Noise Reduction Bound
nr
Sets the threshold for noise reduction.
Valid values are in the range -100 to 100. If no value is supplied, the default is 20.
The nr value sets the threshold at which noise is removed. Pixels with a luminance value that falls below the threshold are considered noise and will be blurred. Pixels above the threshold will be sharpened according to the nrs parameter value.
If only nr is set, then nrs will be set to its default, and vice versa. If neither value is supplied, then no noise reduction will be applied. Try setting nr=60 and nrs=40 below to see the difference from the defaults.