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OpenCV245之SURF源码分析
一、fastHessianDetector函数分析
（1）参数
const Mat& sum 积分图片
const Mat& mask_sum
vector<KeyPoint>& keypoints 关键点
int nOctaves 金字塔的阶数
int nOctaveLayers 每阶金字塔的中间层数
float hessianThreshold Hessian阀值
（2）函数体
static void fastHessianDetector( const Mat& sum, const Mat& mask_sum, vector<KeyPoint>& keypoints,
int nOctaves, int nOctaveLayers, float hessianThreshold )
{
const int SAMPLE_STEP0 = 1;
int nTotalLayers = (nOctaveLayers+2)*nOctaves;//所有层的数目
int nMiddleLayers = nOctaveLayers*nOctaves;//所有中间层的数目
vector<Mat> dets(nTotalLayers);//所有层dets
vector<Mat> traces(nTotalLayers);//所有层tra
vector<int> sizes(nTotalLayers);//所有层滤波器尺寸
vector<int> sampleSteps(nTotalLayers);//所有层采样比例
vector<int> middleIndices(nMiddleLayers);//所有中间层的索引
keypoints.clear();
// Allocate space and calculate properties of each layer
int index = 0, middleIndex = 0, step = SAMPLE_STEP0;//第一阶梯的采样比例设置为1
for( int octave = 0; octave < nOctaves; octave++ )//阶梯循环
{
for( int layer = 0; layer < nOctaveLayers+2; layer++ )//层循环
{
//申请内存
dets[index].create( (sum.rows-1)/step, (sum.cols-1)/step, CV_32F );
traces[index].create( (sum.rows-1)/step, (sum.cols-1)/step, CV_32F );
sizes[index] = (SURF_HAAR_SIZE0 + SURF_HAAR_SIZE_INC*layer) << octave;//滤波器大小
sampleSteps[index] = step;//图片比例尺寸
if( 0 < layer && layer <= nOctaveLayers )//记录中间层索引
middleIndices[middleIndex++] = index;
index++;
}
step *= 2;//进入下一阶梯时，比例放大两倍
}
//生成所有层数据，SURFBuildInvoker函数在第（3）点分析
parallel_for( BlockedRange(0, nTotalLayers),
SURFBuildInvoker(sum, sizes, sampleSteps, dets, traces) );
// Find maxima in the determinant of the hessian
parallel_for( BlockedRange(0, nMiddleLayers),
SURFFindInvoker(sum, mask_sum, dets, traces, sizes,
sampleSteps, middleIndices, keypoints,
nOctaveLayers, hessianThreshold) );
std::sort(keypoints.begin(), keypoints.end(), KeypointGreater());
} （3）SURFBuildInvoker
构建每一阶各层的数据，具体函数calcLayerDetAndTrace看第（4）点
struct SURFBuildInvoker
{
SURFBuildInvoker( const Mat& _sum, const vector<int>& _sizes,
const vector<int>& _sampleSteps,
vector<Mat>& _dets, vector<Mat>& _traces )
{
sum = &_sum;
sizes = &_sizes;
sampleSteps = &_sampleSteps;
dets = &_dets;
traces = &_traces;
}
void operator()(const BlockedRange& range) const
{
for( int i=range.begin(); i<range.end(); i++ )
calcLayerDetAndTrace( *sum, (*sizes)[i], (*sampleSteps)[i], (*dets)[i], (*traces)[i] );
}
const Mat *sum;
const vector<int> *sizes;
const vector<int> *sampleSteps;
vector<Mat>* dets;
vector<Mat>* traces;
};（4）calcLayerDetAndTrace
功能：计算每一层的数据
参数：
const Mat& sum 积分图像
int size 滤波器大小
sampleStep 图像采样比例
Mat& det det数据
Mat& trace trace数据
函数体：
static void calcLayerDetAndTrace( const Mat& sum, int size, int sampleStep,
Mat& det, Mat& trace )
{
const int NX=3, NY=3, NXY=4;//3个滤波器的各个子块
//滤波器各个子块在起始位置的X起始坐标，Y起始坐标，X结束坐标，Y结束坐标，以及加权数
const int dx_s[NX][5] = { {0, 2, 3, 7, 1}, {3, 2, 6, 7, -2}, {6, 2, 9, 7, 1} };
const int dy_s[NY][5] = { {2, 0, 7, 3, 1}, {2, 3, 7, 6, -2}, {2, 6, 7, 9, 1} };
const int dxy_s[NXY][5] = { {1, 1, 4, 4, 1}, {5, 1, 8, 4, -1}, {1, 5, 4, 8, -1}, {5, 5, 8, 8, 1} };
/*
SurfHF结构体：滤波器各个子块4个坐标
struct SurfHF
{
int p0, p1, p2, p3;
float w;
SurfHF(): p0(0), p1(0), p2(0), p3(0), w(0) {}
};
*/
SurfHF Dx[NX], Dy[NY], Dxy[NXY];
if( size > sum.rows-1 || size > sum.cols-1 )
return;
//计算出每一层滤波器各个子块在原积分图像中的起始坐标，在第（5）点分析
resizeHaarPattern( dx_s , Dx , NX , 9, size, sum.cols );
resizeHaarPattern( dy_s , Dy , NY , 9, size, sum.cols );
resizeHaarPattern( dxy_s, Dxy, NXY, 9, size, sum.cols );
/* The integral image 'sum' is one pixel bigger than the source image */
//在sampleStep比例下计算的长宽
//减去滤波器大小size防止越界，之所以是size而不是size/2,是因为把起始与结束的越界范围一起算进来
int samples_i = 1+(sum.rows-1-size)/sampleStep;
int samples_j = 1+(sum.cols-1-size)/sampleStep;
/* Ignore pixels where some of the kernel is outside the image */
//计算在缩小sampleStep比例后得越界大小
int margin = (size/2)/sampleStep;
for( int i = 0; i < samples_i; i++ )
{
//获取原积分图像在第i*sampleStep行的指针
const int* sum_ptr = sum.ptr<int>(i*sampleStep);
//获取det、trace在第i+margin行、第margin个的数据指针
float* det_ptr = &det.at<float>(i+margin, margin);
float* trace_ptr = &trace.at<float>(i+margin, margin);
for( int j = 0; j < samples_j; j++ )
{
//计算滤波器子块所覆盖区域的加权后的积分值，在第（6）点分析
float dx = calcHaarPattern( sum_ptr, Dx , 3 );
float dy = calcHaarPattern( sum_ptr, Dy , 3 );
float dxy = calcHaarPattern( sum_ptr, Dxy, 4 );
//调至原积分图像中要计算下一个坐标点
sum_ptr += sampleStep;
//计算det_ptr、trace_ptr
det_ptr[j] = dx*dy - 0.81f*dxy*dxy;
trace_ptr[j] = dx + dy;
}
}
}（5）resizeHaarPattern
功能：生成新滤波器子块在原图像中的坐标点
参数：
const int src[][5]
SurfHF* dst
int n 滤波器子块数目
int oldSize 旧滤波器大小
int newSize 新滤波器大小
int widthStep 原图像宽度
函数体：
static void
resizeHaarPattern( const int src[][5], SurfHF* dst, int n, int oldSize, int newSize, int widthStep )
{
float ratio = (float)newSize/oldSize;
for( int k = 0; k < n; k++ )
{
//计算新滤波器子块4个点坐标
int dx1 = cvRound( ratio*src[k][0] );//四舍五入
int dy1 = cvRound( ratio*src[k][1] );
int dx2 = cvRound( ratio*src[k][2] );
int dy2 = cvRound( ratio*src[k][3] );
//计算4个点在原图像中的坐标
dst[k].p0 = dy1*widthStep + dx1;
dst[k].p1 = dy2*widthStep + dx1;
dst[k].p2 = dy1*widthStep + dx2;
dst[k].p3 = dy2*widthStep + dx2;
//src[k][4]为加权数，1.0 /((float)(dx2-dx1)*(dy2-dy1))求此子块的均值
dst[k].w = src[k][4]/((float)(dx2-dx1)*(dy2-dy1));
}
}（6）calcHaarPattern
功能：计算滤波器子块所覆盖区域的加权后的积分值
参数：
const int* origin 原积分图像要计算的坐标点指针
const SurfHF* f resizeHaarPattern中计算出的坐标定位点，或者说滤波器子块的相对于中心点的坐标关系
int n 子块数目
函数体：
inline float calcHaarPattern( const int* origin, const SurfHF* f, int n )
{
double d = 0;
for( int k = 0; k < n; k++ )
d += (origin[f[k].p0] + origin[f[k].p3] - origin[f[k].p1] - origin[f[k].p2])*f[k].w;
return (float)d;
}分析完SURFBuildInvoker以及其各个子函数，现在下面开始分析利用上面得到的金字塔数据来进行非极大值抑制
（7）SURFFindInvoker
功能：金字塔数据来进行非极大值抑制，找到极值点并插值
findMaximaInLayer在第（8）点分析
参数：
const Mat& _sum 原积分图数据
const Mat& _mask_sum,
const vector<Mat>& _dets
const vector<Mat>& _traces
const vector<int>& _sizes 滤波器大小
const vector<int>& _sampleSteps 图片比例尺寸
const vector<int>& _middleIndices 中间层索引
vector<KeyPoint>& _keypoints 关键点
int _nOctaveLayers 每一阶中间层数目
float _hessianThreshold hessian阀值
函数体：
struct SURFFindInvoker
{
SURFFindInvoker( const Mat& _sum, const Mat& _mask_sum,
const vector<Mat>& _dets, const vector<Mat>& _traces,
const vector<int>& _sizes, const vector<int>& _sampleSteps,
const vector<int>& _middleIndices, vector<KeyPoint>& _keypoints,
int _nOctaveLayers, float _hessianThreshold )
{
sum = &_sum;
mask_sum = &_mask_sum;
dets = &_dets;
traces = &_traces;
sizes = &_sizes;
sampleSteps = &_sampleSteps;
middleIndices = &_middleIndices;
keypoints = &_keypoints;
nOctaveLayers = _nOctaveLayers;
hessianThreshold = _hessianThreshold;
}
//在第（8）点分析
static void findMaximaInLayer( const Mat& sum, const Mat& mask_sum,
const vector<Mat>& dets, const vector<Mat>& traces,
const vector<int>& sizes, vector<KeyPoint>& keypoints,
int octave, int layer, float hessianThreshold, int sampleStep );
void operator()(const BlockedRange& range) const
{
for( int i=range.begin(); i<range.end(); i++ )
{
int layer = (*middleIndices)[i];
int octave = i / nOctaveLayers;
findMaximaInLayer( *sum, *mask_sum, *dets, *traces, *sizes,
*keypoints, octave, layer, hessianThreshold,
(*sampleSteps)[layer] );
}
}
const Mat *sum;
const Mat *mask_sum;
const vector<Mat>* dets;
const vector<Mat>* traces;
const vector<int>* sizes;
const vector<int>* sampleSteps;
const vector<int>* middleIndices;
vector<KeyPoint>* keypoints;
int nOctaveLayers;
float hessianThreshold;
#ifdef HAVE_TBB
static tbb::mutex findMaximaInLayer_m;
#endif
};（8）findMaximaInLayer
功能：确定是否为3X3X3中的极大值
参数：
const Mat& sum 原积分图像
const Mat& mask_sum
const vector<Mat>& dets
const vector<Mat>& traces
const vector<int>& sizes 滤波器大小数组
vector<KeyPoint>& keypoints
int octave 中间层所在的阶
int layer 中间层索引
float hessianThreshold hessian阀值
int sampleStep 所在中间层的图像比例
函数体：
void SURFFindInvoker::findMaximaInLayer( const Mat& sum, const Mat& mask_sum,
const vector<Mat>& dets, const vector<Mat>& traces,
const vector<int>& sizes, vector<KeyPoint>& keypoints,
int octave, int layer, float hessianThreshold, int sampleStep )
{
// Wavelet Data
const int NM=1;
const int dm[NM][5] = { {0, 0, 9, 9, 1} };
SurfHF Dm;
int size = sizes[layer];//获取所在层的滤波器大小
// The integral image 'sum' is one pixel bigger than the source image
//计算缩小sampleStep比例后的图像的cols,rows
int layer_rows = (sum.rows-1)/sampleStep;
int layer_cols = (sum.cols-1)/sampleStep;
// Ignore pixels without a 3x3x3 neighbourhood in the layer above
//通过最顶层滤波器所在越界区域大小，因为最顶层滤波器在比较的3层中是最大的
int margin = (sizes[layer+1]/2)/sampleStep+1;
if( !mask_sum.empty() )
resizeHaarPattern( dm, &Dm, NM, 9, size, mask_sum.cols );
int step = (int)(dets[layer].step/dets[layer].elemSize());//获取所在层的像素宽
for( int i = margin; i < layer_rows - margin; i++ )
{
const float* det_ptr = dets[layer].ptr<float>(i);
const float* trace_ptr = traces[layer].ptr<float>(i);
for( int j = margin; j < layer_cols-margin; j++ )
{
float val0 = det_ptr[j];
if( val0 > hessianThreshold )//剔除对比度低的点
{
/* Coordinates for the start of the wavelet in the sum image. There
is some integer division involved, so don't try to simplify this
(cancel out sampleStep) without checking the result is the same */
//计算小波变换在原积分图像中的起始坐标
int sum_i = sampleStep*(i-(size/2)/sampleStep);
int sum_j = sampleStep*(j-(size/2)/sampleStep);
/* The 3x3x3 neighbouring samples around the maxima.
The maxima is included at N9[1][4] */
const float *det1 = &dets[layer-1].at<float>(i, j);//比较的最底层
const float *det2 = &dets[layer].at<float>(i, j);//比较的中间层
const float *det3 = &dets[layer+1].at<float>(i, j);//比较的最顶层
//获取要比较的26个点数据
float N9[3][9] = { { det1[-step-1], det1[-step], det1[-step+1],
det1[-1] , det1[0] , det1[1],
det1[step-1] , det1[step] , det1[step+1] },
{ det2[-step-1], det2[-step], det2[-step+1],
det2[-1] , det2[0] , det2[1],
det2[step-1] , det2[step] , det2[step+1] },
{ det3[-step-1], det3[-step], det3[-step+1],
det3[-1] , det3[0] , det3[1],
det3[step-1] , det3[step] , det3[step+1] } };
/* Check the mask - why not just check the mask at the center of the wavelet? */
if( !mask_sum.empty() )
{
const int* mask_ptr = &mask_sum.at<int>(sum_i, sum_j);
float mval = calcHaarPattern( mask_ptr, &Dm, 1 );
if( mval < 0.5 )
continue;
}//!mask_sum.empty()
/* Non-maxima suppression. val0 is at N9[1][4]*/
//比较val0是不是3X3区域内的极大值
if( val0 > N9[0][0] && val0 > N9[0][1] && val0 > N9[0][2] &&
val0 > N9[0][3] && val0 > N9[0][4] && val0 > N9[0][5] &&
val0 > N9[0][6] && val0 > N9[0][7] && val0 > N9[0][8] &&
val0 > N9[1][0] && val0 > N9[1][1] && val0 > N9[1][2] &&
val0 > N9[1][3] && val0 > N9[1][5] &&
val0 > N9[1][6] && val0 > N9[1][7] && val0 > N9[1][8] &&
val0 > N9[2][0] && val0 > N9[2][1] && val0 > N9[2][2] &&
val0 > N9[2][3] && val0 > N9[2][4] && val0 > N9[2][5] &&
val0 > N9[2][6] && val0 > N9[2][7] && val0 > N9[2][8] )
{
/* Calculate the wavelet center coordinates for the maxima */
//获取小波变化的中心坐标
float center_i = sum_i + (size-1)*0.5f;//y
float center_j = sum_j + (size-1)*0.5f;//x
/*
kpt.pt.x = center_j;//关键点在原图像中的坐标
kpt.pt.y = center_i;
kpt.size = (float)sizes[layer]//获取所在层的滤波器大小
kpt.angle = -1;//方向未定
kpt.response = val0;//海参响应值
kpt.class_id = CV_SIGN(trace_ptr[j]);//CV_SIGN(trace_ptr[j])判断dx+dy是否大于0,是为1，不是为-1
*/
KeyPoint kpt( center_j, center_i, (float)sizes[layer],
-1, val0, octave, CV_SIGN(trace_ptr[j]) );
/* Interpolate maxima location within the 3x3x3 neighbourhood */
int ds = size - sizes[layer-1];//中间层与底层的滤波器尺寸差
//插值，在第（9）点分析
int interp_ok = interpolateKeypoint( N9, sampleStep, sampleStep, ds, kpt );
/* Sometimes the interpolation step gives a negative size etc. */
if( interp_ok )
{
/*printf( "KeyPoint %f %f %d\n", point.pt.x, point.pt.y, point.size );*/
//#ifdef HAVE_TBB
// tbb::mutex::scoped_lock lock(findMaximaInLayer_m);
//#endif
keypoints.push_back(kpt);
}//interp_ok
}//Non-maxima suppression
}//val0 > hessianThreshold
}//j
}//i
}（9）interpolateKeypoint
功能：进行像素点插值，并判断是否合格
参数：
float N9[3][9] 3X3X3个数据
int dx X方向上的图像比例
int dy Y方向上的图像比例
int ds 与邻近层的滤波器尺寸差
KeyPoint& kpt 关键点
函数体：
static int
interpolateKeypoint( float N9[3][9], int dx, int dy, int ds, KeyPoint& kpt )
{
// 像素在x，y，s三个方向的偏导数
Vec3f b(-(N9[1][5]-N9[1][3])/2, // Negative 1st deriv with respect to x
-(N9[1][7]-N9[1][1])/2, // Negative 1st deriv with respect to y
-(N9[2][4]-N9[0][4])/2); // Negative 1st deriv with respect to s
// 像素的Hessian矩阵
Matx33f A(
N9[1][3]-2*N9[1][4]+N9[1][5], // 2nd deriv x, x
(N9[1][8]-N9[1][6]-N9[1][2]+N9[1][0])/4, // 2nd deriv x, y
(N9[2][5]-N9[2][3]-N9[0][5]+N9[0][3])/4, // 2nd deriv x, s
(N9[1][8]-N9[1][6]-N9[1][2]+N9[1][0])/4, // 2nd deriv x, y
N9[1][1]-2*N9[1][4]+N9[1][7], // 2nd deriv y, y
(N9[2][7]-N9[2][1]-N9[0][7]+N9[0][1])/4, // 2nd deriv y, s
(N9[2][5]-N9[2][3]-N9[0][5]+N9[0][3])/4, // 2nd deriv x, s
(N9[2][7]-N9[2][1]-N9[0][7]+N9[0][1])/4, // 2nd deriv y, s
N9[0][4]-2*N9[1][4]+N9[2][4]); // 2nd deriv s, s
Vec3f x = A.solve(b, DECOMP_LU);
bool ok = (x[0] != 0 || x[1] != 0 || x[2] != 0) &&
std::abs(x[0]) <= 1 && std::abs(x[1]) <= 1 && std::abs(x[2]) <= 1;
if( ok )
{
//计算插值后的坐标与滤波器尺寸
kpt.pt.x += x[0]*dx;
kpt.pt.y += x[1]*dy;
kpt.size = (float)cvRound( kpt.size + x[2]*ds );
}
return ok;
}SURF的金字塔构建、局部极大值提取至此完成，下面是候选点的描述符提取算法分析。
二、SURFInvoker
参数：
const Mat& _img
const Mat& _sum 原积分图像
vector<KeyPoint>& _keypoints 关键点
Mat& _descriptors 描述符
bool _extended 是否把64维描述符拓展到128维
bool _upright 是否考虑旋转,false表示要考虑，upright为垂直的意思
函数体：
struct SURFInvoker
{
enum { ORI_RADIUS = 6, ORI_WIN = 60, PATCH_SZ = 20 };
SURFInvoker( const Mat& _img, const Mat& _sum,
vector<KeyPoint>& _keypoints, Mat& _descriptors,
bool _extended, bool _upright )
{
keypoints = &_keypoints;
descriptors = &_descriptors;
img = &_img;
sum = &_sum;
extended = _extended;
upright = _upright;
// Simple bound for number of grid points in circle of radius ORI_RADIUS
const int nOriSampleBound = (2*ORI_RADIUS+1)*(2*ORI_RADIUS+1);
// Allocate arrays
apt.resize(nOriSampleBound);// 特征点周围用于描述方向的邻域的点
aptw.resize(nOriSampleBound);// 描述方向时的高斯权
//以20s为边长的矩形窗口为特征描述子计算使用的窗口
DW.resize(PATCH_SZ*PATCH_SZ); //特征描述子的高斯加权，PATHC_SZ为特征描述子的区域大小 20s(s这里初始为1了)
/* Coordinates and weights of samples used to calculate orientation */
//生成高斯模板，用于描述方向时的高斯权
//（s=1.2?L/9为特征点的尺度）这里把s近似为1 ORI_DADIUS = 6s
//nOriSampleBound为 矩形框内点的个数
Mat G_ori = getGaussianKernel( 2*ORI_RADIUS+1, SURF_ORI_SIGMA, CV_32F );
nOriSamples = 0;
for( int i = -ORI_RADIUS; i <= ORI_RADIUS; i++ )
{
for( int j = -ORI_RADIUS; j <= ORI_RADIUS; j++ )
{
if( i*i + j*j <= ORI_RADIUS*ORI_RADIUS )
{
apt[nOriSamples] = cvPoint(i,j);
aptw[nOriSamples++] = G_ori.at<float>(i+ORI_RADIUS,0) * G_ori.at<float>(j+ORI_RADIUS,0);
}
}
}
CV_Assert( nOriSamples <= nOriSampleBound );
/* Gaussian used to weight descriptor samples */
//用于生成特征描述子的 高斯加权 sigma = 3.3s(s初取1)
Mat G_desc = getGaussianKernel( PATCH_SZ, SURF_DESC_SIGMA, CV_32F );
for( int i = 0; i < PATCH_SZ; i++ )
{
for( int j = 0; j < PATCH_SZ; j++ )
DW[i*PATCH_SZ+j] = G_desc.at<float>(i,0) * G_desc.at<float>(j,0);
}
}
void operator()(const BlockedRange& range) const
{
/* X and Y gradient wavelet data */
/* x与y方向上的 Harr小波,参数为4s */
const int NX=2, NY=2;
const int dx_s[NX][5] = {{0, 0, 2, 4, -1}, {2, 0, 4, 4, 1}};
const int dy_s[NY][5] = {{0, 0, 4, 2, 1}, {0, 2, 4, 4, -1}};
// Optimisation is better using nOriSampleBound than nOriSamples for
// array lengths. Maybe because it is a constant known at compile time
const int nOriSampleBound =(2*ORI_RADIUS+1)*(2*ORI_RADIUS+1);
//用于计算HarrX响应、HarrY响应、特征点主方向
float X[nOriSampleBound], Y[nOriSampleBound], angle[nOriSampleBound];
uchar PATCH[PATCH_SZ+1][PATCH_SZ+1];
// 20s * 20s区域的 梯度值
float DX[PATCH_SZ][PATCH_SZ], DY[PATCH_SZ][PATCH_SZ];
CvMat matX = cvMat(1, nOriSampleBound, CV_32F, X);
CvMat matY = cvMat(1, nOriSampleBound, CV_32F, Y);
CvMat _angle = cvMat(1, nOriSampleBound, CV_32F, angle);
Mat _patch(PATCH_SZ+1, PATCH_SZ+1, CV_8U, PATCH);
int dsize = extended ? 128 : 64;//是否把64维描述符拓展到128维
int k, k1 = range.begin(), k2 = range.end();//keypoints数目
float maxSize = 0;
for( k = k1; k < k2; k++ )
{
//找出关键点中Harr小波模板最大尺寸
maxSize = std::max(maxSize, (*keypoints)[k].size);
}
// maxSize*1.2/9 表示最大的尺度 s
int imaxSize = std::max(cvCeil((PATCH_SZ+1)*maxSize*1.2f/9.0f), 1);
//用于20*s邻域窗口缓存
Ptr<CvMat> winbuf = cvCreateMat( 1, imaxSize*imaxSize, CV_8U );
for( k = k1; k < k2; k++ )
{
int i, j, kk, nangle;
float* vec;
SurfHF dx_t[NX], dy_t[NY];
KeyPoint& kp = (*keypoints)[k];
float size = kp.size;//此关键点的Harr小波模板尺寸
Point2f center = kp.pt;//获取关键点坐标，座位模板中心点
/* The sampling intervals and wavelet sized for selecting an orientation
and building the keypoint descriptor are defined relative to 's' */
float s = size*1.2f/9.0f;
/* To find the dominant orientation, the gradients in x and y are
sampled in a circle of radius 6s using wavelets of size 4s.
We ensure the gradient wavelet size is even to ensure the
wavelet pattern is balanced and symmetric around its center */
int grad_wav_size = 2*cvRound( 2*s );
//小波模板尺寸超出图像范围，把关键点设为无意义的点
if( sum->rows < grad_wav_size || sum->cols < grad_wav_size )
{
/* when grad_wav_size is too big,
* the sampling of gradient will be meaningless
* mark keypoint for deletion. */
kp.size = -1;
continue;
}
float descriptor_dir = 360.f - 90.f;
//考虑旋转
if (upright == 0)
{
//生成新滤波器子块在原图像中的坐标点
resizeHaarPattern( dx_s, dx_t, NX, 4, grad_wav_size, sum->cols );
resizeHaarPattern( dy_s, dy_t, NY, 4, grad_wav_size, sum->cols );
for( kk = 0, nangle = 0; kk < nOriSamples; kk++ )//nOriSamples = 169
{
//定位相关扫描角度内的坐标点
int x = cvRound( center.x + apt[kk].x*s - (float)(grad_wav_size-1)/2 );
int y = cvRound( center.y + apt[kk].y*s - (float)(grad_wav_size-1)/2 );
if( y < 0 || y >= sum->rows - grad_wav_size ||
x < 0 || x >= sum->cols - grad_wav_size )
continue;
const int* ptr = &sum->at<int>(y, x);//获取原积分图像素值
float vx = calcHaarPattern( ptr, dx_t, 2 );//计算HarrX响应
float vy = calcHaarPattern( ptr, dy_t, 2 );//计算HarrY响应
//加权滤波
X[nangle] = vx*aptw[kk];
Y[nangle] = vy*aptw[kk];
nangle++;
}//k
if( nangle == 0 )
{
// No gradient could be sampled because the keypoint is too
// near too one or more of the sides of the image. As we
// therefore cannot find a dominant direction, we skip this
// keypoint and mark it for later deletion from the sequence.
kp.size = -1;
continue;
}//nangle == 0
matX.cols = matY.cols = _angle.cols = nangle;
cvCartToPolar( &matX, &matY, 0, &_angle, 1 );
//求出主方向
float bestx = 0, besty = 0, descriptor_mod = 0;
for( i = 0; i < 360; i += SURF_ORI_SEARCH_INC )
{
float sumx = 0, sumy = 0, temp_mod;
for( j = 0; j < nangle; j++ )
{
int d = std::abs(cvRound(angle[j]) - i);
if( d < ORI_WIN/2 || d > 360-ORI_WIN/2 )
{
sumx += X[j];
sumy += Y[j];
}//d
}//j
temp_mod = sumx*sumx + sumy*sumy;
if( temp_mod > descriptor_mod )
{
descriptor_mod = temp_mod;
bestx = sumx;
besty = sumy;
}//temp_mod > descriptor_mod
}//i
descriptor_dir = fastAtan2( -besty, bestx );
}//upright == 0
kp.angle = descriptor_dir;
if( !descriptors || !descriptors->data )
continue;
/* Extract a window of pixels around the keypoint of size 20s */
/* 用特征点周围20*s为边长的邻域 计算特征描述子 */
int win_size = (int)((PATCH_SZ+1)*s);//20*s邻域窗口大小
CV_Assert( winbuf->cols >= win_size*win_size );
Mat win(win_size, win_size, CV_8U, winbuf->data.ptr);
if( !upright )//考虑旋转
{
descriptor_dir *= (float)(CV_PI/180);//转为弧度
float sin_dir = -std::sin(descriptor_dir);
float cos_dir = std::cos(descriptor_dir);
/* Subpixel interpolation version (slower). Subpixel not required since
the pixels will all get averaged when we scale down to 20 pixels */
/*
float w[] = { cos_dir, sin_dir, center.x,
-sin_dir, cos_dir , center.y };
CvMat W = cvMat(2, 3, CV_32F, w);
cvGetQuadrangleSubPix( img, &win, &W );
*/
//旋转到主方向
float win_offset = -(float)(win_size-1)/2;
float start_x = center.x + win_offset*cos_dir + win_offset*sin_dir;
float start_y = center.y - win_offset*sin_dir + win_offset*cos_dir;
uchar* WIN = win.data;
//#if 0
// // Nearest neighbour version (faster)
// for( i = 0; i < win_size; i++, start_x += sin_dir, start_y += cos_dir )
// {
// float pixel_x = start_x;
// float pixel_y = start_y;
// for( j = 0; j < win_size; j++, pixel_x += cos_dir, pixel_y -= sin_dir )
// {
// int x = std::min(std::max(cvRound(pixel_x), 0), img->cols-1);
// int y = std::min(std::max(cvRound(pixel_y), 0), img->rows-1);
// WIN[i*win_size + j] = img->at<uchar>(y, x);
// }
// }
//#else
int ncols1 = img->cols-1, nrows1 = img->rows-1;
size_t imgstep = img->step;
for( i = 0; i < win_size; i++, start_x += sin_dir, start_y += cos_dir )
{
double pixel_x = start_x;
double pixel_y = start_y;
for( j = 0; j < win_size; j++, pixel_x += cos_dir, pixel_y -= sin_dir )
{
int ix = cvFloor(pixel_x), iy = cvFloor(pixel_y);
if( (unsigned)ix < (unsigned)ncols1 &&
(unsigned)iy < (unsigned)nrows1 )
{
float a = (float)(pixel_x - ix), b = (float)(pixel_y - iy);
const uchar* imgptr = &img->at<uchar>(iy, ix);
WIN[i*win_size + j] = (uchar)
cvRound(imgptr[0]*(1.f - a)*(1.f - b) +
imgptr[1]*a*(1.f - b) +
imgptr[imgstep]*(1.f - a)*b +
imgptr[imgstep+1]*a*b);
}
else
{
int x = std::min(std::max(cvRound(pixel_x), 0), ncols1);
int y = std::min(std::max(cvRound(pixel_y), 0), nrows1);
WIN[i*win_size + j] = img->at<uchar>(y, x);
}
}//j
}//i
#endif
}//考虑旋转
else//不考虑旋转
{
// extract rect - slightly optimized version of the code above
// TODO: find faster code, as this is simply an extract rect operation,
// e.g. by using cvGetSubRect, problem is the border processing
// descriptor_dir == 90 grad
// sin_dir == 1
// cos_dir == 0
float win_offset = -(float)(win_size-1)/2;
int start_x = cvRound(center.x + win_offset);
int start_y = cvRound(center.y - win_offset);
uchar* WIN = win.data;
for( i = 0; i < win_size; i++, start_x++ )
{
int pixel_x = start_x;
int pixel_y = start_y;
for( j = 0; j < win_size; j++, pixel_y-- )
{
int x = MAX( pixel_x, 0 );
int y = MAX( pixel_y, 0 );
x = MIN( x, img->cols-1 );
y = MIN( y, img->rows-1 );
WIN[i*win_size + j] = img->at<uchar>(y, x);
}//j
}//i
}//不考虑旋转
// Scale the window to size PATCH_SZ so each pixel's size is s. This
// makes calculating the gradients with wavelets of size 2s easy
resize(win, _patch, _patch.size(), 0, 0, INTER_AREA);
// Calculate gradients in x and y with wavelets of size 2s
for( i = 0; i < PATCH_SZ; i++ )
for( j = 0; j < PATCH_SZ; j++ )
{
float dw = DW[i*PATCH_SZ + j];
float vx = (PATCH[i][j+1] - PATCH[i][j] + PATCH[i+1][j+1] - PATCH[i+1][j])*dw;
float vy = (PATCH[i+1][j] - PATCH[i][j] + PATCH[i+1][j+1] - PATCH[i][j+1])*dw;
DX[i][j] = vx;
DY[i][j] = vy;
}
// Construct the descriptor
vec = descriptors->ptr<float>(k);
for( kk = 0; kk < dsize; kk++ )
vec[kk] = 0;
double square_mag = 0;
if( extended )// 128-bin descriptor
{
// 128-bin descriptor
for( i = 0; i < 4; i++ )
for( j = 0; j < 4; j++ )
{
for(int y = i*5; y < i*5+5; y++ )
{
for(int x = j*5; x < j*5+5; x++ )
{
float tx = DX[y][x], ty = DY[y][x];
if( ty >= 0 )
{
vec[0] += tx;
vec[1] += (float)fabs(tx);
} else {
vec[2] += tx;
vec[3] += (float)fabs(tx);
}
if ( tx >= 0 )
{
vec[4] += ty;
vec[5] += (float)fabs(ty);
} else {
vec[6] += ty;
vec[7] += (float)fabs(ty);
}
}
}
for( kk = 0; kk < 8; kk++ )
square_mag += vec[kk]*vec[kk];
vec += 8;
}
}
else// 64-bin descriptor
{
// 64-bin descriptor
for( i = 0; i < 4; i++ )
for( j = 0; j < 4; j++ )
{
for(int y = i*5; y < i*5+5; y++ )
{
for(int x = j*5; x < j*5+5; x++ )
{
float tx = DX[y][x], ty = DY[y][x];
vec[0] += tx; vec[1] += ty;
vec[2] += (float)fabs(tx); vec[3] += (float)fabs(ty);
}
}
for( kk = 0; kk < 4; kk++ )
square_mag += vec[kk]*vec[kk];
vec+=4;
}
}
// unit vector is essential for contrast invariance
// 归一化描述子以满足光照不变性
vec = descriptors->ptr<float>(k);
float scale = (float)(1./(sqrt(square_mag) + DBL_EPSILON));
for( kk = 0; kk < dsize; kk++ )
vec[kk] *= scale;
}
}
// Parameters
const Mat* img;
const Mat* sum;
vector<KeyPoint>* keypoints;
Mat* descriptors;
bool extended;
bool upright;
// Pre-calculated values
int nOriSamples;
vector<Point> apt;
vector<float> aptw;
vector<float> DW;
};OpenCV245之SURF源码分析
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