26.OpenGL--分格化

• 理论基础 
  分格化：OpenGL渲染的都是一些简单的凸多边形，对于那些复杂的多边形，如凹多边形，含孔多边形以及自交叉的多边形，OpenGL函数库不能对他们进行渲染。不过OpenGL实用库(GLU)提供了一系列分格化函数，它们可以将一个复杂的多边形划分为一系列简单凸多边形，我们就把这个过程叫做分格化(又称网格化)。

分格化一般步骤：1，创建分格化对象 2，注册对应阶段分格处理的回调函数(复杂点的是相交处回调处理) 3，指定分格化属性(主要是环绕数规则设定，以确认是内部还是外部) 4，指定一个或多个封闭多边形的轮廓数据 5，删除分格化对象。

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• 代码示例

#include "GLTools.h"#ifdef __APPLE__#include <glut/glut.h>#else#define FREEGLUT_STATIC#include <GL/glut.h>#endif#ifndef CALLBACK#define CALLBACK#endifGLuint startList;void display (void) {    glClear(GL_COLOR_BUFFER_BIT);    glColor3f(1.0, 1.0, 1.0);    glCallList(startList);    glCallList(startList + 1);    glFlush();}/*注册对应阶段回调函数*///开始绘制回调(类似glBegin())void CALLBACK beginCallback(GLenum which){    glBegin(which);}//错误回调void CALLBACK errorCallback(GLenum errorCode){    const GLubyte *estring;    estring = gluErrorString(errorCode);    fprintf(stderr, "Tessellation Error: %s\n", estring);    exit(0);}//结束绘制回调(类似glEnd())void CALLBACK endCallback(void){    glEnd();}//顶点数据回调void CALLBACK vertexCallback(GLdouble *vertex){    const GLdouble *pointer;    pointer = (GLdouble *) vertex;    glColor3dv(pointer+3);    glVertex3dv(vertex);}//相交处创建新顶点回调void CALLBACK combineCallback(GLdouble coords[3],                              GLdouble *vertex_data[4],                              GLfloat weight[4], GLdouble **dataOut ){    GLdouble *vertex;    int i;    vertex = (GLdouble *) malloc(6 * sizeof(GLdouble));    vertex[0] = coords[0];    vertex[1] = coords[1];    vertex[2] = coords[2];    for (i = 3; i < 6; i++)        vertex[i] = weight[0] * vertex_data[0][i]        + weight[1] * vertex_data[1][i]        + weight[2] * vertex_data[2][i]        + weight[3] * vertex_data[3][i];    *dataOut = vertex;}void init (void){    GLUtesselator *tobj;    //矩形，三角形，五星顶点数据    GLdouble rect[4][3] = {50.0, 50.0, 0.0,        200.0, 50.0, 0.0,        200.0, 200.0, 0.0,        50.0, 200.0, 0.0};    GLdouble tri[3][3] = {75.0, 75.0, 0.0,        125.0, 175.0, 0.0,        175.0, 75.0, 0.0};    GLdouble star[5][6] = {250.0, 50.0, 0.0, 1.0, 0.0, 1.0,        325.0, 200.0, 0.0, 1.0, 1.0, 0.0,        400.0, 50.0, 0.0, 0.0, 1.0, 1.0,        250.0, 150.0, 0.0, 1.0, 0.0, 0.0,        400.0, 150.0, 0.0, 0.0, 1.0, 0.0};    glClearColor(0.0, 0.0, 0.0, 0.0);    startList = glGenLists(2);    //创建分格化对象    tobj = gluNewTess();    //对应阶段的回调    gluTessCallback(tobj, GLU_TESS_VERTEX,                    (void(__stdcall*)())glVertex3dv);    gluTessCallback(tobj, GLU_TESS_BEGIN,                    (void(__stdcall*)())beginCallback);    gluTessCallback(tobj, GLU_TESS_END,                    (void(__stdcall*)())endCallback);    gluTessCallback(tobj, GLU_TESS_ERROR,                    (void(__stdcall*)())errorCallback);    glNewList(startList, GL_COMPILE);    glShadeModel(GL_FLAT);    gluTessBeginPolygon(tobj, NULL);//类似glBegin()    //矩形    gluTessBeginContour(tobj);//指定一条闭合的轮廓线的开始(即需要进行分格化的凹多边形数据)    gluTessVertex(tobj, rect[0], rect[0]);    gluTessVertex(tobj, rect[1], rect[1]);    gluTessVertex(tobj, rect[2], rect[2]);    gluTessVertex(tobj, rect[3], rect[3]);    gluTessEndContour(tobj);//指定轮廓线结束    //三角形    gluTessBeginContour(tobj);    gluTessVertex(tobj, tri[0], tri[0]);    gluTessVertex(tobj, tri[1], tri[1]);    gluTessVertex(tobj, tri[2], tri[2]);    gluTessEndContour(tobj);    gluTessEndPolygon(tobj);//类似glEnd()    glEndList();    gluTessCallback(tobj, GLU_TESS_VERTEX,                    (void(__stdcall*)())vertexCallback);    gluTessCallback(tobj, GLU_TESS_BEGIN,                    (void(__stdcall*)())beginCallback);    gluTessCallback(tobj, GLU_TESS_END,                    (void(__stdcall*)())endCallback);    gluTessCallback(tobj, GLU_TESS_ERROR,                    (void(__stdcall*)())errorCallback);    gluTessCallback(tobj, GLU_TESS_COMBINE,                    (void(__stdcall*)())combineCallback);    glNewList(startList + 1, GL_COMPILE);    glShadeModel(GL_SMOOTH);    //指定分格化属性    gluTessProperty(tobj, GLU_TESS_WINDING_RULE,                    GLU_TESS_WINDING_POSITIVE);//环绕数为正数则为内部区域    gluTessBeginPolygon(tobj, NULL);    gluTessBeginContour(tobj);    gluTessVertex(tobj, star[0], star[0]);    gluTessVertex(tobj, star[1], star[1]);    gluTessVertex(tobj, star[2], star[2]);    gluTessVertex(tobj, star[3], star[3]);    gluTessVertex(tobj, star[4], star[4]);    gluTessEndContour(tobj);    gluTessEndPolygon(tobj);    glEndList();    gluDeleteTess(tobj);//删除分格对象}void reshape (int w, int h){    glViewport(0, 0, (GLsizei) w, (GLsizei) h);    glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluOrtho2D(0.0, (GLdouble) w, 0.0, (GLdouble) h);}void keyboard(unsigned char key, int x, int y){    switch (key) {        case 27:            exit(0);            break;    }}int main(int argc, char** argv){    glutInit(&argc, argv);    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);    glutInitWindowSize(500, 500);    glutCreateWindow(argv[0]);    init();    glutDisplayFunc(display);    glutReshapeFunc(reshape);    glutKeyboardFunc(keyboard);    glutMainLoop();    return 0;}
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