcdFilter

Subpixel 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. Since these subpixels are color pixels, using them unfiltered creates severe color fringes. 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 sacrifices some of the higher resolution to reduce color fringes, making the glyph image slightly blurrier. Positional improvements will remain.

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 and explicitly call FT_Library_SetLcdFilter to enable it.

A filter should have two properties:

1) It should be normalized, meaning the sum of the 5 components should be 256 (0x100). It is possible to go above or under this target sum, however: going under means tossing out contrast, going over means invoking clamping and thereby non-linearities that increase contrast somewhat at the expense of greater distortion and color-fringing. Contrast is better enhanced through stem darkening.

2) It should be color-balanced, meaning a filter ‘{ a, b, c, b, a }’ where a + b = c. It distributes the computed coverage for one subpixel to all subpixels equally, sacrificing some won resolution but drastically reducing color-fringing. Positioning improvements remain! Note that color-fringing can only really be minimized when using a color-balanced filter and alpha-blending the glyph onto a surface in linear space; see FT_Render_Glyph.

Regarding the form, a filter can be a ‘boxy’ filter or a ‘beveled’ filter. Boxy filters are sharper but are less forgiving of non-ideal gamma curves of a screen (viewing angles!), beveled filters are fuzzier but more tolerant.

Examples:

- [0x10 0x40 0x70 0x40 0x10] is beveled and neither balanced nor normalized.

- [0x1A 0x33 0x4D 0x33 0x1A] is beveled and balanced but not normalized.

- [0x19 0x33 0x66 0x4c 0x19] is beveled and normalized but not balanced.

- [0x00 0x4c 0x66 0x4c 0x00] is boxily beveled and normalized but not balanced.

- [0x00 0x55 0x56 0x55 0x00] is boxy, normalized, and almost balanced.

- [0x08 0x4D 0x56 0x4D 0x08] is beveled, normalized and, almost balanced.

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 3 subpixels to the left, and 3 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.

It is important to understand that linear alpha blending and gamma correction is critical for correctly rendering glyphs onto surfaces without artifacts and even more critical when subpixel rendering is involved.

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.

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

The default filter reduces color fringes considerably, at the cost of a slight blurriness in the output.

It is a beveled, normalized, and color-balanced five-tap filter that is more forgiving to screens with non-ideal gamma curves and viewing angles. Note that while color-fringing is reduced, it can only be minimized by using linear alpha blending and gamma correction to render glyphs onto surfaces. The default filter weights are [0x08 0x4D 0x56 0x4D 0x08].

FT_LCD_FILTER_LIGHT

The light filter is a variant that is sharper at the cost of slightly more color fringes than the default one.

It is a boxy, normalized, and color-balanced three-tap filter that is less forgiving to screens with non-ideal gamma curves and viewing angles. This filter works best when the rendering system uses linear alpha blending and gamma correction to render glyphs onto surfaces. The light filter weights are [0x00 0x55 0x56 0x55 0x00].

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.

FT_LCD_FILTER_LEGACY1

For historical reasons, the FontConfig library returns a different enumeration value for legacy LCD filtering. To make code work that (incorrectly) forwards FontConfig's enumeration value to FT_Library_SetLcdFilter without proper mapping, it is thus easiest to have another enumeration value, which is completely equal to ‘FT_LCD_FILTER_LEGACY’.

since

2.3.0 (‘FT_LCD_FILTER_LEGACY1’ since 2.6.2)


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 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.

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.

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.

since

2.4.0