# Edge Model

To evaluate whether an edge pattern candidate in the image is a good match for the edge pattern that you are seeking, you must define a model that describes the edge pattern of interest. You define an edge model by specifying the polarity (light-to-dark or dark-to-light) and position of each edge in the model. You specify the positions of edges relative to a model origin. The Caliper tool lets you define edge models with a single edge or a pair of edges.

The following table describes an edge model with a symmetric pair of edges. The origin of the model is centered between the two edges.

Edge | Position | Polarity
--- | --- | ---
1 | -20 | Light to dark
2 | +20 | Dark to light

The following figure shows an idealized graphical representation of the edge model described in the previous table. Note that the position of the model origin is implied by the values of the edge positions.