点云操作过程中经常会需要提取点云子集，包括一些点云滤波算法也会经常得到点云的索引，然后根据这些点云索引来提取点云子集，下面代码示例了如何利用索引向量来构建点云索引并提取点云子集。
/*======================================================================
* Author : Haiming Zhang
* Email : zhanghm_1995@qq.com
* Version : 2018年10月29日
* Copyright :
* Descriptoin :
* References :
======================================================================*/
#include <iostream>
#include <vector>
#include <pcl/io/pcd_io.h>
#include <pcl/point_types.h>
#include <pcl/ModelCoefficients.h>
#include <pcl/filters/extract_indices.h>
using std::cout; using std::endl;
int main()
{
pcl::PointCloud<pcl::PointXYZ>::Ptr cloud(new pcl::PointCloud<pcl::PointXYZ>);
pcl::PointCloud<pcl::PointXYZ>::Ptr cloud_p(new pcl::PointCloud<pcl::PointXYZ>);
cloud->width = 5;
cloud->height = 1;
cloud->points.resize (cloud->width * cloud->height);
for (size_t i = 0; i < cloud->points.size (); ++i)
{
cloud->points[i].x = 1024 * rand () / (RAND_MAX + 1.0f);
cloud->points[i].y = 1024 * rand () / (RAND_MAX + 1.0f);
cloud->points[i].z = 1024 * rand () / (RAND_MAX + 1.0f);
}
std::cerr << "Cloud before extract: " << std::endl;
for (size_t i = 0; i < cloud->points.size (); ++i)
std::cerr << " " << cloud->points[i].x << " "
<< cloud->points[i].y << " "
<< cloud->points[i].z << std::endl;
std::vector<int> index = {1,3,4};//提取1,3,4位置处点云
boost::shared_ptr<std::vector<int>> index_ptr = boost::make_shared<std::vector<int>>(index);
// Create the filtering object
pcl::ExtractIndices<pcl::PointXYZ> extract;
// Extract the inliers
extract.setInputCloud (cloud);
extract.setIndices (index_ptr);
extract.setNegative (false);//如果设为true,可以提取指定index之外的点云
extract.filter (*cloud_p);
cout<<"----------------"<<endl;
for (size_t i = 0; i < cloud_p->points.size (); ++i)
std::cerr << " " << cloud_p->points[i].x << " "
<< cloud_p->points[i].y << " "
<< cloud_p->points[i].z << std::endl;
}
输出结果：
Cloud before extract:
0.352222 -0.151883 -0.106395
-0.397406 -0.473106 0.292602
-0.731898 0.667105 0.441304
-0.734766 0.854581 -0.0361733
-0.4607 -0.277468 -0.916762
----------------
-0.397406 -0.473106 0.292602
-0.734766 0.854581 -0.0361733
-0.4607 -0.277468 -0.916762
ExtractIndices类可以提供多种功能的点云子集提取，如果只是提取指定索引的点云组成一个新的点云，还可以用pcl::copyPointCloud函数：
std::vector<int > indexs = { 1, 2, 5 };//声明索引值
pcl::copyPointCloud(*cloud, indexs, *cloud_p);//将对应索引的点存储
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