pcl 用RANSAC算法拟合曲线 并显示

要使用PCL中的RANSAC算法拟合曲线，需要使用`pcl::SampleConsensusModelCurve`类。下面是一个简单的代码示例，演示如何使用RANSAC算法拟合一条直线：

#include <iostream>
#include <pcl/io/pcd_io.h>
#include <pcl/point_types.h>
#include <pcl/sample_consensus/model_types.h>
#include <pcl/sample_consensus/method_types.h>
#include <pcl/segmentation/sac_segmentation.h>

int main (int argc, char** argv)
{
  pcl::PointCloud<pcl::PointXYZ>::Ptr cloud (new pcl::PointCloud<pcl::PointXYZ>);
  pcl::io::loadPCDFile<pcl::PointXYZ> ("table_scene_lms400.pcd", *cloud);

  // 创建一个SampleConsensusModel对象，用于拟合直线
  pcl::SampleConsensusModelLine<pcl::PointXYZ>::Ptr model (new pcl::SampleConsensusModelLine<pcl::PointXYZ> (cloud));

  // 创建一个随机样本一致性对象
  pcl::RandomSampleConsensus<pcl::PointXYZ> ransac (model);
  ransac.setDistanceThreshold (0.01); // 设置距离阈值
  ransac.computeModel();

  // 获取拟合出的直线的参数
  Eigen::VectorXf model_coefficients;
  ransac.getModelCoefficients (model_coefficients);

  // 显示拟合出的直线
  pcl::ModelCoefficients line_coeff;
  line_coeff.values.resize (6);
  line_coeff.values[0] = model_coefficients[0];
  line_coeff.values[1] = model_coefficients[1];
  line_coeff.values[2] = model_coefficients[2];
  line_coeff.values[3] = model_coefficients[3];
  line_coeff.values[4] = model_coefficients[4];
  line_coeff.values[5] = model_coefficients[5];

  pcl::visualization::PCLVisualizer viewer ("line fitting");
  viewer.addPointCloud<pcl::PointXYZ> (cloud, "cloud");
  viewer.addLine (line_coeff, "line");
  viewer.spin ();

  return (0);
}

要拟合其他类型的曲线，比如圆或者椭圆，需要使用`pcl::SampleConsensusModelCircle`或者`pcl::SampleConsensusModelEllipse2D`类。其他步骤与拟合直线相似。