``umbra'' - compute the umbra of a volume

VFX name:

iUmbra.

IMPROPTU name:

Umbra.

purpose:

Reduce the gray-scale value of all voxels by the value of the user-preset intensity cutoff. All voxels that drop below 0 are set to zero. This is the grayscale umbra function that appears in gray-scale mathematical morphology.

input:

An 8-bit volume.

output:

An 8-bit volume.

parameters:

Default parameters menu is

intensity_cutoff = 128

comments:

1.

A very fast function.