AviSynth Syntax - Control functions

They facilitate flow of control (loading of scripts, arguments checks, global settings adjustment, etc.).

Apply calls the function or filter func_string with arguments arg1, arg2, ..., argn (as many as supplied). Thus, it provides a way to call a function or filter by name providing arguments in the usual way as in a typical function call.
Consequently, Apply("f", x) is equivalent to f(x) which in turn is equivalent to Eval("f(" + String(x) + ")").
Examples:
# here the same call to BicubicResize as in the Eval() example is shown 
Apply("BicubicResize", 352, 288)
Eval evaluates an arbitrary expression as if it was placed inside the script at the point of the call to Eval and returns the result of evaluation (either to the variable that is explicitly assigned to or to the last special variable.
You can use Eval to construct and evaluate expressions dynamically inside your scripts, based on variable input data. Below some specific examples are shown but you get the general idea.
Examples:
# this calls BicubicResize(last, 352, 288)
settings = "352, 288"
Eval( "BicubicResize(" + settings + ")" )
...
# this will result in Defined(u) == false
u = Eval("#")   
...
# this increments a global based on a variable's value
dummy = Eval("global my_counter = my_counter + " + String(increment))
Import evaluates the contents of another AviSynth script and returns the imported script's return value. Typically it is used to make available to the calling script library functions and the return value is not used. However this is simply a convention; it is not enforced by the AviSynth Syntax. See also the dedicated Import page in Internal filters for other possible uses.
Possible scenarios (an indicative list) where the return value could be of use is for the library script to:
the value then could be tested by the calling script to decide what action to take next.
Examples:
Import("mylib.avsi")  # here we do not care about the value (mylib.avsi contains only functions)
...
okflag = Import("mysources.avsi")  # mysources loads predetermined filenames from a folder into globals
source = okflag ? global1 + global2 + global3 : BlankClip()
Returns the item selected by the index argument, which must be of int type (0 returns item0, 1 returns item1, ..., etc). Items can be any script variable or expression of any type and can even be mixed.
Examples:
# select a clip-brush from a set of presets
idx = 2
brush = Select(idx, AviSource("round.avi"), rectangle, diagonal, diagonal.FlipHorizontal)
Returns x if Defined(x) is true, d otherwise. x must either be a function's argument or an already declared script variable (ie a variable which has been assigned a value) else an error will occur.
Examples:
function myfunc(clip c, ..., int "strength") {
    ...
    strength = Default(strength, 4) # if not supplied make it 4
    ...
}
Does nothing if condition is true; throws an error, immediately terminating script execution, if condition is false. In the later case err_msg, if supplied, is presented to the user through a dialog box; else the standard message "Assert: assertion failed". shows up.
Examples:
function myfunc(clip c, ..., int "strength") {
    ...
    strength = Default(strength, 4) # if not supplied make it 4
    Assert(strength > 0, "'strength' must be positive")
    ...
}
This is a no-operation function provided mainly for conditional execution with non-return value items such as Import, when no "else" condition is desired. That is, use it whenever the AviSynth Syntax requires an operation (such as with the ?: operator) but your script does not need one.
Return value: 0 (int type).
Examples:
preset = want_presets ? AviSource("c:\presets\any.avi") : NOP
... 
loadlib ? Import("my_useful_functions.avs") : NOP
Sets the maximum memory that AviSynth uses (in MB) to the value of amount. Setting to zero just returns the current Memory Max value. v2, (=0) v2.5.8. In the 2.5 series the default Memory Max value is 25% of the free physical memory, with a minimum of 16MB.
From rev 2.5.8, above 256MB of the free physical memory the default Memory Max is 50% of the free physical memory minus 64MB.
Free <64 128 256 512 1024 2048
Max v2.57 and older 16 32 64 128 256 512
Max v2.58 16 32 64 192 448 960
In some versions there is a default setting of 5MB, which is quite low. If you encounter problems (e.g. low speed) try to set this values to at least 32MB.
Return value: Actual MemoryMax value set.
Examples:
SetMemoryMax(128)
Sets the default directory for AviSynth to the path argument.
This is primarily for easy loading of source clips, importing scripts, etc. It does not affect plugins' autoloading.
Return value is 0 if successful, -1 otherwise.
Examples:
SetWorkingDir("c:\my_presets")
AviSource("border_mask.avi")  # this loads c:\my_presets\border_mask.avi
Set alignment mode for planar frames. mode van either be true or false.
Some older plugins illegally assume the layout of video frames in memory. This special filter forces the memory layout of planar frames to be compatible with prior versions of AviSynth. The filter works on the GetFrame() call stack, so it effects filters before it in the script.
Examples:
Example : Using an older version of Mpeg2Source() (1.10 or older):

LoadPlugin("...\Mpeg2Decode.dll")
Mpeg2Source("test.d2v")         # A plugin that illegally assumes the layout of memory
SetPlanarLegacyAlignment(true)  # Set legacy memory alignment for prior statements
ConvertToYUY2()     # Statements through to the end of the script have
...                             # advanced memory alignment.
This option enables WAVE_FORMAT_IEEE_FLOAT audio output. The default is to autoconvert Float audio to 16 bit.
This option enables WAVE_FORMAT_EXTENSIBLE audio output. The default is WAVE_FORMAT_EX.
Note: The default DirectShow component for .AVS files, "AVI/WAV File Source", does not correctly implement WAVE_FORMAT_EXTENSIBLE processing, so many application may not be able to detect the audio track. There are third party DirectShow readers that do work correctly. Intermediate work files written using the AVIFile interface for later DirectShow processing will work correctly if they use the DirectShow "File Source (async)" component or equivalent.

Back to Internal functions.

$Date: 2008/04/21 20:31:23 $