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.