sift.c

opencv下的图像sift特征提取以及匹配

Calculates the gradient magnitude and orientation at a given pixel.

@param img image
@param r pixel row
@param c pixel col
@param mag output as gradient magnitude at pixel (r,c)
@param ori output as gradient orientation at pixel (r,c)

@return Returns 1 if the specified pixel is a valid one and sets mag and
	ori accordingly; otherwise returns 0
*/
int calc_grad_mag_ori( IplImage* img, int r, int c, double* mag, double* ori )
{
	double dx, dy;

	if( r > 0  &&  r < img->height - 1  &&  c > 0  &&  c < img->width - 1 )
	{
		dx = pixval32f( img, r, c+1 ) - pixval32f( img, r, c-1 );
		dy = pixval32f( img, r-1, c ) - pixval32f( img, r+1, c );
		*mag = sqrt( dx*dx + dy*dy );
		*ori = atan2( dy, dx );
		return 1;
	}

	else
		return 0;
}



/*
Gaussian smooths an orientation histogram.

@param hist an orientation histogram
@param n number of bins
*/
void smooth_ori_hist( double* hist, int n )
{
	double prev, tmp, h0 = hist[0];
	int i;

	prev = hist[n-1];
	for( i = 0; i < n; i++ )
	{
		tmp = hist[i];
		hist[i] = 0.25 * prev + 0.5 * hist[i] + 
			0.25 * ( ( i+1 == n )? h0 : hist[i+1] );
		prev = tmp;
	}
}



/*
Finds the magnitude of the dominant orientation in a histogram

@param hist an orientation histogram
@param n number of bins

@return Returns the value of the largest bin in hist
*/
double dominant_ori( double* hist, int n )
{
	double omax;
	int maxbin, i;

	omax = hist[0];
	maxbin = 0;
	for( i = 1; i < n; i++ )
		if( hist[i] > omax )
		{
			omax = hist[i];
			maxbin = i;
		}
	return omax;
}



/*
Interpolates a histogram peak from left, center, and right values
*/
#define interp_hist_peak( l, c, r ) ( 0.5 * ((l)-(r)) / ((l) - 2.0*(c) + (r)) )



/*
Adds features to an array for every orientation in a histogram greater than
a specified threshold.

@param features new features are added to the end of this array
@param hist orientation histogram
@param n number of bins in hist
@param mag_thr new features are added for entries in hist greater than this
@param feat new features are clones of this with different orientations
*/
void add_good_ori_features( CvSeq* features, double* hist, int n,
						   double mag_thr, struct feature* feat )
{
	struct feature* new_feat;
	double bin, PI2 = CV_PI * 2.0;
	int l, r, i;

	for( i = 0; i < n; i++ )
	{
		l = ( i == 0 )? n - 1 : i-1;
		r = ( i + 1 ) % n;

		if( hist[i] > hist[l]  &&  hist[i] > hist[r]  &&  hist[i] >= mag_thr )
		{
			bin = i + interp_hist_peak( hist[l], hist[i], hist[r] );
			bin = ( bin < 0 )? n + bin : ( bin >= n )? bin - n : bin;
			new_feat = clone_feature( feat );
			new_feat->ori = ( ( PI2 * bin ) / n ) - CV_PI;
			cvSeqPush( features, new_feat );
			free( new_feat );
		}
	}
}



/*
Makes a deep copy of a feature

@param feat feature to be cloned

@return Returns a deep copy of feat
*/
struct feature* clone_feature( struct feature* feat )
{
	struct feature* new_feat;
	struct detection_data* ddata;

	new_feat = new_feature();
	ddata = feat_detection_data( new_feat );
	memcpy( new_feat, feat, sizeof( struct feature ) );
	memcpy( ddata, feat_detection_data(feat), sizeof( struct detection_data ) );
	new_feat->feature_data = ddata;

	return new_feat;
}



/*
Computes feature descriptors for features in an array.  Based on Section 6
of Lowe's paper.

@param features array of features
@param gauss_pyr Gaussian scale space pyramid
@param d width of 2D array of orientation histograms
@param n number of bins per orientation histogram
*/
void compute_descriptors( CvSeq* features, IplImage*** gauss_pyr, int d, int n)
{
	struct feature* feat;
	struct detection_data* ddata;
	double*** hist;
	int i, k = features->total;

	for( i = 0; i < k; i++ )
	{
		feat = CV_GET_SEQ_ELEM( struct feature, features, i );
		ddata = feat_detection_data( feat );
		hist = descr_hist( gauss_pyr[ddata->octv][ddata->intvl], ddata->r,
			ddata->c, feat->ori, ddata->scl_octv, d, n );
		hist_to_descr( hist, d, n, feat );
		release_descr_hist( &hist, d );
	}
}



/*
Computes the 2D array of orientation histograms that form the feature
descriptor.  Based on Section 6.1 of Lowe's paper.

@param img image used in descriptor computation
@param r row coord of center of orientation histogram array
@param c column coord of center of orientation histogram array
@param ori canonical orientation of feature whose descr is being computed
@param scl scale relative to img of feature whose descr is being computed
@param d width of 2d array of orientation histograms
@param n bins per orientation histogram

@return Returns a d x d array of n-bin orientation histograms.
*/
double*** descr_hist( IplImage* img, int r, int c, double ori,
					 double scl, int d, int n )
{
	double*** hist;
	double cos_t, sin_t, hist_width, exp_denom, r_rot, c_rot, grad_mag,
		grad_ori, w, rbin, cbin, obin, bins_per_rad, PI2 = 2.0 * CV_PI;
	int radius, i, j;

	hist = calloc( d, sizeof( double** ) );
	for( i = 0; i < d; i++ )
	{
		hist[i] = calloc( d, sizeof( double* ) );
		for( j = 0; j < d; j++ )
			hist[i][j] = calloc( n, sizeof( double ) );
	}

	cos_t = cos( ori );
	sin_t = sin( ori );
	bins_per_rad = n / PI2;
	exp_denom = d * d * 0.5;
	hist_width = SIFT_DESCR_SCL_FCTR * scl;
	radius = hist_width * sqrt(2) * ( d + 1.0 ) * 0.5 + 0.5;
	for( i = -radius; i <= radius; i++ )
		for( j = -radius; j <= radius; j++ )
		{
			/*
			Calculate sample's histogram array coords rotated relative to ori.
			Subtract 0.5 so samples that fall e.g. in the center of row 1 (i.e.
			r_rot = 1.5) have full weight placed in row 1 after interpolation.
			*/
			c_rot = ( j * cos_t - i * sin_t ) / hist_width;
			r_rot = ( j * sin_t + i * cos_t ) / hist_width;
			rbin = r_rot + d / 2 - 0.5;
			cbin = c_rot + d / 2 - 0.5;

			if( rbin > -1.0  &&  rbin < d  &&  cbin > -1.0  &&  cbin < d )
				if( calc_grad_mag_ori( img, r + i, c + j, &grad_mag, &grad_ori ))
				{
					grad_ori -= ori;
					while( grad_ori < 0.0 )
						grad_ori += PI2;
					while( grad_ori >= PI2 )
						grad_ori -= PI2;

					obin = grad_ori * bins_per_rad;
					w = exp( -(c_rot * c_rot + r_rot * r_rot) / exp_denom );
					interp_hist_entry( hist, rbin, cbin, obin, grad_mag * w, d, n );
				}
		}

	return hist;
}



/*
Interpolates an entry into the array of orientation histograms that form
the feature descriptor.

@param hist 2D array of orientation histograms
@param rbin sub-bin row coordinate of entry
@param cbin sub-bin column coordinate of entry
@param obin sub-bin orientation coordinate of entry
@param mag size of entry
@param d width of 2D array of orientation histograms
@param n number of bins per orientation histogram
*/
void interp_hist_entry( double*** hist, double rbin, double cbin,
					   double obin, double mag, int d, int n )
{
	double d_r, d_c, d_o, v_r, v_c, v_o;
	double** row, * h;
	int r0, c0, o0, rb, cb, ob, r, c, o;

	r0 = cvFloor( rbin );
	c0 = cvFloor( cbin );
	o0 = cvFloor( obin );
	d_r = rbin - r0;
	d_c = cbin - c0;
	d_o = obin - o0;

	/*
	The entry is distributed into up to 8 bins.  Each entry into a bin
	is multiplied by a weight of 1 - d for each dimension, where d is the
	distance from the center value of the bin measured in bin units.
	*/
	for( r = 0; r <= 1; r++ )
	{
		rb = r0 + r;
		if( rb >= 0  &&  rb < d )
		{
			v_r = mag * ( ( r == 0 )? 1.0 - d_r : d_r );
			row = hist[rb];
			for( c = 0; c <= 1; c++ )
			{
				cb = c0 + c;
				if( cb >= 0  &&  cb < d )
				{
					v_c = v_r * ( ( c == 0 )? 1.0 - d_c : d_c );
					h = row[cb];
					for( o = 0; o <= 1; o++ )
					{
						ob = ( o0 + o ) % n;
						v_o = v_c * ( ( o == 0 )? 1.0 - d_o : d_o );
						h[ob] += v_o;
					}
				}
			}
		}
	}
}



/*
Converts the 2D array of orientation histograms into a feature's descriptor
vector.

@param hist 2D array of orientation histograms
@param d width of hist
@param n bins per histogram
@param feat feature into which to store descriptor
*/
void hist_to_descr( double*** hist, int d, int n, struct feature* feat )
{
	int int_val, i, r, c, o, k = 0;

	for( r = 0; r < d; r++ )
		for( c = 0; c < d; c++ )
			for( o = 0; o < n; o++ )
				feat->descr[k++] = hist[r][c][o];

	feat->d = k;
	normalize_descr( feat );
	for( i = 0; i < k; i++ )
		if( feat->descr[i] > SIFT_DESCR_MAG_THR )
			feat->descr[i] = SIFT_DESCR_MAG_THR;
	normalize_descr( feat );

	/* convert floating-point descriptor to integer valued descriptor */
	for( i = 0; i < k; i++ )
	{
		int_val = SIFT_INT_DESCR_FCTR * feat->descr[i];
		feat->descr[i] = MIN( 255, int_val );
	}
}



/*
Normalizes a feature's descriptor vector to unitl length

@param feat feature
*/
void normalize_descr( struct feature* feat )
{
	double cur, len_inv, len_sq = 0.0;
	int i, d = feat->d;

	for( i = 0; i < d; i++ )
	{
		cur = feat->descr[i];
		len_sq += cur*cur;
	}
	len_inv = 1.0 / sqrt( len_sq );
	for( i = 0; i < d; i++ )
		feat->descr[i] *= len_inv;
}



/*
Compares features for a decreasing-scale ordering.  Intended for use with
CvSeqSort

@param feat1 first feature
@param feat2 second feature
@param param unused

@return Returns 1 if feat1's scale is greater than feat2's, -1 if vice versa,
and 0 if their scales are equal
*/
int feature_cmp( void* feat1, void* feat2, void* param )
{
	struct feature* f1 = (struct feature*) feat1;
	struct feature* f2 = (struct feature*) feat2;

	if( f1->scl < f2->scl )
		return 1;
	if( f1->scl > f2->scl )
		return -1;
	return 0;
}



/*
De-allocates memory held by a descriptor histogram

@param hist pointer to a 2D array of orientation histograms
@param d width of hist
*/
void release_descr_hist( double**** hist, int d )
{
	int i, j;

	for( i = 0; i < d; i++)
	{
		for( j = 0; j < d; j++ )
			free( (*hist)[i][j] );
		free( (*hist)[i] );
	}
	free( *hist );
	*hist = NULL;
}


/*
De-allocates memory held by a scale space pyramid

@param pyr scale space pyramid
@param octvs number of octaves of scale space
@param n number of images per octave
*/
void release_pyr( IplImage**** pyr, int octvs, int n )
{
	int i, j;
	for( i = 0; i < octvs; i++ )
	{
		for( j = 0; j < n; j++ )
			cvReleaseImage( &(*pyr)[i][j] );
		free( (*pyr)[i] );
	}
	free( *pyr );
	*pyr = NULL;
}