From
Avisynth documentation:
Basically,
AviSynth works like this: First, you create a simple text document with special commands, called a script. These commands make references to one or more videos and the filters you wish to run on them. Then, you run a video application, such as VirtualDub
?, and open the script file. This is when
AviSynth takes action. It opens the videos you referenced in the script, runs the specified filters, and feeds the output to video application. The application, however, is not aware that
AviSynth is working in the background. Instead, the application thinks that it is directly opening a filtered AVI file that resides on your hard drive.
ffdshow uses Avisynth precise colorspace conversion routines and few image processing filters. Additionally it is possible to write simple Avisynth scripts directly inside ffdshow.
ffdshow distribution contains makeAVIS
? utility to produce "fake" AVI files from Avisynth scripts to use them in application which don't support avs files directly.