std.FrameEval(clip clip, func eval[, clip[] prop_src])

Allows an arbitrary function to be evaluated every frame. The function gets the frame number, n, as input and should return a clip the output frame can be requested from.

The clip argument is only used to get the output format from since there is no reliable automatic way to deduce it.

When using the argument prop_src the function will also have an argument, f, containing the current frames. This is mainly so frame properties can be accessed and used to make decisions. Note that f will only be a list if more than one prop_src clip is provided.

This function can be used to accomplish the same things as Animate, ScriptClip and all the other conditional filters in Avisynth. Note that to modify per frame properties you should use ModifyFrame.

How to animate a BlankClip to fade from white to black. This is the simplest use case without using the prop_src argument:

import vapoursynth as vs
import functools

core = vs.core
base_clip = core.std.BlankClip(format=vs.YUV420P8, length=1000, color=[255, 128, 128])

def animator(n, clip):
   if n > 255:
      return clip
      return core.std.BlankClip(format=vs.YUV420P8, length=1000, color=[n, 128, 128])

animated_clip = core.std.FrameEval(base_clip, functools.partial(animator, clip=base_clip))

How to perform a simple per frame auto white balance. It shows how to access calculated frame properties and use them for conditional filtering:

import vapoursynth as vs
import functools
import math

def GrayWorld1Adjust(n, f, clip, core):
   small_number = 0.000000001
   red   = f[0].props['PlaneStatsAverage']
   green = f[1].props['PlaneStatsAverage']
   blue  = f[2].props['PlaneStatsAverage']
   max_rgb = max(red, green, blue)
   red_corr   = max_rgb/max(red, small_number)
   green_corr = max_rgb/max(green, small_number)
   blue_corr  = max_rgb/max(blue, small_number)
   norm = max(blue, math.sqrt(red_corr*red_corr + green_corr*green_corr + blue_corr*blue_corr) / math.sqrt(3), small_number)
   r_gain = red_corr/norm
   g_gain = green_corr/norm
   b_gain = blue_corr/norm
   return core.std.Expr(clip, expr=['x ' + repr(r_gain) + ' *', 'x ' + repr(g_gain) + ' *', 'x ' + repr(b_gain) + ' *'])

def GrayWorld1(clip, matrix_s=None):
   core = vs.core
   rgb_clip = core.resize.Bilinear(clip, format=vs.RGB24)
   r_avg = core.std.PlaneStats(rgb_clip, plane=0)
   g_avg = core.std.PlaneStats(rgb_clip, plane=1)
   b_avg = core.std.PlaneStats(rgb_clip, plane=2)
   adjusted_clip = core.std.FrameEval(rgb_clip, functools.partial(GrayWorld1Adjust, clip=rgb_clip, core=core), prop_src=[r_avg, g_avg, b_avg])
   return core.resize.Bilinear(adjusted_clip,, matrix_s=matrix_s)

core = vs.core
main = core.ffms2.Source(source='...')
main = GrayWorld1(main)