vmediaxis(1)

PROGRAM
vmediaxis - Computes the medial axis transform

SYNTAX

vmediaxis

-i  input image

-o  output image

-w  if flag=1 the medial axis has the value 255, otherwise (default) the value is the distance from the border

[-V]
    Gives the version for vmediaxis

[-U]
    Gives the usage for vmediaxis

[-P]
    Prompts for command line options

[-A [file1]]
    Creates the answer file called vmediaxis.ans or file1

[-a [file1]]
    Uses vmediaxis.ans or file1 as the answer file

DESCRIPTION
vmediaxis Computes the medial axis transform on a byte binary image.

The median axis of a region in a binary image is the set of pixels whose minimum distance from the edge is a local maximum. It is a thinning algorithm that is comparable to the morphological skeletonization. The median axis is generally not connected.

An optional flag allows the user to obtain the medial axis pixel values set either to their distance from the border, or to 255. If the flag is set to one, it leads to a better visualization of the median axis but the information that permits the user to reconstruct the original binary image is lost.
If the flag is not set to 1, it is possible to reconstruct the original image from the medial axis.

Presentation of the medial axis transform algorithm :

U(m,n) : original image. This image must be a two-valued byte image, 0 for the background, and any other value for the objects.

Uk(i,j) : image after k iterations.

U0(m,n)= U(m,n) , and for k= 1, 2, 3, ...
Uk(m,n)= U(m,n)+min{U0(i,j);((i,j):dist(m,n;i,j)<= 1)},
dist(m,n;i,j) is the mediaxis between (m,n) and (i,j).
dist(i,j-1;i,j)=dist(i,j+1;i,j)=dist(i-1,j;i,j)=dist(i+1,j;i,j)=1.
dist(i-1,j-1;i,j)=dist(i+1,j-1;i,j)=dist(i-1,j+1;i,j)=dist(i+1,j+1;i,j)=2.

The transform is done when k equals the maximum thickness of the region, that is when Uk+1 equals Uk.

The last part consists of using this image and keeping only the pixels that are a local maximum in the 4 neighborhood.
The Medial Axis image is a form of skeletonization.

Functions that can be useful when using vmediaxis:

vthresh
    generates a binary image.

ADVICE:

    For this routine, the image must be a byte binary image, with the gray level value 0 for the background. So it is advisable to use vthresh on the image before using vmediaxis.

REQUIRED ARGUMENTS

-i  type: infile
    desc: input image

-o  type: outfile
    desc: output image

-w  type: boolean
    desc: if flag=1 the medial axis has the value 255, otherwise (default) the value is the distance from the border

OPTIONAL ARGUMENTS
none

EXAMPLES

vmediaxis -i image1 [-w wflag] -o image2
Computes the medial axis transform.
For this routine, the image must be a byte binary image, with the gray-level value 0 for the background. So it is advisable to use vthresh on the image before using vmediaxis.

SEE ALSO
intro(1), verror(3), vgparms(3), vutils(3), lvmediaxis(3) lvmediaxis(3), lvthresh(3), vthresh(1).

RESTRICTIONS
vmediaxis works only with binary BYTE images.

COPYRIGHT
Copyright 1991, University of New Mexico. All rights reserved.