如何输出*.ply文件

所谓ply文件格式，是由斯坦福大学开发的一套三维mesh模型数据格式，图形学领域内很多著名的模型数据，比如斯坦福的三维扫描数据库（其中包括很多文章中会见到的Happy Buddha, Dragon, Bunny兔子）最初的模型都是基于这个格式的。其开发目标是，建立一套针对多边形模型的，结构简单但是能够满足大多数图形应用需要的模型格式，而且它允许以ASCII码格式或二进制形式存储文件。这样一套既简单又灵活的文件格式，能够帮助开发人员避免重复开发文件格式的问题。尽管在工业领域内，新的文件格式仍然在不断的出现，但是在图形学的研究领域中，PLY还是种常用且重要的文件格式。

ply数据文件格式中包含用来描述多边形的一系列点的信息：

<顶点列表>

<面片列表>

<其他元素列表>

头部是一系列以回车结尾的文本行，用来描述文件的剩余部分。头部包含一个对每个元素类型的描述，包括元素名（如“边”），这个元素在工程里有多少，以及一个与这个元素关联的不同属性的列表。头部还说明这个文件是二进制的或者是ASCII的。头部后面的是一个每个元素类型的元素列表，按照在头部中描述的顺序出现。

以下是一个立方体的完整ASCII描述。相同工程的二进制版本头部的唯一不同是用词“binary_little_endian”或者“binary_big_endian”替换词“ascii”。大括号中的注释不是文件的一部分，它们是这个例子的注解。文件中的注释一般在“comment”开始的关键词。

format ascii 1.0;
comment made by anonymous;
/*comment this is a cube*/ 
element vertex 8; 
property float32 x ;
property float32 y ;
property float32 z ;
element face 6 ; 
property list uint8 int32 vertex_index ;
end_header;
/*list vertex*/
0 0 0 
0 0 1 
0 1 1 
0 1 0 
1 0 0 
1 0 1 
1 1 1 
1 1 0 
/*list face*/
4 0 1 2 3
4 7 6 5 4 
4 0 4 5 1 
4 1 5 6 2 
4 2 6 7 3 
4 3 7 4 0 

这个例子说明头部的基本组成。头部的每个部分都是以一个关键词开头以回车结尾的ASCII串。即使是头部的开始和结尾（“ply”和“end_header”）也是以这种形式。因为字符“ply”是文件的魔法数字，必须是文件的头四个字符。跟在文件头部开头之后的是关键词“format”和一个特定的ASCII或者二进制的格式，接下来是一个版本号。再下面是多边形文件中每个元素的描述，在每个元素里还有多属性的说明。一般元素以下面的格式描述：

element <元素名> <在文件中的个数>

property <数据类型> <属性名-1>

property <数据类型> <属性名-2>

property <数据类型> <属性名-3>

...

属性罗列在“element”（元素）行后面定义，既包含属性的数据类型也包含属性在每个元素中出现的次序。一个属性可以有三种数据类型：标量，字符串和列表。属性可能具有的标量数据类型列表如下：

int8 字符 1

uint8 非负字符 1

int16 短整型 2

uint16 非负短整型 2

int32 整型 4

uint32 非负整型 4

float32 单精度浮点数 4

float64 双精度浮点数 8

这些字节计数很重要，而且在实现过程中不能修改以使这些文件可移植。使用列表数据类型的属性定义有一种特殊的格式：

 property list <数值类型> <数值类型> <属性名>

这种格式的一类例子是上面的立方体文件中的property list uint8 int32 vertex_index，这表示属性“vertex_index”首先包含一个非负字符报苏在属性里包含多少索引，接下来是一个列表包含许多整数。在这个边长列表里的每个整数都是一个顶点的索引。

以下是一个基于C++的读入ASCII文件或二进制文件，输出ply文件的程序。首先定义头文件ply.h

/*

Header for PLY polygon files.

- Greg Turk, March 1994

A PLY file contains a single polygonal _object_.

An object is composed of lists of _elements_.  Typical elements are
vertices, faces, edges and materials.

Each type of element for a given object has one or more _properties_
associated with the element type.  For instance, a vertex element may
have as properties three floating-point values x,y,z and three unsigned
chars for red, green and blue.

*/

#ifndef __PLY_H__
#define __PLY_H__

#ifdef __cplusplus
extern "C" {
#endif

#include 
#include 

#define PLY_ASCII      1        /* ascii PLY file */
#define PLY_BINARY_BE  2        /* binary PLY file, big endian */
#define PLY_BINARY_LE  3        /* binary PLY file, little endian */

#define PLY_OKAY    0           /* ply routine worked okay */
#define PLY_ERROR  -1           /* error in ply routine */

/* scalar data types supported by PLY format */

#define PLY_START_TYPE 0
#define PLY_CHAR       1
#define PLY_SHORT      2
#define PLY_INT        3
#define PLY_UCHAR      4
#define PLY_USHORT     5
#define PLY_UINT       6
#define PLY_FLOAT      7
#define PLY_DOUBLE     8
#define PLY_END_TYPE   9

#define  PLY_SCALAR  0
#define  PLY_LIST    1


typedef struct PlyProperty {    /* description of a property */

  char *name;                           /* property name */
  int external_type;                    /* file's data type */
  int internal_type;                    /* program's data type */
  int offset;                           /* offset bytes of prop in a struct */

  int is_list;                          /* 1 = list, 0 = scalar */
  int count_external;                   /* file's count type */
  int count_internal;                   /* program's count type */
  int count_offset;                     /* offset byte for list count */

} PlyProperty;

typedef struct PlyElement {     /* description of an element */
  char *name;                   /* element name */
  int num;                      /* number of elements in this object */
  int size;                     /* size of element (bytes) or -1 if variable */
  int nprops;                   /* number of properties for this element */
  PlyProperty **props;          /* list of properties in the file */
  char *store_prop;             /* flags: property wanted by user? */
  int other_offset;             /* offset to un-asked-for props, or -1 if none*/
  int other_size;               /* size of other_props structure */
} PlyElement;

typedef struct PlyOtherProp {   /* describes other properties in an element */
  char *name;                   /* element name */
  int size;                     /* size of other_props */
  int nprops;                   /* number of properties in other_props */
  PlyProperty **props;          /* list of properties in other_props */
} PlyOtherProp;

typedef struct OtherData { /* for storing other_props for an other element */
  void *other_props;
} OtherData;

typedef struct OtherElem {     /* data for one "other" element */
  char *elem_name;             /* names of other elements */
  int elem_count;              /* count of instances of each element */
  OtherData **other_data;      /* actual property data for the elements */
  PlyOtherProp *other_props;   /* description of the property data */
} OtherElem;

typedef struct PlyOtherElems {  /* "other" elements, not interpreted by user */
  int num_elems;                /* number of other elements */
  OtherElem *other_list;        /* list of data for other elements */
} PlyOtherElems;

typedef struct PlyFile {        /* description of PLY file */
  FILE *fp;                     /* file pointer */
  int file_type;                /* ascii or binary */
  float version;                /* version number of file */
  int nelems;                   /* number of elements of object */
  PlyElement **elems;           /* list of elements */
  int num_comments;             /* number of comments */
  char **comments;              /* list of comments */
  int num_obj_info;             /* number of items of object information */
  char **obj_info;              /* list of object info items */
  PlyElement *which_elem;       /* which element we're currently writing */
  PlyOtherElems *other_elems;   /* "other" elements from a PLY file */
} PlyFile;

/* memory allocation */
extern char *my_alloc();
#define myalloc(mem_size) my_alloc((mem_size), __LINE__, __FILE__)


/*** delcaration of routines ***/

extern PlyFile *ply_write(FILE *, int, char **, int);
extern PlyFile *ply_open_for_writing(char *, int, char **, int, float *);
extern void ply_describe_element(PlyFile *, char *, int, int, PlyProperty *);
extern void ply_describe_property(PlyFile *, char *, PlyProperty *);
extern void ply_element_count(PlyFile *, char *, int);
extern void ply_header_complete(PlyFile *);
extern void ply_put_element_setup(PlyFile *, char *);
extern void ply_put_element(PlyFile *, void *);
extern void ply_put_comment(PlyFile *, char *);
extern void ply_put_obj_info(PlyFile *, char *);
extern PlyFile *ply_read(FILE *, int *, char ***);
extern PlyFile *ply_open_for_reading( char *, int *, char ***, int *, float *);
extern PlyProperty **ply_get_element_description(PlyFile *, char *, int*, int*);
extern void ply_get_element_setup( PlyFile *, char *, int, PlyProperty *);
extern void ply_get_property(PlyFile *, char *, PlyProperty *);
extern PlyOtherProp *ply_get_other_properties(PlyFile *, char *, int);
extern ply_get_element(PlyFile *, void *);
extern char **ply_get_comments(PlyFile *, int *);
extern char **ply_get_obj_info(PlyFile *, int *);
extern void ply_close(PlyFile *);
extern void ply_get_info(PlyFile *, float *, int *);
extern PlyOtherElems *ply_get_other_element (PlyFile *, char *, int);
extern void ply_describe_other_elements ( PlyFile *, PlyOtherElems *);
extern void ply_put_other_elements (PlyFile *);
extern void ply_free_other_elements (PlyOtherElems *);


#ifdef __cplusplus
}
#endif
#endif /* !__PLY_H__ */

Part 1 preparation

/*

The interface routines for reading and writing PLY polygon files.

Greg Turk, February 1994

---------------------------------------------------------------

A PLY file contains a single polygonal _object_.

An object is composed of lists of _elements_.  Typical elements are
vertices, faces, edges and materials.

Each type of element for a given object has one or more _properties_
associated with the element type.  For instance, a vertex element may
have as properties the floating-point values x,y,z and the three unsigned
chars representing red, green and blue.

*/

#include 
#include 
#include 
#include 
#include 
#include 
#include "ply.h"
#include 
using namespace std;

char *type_names[] = {
"invalid",
"char", "short", "int",
"uchar", "ushort", "uint",
"float", "double",
};

int ply_type_size[] = {
  0, 1, 2, 4, 1, 2, 4, 4, 8
};

#define NO_OTHER_PROPS  -1

#define DONT_STORE_PROP  0
#define STORE_PROP       1

#define OTHER_PROP       0
#define NAMED_PROP       1


/* returns 1 if strings are equal, 0 if not */

/* find an element in a plyfile's list */
PlyElement *find_element(PlyFile *, char *);

/* find a property in an element's list */
PlyProperty *find_property(PlyElement *, char *, int *);

/* write to a file the word describing a PLY file data type */
void write_scalar_type (FILE *, int);

/* read a line from a file and break it up into separate words */
char **get_words(FILE *, int *, char **);
char **old_get_words(FILE *, int *);

/* write an item to a file */
void write_binary_item(FILE *, int, unsigned int, double, int);
void write_ascii_item(FILE *, int, unsigned int, double, int);
double old_write_ascii_item(FILE *, char *, int);

/* add information to a PLY file descriptor */
void add_element(PlyFile *, char **, int);
void add_property(PlyFile *, char **, int);
void add_comment(PlyFile *, char *);
void add_obj_info(PlyFile *, char *);

/* copy a property */
void copy_property(PlyProperty *, PlyProperty *);

/* store a value into where a pointer and a type specify */
void store_item(char *, int, int, unsigned int, double);

/* return the value of a stored item */
void get_stored_item( void *, int, int *, unsigned int *, double *);

/* return the value stored in an item, given ptr to it and its type */
double get_item_value(char *, int);

/* get binary or ascii item and store it according to ptr and type */
void get_ascii_item(char *, int, int *, unsigned int *, double *);
void get_binary_item(FILE *, int, int *, unsigned int *, double *);

/* get a bunch of elements from a file */
void ascii_get_element(PlyFile *, char *);
void binary_get_element(PlyFile *, char *);

/* memory allocation */
char *my_alloc(int, int, char *);


/*************/
/*  Writing  */
/*************/


/******************************************************************************
Given a file pointer, get ready to write PLY data to the file.

Entry:
  fp         - the given file pointer
  nelems     - number of elements in object
  elem_names - list of element names
  file_type  - file type, either ascii or binary

Exit:
  returns a pointer to a PlyFile, used to refer to this file, or NULL if error
******************************************************************************/

PlyFile *ply_write(
  FILE *fp,
  int nelems,
  char **elem_names,
  int file_type
)
{
  int i;
  PlyFile *plyfile;
  PlyElement *elem;

  /* check for NULL file pointer */
  if (fp == NULL)
    return (NULL);

  /* create a record for this object */

  plyfile = (PlyFile *) myalloc (sizeof (PlyFile));
  plyfile->file_type = file_type;
  plyfile->num_comments = 0;
  plyfile->num_obj_info = 0;
  plyfile->nelems = nelems;
  plyfile->version = 1.0;
  plyfile->fp = fp;
  plyfile->other_elems = NULL;

  /* tuck aside the names of the elements */

  plyfile->elems = (PlyElement **) myalloc (sizeof (PlyElement *) * nelems);
  for (i = 0; i < nelems; i++) {
    elem = (PlyElement *) myalloc (sizeof (PlyElement));
    plyfile->elems[i] = elem;
    elem->name = strdup (elem_names[i]);
    elem->num = 0;
    elem->nprops = 0;
  }

  /* return pointer to the file descriptor */
  return (plyfile);
}


/******************************************************************************
Open a polygon file for writing.

Entry:
  filename   - name of file to read from
  nelems     - number of elements in object
  elem_names - list of element names
  file_type  - file type, either ascii or binary

Exit:
  version - version number of PLY file
  returns a file identifier, used to refer to this file, or NULL if error
******************************************************************************/

PlyFile *ply_open_for_writing(
  char *filename,
  int nelems,
  char **elem_names,
  int file_type,
  float *version
)
{
  int i;
  PlyFile *plyfile;
  PlyElement *elem;
  char *name;
  FILE *fp;

  /* tack on the extension .ply, if necessary */

  name = (char *) myalloc (sizeof (char) * (strlen (filename) + 5));
  strcpy (name, filename);
  if (strlen (name) < 4 ||
      strcmp (name + strlen (name) - 4, ".ply") != 0)
      strcat (name, ".ply");

  /* open the file for writing */

  fp = inFile (name, "w");
  if (fp == NULL) {
    return (NULL);
  }

  /* create the actual PlyFile structure */

  plyfile = ply_write (fp, nelems, elem_names, file_type);
  if (plyfile == NULL)
    return (NULL);

  /* say what PLY file version number we're writing */
  *version = plyfile->version;

  /* return pointer to the file descriptor */
  return (plyfile);
}

Part 2 description

/******************************************************************************
Describe an element, including its properties and how many will be written
to the file.

Entry:
  plyfile   - file identifier
  elem_name - name of element that information is being specified about
  nelems    - number of elements of this type to be written
  nprops    - number of properties contained in the element
  prop_list - list of properties
******************************************************************************/

void ply_describe_element(
  PlyFile *plyfile,
  char *elem_name,
  int nelems,
  int nprops,
  PlyProperty *prop_list
)
{
  int i;
  PlyElement *elem;
  PlyProperty *prop;

  /* look for appropriate element */
  elem = find_element (plyfile, elem_name);
  if (elem == NULL) {
    cerr << " ply_describe_element: can't find element" << elem_name << endl;
    exit (-1);
  }

  elem->num = nelems;

  /* copy the list of properties */

  elem->nprops = nprops;
  elem->props = (PlyProperty *