CGAL是一個優秀的幾何處理庫,對於三維網格採用半邊格式存儲。
其實對於網格而言,無外乎定義它的邊,頂點,面,數據存儲。
問題是用戶可能會有不同的需求,比如做模型簡化,需要對每個頂點加一個cost域,而對其它應用則不需要,
也許你會說可以給基本的定點數據結構加一個指針,用戶自己定義其它的數據都由該指針指向,但這種設計並不好。
顯然模板化處理是更好的方法,不要把頂點類型定死。
這樣一來我們可以通過讓一個網格類,擁有不同的頂點類型,邊類型,面類型,具體類型由用戶決定,用戶可以通過
繼承底層提供的基本類型,加入自己 的需要的屬性,方法,生成新的類型,作爲template參數,並傳給網格類。從而實現
了網格的靈活設計。
CGAL就是採用了這一方法,但是它有大量的模板技巧,wrapper class 包裝等等,從而對於想要了解底層具體實現的而言
顯得比較複雜難於理解,雖然它的使用還是很簡單方便的。 下面我將根據參考文獻 Using Generic Programming for Designing a Data Structure for Polyhedral Surface
介紹其模板類設計的非常簡潔的核心的思想,並給出代碼實例,注意和實際CGAL實現代碼是有出入的,這裏做了簡化
(比如CGAL對頂點,邊,面數據結構進行了wrapper class封裝,使得wrapper class沒有任何模板參數),但是思想是一致的。
這裏爲了簡單,去掉面,僅僅考慮邊和頂點,對於頂點我們要記錄它的對應的半邊信息,對於半邊我們也要記錄頂點信息,
問題是對於特定的item type,比如頂點,它不知道其相關聯的其它item的具體類型,如半邊的具體類型 。
在CGAL的設計中,vertex頂點,通過一個placeholder來獲得其它item的類型信息。(在下面的小程序演示中,HalfedgeDS作爲那個placeholder,存儲所有iterm的類型信息)
CGAL將所有的局部類型信息,如頂點,半邊,面放在一個單一的模板參數Refs中。
在頂點中僅僅用到 Halfedge_handel作爲對應的halfedge的引用。
其他如半邊,面的設計類似。
對於整體的半邊數據結構,它將會由如頂點,半邊,面的類型來模板化,(parameterized with the item types)
但是item types如頂點,已經是 class template模板類了,所以我們需要 template as template arguments 即模板參數本身是模板類
這種方法還是很常見的,比如考慮二叉樹的設計,允許二叉樹類裝配不同類型的節點,即節點是模板參數
而節點類本身允許有不同的存儲數據類型,也就是說節點本身就是模板類。
可以採用下面的設計
二叉樹節點類
template <typename ElemType>
class Node {
private:
ElemType m_data;
};
二叉樹類
template <template<type Elem> class Node, typename U>
class BinaryTree {
private:
Node<U> *m_root;
};
具體使用
BinaryTree< Node, int> MyBinaryTree;
即可。
好回到CGAL中,這 裏的類型依賴構成了一個循環,半邊數據結構需要vertex,halfedge類型參數的實化,而半邊數據結構類halfedge ds知道 handle 類型,可以用做Refs 參數的實際類型, 儘管當前handles還沒有被聲明(vertex,halfedge同樣需要半邊數據結構類來實化)。聲明和定義的不同使得這一切成爲可能。
struct Edge;
struct Node {
Edge * edge;
// .... maybe more than one edge ....
};
struct Edge {
Node * source;
Node * dest;
};
//最簡單的一個示例 template <class Graph> struct Node { typedef typename Graph::Edge Edge; Edge* edge; // .... maybe some more edges .... }; template <class Graph> struct Edge { typedef typename Graph::Node Node; Node* node; }; template < template <class G> class T_Node, template <class G> class T_Edge> struct Graph { typedef Graph< T_Node, T_Edge> Self; typedef T_Node<Self> Node; typedef T_Edge<Self> Edge; }; int main() { typedef Graph< Node, Edge> G; G::Node node; G::Edge edge; node.edge = &edge; edge.node = &node; }
template <class Graph>
struct Colored_node : public Node<Graph> {
int color;
};
int main() {
typedef Graph< Colored_node, Edge> G;
G::Node node;
G::Edge edge;
node.edge = &edge;
edge.node = &node;
node.color = 3;
}
注意上面代碼用的是指針,本質上還是利用前置聲明,如果改爲Edge edge,就不行了,因爲那樣編譯器需要知道Edge的定義,前置聲明不行。
It is important to understand that these cyclic definitions work -- as for the C example -- because we can make use of a declared type to define pointers and references to this type before this type is defined itself. For example, we cannot change the pointer member Edge * edge of the node class to a value Edge edge .
好了,說了這麼多看實際的代碼吧。
我寫了個小程序測試了一下,爲了測試加入了顯示PrintName的代碼。
//simple_cgal.h
#include <list>
#include <string>
#include <iostream>
using std::list;
using std::string;
using std::cout;
using std::endl;
template <typename Refs>
struct Vertex {
typedef typename Refs::Halfedge_handle Halfedge_handle;
Vertex(string name = "vertex0") { m_name = name;}
void PrintName() const {
cout << "This is " << m_name << endl;
}
Halfedge_handle halfedge() const { return h; }
void set_halfedge (Halfedge_handle g) { h = g; }
private:
Halfedge_handle h;
string m_name;
};
template <typename Refs>
struct Halfedge {
typedef typename Refs::Vertex_handle Vertex_handle;
Halfedge(string name = "halfedge0") { m_name = name;}
void PrintName() const {
cout << "This is " << m_name << endl;
}
Vertex_handle vertex() const { return v;}
void set_vertex (Vertex_handle vv) { v = vv;}
private:
Vertex_handle v;
string m_name;
};
template < template <typename Ref> class Vertex, template <typename Ref> class Halfedge>
struct HalfedgeDS {
typedef HalfedgeDS< Vertex, Halfedge> Self;
typedef Vertex<Self> V;
typedef Halfedge<Self> H;
typedef list<V> Vlist;
typedef list<H> Hlist;
typedef typename Vlist::iterator Vertex_handle;
typedef typename Hlist::iterator Halfedge_handle;
};
//test_simple_cgal.cc
#include "simple_cgal.h"
#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
typedef HalfedgeDS< Vertex, Halfedge> HDS;
typedef HDS::V Vert;
typedef HDS::H HalfEdge;
typedef HDS::Vlist Vlist;
typedef HDS::Hlist Hlist;
Vert v("vertex a");
HalfEdge h("Halfedge b");
Vlist vlist;
Hlist hlist;
vlist.push_back(v);
hlist.push_back(h);
v.set_halfedge(hlist.begin());
h.set_vertex(vlist.begin());
v.halfedge()->PrintName();
h.vertex()->PrintName();
return 0;
}
運行結果:
#include <list>
#include <string>
#include <iostream>
using std::list;
using std::string;
using std::cout;
using std::endl;
template <typename Refs>
struct Vertex {
typedef typename Refs::Halfedge_handle Halfedge_handle;
Vertex(string name = "vertex0") { m_name = name;}
void PrintName() const {
cout << "This is " << m_name << endl;
}
Halfedge_handle halfedge() const { return h; }
void set_halfedge (Halfedge_handle g) { h = g; }
private:
Halfedge_handle h;
string m_name;
};
template <typename Refs>
struct Halfedge {
typedef typename Refs::Vertex_handle Vertex_handle;
Halfedge(string name = "halfedge0") { m_name = name;}
void PrintName() const {
cout << "This is " << m_name << endl;
}
Vertex_handle vertex() const { return v;}
void set_vertex (Vertex_handle vv) { v = vv;}
private:
Vertex_handle v;
string m_name;
};
struct HalfedgeDS_iems {
template <class Refs>
struct Vertex_wrapper {
typedef Vertex<Refs> Vertex;
};
template <class Refs>
struct Halfedge_wrapper {
typedef Halfedge<Refs> Halfedge;
};
};
template < typename HalfedgeDSItems>
struct HalfedgeDS {
typedef HalfedgeDS< HalfedgeDSItems> Self;
typedef HalfedgeDSItems Items;
typedef typename Items::template Vertex_wrapper<Self> Vertex_wrapper;
typedef typename Items::template Halfedge_wrapper<Self>
Halfedge_wrapper;
typedef typename Vertex_wrapper::Vertex Vertex;
typedef typename Halfedge_wrapper::Halfedge Halfedge;
typedef list<Vertex> Vlist;
typedef list<Halfedge> Hlist;
typedef typename Vlist::iterator Vertex_handle;
typedef typename Hlist::iterator Halfedge_handle;
};
//test_simgple_cgal2.cc
#include "simple_cgal2.h"
#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
typedef HalfedgeDS<HalfedgeDS_iems> HDS;
typedef HDS::Vertex Vert;
typedef HDS::Halfedge HalfEdge;
typedef HDS::Vlist Vlist;
typedef HDS::Hlist Hlist;
Vert v("vertex a");
HalfEdge h("Halfedge b");
Vlist vlist;
Hlist hlist;
vlist.push_back(v);
hlist.push_back(h);
v.set_halfedge(hlist.begin());
h.set_vertex(vlist.begin());
v.halfedge()->PrintName();
h.vertex()->PrintName();
return 0;
}
運行結果和上面程序一致。多了一個HalfedgeDS_iems包裝的一個好處是由於HalfedgeDS_iems沒有模板參數,所以下面的定義是可能的,CGAL中定義有Polyhedron它的一個模板參數是HDS,即半邊結構,考慮上面沒有封裝到HalfedgeItesms的方案,
template < template <typename Ref> class Vertex, template <typename Ref> class Halfedge>
struct HalfedgeDS
HalfedgeDS的定義已經有兩級模板參數了,如果再被傳遞會有三級模板參數,C++是不允許這樣的,所以HalfedgeDS_iems方法不失爲一種繞過的技巧。關於Polyhedron_items<Items>下面給予介紹。先看一個用戶自己擴充面的例子,完全繼承已有的邊和頂點,用戶當然也可以全部擴充,總之通過wrapper是很靈活的。template < class Traits,
class Items = Items_default,
template < class, class> calss HDS = HalfedgeDS_default>
struct Polyhedron {
typedef Polyhedron_items<Items> Deerived_items;
typedef HDS< Traits, Derived_items> HDS;
typedef typename HDS::Vertex Vertex;
typedef typename HDS::Halfedge Halfedge;
typedef typename HDS::Face Face;
.....
};
template <class Refs, class Triats>
class My_facet : public CGAL_HalfedgeDS_facet_base<Refs> {
int face_cost; // we need to sore addtional info
public:
typename Traits::Vector_3 normal;
};
struct My_items : public CGAL_Polyhedron_items_3 {
template <class Refs, class Traits>
struct Face_wrapper {
typedef My_facet< Refs, Traits> Facet;
};
};
typedef CGAL_Polyhedron_3<Traits, My_items> Polyhedron;
OK,用戶使用起來就是這麼簡單,這樣我們的Polyhedron就是應用用戶定義的facet了同時保留了CGAL_Polyhedron_items_3中定義的facet的所以屬性函數,如set_handle.由於用戶沒有定義Vertex和Halfedge所以它們完全用CGAL_Polyhedron_items_3中提供的Vertex和Halfege不加任何擴充。//CGAL源代碼class Polyhedron_items_3 {
public:
template < class Refs, class Traits>
struct Vertex_wrapper {
typedef typename Traits::Point_3 Point;
typedef HalfedgeDS_vertex_base< Refs, Tag_true, Point> Vertex;
};
template < class Refs, class Traits>
struct Halfedge_wrapper {
typedef HalfedgeDS_halfedge_base< Refs> Halfedge;
};
template < class Refs, class Traits>
struct Face_wrapper {
typedef typename Traits::Plane_3 Plane;
typedef HalfedgeDS_face_base< Refs, Tag_true, Plane> Face;
};
};
template < class Refs >
class HalfedgeDS_face_base< Refs, Tag_true, Tag_false> {
public:
typedef Refs HalfedgeDS;
typedef HalfedgeDS_face_base< Refs, Tag_true, Tag_false> Base;
typedef Tag_true Supports_face_halfedge;
typedef typename Refs::Vertex_handle Vertex_handle;
typedef typename Refs::Vertex_const_handle Vertex_const_handle;
typedef typename Refs::Halfedge_handle Halfedge_handle;
typedef typename Refs::Halfedge_const_handle Halfedge_const_handle;
typedef typename Refs::Face_handle Face_handle;
typedef typename Refs::Face_const_handle Face_const_handle;
typedef typename Refs::Vertex Vertex;
typedef typename Refs::Halfedge Halfedge;
// Additional tags required by Polyhedron.
typedef Tag_false Supports_face_plane;
struct Plane_not_supported {};
typedef Plane_not_supported Plane;
// No longer required.
//typedef Tag_false Supports_face_normal;
private:
Halfedge_handle hdg;
public:
Halfedge_handle halfedge() { return hdg; }
Halfedge_const_handle halfedge() const { return hdg; }
void set_halfedge( Halfedge_handle h) { hdg = h; }
};
這裏面實現還是有另外的技巧,原文章中又以vertex舉例了,就按照vertex繼續吧。
template <class Vertex_base>
struct Polyhedron_vertex : public Vertex_base {
typedef Vertex_base Base;
private:
void set_halfedge(typename Base::Halfedge_handle g) {
Base::set_halfedge(g);
}
};
template <class Items>
struct Polyhderon_items {
template <class Refs, class Traits>
struct Vertex_wrapper {
typedef typename Items::Vertex_wrapper<Refs, Traits> Wrapper;
typedef typename Wrapper::Vertex Vertex_base;
typedef Polyhedron_vertex<Vertex_base> Vertex;
};
//similar for facet and halfedge
};
template < class Traits,
class Items = Items_default,
template < class, class> calss HDS = HalfedgeDS_default>
struct Polyhedron {
typedef Polyhedron_items<Items> Deerived_items;
typedef HDS< Traits, Derived_items> HDS;
typedef typename HDS::Vertex Vertex;
typedef typename HDS::Halfedge Halfedge;
typedef typename HDS::Face Face;
.....
};
用戶自定義的Items,Vetex會被當作模板參數傳遞如下。也就是說最後實際用的是Polyhedron_items<Items> 中定義的Vertex Polyhedron_vertex<Vertex_base>而 Items即爲用戶定義並傳遞的點面邊數據類型類。Vertex_base是用戶定義的Vertex類型。下面看一下CGAL實際的代碼,這裏的I_Polyhedron_vertex其實就是上面的Polyhedron_vertex。
template <class VertexBase>
class I_Polyhedron_vertex : public VertexBase {
public:
typedef VertexBase Base;
//typedef typename Base::HalfedgeDS HDS;
typedef typename Base::Point Point;
typedef Point Point_3;
// Handles have to explicitly repeated, although they are derived
typedef typename Base::Vertex_handle Vertex_handle;
typedef typename Base::Halfedge_handle Halfedge_handle;
typedef typename Base::Face_handle Face_handle;
typedef typename Base::Face_handle Facet_handle;
typedef typename Base::Vertex_const_handle Vertex_const_handle;
typedef typename Base::Halfedge_const_handle Halfedge_const_handle;
typedef typename Base::Face_const_handle Face_const_handle;
typedef typename Base::Face_const_handle Facet_const_handle;
typedef typename Base::Halfedge Halfedge;
typedef typename Base::Face Face;
typedef typename Base::Face Facet;
// Supported options by HDS.
typedef typename Base::Supports_vertex_halfedge
Supports_vertex_halfedge;
typedef typename Base::Supports_vertex_point Supports_vertex_point;
// Circulator category.
typedef typename Halfedge::Supports_halfedge_prev Supports_prev;
public:
// Circulator category.
typedef HalfedgeDS_circulator_traits<Supports_prev> Ctr;
typedef typename Ctr::iterator_category circulator_category;
// Circulators around a vertex and around a facet.
typedef I_HalfedgeDS_facet_circ< Halfedge_handle, circulator_category>
Halfedge_around_facet_circulator;
typedef I_HalfedgeDS_vertex_circ< Halfedge_handle, circulator_category>
Halfedge_around_vertex_circulator;
typedef I_HalfedgeDS_facet_circ<
Halfedge_const_handle,
circulator_category> Halfedge_around_facet_const_circulator;
typedef I_HalfedgeDS_vertex_circ<
Halfedge_const_handle,
circulator_category> Halfedge_around_vertex_const_circulator;
typedef typename Halfedge_around_vertex_circulator::size_type
size_type;
typedef typename Halfedge_around_vertex_circulator::difference_type
difference_type;
public:
// We need to repeat the constructors here.
I_Polyhedron_vertex() {}
I_Polyhedron_vertex( const VertexBase& b) : VertexBase(b) {}
I_Polyhedron_vertex( const Point_3& p) : VertexBase(p) {}
// New Access Functions (not provided in VertexBase).
Halfedge_around_vertex_circulator vertex_begin() {
// a circulator of halfedges around the vertex (clockwise).
return Halfedge_around_vertex_circulator( this->halfedge());
}
Halfedge_around_vertex_const_circulator vertex_begin() const {
// a circulator of halfedges around the vertex (clockwise).
return Halfedge_around_vertex_const_circulator( this->halfedge());
}
// the degree of the vertex, i.e., edges emanating from this vertex
std::size_t vertex_degree() const {
return this->halfedge()->vertex_degree();
}
size_type degree() const { return vertex_degree(); } //backwards compatible
// returns true if the vertex has exactly two incident edges
bool is_bivalent() const { return this->halfedge()->is_bivalent(); }
// returns true if the vertex has exactly three incident edges
bool is_trivalent() const { return this->halfedge()->is_trivalent(); }
// No longer hidden. Now the restricted version with precondition.
// sets incident halfedge to h. Precondition: h is incident, i.e.,
// h->vertex() == v.
void set_halfedge( Halfedge_handle hh) {
CGAL_assertion( &*(hh->vertex()) == this);
Base::set_halfedge(hh);
}
};
//所有這些內容對應的具體CGAL源代碼參照/usr/local/include/CGAL