MDegrain, protect fast motion dark areas

[bxyhxyh]

MDegrain oversmooths dark areas of the video.
We use our own tricks to protect dark areas.
something like
source=source()
mask=mt_lut("expression to protect dark areas")
normaldegrain=mdegrain(source,plane=4)
mt_merge(contdegrain,source,mask,u=2,v=2)
It doesn't denoise much on dark areas.

Mine is:
source=source()
normaldegrain=mdegrain(source,plane=4)
cont=s.mt_lut(yexpr="x 2.5 *",u=2,v=2)
contdegrain=mdegrain(cont,plane=0)
mask=contdegrain.mt_lut("x 252 - 255 *").grayscale.invert()
contdegrain=contdegrain.mt_lut(yexpr="x 2.5 /",u=2,v=2)
mt_merge(normaldegain,contdegrain,mask,u=2,v=2)
It still smooths dark areas. But not as much as normaldegrain.

So I created another mask to protect dark areas
mt_lutxy(denoised,source,yepxr="x y - 0 < x y ?")
but it protects "black" noises too. What should i do to eliminate "black" noises? Or you can give me better methods.