## AviSynth Syntax

All basic AviSynth scripting statements have one of these forms:

1. variable_name = expression
2. expression
3. return expression

(Two higher-level constructs also exist - the function declaration and the try..catch statement.)

In the first case, expression is evaluated and the result is assigned to variable_name. In the second case, expression is evaluated and the result, if a clip, is assigned to the special variable last. In the third case, expression is evaluated and is used as the "return value" of the active script block--that is either a function or the entire script. In the latter case, the return value is typically the video clip that will be seen by the application which opens the AVS file. As a shorthand, a bare expression as the final statement in a script (or script block) is treated as if the keyword return was present.

Most of the time the result of an expression will be a video clip; however an expression's result can be of any type supported by the scripting language (clip, int, float, bool, string) and this is how utility functions such as internal script functions operate.

An expression can have one of these forms:

1. numeric_constant, string_constant or boolean_constant
2. variable_name or clip_property
3. Function(args)
4. expression.Function(args)
5. expression1 operator expression2
6. bool_expression ? expression1 : expression2

In the first case, the value of the expression is the value of the constant. In the second case, the values correspond to clip properties or script variables (which must have been previously initialized). In the third case, the value is the return value of an AVS function (see below). The fourth case is an alternate syntax (called "OOP notation") which is equivalent to Function(expression, args).

The final two cases show that one can manipulate expressions using all of the usual arithmetic and logical operators (from C) as you'd expect on ints, floats and bools, as well as execute code conditionally with the ternary operator. Strings can be compared with relational operators and concatenated with '+'. The following operators are also defined on video clips: a + b is equivalent to UnalignedSplice(a, b), and a ++ b is equivalent to AlignedSplice(a, b).

The functions in AviSynth's scripting language are, by and large, video filters. Although a function can return any type it chooses (this is a useful feature for creating utility code to reuse in scripts; you can define your own script functions) functions which do not return a clip are always limited to intermediate processing of variables to pass as arguments to filters (functions that do return a clip). The script should always return a clip as its final value. After all, AviSynth is a video processing application.

Functions can take up to sixty arguments (hope that's enough), and the return value can be of any type supported by the scripting language (clip, int, float, bool, string). Functions always produce a new value and never modify an existing one. What that means is that all arguments to a function are passed "by value" and not "by reference"; in order to alter a variable's value in AviSynth script language you must assign to it a new value.

To see the syntax of the function call for each built-in filter, view the internal filters. There are also built-in internal functions that perform common operations on non-clip variables.

Args is a list of function arguments separated by commas. The list can be empty. Each argument must be an expression whose type (eg text string, integer, floating-point number, boolean value or video clip) matches the one expected by the function. If the function expects a video clip as its first argument, and that argument is not supplied, then the clip in the special variable last will be used.

AviSynth functions can take named arguments. The named arguments can be specified in any order, and the filter will choose default values for any that you leave off. This makes certain filters much easier to use. For example, you can now write Subtitle("Hello, World!", text_color=$00FF00, x=100, y=200) instead of Subtitle("Hello, World!", 100, 200, 0, 999999, "Arial", 24, $00FF00). Colors can be specified in hexadecimal as in the example above or in decimal. In both cases it should be specified as RGB value, even if the clip itself is YUV.

If no arguments are being passed to the function, you can also make the function call without parentheses, e.g. FilterName. The primary reason for this is to retain compatibility with old scripts. However, it's sometimes convenient to leave off the parentheses when there's no possibility of confusion.

Avisynth ignores anything from a # character to the end of that line. This can be used to add comments to a script.

    # comment

In v2.58 it is possible to add block and nested block comments in the following way:

    # block comment:
    /*
    comment 1
    comment 2
    */

    # nested block comments:
    [* [* a meaningful example will follow later :) *] *]

Avisynth ignores anything from an __END__ keyword (with double underscores) to the end of the script file. This can be used to disable some last commands of script.

    Version()
    __END__
    ReduceBy2()
    Result is not reduced and we can write any text here

Avisynth ignores case: aViSouRCe is just as good as AVISource.

Multiple Avisynth statements on a single line can only be achieved in the context of OOP notation or embedding filters as parameters of another function such as:

    AviSource("c:\video.avi").Trim(0, 499)
    -or-
    AudioDub(AviSource("c:\video.avi"), WavSource("c:\audio.wav"))

Avisynth statements can be split across multiple lines by placing a backslash ("\") either as the last non-space character of the line being extended, or as the first non-space character on the next line.

Line splitting examples (both valid and equal):

    Subtitle("Hello, World!", 100, 200, 0, \
      999999, "Arial", 24, $00FF00)

-or-

    Subtitle("Hello, World!", 100, 200, 0,
      \ 999999, "Arial", 24, $00FF00)

When splitting across multiple lines you may place comments only at the end of the last line. Mixing comments with backslashes at an intermediate line of the line-split will either produce an error message or result at hard to trace bugs.

Example of a not-signaled bug by improper mixing of comments and line separation:

    ColorBars
    ShowFrameNumber
    Trim(0,9) # select some frames  \
      + Trim(20,29)

The above example does not return frames [0..9,20..29] as intended because the "\" is masked by the "#" character before it; thus the line continuation never happens.

`$Date: 2008/12/21 09:23:02 $`