每个物体在不同的环境在会有不同的视觉效果。为了使效果更加真实需要增加特殊效果。
例8：绘制三个互相重叠的矩形，这三个矩形从左到右分别为绿、红、蓝。其中红色、蓝色矩形的透明度分别为50%的透明度，即alpha值为0.5，如图九所示。
#include <stdlib.h>
#include <GL/glut.h>
void myInit(void)
{
glClearColor(0.2,0.8,0.8,0.0);//将背景设置为湖蓝色
glEnable(GL_BLEND);//激活GL_BLEND
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);//指定像素的数学算法
glEnable(GL_FLAT);
}
void myDisplay(void)
{
glClear(GL_COLOR_BUFFER_BIT);
/*绘制一个绿色的矩形 alpha = 1.0*/
glColor4f(0.0,1.0,0.0,1.0);
glBegin(GL_POLYGON);
glVertex3f(-0.75,0.5,0.0);
glVertex3f(-0.75,-0.75,0.0);
glVertex3f(0.5,-0.75,0.0);
glVertex3f(0.5,0.5,0.0);
glEnd();
/*绘制一个红色的矩形 alpha = 0.5*/
glColor4f(1.0,0.0,0.0,0.5);
glBegin(GL_POLYGON);
glVertex3f(-0.25,1.0,0.0);
glVertex3f(-0.25,-0.25,0.0);
glVertex3f(1.0,-0.25,0.0);
glVertex3f(1.0,1.0,0.0);
glEnd();
/*绘制一个蓝色的矩形 alpha = 0.5*/
glColor4f(0.0,0.0,1.0,0.5);
glBegin(GL_POLYGON);
glVertex3f(0.25,1.5,0.0);
glVertex3f(0.25,0.25,0.0);
glVertex3f(1.5,0.25,0.0);
glVertex3f(1.5,1.5,0.0);
glEnd();
glFlush();
}
void myReshape(int w,int h)
{
glViewport(0,0,w,h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
if(w<h)
glOrtho(-1.5,1.5,-1.5*(GLfloat)h/(GLfloat)w,1.5*(GLfloat)h/(GLfloat)w,-1.5,1.5);
else
glOrtho(-1.5*(GLfloat)w/(GLfloat)h,1.5*(GLfloat)w/(GLfloat)h,-1.5,1.5,-1.5,1.5);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(-0.4,0.0,0.0);
}
int main(int argc,char ** argv)
{
/*初始化*/
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(300,400);
glutInitWindowPosition(200,200);
/*创建窗口*/
glutCreateWindow("BLEND POLYGON");
/*绘制与显示*/
myInit();
glutReshapeFunc(myReshape);
glutDisplayFunc(myDisplay);
/*进入GLUT事件处理循环*/
glutMainLoop();
return(0);
} |
 图九：三个互相重叠的矩形 |
·glBlendFunc（GLenum sfactor，GLenum dfactor） 指定像素算法。sfactor指定红，绿，蓝及alpha源混合因素是如何计算的。dfactor指定红，绿，蓝及alpha目标混合因素是如何计算的。
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