# Specifying the Caliper Input Region

The Affine Rectangle you specify the Caliper tool determines how the tool projects the image to form a one-dimensional image and detect edges.

If you add an interactive RectangleAffine to a 2D Display control, you position the rectangle with rotation and skew handles:

The pixels are projected from the top of the rectangle to the bottom, where the top side is defined as the side opposite the skew handle. Edges are detected from left to right in the resulting projection image, where the left side is defined as the side opposite from the rotation handle.

If you specify the RectangleAffine programmatically, the projection is performed as shown in the following figure: