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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。 例9：绘制一个被雾化的圆锥体，如图十所示，为了观察不同的雾化参数，中加入了键盘操作。"shift+1"，" shift+2"键分别增加和减弱雾化浓度，"shift+3"设置雾化的起始点和终止点，"shift+4"和"shift+5"键改变雾化方程，"shift+6"将雾化颜色由白改为绿色，如图十一所示。
#include <stdlib.h>
#include <GL/glut.h>
GLfloat light_ambient[] = {0.1,0.1,0.1,0.0};
GLfloat light_diffuse[] = {1.0,1.0,1.0,0.0};
GLfloat light_specular[] = {1.0,1.0,1.0,0.0};
GLfloat light_position[] = {-10.0,0.0,5.0,0.0};
GLfloat material_ambient[] = {0.1745,0.01175,0.01175};
GLfloat material_diffuse[] = {0.61424,0.04136,0.04136};
GLfloat material_specular[] = {0.727811,0.626959,0.62659};
GLfloat fogColorWhite[] = {1.0,1.0,1.0,1.0};
GLfloat fogColorGreen[] = {0.0,1.0,0.0,1.0};
float fogDensity = 0.02;
void myInit(void)
{
/*设置背景色*/
glClearColor(0.5,0.5,0.5,1.0);
/*设置光照*/
glLightfv(GL_LIGHT0,GL_AMBIENT,light_ambient);
glLightfv(GL_LIGHT0,GL_DIFFUSE,light_diffuse);
glLightfv(GL_LIGHT0,GL_SPECULAR,light_specular);
glLightfv(GL_LIGHT0,GL_POSITION,light_position);
/*设置材质*/
glMaterialfv(GL_FRONT,GL_AMBIENT,material_ambient);
glMaterialfv(GL_FRONT,GL_DIFFUSE,material_diffuse);
glMaterialfv(GL_FRONT,GL_SPECULAR,material_specular);
glMaterialf(GL_FRONT,GL_SHININESS,0.6*128);
glShadeModel(GL_SMOOTH);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glEnable(GL_AUTO_NORMAL);
glEnable(GL_NORMALIZE);
glFrontFace(GL_CW);
/*设置雾化*/
glEnable(GL_FOG);
glFogi(GL_FOG_MODE,GL_LINEAR);
glFogfv(GL_FOG_COLOR,fogColorWhite);
glFogf(GL_FOG_DENSITY,fogDensity);
glFogf(GL_FOG_START,0.0);
glFogf(GL_FOG_END,15.0);
glHint(GL_FOG_HINT,GL_DONT_CARE);
}
/*根据不同的键值设置不同的雾化效果*/
static void myKey(unsigned char key,int x,int y)
{
switch(key)
{
case 33: //shift+1
fogDensity *= 1.10;
glFogi(GL_FOG_MODE,GL_EXP);
glFogf(GL_FOG_DENSITY,fogDensity);
glutPostRedisplay();
break;
case 64: //shift+2
fogDensity /= 1.10;
glFogi(GL_FOG_MODE,GL_EXP);
glFogf(GL_FOG_DENSITY,fogDensity);
glutPostRedisplay();
break;
case 35: //shift+3
glFogi(GL_FOG_START,0.0);
glFogi(GL_FOG_END,8.0);
glutPostRedisplay();
break;
case 36: //shift+4
glFogi(GL_FOG_MODE,GL_EXP2);
glutPostRedisplay();
break;
case 37: //shift+5
glFogi(GL_FOG_MODE,GL_LINEAR);
glutPostRedisplay();
break;
case 94: //shift+6
glFogfv(GL_FOG_COLOR,fogColorGreen);
glutPostRedisplay();
break;
case 27: //Esc
exit(0);
default:
break;
}
}
/*绘制圆锥体*/
void myDisplay(void)
{
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glPushMatrix();
glTranslatef(-4.0,4.0,0.0);
glRotatef(30.0,1.0,1.0,0.0);
glutSolidCone(1.0,50.0,20.0,20.0);
glPopMatrix();
glutSwapBuffers();
}
void myReshape(int w,int h)
{
glViewport(0,0,(GLsizei)w,(GLsizei)h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(100.0,1.0,1.0,20.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0,0.0,-15.5);
}
int main(int argc,char ** argv)
{
/*初始化*/
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB|GLUT_DEPTH);
glutInitWindowSize(300,300);
glutInitWindowPosition(100,100);
/*创建窗口*/
glutCreateWindow(" FOG ");
/*绘制与显示*/
myInit();
glutKeyboardFunc(myKey);
glutReshapeFunc(myReshape);
glutDisplayFunc(myDisplay);
/*进入GLUT事件处理循环*/
glutMainLoop();
return 0;
} |
 图十：被雾化的圆锥体 | ·glFog指定雾化参数。后面不同的字母表明参数的数据类型。f表示float，i表示integer，v表示vector，也就是表明一个指针。 GL_FOG_MODE是一个单值整数或浮点数，该数值指定了用来计算雾化融合因子f的方程。 GL_DENSITY 是一个单值整数或浮点数，该值指定雾化浓度。 GL_FOG_START 是一个单值整数或浮点数，该值指定雾化的起始值。 GL_FOG_END是一个单值整数或浮点数，该值指定雾化的终止值。 GL_FOG_INDEX是一个单值整数或浮点数，该值指定雾化索引值if，缺省的雾化索引值为0.0。 GL_FOG_COLOR包含4个整数值或浮点数值，这些数值指定的是雾化颜色Cf，整数值进行线性映射。 ·glHint指定实现的线索。本例中GL_FOG_HINT指定雾化计算精度。GL_DONT_CARE指对选项不做考虑。
 图十一：雾化为绿色的圆锥体 | 本例中还涉及到键盘操作，glutKeyboardFun。此函数注册当前窗口的键盘回调函数。因为这不是本节的主要内容，我们只简单提一下。此函数的原形为void glutKeyboardFunc （void（*func）（unsigned char key，int x，int y））所以我们自己写的回调函数必须按照原形上规定的写。
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