ApplyEvery

ApplyEvery(clip c, int period, string thunk)

A general function to apply a filter at regular intervals in a clip. This function is not very efficient, so it's recommended that this function be used only if a combination of SelectEvery, DeleteEvery, and InterleaveEvery cannot suit your needs.

If the total number of frames is not a multiple of period, then silent, blank frames will be appended to the end of the output clip. It is the caller's responsibility to remove these afterward as necessary. (One of the examples below demonstrates how to remove these extra frames.)

Examples:

# deletes the 0th frame of every 100
#
# (NOTE: if you want to do this, DeleteEvery is better)
#
ApplyEvery(clip, 100, "DeleteFrame(0)")

# inverts the first two frames of every 5
#
# (NOTE: if you want to do this, a combination of SelectEvery, DeleteEvery,
#        and InterleaveEvery is better)
#
ApplyEvery(clip, 5, """ApplyRange(0, 1, "Levels", 0, 1.0, 255, 255, 0)""")

# reverses every group of 4 frames
ApplyEvery(clip, 4, "Reverse()")

# how to correct the frame-length
# (assumes that thunk does not itself modify the frame-count)
#
n = clip.FrameCount()
ApplyEvery(clip, period, thunk)
Trim(0, -n)

DeleteEvery(clip c, int period [, int offset1 [, int offset2 [, ...]]])

DeleteEvery reduces the frame-rate proportionately. For example, DeleteEvery(clip, 3, 0, 1) will reduce the frame-rate to one-third of its original value. DeleteEvery does not affect the audio track.

DeleteEvery(clip, 9, 0) will delete frames 0, 9, 18, 27, … from clip.

DeleteEvery(clip, 9) is equivalent to DeleteEvery(clip, 9, 0).

DeleteEvery(clip, 10, 0, 3) will delete frames 0, 3, 10, 13, 20, 23, … from clip.

DeleteEvery complements the built-in SelectEvery function; SelectEvery(4, 0, 1, 2) is equivalent to DeleteEvery(4, 3).

DeleteEvery(clip, period, 0) is conceptually equivalent to ApplyEvery(clip, period, "DeleteFrame(0)"). In practice, DeleteEvery is much more efficient than ApplyEvery. (Another difference is that DeleteEvery adjusts the frame-rate.)

DeleteFrames(clip c, int frame1, [, int frame2 [, ...]])

Like the built-in DeleteFrame function, DeleteFrames does not affect the audio track nor the frame-rate. Therefore, if enough frames are deleted, it may cause significant audio/video desynchronization.

Advantages of DeleteFrames over the built-in DeleteFrame function:

• Repeatedly calling the built-in DeleteFrame function is tedious.
• Repeatedly calling the built-in DeleteFrame function is inefficient; each call generates a new clip, wasting resources.
• Repeated calls to the built-in DeleteFrame function must be performed in reverse order to prevent each call from affecting the frame numbers in subsequent calls. In contrast, the frame numbers passed to DeleteFrames may be in any order.

DeleteFrames(clip, ...) essentially is shorthand for DeleteEvery(clip, clip.FrameCount(), ...). (They are not exactly the same, however; DeleteFrames does not affect the frame-rate.)

InterleaveEvery(clip baseClip, clip shimClip,
                int period [, int offset1 [, int offset2 [, ...]]])

InterleaveEvery inserts frames at regular intervals from shimClip into baseClip at the specified offsets. For example:

InterleaveEvery continues until there are no more frames in baseClip:

If shimClip does not have enough frames to insert into baseClip, the last frame of shimClip will be repeated:

Offset order determines where particular frames from shimClip are inserted:

InterleaveEvery increases the frame-rate proportionately to preserve the total duration of baseClip. For example, InterleaveEvery(baseClip, shimClip, 5, 0, 1) will return a clip with a frame-rate that is five-thirds of baseClip's.

The output clip uses the unaltered audio track of baseClip.

Note that the period and offset parameters specify the desired periodicity of the output clip, not of the base clip. This syntax is designed to ease interoperability with SelectEvery and DeleteEvery. The same arguments used to extract frames with SelectEvery/DeleteEvery can be used to put them back with InterleaveEvery.

For example, to invert the first two frames of every group of five, we could use:

src = AVISource( ... )

# pull out the frames we want
subClip = src.SelectEvery(5, 0, 1)
src = src.DeleteEvery(5, 0, 1)

# invert subClip
subClip = subClip.Levels(0, 1.0, 255, 255, 0)

# inject the frames back in
src.InterleaveEvery(subClip, 5, 0, 1)

My JDL_SimpleApplyEvery function (part of jdl-range.avsi) automates the above.

RepeatEveryFrame(clip c, int count)

LengthenClip(clip c, int minLength, bool "copy", int "color")

If the input clip already meets the minimum length, LengthenClip has no effect.

(This function doesn't belong with the rest, but it's provided because it's used by ApplyEvery.)

WhileEval(string condition, string loopBody)

WhileEval provides a very crude, static looping construct. That is, the loop is processed when the script is initially loaded.

WhileEval is not very efficient. A large number of iterations may cause a script to be slow to load; too many iterations may prevent AviSynth from loading the script at all. Although WhileEval should consume fewer resources than recursive looping constructs (AviSynth currently does not optimize tail-recursive calls), ApplyEvery, SelectEvery/DeleteEvery/InterleaveEvery, Animate, or ScriptClip/FrameEvaluate should be considered first when possible.

The example below is yet another way to delete the 0th frame of every 100. (Again, for this specific task, using DeleteEvery would be better.)

c = AVISource( ... )

i = c.FrameCount() - 1

# this is NOT a no-op; this rounds i down to the next highest multiple
# of 100
i = i / 100 * 100
WhileEval("i >= 0",
\         "c = c.DeleteFrame(i)
           i = i - 100")
c