[HELP] seeking paid filter to remove purple fringing in old videos

[Selur]

Here's a quick fix:

• Start Hybrid
  
• load source
  
• enable "Filtering->Vapoursynth->Custom->Insert before DeCross" and enter:
  
  # create edgemask
edges = core.std.Prewitt(clip.std.ShufflePlanes(0, vs.GRAY))

bits = clip.format.bits_per_sample
thr = 32 << (bits - 8)  # scale threshold to bitdepth

mask = core.std.Binarize(edges, threshold=thr)

# increase width of mask, by using higher integer values, depending on source and resolution
increase = 0
for _ in range(increase):
    mask = core.std.Maximum(mask)

# optional: soften mask edges
mask = core.std.BoxBlur(mask, hradius=1, vradius=1)

# create desaturated version
mid = 1 << (bits - 1)
mult = 0.5  # 0.0 = grayscale chroma, 1.0 = unchanged

desat = core.std.Expr(
    [clip],
    expr=[
        "",  # keep luma unchanged
        f"x {mid} - {mult} * {mid} +",
        f"x {mid} - {mult} * {mid} +"
    ]
)

# merge original with desaturated version around edges
clip = core.std.MaskedMerge(clip, desat, mask)

To see what happens and tweaking the values depending on your source:

• enable "Filtering->Vapoursynth->Misc->Preview->Split Compare View" and set "FilterView mode" to "orginal|filtered|diff"
  
• enabled "Filtering->Compare view" (lower right corner)
  
• enabled "Filtering->Synth auto refresh" (lower right corner)
  
• open Vapoursynth Preview (lower right corner)
  

then adjust the parameters, i.e. thr, increase, mult, if your source differs from the sample.

The filtering should be fast, on any machine.
No filters used that do no come with Vapoursynth itself, so any tool which allows you to add custom Vapoursynth code should work fine.
(+ works on Linux, Mac, Windows)
Side note: if processing speed is an issue, external filters can replace the used filters:

• core.std.Prewitt => core.edgemasks.Prewitt
  
• core.std.BoxBlur => core.vszip.BoxBlur
  
• core.std.Expr => core.akarin.Expr
  

Cu Selur