FreeType-2.5.4 API Reference

LCD Filtering

Synopsis

FT_LcdFilterFT_Library_SetLcdFilterWeights
FT_Library_SetLcdFilter

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.

FT_LcdFilter

Defined in FT_LCD_FILTER_H (ftlcdfil.h).

  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;

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


FT_Library_SetLcdFilter

Defined in FT_LCD_FILTER_H (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.

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_Library_SetLcdFilterWeights

Defined in FT_LCD_FILTER_H (ftlcdfil.h).

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

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