沿着z轴看，gluLookAt在theta = 360时工作，但在theta = 0时不工作

Question

我正在玩OpenGL渲染三角形，我遇到了一个有趣的问题。显然，当你沿着“向上看”时，方向，在我的情况下意味着Eye_z = Up_z（并且由于我在球坐标中工作，这意味着theta = 0），因此不显示对象（请参阅Google Fonts FAQs的最佳答案以获得数学解释）。 但是，当我将角度设置为360度而不是0时，对象显示没有任何问题。有谁知道为什么？我已经附上了我的代码片段以及截图：

triangle.cpp

void Triangle::render(Camera C){
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glEnable(GL_DEPTH_TEST);
        glLoadIdentity(); 
        float th = C.Getth();
        float ph = C.Getph();

        float Eye_x = r*sin(th*DEGtoRAD)*cos(ph*DEGtoRAD);
        float Eye_y = r*sin(th*DEGtoRAD)*sin(ph*DEGtoRAD);
        float Eye_z = r*cos(th*DEGtoRAD);

        float Up_z = (th>=180.0f && th<360.0f)?-1.0f:1.0f;

        gluLookAt(Eye_x,Eye_y,Eye_z,0.0,0.0,0.0,0.0,0.0,Up_z); 
        glRotatef(m_rotationAngle, 0.0f, 0.0f, 1.0f);

        // Draw Triangle
        glBegin(GL_TRIANGLES);
                glColor3f(1.0f,0.0f,0.0f);
                glVertex3f(-0.5f,-0.5f,0.0f);
                glColor3f(1.0f,1.0f,0.0f);
                glVertex3f(0.5f,-0.5f,0.0f);
                glColor3f(0.0f,0.0f,1.0f);
                glVertex3f(0.0f,0.5f,0.0f);
        glEnd();

        // Draw Axes
        glBegin(GL_LINES);
                glColor3f(1.0f,0.0f,0.0f);
                glVertex3f(0.0f,0.0f,0.0f);
                glVertex3f(1.0f,0.0f,0.0f);
                glColor3f(0.0f,1.0f,0.0f);
                glVertex3f(0.0f,0.0f,0.0f);
                glVertex3f(0.0f,1.0f,0.0f);
                glColor3f(0.0f,0.0f,1.0f);
                glVertex3f(0.0f,0.0f,0.0f);
                glVertex3f(0.0f,0.0f,1.0f);
        glEnd();
}

camera.cpp：

void Camera::Move(int dir){
        switch(dir){
                case 1: // +th
                        th += move_amt;
                        if(th>360.0f){
                                th-=360.0f;
                        }
                        break;
                case -1: // -th
                        th -= move_amt;
                        if(th<=0.0f){
                                th+=360.0f;
                        }
                        break;
                case 2: // ph
                        ph += move_amt;
                        if(ph>=360.0f){
                                ph-=360.0f;
                        }
                        break;
                case -2: // -ph
                        ph -= move_amt;
                        if(ph<=0.0f){
                                ph+=360.0f;
                        }

                        break;
                default:
                        break;
        }
}

OpenGL is not displaying anything when looking along the y axis

This is what I get when Theta=0