OpenGL+GLUT没有填充最左边的多边形

我遇到了一个奇怪的OpenGL Bug。OpenGL对我来说很新，但我们被要求在我的人工智能课上使用它(因为老师是真正的图形学教授)。

无论哪种方式，都发生了:http://img818.imageshack.us/img818/422/reversiothello.png

它只发生在最上面，最左边的多边形。换句话说，它先找到左边最远的多边形，然后再找到上面最远的多边形，然后对它进行处理。(目前没有任何东西可以从板上擦除多边形)。

我的显示功能是:

void display_func(void)
{
    glClearColor(0.0, 0.45, 0.0, 1.0); // Background Color (Forest Green :3)
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 0.0, 0.0);
    draw_board();
    glFlush();
    glutSwapBuffers();
};

我的draw_board函数是:

void draw_board()
{
    int size = 8;
    int stepX = WINDOW_XS / size;
    int stepY = WINDOW_YS / size;
    glColor3f(0.0,0.0,0.0); // line color black
    glBegin(GL_LINES);
    // Draw Columns
    for(int i = 0;i <= WINDOW_XS;i += stepX)
    {
        glVertex2i(i,0);
        glVertex2i(i, WINDOW_YS);
    }
    // Draw Rows
    for(int j = 0;j <= WINDOW_YS;j += stepY)
    {
        glVertex2i(0, j);
        glVertex2i(WINDOW_XS, j);
    }
    // Draw Circles
    for(int i = 0;i < 8;++i)
    {
        for(int j = 0;j < 8;++j)
        {
            if(engine->getOnBoard(i,j) == Reversi::PIECE_NONE) continue;
            if(engine->getOnBoard(i,j) == Reversi::PIECE_WHITE)
                glColor3f(1.0,1.0,1.0);
            if(engine->getOnBoard(i,j) == Reversi::PIECE_BLACK)
                glColor3f(0.0,0.0,0.0);
            int drawX = ((i+1)*64)-32;
            int drawY = 512-((j+1)*64)+32;
            gl_drawCircle(drawX,drawY,30);
        }
    }
    glEnd();
};

我的鼠标功能如下:

void mouse_func(int button, int state, int x, int y)
{
    if(button == GLUT_LEFT_BUTTON && state == GLUT_DOWN && x < WINDOW_XS)
    {    
        // row and column index
        x = (int)( x / (WINDOW_XS/8) );
        y = (int)( y / (WINDOW_YS/8) );
        std::cout << "Attempting to make a move at " << x << "," << y << std::endl;
        if(engine->makeMove(x,y))
        {
            glutPostRedisplay();
        }
    }
};

我的gl_drawCircle函数是这样的

void gl_drawCircle(float x, float y, float r)
{
    // http://stackoverflow.com/questions/5094992/c-drawing-a-2d-circle-in-opengl/5095188#5095188
    glBegin( GL_POLYGON );
    float t;
    int n;
    for(t = 0,n = 0; n <= 90; ++n, t = float(n)/90.f ) // increment by a fraction of the maximum 
    {
        glVertex2f( x + sin( t * 2 * PI ) * r, y + cos( t * 2 * PI ) * r );
    }
    glEnd();
}