HomeVolocityGeneralWhy is the length of the 'Skeleton' not always equal to the skeletal length measurement stored in the Measurement Item?

# 3.5. Why is the length of the 'Skeleton' not always equal to the skeletal length measurement stored in the Measurement Item?

A description as to how the Skeletal length measurement is calculated can be found in the Volocity Help menu and is defined as follows:

"Skeletal length is calculated by first thinning or skeletonising the object from all sides until a single voxel line is left. This measurement is best suited to elongated shapes. The direction of the line is elongated to meet the edges of the object and the length of this line is the skeletal length. For objects where the skeleton branches the skeletal length is measured as the longest traverse across the object without crossing a point more than once.
Skeletal diameter is the diameter of a cylinder if it had a length equal to the skeletal length of the object determined as above and a volume equal to the object's measured volume.

Derived using the equation for the volume of a cylinder.

V = Pi r2 *l

Measure skeletal length populates the measurement columns skeletal length and skeletal diameter for each measured object. For each skeletal length a measurement row is added. This row is a line measurement representing the skeletal length for the purpose of viewing feedback. The parent ID column of the row representing the skeleton will reflect the ID of the source object."

The Skeleton (shown as feedback on the image) should only be used to visually represent the path taken through the object after it has been skeletonised, the path of the line does not necessarily reflect the actual measured distance.

In order to create the 'Skeleton' the path of the line has to be sub-sampled i.e. Volocity uses a subset of the total number of voxels (nodes) in the skeletonised object to create the skeleton, typically for elongated structures this can lead to what we are seeing in the image below.

The thicker Red line/'object' represents the perimeter of the top half of the green structure, in this instance when the Skeletal Length of the red object is measured and the Skeleton is applied as feedback (yellow line) the skeleton appears to deviate from the path of the original object i.e. the skeleton appears to cut corners. This is a consequence of sub-sampling. The Skeleton should only be used as a visual reference, the measured skeletal length is not necessarily the same as the distance along the skeleton. The Skeletal length measurement is accurate based on the algorithm used to calculate it.

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