Convex hull detection of point cloud is more commonly used. PCL comes with Convex hull detection ConvexHull, which is a relatively simple function. Set the dimension setDimension of ConvexHull, and record it here
Header: #include <; pcl/surface/convex_hull.h>

	pcl::PointCloud<pcl::PointXYZ>::Ptr cloud(new pcl::PointCloud<pcl::PointXYZ>);
	pcl::io::loadPCDFile<pcl::PointXYZ>("bun_45.pcd", *cloud);

	pcl::ConvexHull<pcl::PointXYZ> hull;                  
	hull.setInputCloud(cloud);                   
	hull.setDimension(3);

	std::vector<pcl::Vertices> polygons;                 
	pcl::PointCloud<pcl::PointXYZ>::Ptr surface_hull(new pcl::PointCloud<pcl::PointXYZ>);
	hull.reconstruct(*surface_hull, polygons);           

	cout << surface_hull->size() << endl;

	// ---------------------- Visualizer -------------------------------------------
	boost::shared_ptr<pcl::visualization::PCLVisualizer> viewer(new pcl::visualization::PCLVisualizer);
	viewer->setBackgroundColor(255,255,255);

	pcl::visualization::PointCloudColorHandlerCustom<pcl::PointXYZ> color_handler(cloud, 255, 255, 0);
	viewer->addPointCloud(cloud, color_handler, "sample cloud");
	viewer->setPointCloudRenderingProperties(pcl::visualization::PCL_VISUALIZER_POINT_SIZE, 6, "sample cloud");

	pcl::visualization::PointCloudColorHandlerCustom<pcl::PointXYZ> color_handlerK(surface_hull, 255, 0, 0);
	viewer->addPointCloud(surface_hull, color_handlerK, "point");
	viewer->setPointCloudRenderingProperties(pcl::visualization::PCL_VISUALIZER_POINT_SIZE, 6, "point");

	//viewer->addPolygon<pcl::PointXYZ>(surface_hull, 0, 0, 255, "polyline");

	while (!viewer->wasStopped())
	{
		viewer->spinOnce(100);
	}

Adopting Stanford Rabbit Point Cloud, the effect is shown in the figure below:

If the point cloud in a certain convex hull is further separated from the scene, Crophull can be used for filtering
CropHull filtering can refer to: https://blog.csdn.net/zfjBIT/article/details/92795689
Thanks for the correction of Xiao snail. If we get the convex hull curve, further triangulation is needed. It is not correct to connect the convex hull points only