xform.h

利用VC 编写的SIFT 特征匹配程序,可以在影像 旋转,缩放时仍能成功匹配

/**@file
Functions for computing transforms from image feature correspondences

Copyright (C) 2006  Rob Hess <[e-mail]>
@version 1.1.1-20070330
*/

#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* M );


/***************************** 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 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