# Projections

The first step in using the Caliper tool is to specify a projection region within the image to which you want to apply the tool. The Caliper tool depends on a carefully defined projection region to isolate just the edge information from a small section of the image.

You specify a projection region for the Caliper tool in either of the following ways:

- Supply an affine rectangle and affine sampling parameters
- Define the location, size, and angles of skew and rotation for the projection region

Typically, the projection region contains the portion of the image with the features of interest. The projection operation sums all the information in the projection region, accentuating the strength of edges that lie parallel to the projection rays and reducing the effects of noise.

The following figure shows how a projection can accentuate the edge information in a two-dimensional image. Notice how the strength of the edge is stronger in the projected image than in the initial image.

Once the Caliper tool has produced a projected image, it locates edges using edge mode.