Unfortunately, freetype does not seem to render directly to a distance field as of version 2.9 (check https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#FT_Render_Mode)

You could calculate the distance from the alpha value of the freetype generated bitmaps using something like this http://weber.itn.liu.se/~stegu/aadist/edtaa_preprint.pdf

Unfortunately, freetype does not seem to render directly to a distance field as of version 2.9 (check the render modes listed here)

You could calculate the distance from the alpha value of the freetype generated bitmaps using something like the techniques described in the paper "Anti-aliased Euclidean distance transform"