轮廓匹配在定位测量应用中对其匹配的精度有更高的要求,通常的像素级的匹配结果难以满足其要求。本文给出了一种具有亚像素精度的快速轮廓匹配定位方法,其进行数学计算的基础为二值图像的距离变换。
二值图像距离变换的概念由Rosenfeld和Pfaltz于1966年其论文中提出,目前广泛应用于计算机图形学,计算机视觉及GIS空间分析等领域,其基本含义是计算一个图像中非零像素点到最近的零像素点的距离,也就是对每一各非零像素点计算其到零像素点的最短距离,并将该距离值赋值给该非零像素位置,从而将一幅二值图像变换为一幅距离图像。OpenCV通过cv::distanceTransform()函数,给出了该功能的快速实现方法。并通过参数的方式给出了距离的几种定义方式,如欧式距离、马氏距离、倒角距离等。
高精度轮廓匹配的算法实现步骤:
假设待匹配的两个轮廓分别为A和B
(1)将轮廓A栅格化并生成为一幅二值图像Ia,其中轮廓点为255,背景点为0;
(2)将二值图Ia进行距离变换得到距离图Id;
(3)通过一个变换描述轮廓B和轮廓A之间的关系,如平移变换、刚体变换、仿射变换等;
(4)通过一定的搜索策略或者最优化方法搜索轮廓B在距离图Id中的最佳变换参数,并输出。
在上述过程中(4)是最关键的一步,具体的搜索策略可考虑使用Powell搜索、分支定界搜索或者Gauss-Newton方法。下面给出了使用Gauss-Newton方法实现求解平移变换的代码,仅供参考。其它复杂的变换可参考此过程完成。
1 数据栅格化,并距离变换
int GetRasterDistanceImage(cv::Point2f* refPoints, int pntsNum, double resolution,
int& offX, int& offY, cv::Mat& rasterDistImg)
{
double maxX, minX, maxY, minY;
int i;
for (i = 0;
i < pntsNum;
++ i)
{
maxX = refPoints[i].x;
minX = refPoints[i].x;
maxY = refPoints[i].y;
minY = refPoints[i].y;
break;
} for ( ;
i < pntsNum;
++ i)
{
if (refPoints[i].x > maxX)
maxX = refPoints[i].x;
if (refPoints[i].x < minX)
minX = refPoints[i].x;
if (refPoints[i].y > maxY)
maxY = refPoints[i].y;
if (refPoints[i].y < minY)
minY = refPoints[i].y;
} int nX = int((maxX - minX) / resolution) + 200;
int nY = int((maxY - minY) / resolution) + 200;
cv::Mat rasterImg;
rasterImg.create(nY, nX, CV_8UC1);
memset(rasterImg.data, 0xFF, nY * nX);
double realCenterX = (maxX + minX) / 2.0f;
double realCenterY = (maxY + minY) / 2.0f;
int offsetX = nX / 2 - int(realCenterX / resolution + 0.5f);
int offsetY = nY / 2 - int(realCenterY / resolution + 0.5f);
uchar* dat = (uchar*)(rasterImg.data);
for (i = 0;
i < pntsNum;
++ i)
{
if (validateMask[i] == 0)
continue;
int x = (int)floor(refPoints[i].x / resolution + 0.5f) + offsetX;
int y = nY - ((int)floor(refPoints[i].y / resolution + 0.5f) + offsetY);
*(dat + nX * y + x) = 0;
} cv::distanceTransform(rasterImg, rasterDistImg, CV_DIST_C, 3);
offX = offsetX;
offY = offsetY;
return 1;
}
2高精度轮廓匹配,并输出平移量 offX,offY
void MatchReferencePoints(cv::Mat& rasterDistImg, cv::Point2f* rasterPoints, int pointsNum , double& offX, double& offY)
{
offX = 0.0f;
offY = 0.0f;
cv::Mat imgB = rasterDistImg;
double* pL = new double[pointsNum];
double* pA = new double[pointsNum * 2];
double a_b[2] = {0};
double deltaX[2];
double AL[2];
double MM[4];
double va0, va1, va2, va3, va4;
double sx, sy;
double pError = 0.0f;
for (int i = 0;
i < 100;
++ i)
{
double error = 0.0f;
for (int j = 0;
j < pointsNum;
++ j)
{
sx = myPoints[j].x + a_b[0];
sy = myPoints[j].y + a_b[1];
va0 = GetDataValue(imgB, sx, sy);
va1 = GetDataValue(imgB, sx + 0.5, sy);
va2 = GetDataValue(imgB, sx - 0.5, sy);
va3 = GetDataValue(imgB, sx, sy + 0.5);
va4 = GetDataValue(imgB, sx, sy - 0.5);
pA[j] = va1 - va2;
pA[j + pointsNum] = va3 - va4;
pL[j] = 0.0f - va0;
error += pL[j] * pL[j];
}if (i == 0)
pError = error;
else if (error > pError)
break;
pError = error;
int nOff_M = 0;
for(int j = 0;
j < 2;
++ j){
for(int jj = 0;
jj < 2;
++ jj){
MM[nOff_M + jj] = 0.0;
for(int kk = 0;
kk < pointsNum;
++ kk)
MM[nOff_M + jj] += pA[j * pointsNum + kk] * pA[jj * pointsNum + kk];
}// M = A * A(T)
nOff_M += 2;
}cv::Mat cvMatM(2, 2, CV_64FC1, MM);
cv::invert(cvMatM, cvMatM);
for (int j = 0;
j < 2;
++ j)
{
AL[j] = 0.0f;
for (int kk = 0;
kk < pointsNum;
++ kk)
AL[j] += pA[j * pointsNum + kk] * pL[kk];
}for (int j = 0;
j < 2;
++ j)
{
deltaX[j] = 0.0f;
for (int jj = 0;
jj < 2;
++ jj)
deltaX[j] += MM[j * 2+ jj] * AL[jj];
}int s = 0;
for (;
s < 2;
++ s)
{
if (fabs(deltaX[s]) > 0.0001)
break;
}if (s == 2)
break;
a_b[0] += deltaX[0];
a_b[1] += deltaX[1];
} offX = a_b[0];
offY = a_b[1];
delete[] pL;
delete[] pA;
delete[] myPoints;
double temp = sqrt(pError / pointsNum);
}
【OpenCV|OpenCV 基于距离变换的高精度轮廓匹配】
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