How to cut colors with low luma values?

[JELSTUDIO]

Hi guys :)

I'm hoping someone can give me some ideas on how to do this.

I would like to cut all colors (desaturate to gray scale) in only the dark areas of an image.

So far I can only figure out how to desaturate based on saturation levels or hue levels, but not luma value levels.

Is there a smart way to do this?

Something like: cut all colors on pixels where luma value is below a certain threshold.

Thanks.

[Gavino]

Using MaskTools:
... # load source clip
thr = ... # set required threshold
mt_merge(Greyscale(), last, mt_binarize(thr), luma=true, U=3,V=3)

[JELSTUDIO]

@Gavino; Perfect :) Thank you!