使用PCL中IO自带的接口读取OBJ文件，然后保存成PLY文件
loadOBJFile
savePLYFile，savePLYFileBinary

代码如下：

#include <pcl/io/obj_io.h>
#include <pcl/io/ply_io.h>
#include <pcl/console/print.h>
#include <pcl/console/parse.h>
#include <pcl/console/time.h>

void
printHelp (int,
           char **argv)
{
  PCL_ERROR ("Syntax is: %s [-format 0|1] [-use_camera 0|1] input.obj output.ply\n", argv[0]);
  PCL_ERROR ("format = 0 writes a binary file (default behavior), format = 1 writes an ASCII file.\n");
}

int
main (int argc,
      char** argv)
{
  PCL_INFO ("Convert a OBJ file to PLY format. For more information, use: %s -h\n", argv[0]);

  if (argc < 3)
  {
    printHelp (argc, argv);
    return (-1);
  }

  // Parse the command line arguments for files
  std::vector<int> obj_file_indices = pcl::console::parse_file_extension_argument (argc, argv, ".obj");
  std::vector<int> ply_file_indices = pcl::console::parse_file_extension_argument (argc, argv, ".ply");
  if (obj_file_indices.size () != 1 || ply_file_indices.size () != 1)
  {
    PCL_ERROR ("Need one input OBJ file and one output PLY file.\n");
    return (-1);
  }

  // Command line parsing
  bool ascii = false;
  pcl::console::parse_argument (argc, argv, "-format", ascii);
  PCL_INFO ("PLY output format: %s\n", ascii ? "ascii" : "binary");

  // Load the first file
  pcl::PolygonMesh mesh;
  if (pcl::io::loadOBJFile (argv[obj_file_indices[0]], mesh) != 0)
  {
    PCL_ERROR ("Error loading OBJ file\n");
    return (-1);
  }

  if (ascii)
  {
    if (pcl::io::savePLYFile (argv[ply_file_indices[0]], mesh) != 0)
    {
      PCL_ERROR ("Error writing PLY file\n");
      return (-1);
    }
  }
  else if (pcl::io::savePLYFileBinary (argv[ply_file_indices[0]], mesh) != 0)
  {
    PCL_ERROR ("Error writing PLY file\n");
    return (-1);
  }

  return (0);
}

来源：PCL官方示例