AviSynth is a free, open source frameserver: it will serve frames to any application that uses the Video for Windows or DirectShow interfaces. Like VirtualDub, AviSynth has a modular structure that allows the use of plug-ins; in fact, you can load VirtualDub filters in AviSynth! So why opt for it?
Well, the first versions of AviSynth were only able to open AVI files, but it has matured over the years and is now able to open any media file supported by DirectShow, and there are plug-ins for many file formats such as MPEG-2. Furthermore, the open source community has contributed to the adoption of AviSynth with the development of plug-ins for all sorts of uses, from de-interlacing to quality assessment. On the other hand, AviSynth is not as easy to use as VirtualDub; it incorporates a scripting language versus the intuitive graphical user interface of VirtualDub. In short, the main reasons to use AviSynth over VirtualDub are:
To process a video format that is not supported...