Runtime environment

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The runtime environment is an extension to the normal AviSynth script execution environment that is available to scripts executed by runtime filters. Its basic characteristic is that the runtime filters' scripts are evaluated (executed) at every frame. This allows for complex video processing that it would be difficult or impossible to perform in a normal AviSynth script.

Let's now look at the above definition in more detail. The runtime environment is:

  1. An environment, that is a set of available local and global variables and filters / functions to be used by the script.
  2. An extension to the normal AviSynth script execution environment; that is there are additional variables and functions available to runtime scripts.
  3. Available to scripts executed by runtime filters only, that is scripts inside it are executed only during the AviSynth "runtime" (the frame serving phase).

The last is the biggest difference. Normal script code is parsed and evaluated (executed) at the start of the script's execution (the parsing phase) in a linear fashion, from top to bottom; the result is the creation of a filter graph that is used by AviSynth to serve frames to the host video application. Runtime script code is executed after the parsing phase, when frames are served. Moreover it is executed on every frame requested and only for that specific frame, in an event-driven fashion.

Note: Audio is not handled by the runtime environment; it is passed through untouched. This also means that you cannot modify clip audio with runtime filters.

Runtime filters

The following filters are the basic set of the so-called "runtime filters":

  • ConditionalFilter: Selects, on every frame, from one of two (provided) clips based on the evaluation of a condition.
  • ScriptClip: Executes arbitrary script code on every frame and returns a clip.
  • FrameEvaluate: Executes arbitrary script code on every frame but the script's output is ignored.
  • ConditionalReader: Loads input from an external file to a selectable variable on every frame.

In addition, the WriteFile filter can also be considered a runtime filter, because it sets the special variables set by all runtime filters before evaluating the expressions passed to it, which can use the special runtime functions.

Special runtime variables and functions

All runtime filters set and make available to runtime scripts the following special variables.

  • last: The clip passed as argument to the filter
  • current_frame: The frame number (ranging from zero to input clip's Framecount minus one) of the requested frame.

All the above variables are defined at the top-level script local scope. That is you read and write to them in a runtime script as if they where variables that you declare at the script level.

Runtime scripts can also call a rich set of runtime functions that provide various pieces of information for the current frame of their input clip - for example, AverageLuma, YDifferenceFromPrevious, YPlaneMin and YPlaneMinMaxDifference.

How to script inside the runtime environment

Using the runtime environment is simple: you use one of the runtime filters and supply it with the needed arguments. Among them, two are the most important:

  • The input clip
  • The runtime script

The latter is supplied as a string argument which contains the AviSynth script commands. Scripts can contain many statements (you can use a multiline string), local and global variables and function calls, as well as special runtime functions and variables. In general all statements of the AviSynth syntax are permitted, but attention must be paid to avoid overly complex scripts because this has a penalty on speed that is paid at every frame. See the runtime section of scripting reference's performance considerations for details.



  • Runtime functions and variables, such as current_frame, AverageLuma(), etc., are available only at the runtime script's scope. To use them in a function you must pass them as arguments.
  • You can include an explicit return statement in your runtime script to return a clip that is not contained in the special last variable.
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