The documentation included with AviSynth provides this answer to "What is AviSynth?"
AviSynth (AVI SYNTHesizer) is a frame server. This means that it serves up video, frame by frame, to another video application. The core of AviSynth’s power is scripting. Scripts are simple text files that contain a set of commands referencing one or more videos, and the effects you want to run on them.
When you open these script files in VirtualDub or another video application, AviSynth takes action. AviSynth performs the commands in the script, and sends the resulting video—frame by frame—to the video application. The video application has no idea what AviSynth is doing. As far as the application is concerned, it’s opened a standard AVI file with all the filters and other commands already applied.
There are five main reasons why people would want to use AviSynth:
1. AviSynth can join videos together on the fly, or otherwise combine media from two sources (for example dubbing a new soundtrack).
2. AviSynth has a lot of support for filtering video, including built-in filters for resizing, cropping, and sharpening.
3. Many programs do not support video files larger than 2GB. By using AviSynth as the ‘middle-man’ between your video application and the video file, you can break the 2GB barrier.
4. AviSynth can make just about any type of video look like a normal AVI, including MPEGs and some Quicktime MOVs. Therefore, AviSynth is an easy way to get applications to open unsupported video formats.
5. AviSynth scripts take up only a small amount of disc space, yet can seem like completely new videos. Because the filtering and joining take place on the fly, there is no need to waste disk space on temporary files or intermediate videos.
As VirtualDub users, our main interest with AviSynth will be applying filters. But the disk space savings are also a handy feature for the heavy VirtualDubber.
Let’s move on to downloading and installing these three software packages from the world of VirtualDub. We’ll do VirtualDub first, followed by VDubMod and AviSynth.