WriteFile / WriteFileIf / WriteFileStart / WriteFileEnd

WriteFile (clip, string filename, string expression1, ... , string expression16, bool "append", bool "flush")
WriteFileIf (clip, string filename, string expression1, ... , string expression16, bool "append", bool "flush")
WriteFileStart (clip, string filename, string expression1, ... , string expression16, bool "append")
WriteFileEnd (clip, string filename, string expression1, ... , string expression16, bool "append")

WriteFile evaluates expressionN, converts the result to a string and puts the result into a file.

The "run-time" variable current_frame is set so that you can use it in an "expression"
(this works similar as with ScriptClip, look there in the docu for more infos).
current_frame is set to -1 when the script is loaded and to -2 when the script is closed.

WriteFile generates output each frame, WriteFileIf generates output only if the first expression is true, there is no output at script opening or closure.
WriteFileStart and WriteFileEnd generates output only on script opening and closure, there is no action on each frame.

When append = true, the result will be appended on any existing file.
When flush = true, the file is closed and reopened after each operation so you can see the result immidiately (this may be slower).
For WriteFileStart and WriteFileEnd flush is always true.
The default for append is always true, except for WriteFileStart (here it is false).

This is best explained with some simple examples:

filename = "c:\myprojects\output.txt"
# create a test video to get frames
Version()

# the expression here is only a variable, which is evaluated and put in the file
# you will get a file with the framenumber in each line
WriteFile(filename, "current_frame")

# this line is written when the script is opened
WriteFileStart(filename, """ "This is the header" """)

# and this when the script is closed
WriteFileEnd(filename, """ "Now the script was closed" """)

Look how you can use triple-quotes to type a string in a string!

If the expression cannot be evaluated, the error message is written instead.
In case this happens with the If-expression in WriteFileIf the result is assumed to be true.

# will result in "I don't know what "this" means"
WriteFile(filename, "this is nonsense")

There are easier ways to write numbers in a file, BUT:

... with this example you can see how to use the "runtime functions" together with FrameEvaluate:

# create a test video to get different frames
Version.FadeIn(50).ConvertToYV12

# this will print the frame number, a ":" and the average luma for that frame
colon = ": "
WriteFile("F:\text.log", "current_frame", "colon", "AverageLuma")

Or maybe you want the actual time printed too:

# create a test video to get different frames
Version.FadeIn(50).ConvertToYV12

# this will print the frame number, the current time and the average luma for that frame
# the triple quotes are necessary to put quotes inside a string
WriteFile(last, filename, "current_frame", """ time(" %H:%M:%S") """, "AverageLuma")

More examples:

In WriteFileIf the FIRST expression is expected to be boolean (true or false).
Only if it is TRUE the other expressions are evaluated and the line is printed.
(Remember: && is AND, || is OR, == is EQUAL, != is NOT EQUAL)
That way you can ommit lines completely from your file.

# create a test video to get different frames
Version.FadeIn(50).ConvertToYV12

# this will print the frame number, but only of frames where AverageLuma is between 30 and 60
WriteFileIf(last, filename, "(AverageLuma>30) && (AverageLuma<60)", "current_frame", """ ":" """, "AverageLuma")

$Date: 2004/07/04 19:37:46 $