剪裁测试用于限制绘制区域。我们可以指定一个矩形的剪裁窗口，当启用剪裁测试后，只有在这个窗口之内的像素才能被绘制，其它像素则会被丢弃。换句话说，无论怎么绘制，剪裁窗口以外的像素将不会被修改。有的朋友可能玩过《魔兽争霸3》这款游戏。游戏时如果选中一个士兵，则画面下方的一个方框内就会出现该士兵的头像。为了保证该头像无论如何绘制都不会越界而覆盖到外面的像素，就可以使用剪裁测试。可以通过下面的代码来启用或禁用剪裁测试：

glEnable(GL_SCISSOR_TEST);   // 启用剪裁测试
glDisable(GL_SCISSOR_TEST); // 禁用剪裁测试

可以通过下面的代码来指定一个位置在(x, y)，宽度为width，高度为height的剪裁窗口。glScissor(x, y, width, height);注意，glScissor窗口坐标是以左下角为(0, 0)，右上角为(width, height)的，这与Windows系统窗口有所不同。

Demo使用裁剪区来将矩形窗口正中间四分之一刷成红色，正中间十六分之一刷成绿色。

源代码：

// GlutScissorDemo.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include <gl/glut.h>
#include <math.h>
//圆周率宏
#define GL_PI 3.1415f
//获取屏幕的宽度
GLint SCREEN_WIDTH=0;
GLint SCREEN_HEIGHT=0;
//设置程序的窗口大小
GLint windowWidth=400;
GLint windowHeight=300;
//绕x轴旋转角度
GLfloat xRotAngle=0.0f;
//绕y轴旋转角度
GLfloat yRotAngle=0.0f;
//受支持的点大小范围
GLfloat sizes[2];
//受支持的点大小增量
GLfloat step;
//显示回调函数
void renderScreen(void){GLfloat x,y,z,angle;int i;// Clear blue windowglClearColor(0.0f, 0.0f, 1.0f, 0.0f);//把整个窗口清理为当前清理颜色：蓝色glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);//将当前Matrix状态入栈glPushMatrix();//坐标系绕x轴旋转xRotAngleglRotatef(xRotAngle,1.0f,0.0f,0.0f);//坐标系绕y轴旋转yRotAngleglRotatef(yRotAngle,0.0f,1.0f,0.0f);x=0.0f;y=0.0f;z=0.0f;//进行平滑处理　glEnable(GL_POINT_SMOOTH);glHint(GL_POINT_SMOOTH,GL_NICEST);glEnable(GL_LINE_SMOOTH);glHint(GL_LINE_SMOOTH,GL_NICEST);glEnable(GL_POLYGON_SMOOTH);glHint(GL_POLYGON_SMOOTH,GL_NICEST);//绘制坐标系glColor3f(1.0f,1.0f,1.0f);glBegin(GL_LINES);glVertex3f(-80.0f,0.0f,0.0f);glVertex3f(80.0f,0.0f,0.0f);glVertex3f(0.0f,-80.0f,0.0f);glVertex3f(0.0f,80.0f,0.0f);glVertex3f(0.0f,0.0f,-80.0f);glVertex3f(0.0f,0.0f,80.0f);glEnd();glPushMatrix();glTranslatef(80.0f,0.0f,0.0f);glRotatef(90.0f,0.0f,1.0f,0.0f);glutWireCone(3,6,10,10);glPopMatrix();glPushMatrix();glTranslatef(0.0f,80.0f,0.0f);glRotatef(-90.0f,1.0f,0.0f,0.0f);glutWireCone(3,6,10,10);glPopMatrix();glPushMatrix();glTranslatef(0.0f,0.0f,80.0f);glRotatef(90.0f,0.0f,0.0f,1.0f);glutWireCone(3,6,10,10);glPopMatrix();//使能裁剪区glEnable(GL_SCISSOR_TEST);//将窗口中间的四分之一面积清空为红色glClearColor(1.0f, 0.0f, 0.0f, 0.0f);glScissor(windowWidth/4,windowHeight/4, windowWidth/2,windowHeight/2);glClear(GL_COLOR_BUFFER_BIT);//将窗口中间的十六分之一面积清空为绿色glClearColor(0.0f, 1.0f, 0.0f, 0.0f);glScissor(windowWidth*3/8,windowHeight*3/8, windowWidth/4,windowHeight/4);glClear(GL_COLOR_BUFFER_BIT);//禁止裁剪区glDisable(GL_SCISSOR_TEST);//恢复压入栈的MatrixglPopMatrix();//交换两个缓冲区的指针glutSwapBuffers();
}
//设置Redering State
void setupRederingState(void){//设置清理颜色为黑色glClearColor(0.0f,0.0,0.0,1.0f);//设置绘画颜色为绿色glColor3f(0.0f,1.0f,0.0f);//使能深度测试glEnable(GL_DEPTH_TEST);//获取受支持的点大小范围glGetFloatv(GL_POINT_SIZE_RANGE,sizes);//获取受支持的点大小增量glGetFloatv(GL_POINT_SIZE_GRANULARITY,&step);printf("point size range:%f-%f\n",sizes[0],sizes[1]);printf("point step:%f\n",step);
}
//窗口大小变化回调函数
void changSize(GLint w,GLint h){//横宽比率GLfloat ratio;//设置坐标系为x(-100.0f,100.0f)、y(-100.0f,100.0f)、z(-100.0f,100.0f)GLfloat coordinatesize=100.0f;//窗口宽高为零直接返回if((w==0)||(h==0))return;//设置视口和窗口大小一致glViewport(0,0,w,h);//对投影矩阵应用随后的矩阵操作glMatrixMode(GL_PROJECTION);//重置当前指定的矩阵为单位矩阵　glLoadIdentity();ratio=(GLfloat)w/(GLfloat)h;//正交投影if(w<h)glOrtho(-coordinatesize,coordinatesize,-coordinatesize/ratio,coordinatesize/ratio,-coordinatesize,coordinatesize);elseglOrtho(-coordinatesize*ratio,coordinatesize*ratio,-coordinatesize,coordinatesize,-coordinatesize,coordinatesize);//对模型视图矩阵堆栈应用随后的矩阵操作glMatrixMode(GL_MODELVIEW);//重置当前指定的矩阵为单位矩阵　glLoadIdentity();
}//按键输入处理回调函数
void specialKey(int key,int x,int y){if(key==GLUT_KEY_UP){xRotAngle-=5.0f;}else if(key==GLUT_KEY_DOWN){xRotAngle+=5.0f;}else if(key==GLUT_KEY_LEFT){yRotAngle-=5.0f;}else if(key==GLUT_KEY_RIGHT){yRotAngle+=5.0f;}//重新绘制glutPostRedisplay();
}int main(int argc, char* argv[])
{int nModeMenu;int nEdgeMenu;int nDepthMask;int nMainMenu;//初始化glut glutInit(&argc,argv);//使用双缓冲区模式glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA|GLUT_DEPTH);//获取系统的宽像素SCREEN_WIDTH=glutGet(GLUT_SCREEN_WIDTH);//获取系统的高像素SCREEN_HEIGHT=glutGet(GLUT_SCREEN_HEIGHT);//创建窗口，窗口名字为OpenGL Scissor DemoglutCreateWindow("OpenGL Scissor Demo");//设置窗口大小glutReshapeWindow(windowWidth,windowHeight);//窗口居中显示glutPositionWindow((SCREEN_WIDTH-windowWidth)/2,(SCREEN_HEIGHT-windowHeight)/2);//窗口大小变化时的处理函数glutReshapeFunc(changSize);//设置显示回调函数 glutDisplayFunc(renderScreen);//设置按键输入处理回调函数glutSpecialFunc(specialKey);//设置全局渲染参数setupRederingState();glutMainLoop();return 0;
}