WriteFile

WriteFile(clip clip, string filename, string expression1 [, string expression2 [, ...]] [, bool append, bool flush])
WriteFileIf(clip clip, string filename, string expression1 [, string expression2 [, ...]] [, bool append, bool flush])
WriteFileStart(clip clip, string filename, string expression1 [, string expression2 [, ...]] [, bool append])
WriteFileEnd(clip clip, string filename, string expression1 [, string expression2 [, ...]] [, bool append])

WriteFile evaluates each expressionN, converts the result to a string and puts the concatenated results into a file, followed by a newline.

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 docs for more info). current_frame is set to -1 when the script is loaded and to -2 when the script is closed.

WriteFile evaluates the expressions and generates output for each frame rendered by the filter. WriteFileIf is similar, but generates output only if the first expression is true. In both cases, there is no output at script opening or closure. Note that since output is produced only for rendered frames, there will be no output at all if the result of the filter is not used in deriving the final result of the script.

WriteFileStart and WriteFileEnd generate output only on script opening and closure respectively, there is no action on each frame. In both cases, the expressions are evaluated exactly once, at the location of the filter in the script.

When append = true, the result(s) will be appended to any existing file. The default for append is always true, except for WriteFileStart (here it is false).

When flush = true, the file is closed and reopened after each operation so you can see the result immediately (this may be slower). The default for flush (WriteFile and WriteFileIf) is true. For WriteFileStart and WriteFileEnd, the file is always closed immediately after writing.

Usage 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 function" AverageLuma:

# 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 omit 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")

Changes:

v2.60	Number of expressions changed from 16 to nearly unlimited.
v2.55	Initial release.