投影变换的最终目的是定义一个视景体，视景体有两个用途，首先，视景体是决定一个物体是如何映射到屏幕的，

其次，视景体定义了哪些物体（或物体的一部分）被裁剪到最终的图像之外

1.透视投影

void glFrustum (GLdouble left, GLdouble right, GLdouble bottom, GLdouble top, GLdouble zNear, GLdouble zFar);

void APIENTRY gluPerspective (
      GLdouble fovy, 
      GLdouble aspect, 
      GLdouble zNear, 
      GLdouble zFar);
   //fovy:角度 aspect：纵横比 zNear：观测点与近侧裁剪截面的距离 zFar：与远侧裁剪截面的距离
   gluPerspective(60.0, (GLfloat) w/(GLfloat) h, 1.0, 20.0);

2.正射投影

void APIENTRY glOrtho (GLdouble left, GLdouble right, GLdouble bottom, GLdouble top, GLdouble zNear, GLdouble zFar);

void APIENTRY gluOrtho2D (
     GLdouble left, 
     GLdouble right, 
     GLdouble bottom, 
     GLdouble top);