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 blog praises the speed and visual quality of ffdshow's rendering but warns that the filter is not feature-complete . It often "chokes" on complex subtitles (like those found in advanced anime fansubs) that require precise positioning and effects. Ffdshow Subtitle Filter
Are you looking to or just researching the history of video codecs? If you are looking to implement this filter
While the post is older, it offers a technical comparison of subtitle rendering quality that remains a classic reference for media enthusiast communities. Ffdshow Subtitle Filter