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 window
glClearColor(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xRotAngle
glRotatef(xRotAngle,1.0f,0.0f,0.0f);
//坐标系绕y轴旋转yRotAngle
glRotatef(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Matrix
glPopMatrix();
//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
",sizes[0],sizes[1]);
printf("point step:%f
",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);
else
glOrtho(-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 Demo
glutCreateWindow("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;
}