# Nonlinear Calibration Details

Like all calibration tools, the Checkerboard tool considers a set of points in an acquired image and the corresponding set of points measured in a calibrated space. The Checkerboard calibration tool obtains the first set of points by locating the vertices in an acquired image of a checkerboard calibration plate and the second set from the dimensions of the tiles in the real world (based on the actual tile size that you specify):

The tool computes the calibration transformation between these two sets of points. It is this transformation that it uses to warp run-time images.

In the case of the example shown in this section, the calibration image exhibits a form of radial distortion called "barrel distortion". The calibration transformation required to map the acquired pixel space to the raw calibrated space has the effect of pulling the points in the corners of the image outward. The following figure shows a magnified view of the upper-left corner of the image. The blue arrows represent the movement required to transform the pixel space points (red) to their corresponding raw calibrated points (blue).

The tool uses this calibration transformation to warp the run-time image to remove the distortion detected during calibration.