如何让我的灯光相对于我的场景（在立方体中）保持固定？

Question

我一直在画康奈尔盒子。我想在我的灯里放灯（灯是一个放在右边墙上的白色立方体）。它会产生一种灯在发光的错觉。但是我不能把我的灯放到立方体里。我尝试设置视图变换，然后设置灯光位置，但它不起作用。 这是我的简单代码：（灯立方已移动glTranslatef(0.63, 0.3, 0.0)）

 void display(void)
{

    glEnable(GL_DEPTH_TEST);
    glViewport(0, 0, 600, 600);
    glClearColor(0.32, 0.32, 0.32, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    // draw cube
    glPushMatrix();
    glTranslatef(0.2, -0.3, 0.4);
    glColor3d(1.0, 0.38, 0.0);
    glutSolidCube(0.2);
    glPopMatrix();
    // draw sphere
    glPushMatrix();
    glTranslatef(0.2, -0.1, 0.4);
    glColor3d(0.71, 0.0, 0.59);
    glutSolidSphere(0.1,100,100);
    glPopMatrix();
    // draw parallelepiped
    glPushMatrix();
    glTranslatef(-0.2, -0.2, 0.25);
    glRotatef(20, 0.0, 1.0, 0.0);
    glColor3d(1.0, 0.38, 0.0);
    glScalef(1, 1.8, 1);
    glutSolidCube(0.2);
    glPopMatrix();
    // draw lamp
    glPushMatrix();
    glTranslatef(0.63, 0.3, 0.0);
    glScalef(1.5, 0.3, 1);
    glColor3d(1.0, 1.0, 1.0);
    glutSolidCube(0.2);
    glPopMatrix();

    glBegin(GL_QUADS);
    glNormal3f(0, 1.0, 0);
    glColor3d(0.69, 0.69, 0.69); // bottom
    glVertex3d(-0.5, -0.5, 0.5);
    glVertex3d(-0.5, -0.5, -0.5);
    glVertex3d(0.5, -0.5, -0.5);
    glVertex3d(0.5, -0.5, 0.5);
    glEnd();
    glBegin(GL_QUADS);
    glNormal3f(0, -1.0, 0);
    glColor3d(0.69, 0.69, 0.69); // top
    glVertex3d(-0.5, 0.5, 0.5);
    glVertex3d(-0.5, 0.5, -0.5);
    glVertex3d(0.5, 0.5, -0.5);
    glVertex3d(0.5, 0.5, 0.5);
    glEnd();
    glBegin(GL_QUADS);
    glNormal3f(-1.0, 0.0, 0.0);
    glColor3d(0.0, 0.79, 0.05); // right
    glVertex3d(0.5, -0.5, 0.5);
    glVertex3d(0.5, 0.5, 0.5);
    glVertex3d(0.5, 0.5, -0.5);
    glVertex3d(0.5, -0.5, -0.5);
    glEnd();
    glBegin(GL_QUADS);
    glNormal3f(1.0, 0.0, 0.0);
    glColor3d(1.0, 0.16, 0.0); // left
    glVertex3d(-0.5, -0.5, 0.5);
    glVertex3d(-0.5, 0.5, 0.5);
    glVertex3d(-0.5, 0.5, -0.5);
    glVertex3d(-0.5, -0.5, -0.5);
    glEnd();
    glBegin(GL_QUADS);
    glNormal3f(0.0, 0.0, 1.0);
    glColor3d(0.69, 0.69, 0.69); // back
    glVertex3d(-0.5, -0.5, -0.5);
    glVertex3d(-0.5, 0.5, -0.5);
    glVertex3d(0.5, 0.5, -0.5);
    glVertex3d(0.5, -0.5, -0.5);
    glEnd();
    glFlush();
    glPopMatrix();
    glutSwapBuffers();

}

void Initialize() {
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    GLfloat global_ambient[] = { 0.1, 0.1, 0.1, 1.0 };
    glLightModelfv(GL_LIGHT_MODEL_AMBIENT, global_ambient);
    GLfloat ambient[] = { 0.0,0.0,0.0,1.0 };
    GLfloat diffuse[] = { 1.0,1.0,1.0,1.0 };
    GLfloat spec[] = { 1,1,1,1 };
    GLfloat specref[] = { 1,1,1,1 };
    GLfloat lpos[] = { 0.0, 0.0, 0.0, 1.0 };
    glLightfv(GL_LIGHT0, GL_POSITION, lpos);
    glLightfv(GL_LIGHT0, GL_AMBIENT, ambient);
    glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuse);
    glLightfv(GL_LIGHT0, GL_SPECULAR, spec);

    glMaterialfv(GL_FRONT_AND_BACK, GL_SPECULAR, specref);
    glMateriali(GL_FRONT_AND_BACK, GL_SHININESS, 64);
    glEnable(GL_COLOR_MATERIAL);
    glColorMaterial(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE);
}

void exitFunc(unsigned char key, int x, int y) {
    switch (key) {
    case 27:
        exit(0);
        break;
    }
}

void reshape(int w, int h)
{
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-0.35, 0.35, -0.35, 0.35, 1.2, 400.0);
    glMatrixMode(GL_MODELVIEW);
    glTranslatef(0, 0, -2);
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize(600, 600);
    glutInitWindowPosition(0, 0);
    glutCreateWindow("Cornell Box");
    Initialize();
    glutDisplayFunc(display);
    glutKeyboardFunc(exitFunc);
    glutReshapeFunc(reshape);
    glutIdleFunc(display);

    glutMainLoop();
    return 0;   
}

Answer 1

请注意，由glBegin/glEnd 序列绘制、固定函数管道矩阵堆栈和每个顶点光照模型的固定函数管道，几十年来已被弃用。 阅读Fixed Function Pipeline 并查看Vertex Specification 和Shader，了解最先进的渲染方式。

---

无论如何，当灯光位置由glLightfv(GL_LIGHT0, GL_POSITION, pos)设置时，那么 pos 乘以当前模型视图矩阵。
这意味着如果在设置模型视图矩阵之前设置位置，那么灯光位置将放置在世界的绝对坐标中。
如果是在设置模型视图矩阵之后设置，则可以在模型坐标系中设置灯光位置。

在绘制灯时设置灯光位置，到“灯”的中心：

// draw lamp
glPushMatrix();
glTranslatef(0.63, 0.3, 0.0);
glScalef(1.5, 0.3, 1);

GLfloat lpos[] = { 0.0, 0.0, 0.0, 1.0 };
glLightfv(GL_LIGHT0, GL_POSITION, lpos);

glColor3d(1.0, 1.0, 1.0);
glutSolidCube(0.2);
glPopMatrix(); 

OpenGL 固定函数管道计算每个顶点的光：Gouraud shading。
这可能会导致，如果光矢量以相对于表面平面的锐角撞击顶点坐标，则您无法“看到”光。

在您的情况下，绿色墙壁几乎没有点亮，因为光源几乎直接在墙上。灯本身不亮，因为光源在灯内部，是从背面发出的光。

见OpenGL Lighting on texture plane is not working

稍微改变灯光的位置，把它放在墙的前面，把右边的墙镶嵌成4个四边形，“看”我的意思：

 GLfloat lpos[] = { -0.1, 0.0, 0.0, 1.0 }; 

// right
glBegin(GL_QUADS);
glNormal3f(-1.0, 0.0, 0.0);
glColor3d(0.0, 0.79, 0.05); 
glVertex3d(0.5,  0.3,  0.5);
glVertex3d(0.5,  0.5,  0.5);
glVertex3d(0.5,  0.5,  0.0);
glVertex3d(0.5,  0.3,  0.0);

glVertex3d(0.5,  0.3,  0.0);
glVertex3d(0.5,  0.5,  0.0);
glVertex3d(0.5,  0.5, -0.5);
glVertex3d(0.5,  0.3, -0.5);

glVertex3d(0.5, -0.5,  0.5);
glVertex3d(0.5,  0.3,  0.5);
glVertex3d(0.5,  0.3,  0.0);
glVertex3d(0.5, -0.5,  0.0);

glVertex3d(0.5, -0.5,  0.0);
glVertex3d(0.5,  0.3,  0.0);
glVertex3d(0.5,  0.3, -0.5);
glVertex3d(0.5, -0.5, -0.5);

glEnd();