As a structure-based detector, PCBR does not use edges, instead, it uses curvilinear structures, also called . Curvilinear structures detection generates a single response for both lines and edges, producing a clearer structural sketch of an image than is usually provided by the gradient magnitude image. The Steger's algorithm is modified to get the curvilinear images. As only the first step of this algorithm is used which is to calculate the principal curvature images, the principal curvature is adopted as the name of this detector. To get the principal curvature, the Hessian matrix is calculated:

where is second partial derivative of the image evaluated at point x in the direction and is the mixed partial second derivative of the image evaluated at point x in the and directions. The maximum and minimum eigenvalues of this matrix form two images which correspond to white lines on black background and black lines on white background.