[Gavino]

Originally Posted by TheProfileth

try {
  Prewparams = Eval(prefilter + "(c, " + params1 + ")")
}
catch(err_msg) {
  Prewoparams = Eval(prefilter + "(removegrain(0)")
}

prefiltered = (params1=="") ? Prewoparams: Prewparams

There's something wrong with your logic here as (depending on the try) either Prewoparams or Prewparams is undefined, and that is independent of params1 being empty. Also, removegrain is called without a clip and there is an unmatched bracket.
Perhaps what you mean is

try {
  prefiltered = Eval(prefilter + "(c, " + params1 + ")")
}
catch(err_msg) {
  prefiltered = Eval(prefilter + "(c.removegrain(0))")
  # or perhaps just prefiltered = c.removegrain(0) ???
}

Similarly for postfiltering.