xform.h

Rob Hess Linux下的SIFT提取源码

/**@file
   Functions for computing transforms from image feature correspondences.
  
   Copyright (C) 2006-2007  Rob Hess <[e-mail]>

   @version 1.1.1-20070913
*/

#ifndef XFORM_H
#define XFORM_H

#include <cxcore.h>


/********************************** Structures *******************************/

struct feature;

/** holds feature data relevant to ransac */
struct ransac_data
{
  void* orig_feat_data;
  int sampled;
};

/******************************* Defs and macros *****************************/

/* RANSAC error tolerance in pixels */
#define RANSAC_ERR_TOL 3

/** pessimistic estimate of fraction of inlers for RANSAC */
#define RANSAC_INLIER_FRAC_EST 0.25

/** estimate of the probability that a correspondence supports a bad model */
#define RANSAC_PROB_BAD_SUPP 0.10

/* extracts a feature's RANSAC data */
#define feat_ransac_data( feat ) ( (struct ransac_data*) (feat)->feature_data )


/**
   Prototype for transformation functions passed to ransac_xform().  Functions
   of this type should compute a transformation matrix given a set of point
   correspondences.

   @param pts array of points
   @param mpts array of corresponding points; each \a pts[\a i], \a i=0..\a
     n-1, corresponds to \a mpts[\a i]
   @param n number of points in both \a pts and \a mpts
   
   @return Should return a transformation matrix that transforms each point in
     \a pts to the corresponding point in \a mpts or NULL on failure.
*/
typedef CvMat* (*ransac_xform_fn)( CvPoint2D64f* pts, CvPoint2D64f* mpts,
				   int n );


/**
   Prototype for error functions passed to ransac_xform().  For a given
   point, its correspondence, and a transform, functions of this type should
   compute a measure of error between the correspondence and the point after
   the point has been transformed by the transform.

   @param pt a point
   @param mpt \a pt's correspondence
   @param T a transform

   @return Should return a measure of error between \a mpt and \a pt after
     \a pt has been transformed by the transform \a T.
*/
typedef double (*ransac_err_fn)( CvPoint2D64f pt, CvPoint2D64f mpt, CvMat* T );


/***************************** Function Prototypes ***************************/


/**
   Calculates a best-fit image transform from image feature correspondences
   using RANSAC.

   For more information refer to:

   Fischler, M. A. and Bolles, R. C.  Random sample consensus: a paradigm for
   model fitting with applications to image analysis and automated cartography.
   <EM>Communications of the ACM, 24</EM>, 6 (1981), pp. 381--395.

   @param features an array of features; only features with a non-NULL match
     of type \a mtype are used in homography computation
   @param n number of features in \a feat
   @param mtype determines which of each feature's match fields to use
     for transform computation; should be one of FEATURE_FWD_MATCH,
     FEATURE_BCK_MATCH, or FEATURE_MDL_MATCH; if this is FEATURE_MDL_MATCH,
     correspondences are assumed to be between a feature's img_pt field
     and its match's mdl_pt field, otherwise correspondences are assumed to
     be between the the feature's img_pt field and its match's img_pt field
   @param xform_fn pointer to the function used to compute the desired
     transformation from feature correspondences
   @param m minimum number of correspondences necessary to instantiate the
     transform computed by \a xform_fn
   @param p_badxform desired probability that the final transformation
     returned by RANSAC is corrupted by outliers (i.e. the probability that
     no samples of all inliers were drawn)
   @param err_fn pointer to the function used to compute a measure of error
     between putative correspondences and for a given transform
   @param err_tol correspondences within this distance of each other are
     considered as inliers for a given transform
   @param inliers if not NULL, output as an array of pointers to the final
     set of inliers
   @param n_in if not NULL, output as the final number of inliers

   @return Returns a transformation matrix computed using RANSAC or NULL
     on error or if an acceptable transform could not be computed.
*/
extern CvMat* ransac_xform( struct feature* features, int n, int mtype,
			    ransac_xform_fn xform_fn, int m,
			    double p_badxform, ransac_err_fn err_fn,
			    double err_tol, struct feature*** inliers,
			    int* n_in );


/**
   Calculates a least-squares planar homography from point correspondeces.
   Intended for use as a ransac_xform_fn.

   @param pts array of points
   @param mpts array of corresponding points; each \a pts[\a i], \a i=0..\a
     n-1, corresponds to \a mpts[\a i]
   @param n number of points in both \a pts and \a mpts; must be at least 4

   @return Returns the \f$3 \times 3\f$ least-squares planar homography
     matrix that transforms points in \a pts to their corresponding points
     in \a mpts or NULL if fewer than 4 correspondences were provided
*/
extern CvMat* lsq_homog( CvPoint2D64f* pts, CvPoint2D64f* mpts, int n );


/**
   Calculates the transfer error between a point and its correspondence for
   a given homography, i.e. for a point \f$x\f$, it's correspondence \f$x'\f$,
   and homography \f$H\f$, computes \f$d(x', Hx)^2\f$.  Intended for use as
   a ransac_err_fn.

   @param pt a point
   @param mpt \a pt's correspondence
   @param H a homography matrix

   @return Returns the transfer error between \a pt and \a mpt given \a H
*/
extern double homog_xfer_err( CvPoint2D64f pt, CvPoint2D64f mpt, CvMat* H );


/**
   Performs a perspective transformation on a single point.  That is, for a
   point \f$(x, y)\f$ and a \f$3 \times 3\f$ matrix \f$T\f$ this function
   returns the point \f$(u, v)\f$, where<BR>
   
   \f$[x' \ y' \ w']^T = T \times [x \ y \ 1]^T\f$,<BR>

   and<BR>

   \f$(u, v) = (x'/w', y'/w')\f$.

   Note that affine transforms are a subset of perspective transforms.
   
   @param pt a 2D point
   @param T a perspective transformation matrix

   @return Returns the point \f$(u, v)\f$ as above.
*/
extern CvPoint2D64f persp_xform_pt( CvPoint2D64f pt, CvMat* T );


#endif