OpenGL进阶(一) - 多视口

提要

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：

实现二：

void renderGL2(){    //视口一  左下角    glEnable(GL_SCISSOR_TEST);    glScissor(0,0,window_width/2-1,window_height/2-1);    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glDisable(GL_SCISSOR_TEST);    glViewport(0,0,window_width/2-1,window_height/2-1);    glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluPerspective(45.0f,(GLfloat)window_width/(GLfloat)window_height,0.1f,100.0f);    glMatrixMode(GL_MODELVIEW);    glLoadIdentity();    //绘制部分    glTranslatef(0.0f  ,0.0f  ,-5.0f  );      glRotatef(rtri,0.0f  ,1.0f  ,0.0f  );       //绘制一个三角锥      glBegin( GL_TRIANGLE_STRIP );      glColor3f( 1.0, 0.0, 0.0 );      glVertex3f(  0.0,  1.0,  0.0 );      glColor3f( 0.0, 1.0, 0.0 );      glVertex3f(-1.0, -1.0,  1.0 );      glColor3f( 0.0, 0.0, 1.0 );      glVertex3f(  1.0, -1.0,  1.0 );      glColor3f( 0.0, 1.0, 0.0 );      glVertex3f(  1.0, -1.0, -1.0 );      glColor3f( 1.0, 0.0, 0.0 );      glVertex3f(  0.0,  1.0,  0.0 );      glColor3f( 0.0, 1.0, 0.0 );      glVertex3f(-1.0, -1.0,  1.0 );      glEnd();      //视口二 左上角    glEnable(GL_SCISSOR_TEST);    glScissor(0,window_height/2+1,window_width/2-1,window_height/2-1);    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glDisable(GL_SCISSOR_TEST);    glViewport(0,window_height/2+1,window_width/2-1,window_height/2-1);    glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluPerspective(45.0f,(GLfloat)window_width/(GLfloat)window_height,0.1f,100.0f);    glMatrixMode(GL_MODELVIEW);    glLoadIdentity();    //绘制部分    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    //视口三  右下角    glEnable(GL_SCISSOR_TEST);    glScissor(window_width/2+1,0,window_width/2-1,window_height/2-1);    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glDisable(GL_SCISSOR_TEST);    glViewport(window_width/2+1,0,window_width/2-1,window_height/2-1);    glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluPerspective(45.0f,(GLfloat)window_width/(GLfloat)window_height,0.1f,100.0f);    glMatrixMode(GL_MODELVIEW);    glLoadIdentity();    //绘制部分    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();    //视口四  右上角    glEnable(GL_SCISSOR_TEST);    glScissor(window_width/2+1,window_height/2+1, window_width/2-1, window_height/2-1);    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glDisable(GL_SCISSOR_TEST);    glViewport(window_width/2+1,window_height/2+1, window_width/2-1, window_height/2-1);    glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluPerspective(45.0f,(GLfloat)window_width/(GLfloat)window_height,0.1f,100.0f);    glMatrixMode(GL_MODELVIEW);    glLoadIdentity();    //绘制部分    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( );}void initGL( int width, int height ){    float ratio = (float) width / (float) height;    // Our shading model--Gouraud (smooth).     glShadeModel( GL_SMOOTH );    // Set the clear color.     glClearColor( 0, 0, 0, 0 );    // Setup our viewport.     glViewport( 0, 0, width, height );    //Change to the projection matrix and set our viewing volume.    glMatrixMode( GL_PROJECTION );    glLoadIdentity();    gluPerspective( 60.0, ratio, 1.0, 100.0 );}

参考：OpenGL中的多视口-