A function to reduce film grain, included in rksfunc, public here.

defilmgrain.py

from vapoursynth import core, VideoNode

def defilmgrain(clip: VideoNode, s1=16, s2=3, s3=3, g=1.5, dark=10000) -> VideoNode:
    from vapoursynth import YUV, GRAY, Error
    from fvsfunc import Depth
    from havsfunc import QTGMC
    from muvsfunc import MergeChroma
    from vsutil import get_y, iterate
    from dfttest2 import DFTTest
    
    def gamma_curve(clip_y, gamma) -> VideoNode:
        def lut_y(x):
            floa = min(max(int(x / 56283 * 65536), 0), 65535) / 65535
            gammaed = floa ** gamma
            return min(max(int(gammaed * 56283 + 4112), 4112), 60395)
        
        return core.std.Lut(clip_y, planes=[0], function=lut_y)
    
    if clip.format.color_family != YUV and clip.format.color_family != GRAY:
        raise Error("rksfunc.defilmgrain: only YUV and GRAY format are supported.")
    origdep = clip.format.bits_per_sample
    if origdep != 16:
        clip = Depth(clip, 16)
    if clip.format.color_family == YUV:
        clip_y = get_y(clip)
    else:
        clip_y = clip
    clip_yf = Depth(clip_y, 32)
    basic_f = clip_yf.bm3dcuda_rtc.BM3Dv2(clip_yf, s1, 3, 12, 0, 2, 8)
    final_f = clip_yf.bm3dcuda_rtc.BM3Dv2(basic_f, s1, 3, 12, 0, 2, 8)
    line_f = clip_yf.bm3dcuda_rtc.BM3Dv2(final_f, s2, 3, 15, 0, 2, 8)
    line = Depth(line_f, 16)
    clearplane = DFTTest(Depth(final_f, 16), tbsize=1)
    emask = gamma_curve(clearplane, g).tcanny.TCanny(1, mode=0, op=3)
    emask = iterate(emask, core.std.Maximum, 2)
    emask = iterate(emask, core.std.Inflate, 2)
    emask = core.std.Expr([emask, clearplane], f"y {dark} > 0 x ?")
    tref = QTGMC(clip_y, InputType=1, Sharpness=0, SourceMatch=3)
    ttp = clip_y.ttmpsm.TTempSmooth(3, 5, 3, 3, pfclip=tref)
    ttp = core.std.MaskedMerge(ttp, line, emask)
    ttp_f = Depth(ttp, 32)
    vfinal_f = ttp_f.bm3dcuda_rtc.BM3Dv2(clip_yf, s3, 3, 12, 1, 2, 8)
    vfinal = Depth(vfinal_f, 16)
    if clip.format.color_family == YUV:
        vfinal = MergeChroma(vfinal, clip)
    if origdep != 16:
        vfinal = Depth(vfinal, origdep)
    return vfinal