contrib/gel/gevd/gevd_fold.h File Reference

detection of fold profiles in the intensity image More...

#include <vcl_iosfwd.h>

Classes
class	gevd_fold

Detailed Description

detection of fold profiles in the intensity image

Operator to implement Canny edge detector which finds elongated fold/roof contours with ddG. Then junctions are found by extending from end points of dangling contours.

The recipe is:

Convolution with Gaussian with sigma typically = 1, to well-condition the surface before taking second derivative. Result is a smoothed image.

Convolution with Laplacian, or second difference. Result is a gradient image. Canny proves that first-derivative of the Gaussian is the optimum filter to detect elongated step profiles.

Optionally estimate sensor/texture sigma, if given noise sigma is a negative interpolation factor -k in range -[0 1]. noise_sigma = (1-k)*sensor_sigma + k*texture_sigma. Sensor and texture sigmas are estimated from the histogram of weak step edges, detected in an ROI centered on gradient image. (see Step constructor documentation in Step.C - JLM)

Non Maximum suppression in the direction of local Hessian, to detect pixels with maximum local curvature, above a priori noise response. Result is connected contours, width < 2, broken only at junctions with weaker chains. Also obtain subpixel accuracy normal to the contour.

Optionally extend from end points of contours to search for maximum curvature in the direction normal to the dangling contours, above some factor of the noise response. Result is a simple detection of all strong junctions.

Input: Intensity image, smoothing sigma, sensor/texture noise sigma and threshold factors for detecting contour/junction edge elements.

Output: Magnitude, direction, location of fold pixels, forming a connected network of contours.

Complexity: O(|pixels|) time and space for convolutions. O(|edgels|) time for iterative extension to recover junctions.

    Authors
     John Canny      (1986) SM Thesis
     Chris Connolly  (1987) use directional 1st-difference
     Van-Duc Nguyen  (1989) add subpixel location, extension to find junctions
     Arron Heller    (1992) translate from CLOS to C++
     Van-Duc Nguyen  (1995) add noise estimation, use FloatOperators.
     Joe Mundy       (1997) Added strength and direction arrays for transfer to
                            edgel chains. The flag "transfer" controls the
                            setting of these arrays. Note difference with
                            gevd_step.h.. Didn't want to affect global use of
                            DetectEdgels.

Definition in file gevd_fold.h.

Classes

Detailed Description