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.