提要
OpenGL視口（Viewport）可以看作是窗口中OpenGL的繪制輸出區域，可以通過一條簡單的glViewport命令設置。一般的OpenGL教程都是單視口，即整個窗口為惟一的一個視口，Nehe的OpenGL教程是我最早見到涉及多視口的。與單視口程序相比，多視口程序的變化主要是：
（一）多視口程序不能在窗口resize/reshape時直接調用glViewport，而應該在此時記下窗口大小，然後在繪制場景時多次調用glViewport設置每個視口的位置和大小；
（二）對於每個視口，應分別調用glMatrixMode(GL_PROJECTION)和glMatrixMode(GL_MODELVIEW)以設置投影和建模矩陣。
多視口的最常見的應用，就是3DMax中的多視口建模，每一個觀察口都從不同的方向去觀察模型，從而很方便地去獲取模型當前的狀態。
還有就是賽車游戲中的後視鏡等等.
具體實現的思路就是對每個視口進行獨立的渲染。
函數解析
下面是需要調用到的一些OpenGL的函數。
glLoadIdentity()
重置當前指定的矩陣為單位矩陣.
glViewport(GLint x,GLint y,GLsizei width,GLsizei height)
在窗口中定義一個像素矩形，最終將圖像映射到這個矩形中。
void gluOrtho2D(GLdouble left,GLdouble right,GLdouble bottom,GLdouble top)
建立了一個可視的二位平面區域。這個和用glOrtho函數的當near=0，far=1時效果是一樣。
void gluPerspective(GLdouble fovy, GLdouble aspect,GLdouble zNear,GLdouble zFar)
這個函數指定了觀察的視景體（frustum為錐台的意思，通常譯為視景體）在世界坐標系中的具體大小，一般而言，其中的參數aspect應該與窗口的寬高比大小相同。
void glScissor(GLint x,GLint y,GLsizei width,GLsizei height)
設置一個裁剪窗口，前兩個參數為窗口左下角位置，後兩個參數是窗口的寬度和高度。
使用前一定要glEnable(GL_SCISSOR_TEST);
代碼實現
實現一：
void renderGL()
{
glClear (GL_COLOR_BUFFER_BIT);
glLoadIdentity(); // 重置模型觀察矩陣
for (int loop=0; loop<4; loop++) // 循環繪制4個視口
{
if (loop==0) // 繪制左上角的視口
{
// 設置視口區域
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) // 繪制右上角視口
{
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) // 繪制右下角視口
{
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) // 繪制左下角視口
{
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) // 繪制左上角的視圖
{
glBegin(GL_QUADS);
glColor3f(1.0f,0.0f,0.0f); // 設置當前色為紅色
glVertex2i(window_width/2, 0 );
glColor3f(0.0f,1.0f,0.0f); // 設置當前色為綠色
glVertex2i(0, 0 );
glColor3f(0.0f,1.0f,1.0f); // 設置當前色為藍色
glVertex2i(0, window_height/2);
glColor3f(1.0f,1.0f,0.0f); // 設置當前色為紅色
glVertex2i(window_width/2, window_height/2);
glEnd();
}
if (loop==1) // 繪制右上角的視圖
{
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); // 繪制三角形
glVertex3f( 0.0f, 1.0f, 0.0f); // 上頂點
glVertex3f(-1.0f,-1.0f, 0.0f); // 左下
glVertex3f( 1.0f,-1.0f, 0.0f); // 右下
glEnd(); // 三角形繪制結束
}
if (loop==2) // 繪制右下角的視圖
{
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) // 繪制左下角的視圖
{
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; // 旋轉變量
rquad-=0.15f; // 旋轉變量
SDL_GL_SwapBuffers( );
}
效果：