If you are looking to implement this filter in a legacy setup, guides like MKVMania on explain how to force the filter by adding it as a "Preferred" External Filter within players like Media Player Classic.
: The author highlights that subtitles look significantly sharper when the ffdshow resize filter is activated to match desktop resolution, provided the resize filter is positioned above the subtitle filter in the processing chain. Ffdshow Subtitle Filter
: For those needing reliability over raw sharpness, the post suggests the MPC-HC internal subtitle engine as the superior middle ground between the basic DirectVobSub and the high-resolution but unstable ffdshow engine. Technical Setup Context If you are looking to implement this filter