Home → Volocity → General → Why Is the Length of the 'Skeleton' Not Always Equal to the Skeletal Length Measurement in the Measurement Item?

3.33. Why Is the Length of the 'Skeleton' Not Always Equal to the Skeletal Length Measurement in the Measurement Item?

Why Is the Length of the 'Skeleton' Not Always Equal to the Skeletal Length Measurement in the Measurement Item?

The difference arises due to how Volocity calculates skeletal length and how the skeleton is visually represented.

Definition of Skeletal Length

As described in the Volocity Help menu:

• Skeletal length is calculated by thinning or skeletonizing the object from all sides until only a single voxel-wide line remains.
• This measurement is best suited for elongated shapes.
• The line's direction is elongated to meet the edges of the object, and the length of this line is the skeletal length.
• For objects with branching skeletons, skeletal length is measured as the longest path across the object without crossing any point more than once.

Skeletal Diameter

The skeletal diameter is the diameter of a hypothetical cylinder that has:

• A length equal to the skeletal length of the object.
• A volume equal to the object's measured volume.

The skeletal diameter is derived using the equation for the volume of a cylinder:
V = πr²l

Skeleton Feedback

• When the skeletal length is measured, Volocity populates the measurement columns with skeletal length and skeletal diameter for each measured object.
• A measurement row is added, visually representing the skeletal length with a line (the skeleton).
• The skeleton serves as feedback, with the parent ID column reflecting the ID of the source object.

Sub-Sampling and Visual Discrepancies

• To create the skeleton, Volocity uses a sub-sampling method, selecting a subset of the total number of voxels (nodes) in the skeletonized object.
• This process can cause the skeleton to visually "cut corners," as the path of the yellow line (skeleton feedback) does not necessarily align with the red object (original shape).

Key Consideration

• The skeleton shown in the image is intended only as a visual representation of the path taken through the object after skeletonization.
• The measured skeletal length is calculated using the algorithm and is accurate, but it may not match the distance along the visual skeleton line.