The script execution model/Sequence of events

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The sequence of events that occur when you execute (ie load and render to your favorite encoder) an AviSynth script is presented below. The sequence is conceptually divided into three major phases:

  1. Initialisation
  2. Script loading and parsing phase
  3. The video frames serving phase

The initialisation phase

During this phase the following events occur:

  • An AVI stream object is created by the operating system, using the handler code provided by AviSynth.
  • The AviSynth dll is loaded and performs (among other initialization actions) auto-loading of the following files that are placed in the AviSynth autoload folder:
    • All AviSynth plugins (files with .dll extension).
    • All AviSynth include scripts (files with .avsi extension).
During auto-loading:
  • The plugin and script functions are registered in the internal data structures of AviSynth; all plugins are unloaded after this phase. Only if a function is actually used inside the script, the plugin is loaded again.
  • Avisynth include scripts are parsed (executed), as if they where all included at the top of the script source code with Import statements.

The script loading and parsing phase

During this phase the following events occur:

  • The host video application (the editor or the encoder) requests a frame from the AVI stream object (usually frame 0, but you should not count on this always being true).
  • The script is loaded with a call to the Import internal function and its contents are passed to the Eval() control function for parsing.
  • The entire parse process takes place. Expressions are evaluated, variables are set and clips are created and chained together to form the script's filter graph (see next section).
  • At this point, just after the end of the parse process:
    • The filter graph has been formed.
    • Global and script-level variables have attained their final values (for the parsing phase) and, along with all functions defined in the script or imported in the script-level scope, are available for use and possible modification by the runtime scripts.

The video frames serving phase

During this phase the following events occur:

  • The initially requested frame is delivered to the host application. This forces runtime scripts that are possibly included in the source code to be parsed (executed) for that frame.
  • The host video application requests additional frames. Each frame is delivered, possibly forcing runtime scripts that are in the filter chain for that frame to be parsed (executed).

Note: since runtime scripts may be anywhere in the filter chain and frames may be shuffled (AviSynth is an NLE after all!) the statement "for that frame" should be interpreted as "for that frame of the final clip and any linked frames of intermediate clips".

Back to the script execution model.

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