[Point Cloud Library] 显示点云

仅仅是简单的显示点云，可以使用CloudViewer类。这个类非常简单易用。但要注意，它不是线程安全的。如果要用于多线程，还要参考PCLVisualizer。

需要注意的是，PointCloud的数据类型要和PCD文件中或者代码中的PointT一致！

简单的点云可视化，那么只需要如下几行代码：

#include <pcl/visualization/cloud_viewer.h>
 //...
 void
 foo ()
 {
   pcl::PointCloud<pcl::PointXYZRGB>::Ptr cloud;
   //... populate cloud
   pcl::visualization::CloudViewer viewer ("Simple Cloud Viewer");
   viewer.showCloud (cloud);
   while (!viewer.wasStopped ())
   {
   }
 }

一个更完整一些的例子如下。显示了如何在可视化线程上运行代码。PCLVisuializer在CloudViewer之后，但是它运行在自己的线程上。避免出现问题，必须使用回调函数才能访问它。

#include <pcl/visualization/cloud_viewer.h>
#include <iostream>
#include <pcl/io/io.h>
#include <pcl/io/pcd_io.h>
    
int user_data;
    
void 
viewerOneOff (pcl::visualization::PCLVisualizer& viewer)
{
    viewer.setBackgroundColor (1.0, 0.5, 1.0);
    pcl::PointXYZ o;
    o.x = 1.0;
    o.y = 0;
    o.z = 0;
    viewer.addSphere (o, 0.25, "sphere", 0);
    std::cout << "i only run once" << std::endl;
    
}
    
void 
viewerPsycho (pcl::visualization::PCLVisualizer& viewer)
{
    static unsigned count = 0;
    std::stringstream ss;
    ss << "Once per viewer loop: " << count++;
    viewer.removeShape ("text", 0);
    viewer.addText (ss.str(), 200, 300, "text", 0);
    
    //FIXME: possible race condition here:
    user_data++;
}
    
int 
main ()
{
    pcl::PointCloud<pcl::PointXYZRGBA>::Ptr cloud (new pcl::PointCloud<pcl::PointXYZRGBA>);
    pcl::io::loadPCDFile ("my_point_cloud.pcd", *cloud);
    
    pcl::visualization::CloudViewer viewer("Cloud Viewer");
    
    //blocks until the cloud is actually rendered
    viewer.showCloud(cloud);
    
    //use the following functions to get access to the underlying more advanced/powerful
    //PCLVisualizer
    
    //This will only get called once
    viewer.runOnVisualizationThreadOnce (viewerOneOff);
    
    //This will get called once per visualization iteration
    viewer.runOnVisualizationThread (viewerPsycho);
    while (!viewer.wasStopped ())
    {
    //you can also do cool processing here
    //FIXME: Note that this is running in a separate thread from viewerPsycho
    //and you should guard against race conditions yourself...
    user_data++;
    }
    return 0;
}