``Sigma'' -- sigma filter

purpose:

Run the sigma filter on an input volume [17]. The filter smooths noise, preserves edges, and can leave thin lines untouched.

For each voxel in the input volume, the filter calculates the mean value of a set of voxels within 2 graylevel values of the voxel of interest. Only voxels within a prespecified local neighborhood are considered in this calculation. If too few points within the local neighborhood lie within the 2 range, then the voxel of interest is left unchanged; otherwise, the calculated mean value is assigned to the voxel.

input:

An 8-bit volume.

output:

An 8-bit volume.

parameters:

Default parameters menu is

x filter dim = 3
y filter dim = 3
z filter dim = 3
standard dev. = 20
no. of iterations = 1
input from vol # = 0
copy output to vol # = 0

The parameters ``standard dev. = '' is the parameter sigma (in graylevels) mentioned above.

comments:

 

1. This filter is good at saving thin lines.
2. The SNN or maximum-homogeneity filters may reduce noise better.
3. Iterating the filter can improves results.
4. The algorithm takes moderate computation.