OpenGL视口（Viewport）可以看作是窗口中OpenGL的绘制输出区域，可以通过一条简单的glViewport命令设置。一般的OpenGL教程都是单视口，即整个窗口为惟一的一个视口，Nehe的OpenGL教程是我最早见到涉及多视口的。与单视口程序相比，多视口程序的变化主要是：
（一）多视口程序不能在窗口resize/reshape时直接调用glViewport，而应该在此时记下窗口大小，然后在绘制场景时多次调用glViewport设置每个视口的位置和大小；
（二）对于每个视口，应分别调用glMatrixMode(GL_PROJECTION)和glMatrixMode(GL_MODELVIEW)以设置投影和建模矩阵。
多视口的最常见的应用，就是3DMax中的多视口建模，每一个观察口都从不同的方向去观察模型，从而很方便地去获取模型当前的状态。
还有就是赛车游戏中的后视镜等等.
具体实现的思路就是对每个视口进行独立的渲染。
函数解析
下面是需要调用到的一些OpenGL的函数。
glLoadIdentity()
重置当前指定的矩阵为单位矩阵.
glViewport(GLint x,GLint y,GLsizei width,GLsizei height)
在窗口中定义一个像素矩形，最终将图像映射到这个矩形中。
void gluOrtho2D(GLdouble left,GLdouble right,GLdouble bottom,GLdouble top)
建立了一个可视的二位平面区域。这个和用glOrtho函数的当near=0，far=1时效果是一样。
void gluPerspective(GLdouble fovy, GLdouble aspect,GLdouble zNear,GLdouble zFar)
这个函数指定了观察的视景体（frustum为锥台的意思，通常译为视景体）在世界坐标系中的具体大小，一般而言，其中的参数aspect应该与窗口的宽高比大小相同。
void glScissor(GLint x,GLint y,GLsizei width,GLsizei height)
设置一个裁剪窗口，前两个参数为窗口左下角位置，后两个参数是窗口的宽度和高度。
使用前一定要glEnable(GL_SCISSOR_TEST);
代码实现
实现一：
void renderGL()
{
glClear (GL_COLOR_BUFFER_BIT);
glLoadIdentity(); // 重置模型观察矩阵
for (int loop=0; loop<4; loop++) // 循环绘制4个视口
{
if (loop==0) // 绘制左上角的视口
{
// 设置视口区域
glViewport (0, window_height/2, window_width/2, window_height/2);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluOrtho2D(0, window_width/2, window_height/2, 0);
}
if (loop==1) // 绘制右上角视口
{
glViewport (window_width/2, window_height/2, window_width/2, window_height/2);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective( 45.0, 1.0f, 0.1f, 500.0 );
}
if (loop==2) // 绘制右下角视口
{
glViewport (window_width/2, 0, window_width/2, window_height/2);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective( 45.0, 1.0f, 0.1f, 500.0 );
}
if (loop==3) // 绘制左下角视口
{
glViewport (0, 0, window_width/2, window_height/2);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective( 45.0, 1.0f, 0.1f, 500.0 );
}
glMatrixMode (GL_MODELVIEW);
glLoadIdentity ();
glClear (GL_DEPTH_BUFFER_BIT);
if (loop==0) // 绘制左上角的视图
{
glBegin(GL_QUADS);
glColor3f(1.0f,0.0f,0.0f); // 设置当前色为红色
glVertex2i(window_width/2, 0 );
glColor3f(0.0f,1.0f,0.0f); // 设置当前色为绿色
glVertex2i(0, 0 );
glColor3f(0.0f,1.0f,1.0f); // 设置当前色为蓝色
glVertex2i(0, window_height/2);
glColor3f(1.0f,1.0f,0.0f); // 设置当前色为红色
glVertex2i(window_width/2, window_height/2);
glEnd();
}
if (loop==1) // 绘制右上角的视图
{
glColor3f(1.0f,0.0f,1.0f);
glTranslatef(0.0f,0.0f,-9.0f);
glRotatef(rtri,1.0,1.0,0.0);
glBegin(GL_TRIANGLES); // 绘制三角形
glVertex3f( 0.0f, 1.0f, 0.0f); // 上顶点
glVertex3f(-1.0f,-1.0f, 0.0f); // 左下
glVertex3f( 1.0f,-1.0f, 0.0f); // 右下
glEnd(); // 三角形绘制结束
}
if (loop==2) // 绘制右下角的视图
{
glTranslatef(0.0f,0.0f,-2.0f);
glRotatef(-45.0f,1.0f,0.0f,0.0f);
glRotatef(rtri,0.0f,0.0f,1.0f);
glBegin(GL_QUADS);
glColor3f(1.0f,0.0f,0.0f); glVertex3f( 0.5f, 0.5f, 0.0f);
glColor3f(0.0f,1.0f,0.0f); glVertex3f(-0.5f, 0.5f, 0.0f);
glColor3f(0.0f,0.0f,1.0f); glVertex3f(-0.5f, -1.0f, 0.0f);
glColor3f(0.0f,0.0f,0.0f); glVertex3f( 0.5f, -1.0f, 0.0f);
glEnd();
}
if (loop==3) // 绘制左下角的视图
{
glColor3f(0.0f,0.0f,0.0f);
glTranslatef(0.0f,0.0f,-4.0f);
glRotatef(-rtri/2,1.0f,0.0f,0.0f);
glRotatef(-rtri/2,0.0f,1.0f,0.0f);
glRotatef(-rquad/2,0.0f,0.0f,1.0f);
glBegin(GL_QUADS);
glColor3f(1.0f,0.0f,0.0f); glVertex3f( 0.5f, 0.5f, 0.0f);
glColor3f(0.0f,1.0f,0.0f); glVertex3f(-0.3f, 0.5f, 0.0f);
glColor3f(0.0f,0.0f,1.0f); glVertex3f(-0.5f, -1.0f, 0.0f);
glColor3f(0.0f,0.0f,0.0f); glVertex3f( 0.5f, -1.0f, 0.0f);
glEnd();
}
}
rtri+=0.2f; // 旋转变量
rquad-=0.15f; // 旋转变量
SDL_GL_SwapBuffers( );
}
效果：