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== AviSynth and its plugin api's ==

AviSynth exists as an instance of the ScriptEnvironment class, that implements the IScriptEnvironment interface. The IScriptEnvironment interface is defined in avisynth.h (and avisynth_c.h) and it is the only way for plugins and external applications to communicate with AviSynth. A pointer to ScriptEnvironment object is passed along to all plugins, so that they can use AviSynth facilities. Plugins are forbidden from storing this pointer. AviSynth creates one ScriptEnvironment object when Windows attempts to use AviSynth to open a script via AVIFile API.

When ScriptEnvironment is created, it checks for CPU extensions (to provide this information to filters that are able to use CPU extensions), sets up memory limits for itself and performs pre-scanning of all plugins.

AviSynth has the capability to load third-party libraries that include their own video and audio filters. It comes with two language interfaces (or plugin api's):

* C++ API (through avisynth.h) - The classes and miscellaneous constants are described in [[Filter_SDK/Cplusplus_API|here]].
* C API (through avisynth_c.h) - The classes and miscellaneous constants are described in [[Filter_SDK/C_API|here]].

The built-in filters use the C++ API. This Filter SDK (or Source Development Kit) describes how to create plugins using both interfaces. 

Although not included in AviSynth itself, several people wrote other language interfaces in Delphi, Purebasic, NET and Java. They can be found [http://forum.doom9.org/showthread.php?p=566904#post566904 here].