glutDisplayFunc显示垃圾

我第一次尝试将openGL合并到我的c++代码中。作为一个开始，我制作了这个非常原始的代码，它定义了一个名为polygon的类，应该用polygong.draw（）方法显示一个多边形。现在，下面的所有内容都驻留在一个main.cpp文件中，不过为了便于阅读，我在这里分为一节：

问题是，下面的代码编译并运行良好。只有当创建名为"simple"的窗口时，才会显示垃圾（主要是从我的计算机后台屏幕收集的）：（.

首先，类多边形：

#include <GL/glut.h>
#include "utility.hpp"
#include <vector>
void init(void);
class nikPolygon{
public:
    std::vector<nikPosition> m_vertices;
    nikColor m_color;
    double m_alpha;
    // constructors
    // without alpha (default is 1.0)
    nikPolygon(std::vector<nikPosition> vList, nikColor c):
    m_vertices(vList), m_color(c), m_alpha(1.0){
    }
    nikPolygon(std::vector<nikPosition> vList, nikColor c, double a):
    m_vertices(vList), m_color(c), m_alpha(a){
    }
    // default constructor
    nikPolygon(){
    }

    // member functions
    // add vertex
    void addVertex(nikPosition v)              { m_vertices.push_back(v); }
    // remove vertex
    void removeVertex(nikPosition v);
    // adjust vertex
    void modifyVertex(unsigned int vIndex, nikPosition newPosition);
    // fill color
    void setColor(nikColor col)               { m_color = col; }
    // set alpha
    void setAlpha(double a)                   { m_alpha = a; }
    // display
    void drawPolygon(void){
        // color the objet
        glColor4f(m_color.red, 
                  m_color.green, 
                  m_color.blue, 
                  m_alpha);
        // construct polygon
        glBegin(GL_POLYGON);    
        for (std::vector<nikPosition>::iterator it = m_vertices.begin();
             it != m_vertices.end(); it++)
            glVertex2f(it->x, it->y);
        glEnd();
        // send to screen
        glFlush();
    }
    void draw(void);
};

然后是c/c++回调接口（蹦床/thunk）：

 // for c++/c callback
 nikPolygon* currentPolygon;
 extern "C"
 void drawCallback(void){
     currentPolygon->drawPolygon();
 }
 void nikPolygon::draw(){
     currentPolygon = this;
     glutDisplayFunc(drawCallback);
 }

然后是它的其余部分：

 // initialize openGL etc
 void init(void){
     // set clear color to black
     glClearColor(0.0, 0.0, 0.0, 0.0);
     // set fill color to white
     glColor3f(1.0, 1.0, 1.0);
     // enable transperancy
     glEnable(GL_BLEND);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
     // setup standard orthogonal view with clipping
     // box as cube of side 2 centered at origin
     // this is the default view
     glMatrixMode(GL_PROJECTION);
     glLoadIdentity();
     gluOrtho2D(-1.0, 1.0, -1.0, 1.0);
 }

 int main(int argc, char** argv){
     nikPolygon poly;
     poly.addVertex(nikPosition(-0.5, -0.5));
     poly.addVertex(nikPosition(-0.5, 0.5));
     poly.addVertex(nikPosition(0.5, 0.5));
     poly.addVertex(nikPosition(0.5, -0.5));
     poly.setColor(nikColor(0.3, 0.5, 0.1));
     poly.setAlpha(0.4);

     glutInit(&argc, argv);
     glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
     glutInitWindowSize(500, 500);
     glutInitWindowPosition(0, 0);
     glutCreateWindow("simple");
     init();
     poly.draw();
     glutMainLoop();
 }