feretdiameter

Feret diameter, often referred to as Feret's diameter, is a measure used in image analysis to describe the size of a two-dimensional object or particle. It is defined as the distance between two parallel tangents to the object's boundary, measured along a specified direction. For a given orientation theta, the two tangents touch the object’s boundary and the Feret diameter at that angle is the separation of the tangents. By varying theta, a family of Feret diameters is obtained.

The maximum Feret diameter across all orientations is called the maximum Feret diameter or sometimes simply the Feret diameter, while the minimum value is the minimum Feret diameter (also known as Feret min or caliper width). In digital images, these measurements are typically computed using the convex hull of the object and a rotating calipers approach, or by evaluating projections along a set of angles to determine extreme points.

Feret diameter is widely used in particle size and morphology analysis in microscopy, electron microscopy, and materials science. It provides a size metric that is independent of the object's orientation but remains sensitive to shape and boundary quality. It complements other size measures such as the equivalent circle diameter and bounding box dimensions. Measurements are reported in pixels or, after calibration, in physical units, and can be affected by image resolution and segmentation accuracy.