The script execution model/Evaluation of runtime scripts
Evaluation of runtime scripts starts, as already stated, at the frame serving phase of the main script's execution. At that point frames of the final output clip are requested by the host video application. This triggers a sequence of successive calls to the GetFrame / GetAudio methods of all filters along the filter graph. Whenever one of those filters is a runtime filter, the following three-phase sequence of events happens in every frame:
- Runtime environment initialisation.
- Runtime script parsing and evaluation.
- Runtime environment cleanup and delivery of the resulting frame.
The following paragraphs examine each phase in more detail.
 Runtime environment initialisation
The runtime filter code sets (at the top-level script local scope) its special variables for the runtime script. These at the minimum include last, which is set to the filter's source clip and current_frame, which is set to the frame number requested by the filter from the AviSynth code.
As a consequence, those special variables cannot be passed between runtime scripts; whatever value the passing script will set, it will be overwritten by the receiving filter's frame initialisation code.
 Runtime script parsing and evaluation
The runtime script is parsed, as a regular script would be parsed if loaded in AviSynth. The parsing mechanism is the same. Thus everything allowed to a regular script is allowed to a runtime script; what changes is the context of execution. For example, you can:
- Use multi-line scripts; they just have to be contained inside a three-double-quotes pair (this is a requirement only if string literals are used inside the script, else single double quotes can be used also).
- Define / assign variables, both local and global.
- Import other scripts and/or load plugins and/or define functions.
- Call functions and filters.
- Use arrays and block statements.
- Use control structures.
Of course, some of the above are not advisable, because the different execution context poses different constraints regarding performance and resource usage. The main rule of thumb here is: Parsing occurs in every frame requested. Therefore, computationally expensive actions should be avoided. More on the performance considerations section.
 Runtime environment cleanup and delivery of the resulting frame
The runtime filter code receives the result of script parsing and evaluation. If all went well, the result will be a valid filter graph (the runtime filter graph) from which the runtime filter requests to fetch the needed frame. If not, the filter will propagate the error to the caller. When the filter's code will return the final video frame, the runtime filter graph will be destroyed. As part of the cleanup the runtime filter code also restores the last special variable to its previous value.
 The runtime environment in detail
Despite the very thin layer of added features (just a handful of variables and functions) the runtime environment is much more dynamic than the normal (main) script environment. The key-difference is the event-driven model of runtime script execution as opposed to the linear flow of the main script's execution. Execution of a runtime script occurs only in the event of a frame request. In addition, since intermediate filters in the chain may shuffle and combine frames in an arbitrary fashion, the requested frame's number may be different than the final clip's frame number.
 Elements of the runtime environment
At any time during the frame serving phase, the elements of the runtime environment are the following:
- The environment inherited by the main script's parsing phase, that is the main script's top-level local variables, the global variables and all imported script and plugin functions.
- The special variables set on every frame by the runtime filter initialisation code (last, current_frame, etc.)
- A set of runtime functions to assist common information extraction operations.
- The environment created by the successive evaluation of runtime scripts triggered by all the final output clip's frames that have been requested so far by the host video application. This may include modifications to the environment inherited by the parsing phase such as change of variables' values, as well as addition of new locals and globals.
 Runtime functions and current_frame
An interesting feature of runtime functions is that they consult the value of the current_frame special variable in order to determine what frame of their input clip(s) to inspect for extracting information. This provides the ability inside a runtime script to easily request information for any frame of a clip by changing before the call to the function the current_frame variable.
As explained above, setting the current_frame variable has no effect on other runtime scripts in the filter chain because the runtime filters' initialisation code resets current_frame to the proper value before executing the runtime script. It also has no effect on subsequent filter calls in the runtime script. But it does have on runtime functions and anywhere the value of current_frame is used. Therefore, after such a usage it is good practice to restore the variable to its initial value before issuing other script commands.
A skeleton example of a runtime script that computes a weighted second order luma interpolation value (the actions after the computation are omitted) follows:
...previous processing omitted... ScriptClip(""" # this is a multiline string n = current_frame lm_k = AverageLuma() current_frame = n - 1 lm_km1 = AverageLuma() current_frame = n + 1 lm_kp1 = AverageLuma() dvg = (lm_km1 - 2 * lm_k + lm_kp1) / 2 lm_ipl = lm_k + Sign(dvg) * Sqrt(Abs(dvg)) current_frame = n # remember to reset current_frame ...rest of script omitted... """) ...subsequent processing omitted...
 Checklist for developing correct runtime scripts
Despite the dynamic nature of the runtime environment, creating runtime scripts is relatively easy if you follow a simple set of rules:
- Remember that your input (source) clip is stored upon start of script execution in the last special variable.
- If you assign temporary clips to variables, remember to set last at the end or issue a return statement.
- Do not change (with respect to the source clip of the filter) the dimensions, color space or framerate of the final result.
- Do not assume - unless there is a very compeling performance-related reason- a particular ordering of frame requests; try to build ordering-neutral scripts, that depend only on current_frame.
- Inspect the names of all input variables (ie those variables that are not initialised to a value inside the runtime script) of your runtime scripts to ensure that they are not overridden accidentally by a normal, not used for inter-script communication variable in any runtime script along the chain.
- In particular, avoid putting inside functions calls to runtime filters that share state between invocations or with other filters through variables; it is easy to forget that you may only call the function once, else you will end up with multiple filters that share the same variables, thus with a bug in your script.
- In view of the above, runtime filters should be used in functions only if they either:
- do not share state between invocations or with other filters through variables, or
- the function code takes care to create unique names of all the runtime script's shared variables on each function invocation.
- A way to avoid variables is to dynamically build the runtime script using string concatenation and assign the related arguments' values to local variables in the runtime script. See the example code of the bracket_luma function.
Back to the script execution model.