VSFilter

From Aegisub Wiki

VSFilter (presumably meaning VobSub Filter) is gabest's subtitle rendering filter for Windows. VSFilter was formerly known as DirectVobSub, and is also referred to as TextSub. VSFilter is GPL licensed.

VSFilter is a three-in-one library, it can work as either a DirectShow filter, an AviSynth filter or a VirtualDub filter.

The most important feature of VSFilter is its ASS renderer, which is the only complete implementation, and is also the de-facto reference implementation.

VSFilter also has an incomplete USF support (works by transforming USF into ASS), and current SVN revisions have the only implementation of SSF.