画出来的坐标轴只有正半轴，有什么讲究？

画出来的坐标轴只有正半轴，有什么讲究？
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);

glLoadIdentity(); //   重置当前模型观察矩阵

glTranslatef(0.0f,0.0f,-1.0f);

glRotatef(30.0f,1.0f,0.0f,0.0f); //   绕x轴旋转30
glRotatef(-30.0f,0.0f,1.0f,0.0f); //   绕y轴旋转30度

glColor3f(1.0f,0.0f,0.0f);
glBegin(GL_LINES);
glVertex3f(-0.5f,0.0f,0.0f);
glVertex3f(0.5f,0.0f,0.0f);
glEnd();

glColor3f(0.0f,1.0f,0.0f);
glBegin(GL_LINES);
glVertex3f(0.0f,-0.5f,0.0f);
glVertex3f(0.0f,0.5f,0.0f);
glEnd();

glColor3f(0.0f,0.0f,1.0f);
glBegin(GL_LINES);
glVertex3f(0.0f,0.0f,-0.5f);
glVertex3f(0.0f,0.0f,0.5f);
glEnd();

 

 

 

什么说只有负半轴???

  你确定它画的只有半轴而不是全部线的长度...
  如果你确定了的话,    
  会不会是投影体设的太小...     剩下的线跑视见体外面去了..