【新手求助】【代码】【OPENGL】【gluPerspective】

[true1064]

目的：在原点位置画一个向Z轴负方向申得很长的长方体，用gluLookAt和gluPerspective观察。

问题：看不到按Z=1处的浅蓝色的面，而且gluLookAt(0,0,【1.9】,0,0,0,0,1,0)的第三个参数一旦为2或者大于2，就看不到任何对象了。搞了很久弄不明白，请求帮助！谢谢了

代码：


#include<gl/glut.h>

void init(void)
{
glClearColor(1.0,1.0,1.0,0.0);//Set display-window color to white.
glMatrixMode(GL_PROJECTION);// Set projection parameters.

/********************【我的疑问在这两行】**********************/
gluLookAt(0,0,1.9,0,0,0,0,1,0);//【问题】第三个参数为2就看不到任何东西了。为什么？
gluPerspective(30,1,0.1,150); //我把参数怎样改都看不见Z=1的那个浅蓝色的面
/********************我的疑问在这两行**********************/


}



/********************定义用顶点数组画四边形的函数,然后再定义画长方体的函数***************/

GLfloat pt[8][3]={{-0.5,-0.5,1},{-0.5,0.5,1},{0.5,-0.5,1},{0.5,0.5,1},
{-0.5,-0.5,-10},{-0.5,0.5,-10},{0.5,-0.5,-10},{0.5,0.5,-10}};

void quad(GLint n1,GLint n2,GLint n3,GLint n4) //画四边形的函数
{

glBegin(GL_QUADS);
glVertex3fv(pt[n1]);
glVertex3fv(pt[n2]);
glVertex3fv(pt[n3]);
glVertex3fv(pt[n4]);
glEnd();


//glFlush();
}


void cube() //画长方体的函数
{
glColor3f(1.0,1.0,0.0);
quad(6,2,3,7);

glColor3f(1.0,0.0,0.0);
quad(5,1,0,4);

glColor3f(0.0,1.0,0.0);
quad(7,3,1,5);

glColor3f(1.0,0.0,1.0);
quad(4,0,2,6);

glColor3f(0.0,1.0,1.0);//浅蓝，最近面，z=1
quad(2,0,1,3);

glColor3f(0.0,0.0,0.0);//黑色，最远面,z=-50
quad(7,5,4,6);


glBegin(GL_LINES);

//X轴,黑色
glColor3f(0.0,0.0,0.0);
glVertex3f(0,0,0);
glVertex3f(1,0,0);

//y轴，绿色
glColor3f(0.0,1.0,0.0);
glVertex3f(0,0,0);
glVertex3f(0,1,0);

//z轴，蓝色
glColor3f(0.0,0.0,1.0);
glVertex3f(0,0,0);
glVertex3f(0,0,1);

glEnd();

//glFlush();
}


void display( )
{
glClear(GL_COLOR_BUFFER_BIT);
cube();
glFlush();

}


int main(int argc,char** argv)
{
glutInit(&argc,argv);//Initialize GLUT
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);//Set display mode.
glutInitWindowSize(500,500);//Set display-window width and height
glutInitWindowPosition(250,250);//Set top-left display-window position
glutCreateWindow("An Example OpenGL Program");//Create display window.

init(); //Execute initialization procedure.
glutDisplayFunc(display);//Send graphics to display window.
//glutReshapeFunc(reshape);//Reshape display window.
glutMainLoop(); //Display everything and wait.
return 0;
}

[true1064]

谢谢指点！弄出来了

void init(void)
{
glClearColor(1.0,1.0,1.0,0.0);

/*********相机位置在glMatrixMode(GL_MODELVIEW)之后设定************/
glMatrixMode(GL_MODELVIEW);
gluLookAt(1,2,2,0,0,0,0,1,0);

/*********透视投影在glMatrixMode(GL_PROJECTION)之后设定************/
glMatrixMode(GL_PROJECTION);
gluPerspective(60,1,0.2,150);

}

[张赐]

原因很简单
你的gluLookAt(0,0,1.9,0,0,0,0,1,0);是作用于投影变换的，因为在glLookAt前面，你使用了
glMatrixMode(GL_PROJECTION);

但是在openGL管线中，在投影变换阶段会把物体的所有定点坐标转换到单位viewvolume中，你设置相机位置为1.9，这里是在相机坐标系中的位置。

所以要设置相机在世界坐标中的位置，应该在模型变换矩阵中来设置，即使用
glMatrixMode(GL_MODLEVIEW);