c++ - 从相机旋转 openGL 中隔离 "Compass"

Question

我即将完成太阳系动画的原型(prototype)。 (它仍然没有达到应有的效率)。目前我有一个太阳系，有土星/天王星/海王星加上海卫一/普罗透斯(海王星的卫星)。该程序允许用户在 awsd 围绕轴旋转相机的同时为这些行星设置动画。 (0,1,0/1,0,0 等)

我放入了一个原型(prototype)罗盘(实际上是 3 条线)来指示当前的相机方向。 Y = 白色。 X = 红色。 Z = 蓝色。然而，无论何时旋转相机，罗盘似乎都在移动(尽管它实际上并没有移动)，从而使用户感到困惑。有没有一种方法可以锁定罗盘位置，同时允许它模仿用户的相机旋转到位？

我知道指南针是 3D 空间中的一组 3D 线，将其视觉锁定在 2D 场中没有意义，但我希望无论如何都能找到解决方案。谢谢!

此外，我还尝试为指南针制作动画，以便它模仿相机在适当位置的旋转，以帮助用户了解他/她当前的相机方向。

在 display() 中绘制/初始化指南针

案例中声明的相机旋转“wasd”

float triton = 0;
float proteus = 0;
float neptune = 0;
float saturn = 0;
float uranus = 0;
int sun = 0;
int angle = 0;
GLint buf, sbuf;

void init(void)
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glShadeModel(GL_SMOOTH);

    // Stars 
    glNewList(1, GL_COMPILE);
    glBegin(GL_POINTS);
    glColor3f(1.0, 1.0, 1.0);
    for (int i = 0; i < 200; i++){
        for (int j = 0; j < 300; j++){
            if (((i + j) % 2) == 0){
                glVertex3f(100 * i, 100 * j, 0.0);
            }
        }
    }
    glEnd();
    glEndList();

    // Material Specs
    GLfloat mat_specular[] = { 0.8, 0.8, 0.9, 0.1 };
    GLfloat mat_shininess[] = { 128.0 };
    GLfloat lightDiffuse[] = { 1.0, 1.0, 1.0, 0.0 };
    GLfloat lmodel_ambient[] = { 0.1, 0.2, 0.7, 0.0 };

    // Light 0 Initialized.
    GLfloat light0[] = { 1.0, 1.0, 1.0, 0.1 };
    GLfloat light_position[] = { 1.0, 0.5, 0.0, -100.0 };

    // Light 0
    glLightfv(GL_LIGHT0, GL_SPECULAR, light0);
    glLightfv(GL_LIGHT0, GL_POSITION, light_position);

    // Mat Specs Implmentations.
    glMaterialfv(GL_FRONT, GL_DIFFUSE, lightDiffuse);
    glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
    glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);

    //Ambient surrounding light on object.
    glLightModelfv(GL_LIGHT_MODEL_AMBIENT, lmodel_ambient);

    // Enable Lighting and Depth
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    glEnable(GL_DEPTH_TEST);
    // Enable AntiAliased Lines

    glEnable(GL_LINE_SMOOTH);
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glHint(GL_LINE_SMOOTH_HINT, GL_DONT_CARE);
    glLineWidth(1.5);

}
// Orbit values are arbitrary
void orbit(void)
{
    triton = triton - 0.11;
    proteus = proteus - 0.08;
    neptune = neptune - 0.04;
    saturn = saturn - 0.02;
    uranus = uranus - 0.037;
    glutPostRedisplay();
}

void backorbit(void)
{
    triton = triton + 0.11;
    proteus = proteus + 0.08;
    neptune = neptune + 0.04;
    saturn = saturn + 0.02;
    uranus = uranus + 0.037;
    glutPostRedisplay();
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_COLOR_MATERIAL);
    glDisable(GL_LIGHTING);
    // Compass
    glPushMatrix();
        glBegin(GL_LINES);
            glColor3f(1.0, 1.0, 1.0);
            // Y axis
            glVertex3f(-15.0, 12.0, 0.0);
            glVertex3f(-15.0, 17.0, 0.0);
            glColor3f(1.0, 0.0, 0.0);
            // X axis
            glVertex3f(-12.5, 14.5, 0.0);
            glVertex3f(-17.5, 14.5, 0.0);
            glColor3f(0.0, 0.0, 1.0);
            // Z axis
            glVertex3f(-15.0, 14.5, 1.0);
            glVertex3f(-15.0, 14.5, -1.5);
        glEnd();
    glPopMatrix();

    glEnable(GL_LIGHTING);
    // Sun 
    glPushMatrix();
        glColor3f(1.0, 0.35, 0.1);
        glutSolidSphere(2.0, 100, 100);

        // Saturn
        glPushMatrix();
            glRotatef((GLfloat)saturn, 0.5, 0.6, 1.0);
            glColor3f(0.7, 0.5, 0.3);
            glTranslatef(5.0, 0.0, 0.0);
            glutSolidSphere(0.38, 100, 100);
            glRotatef(20.0, 1.0, 0.0, 0.0);
            glutSolidTorus(.11, .60, 2, 50);
        glPopMatrix();

        // Uranus
        glPushMatrix();
            glRotatef((GLfloat)uranus, 0.0, 1.0, 0.0);
            glColor3f(0.5, 0.85, 0.9);
            glTranslatef(6.5, 0.0, 0.0);
            glutSolidSphere(0.37, 100, 100);
        glPopMatrix();
        // End Uranus

        // Neptune 
        glPushMatrix();
            glRotatef((GLfloat)neptune, 0.3, 1.0, 0.8);
            glTranslatef(8.0, 0.0, 0.0);
            glColor3f(0.1, 0.1, 0.3);
            glutSolidSphere(0.3, 100, 100);

            // Neptune(Triton)
            glPushMatrix();
                glColor3f(0.85, 0.7, 0.8);
                glRotatef((GLfloat)triton, 1.0, 1.0, 1.0);
                glTranslatef(1.0, 0.0, 0.0);
                glutSolidSphere(0.07, 100, 100);
            glPopMatrix(); // Ends Triton

            // Neptune(Proteus)
            glPushMatrix();
                glColor3f(1.0, 1.0, 1.0);
                glRotatef((GLfloat)proteus, 0.0, 1.0, 0.0);
                glTranslatef(1.0, 0.0, 0.0);
                glutSolidSphere(0.04, 100, 100);
            glPopMatrix(); // Ends Proteus

        glPopMatrix(); // Ends Neptune

    glPopMatrix(); // Ends Sun

    glEnable(GL_MULTISAMPLE);

    glDisable(GL_LIGHTING);
    // Stars
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
        glLoadIdentity();
        glPushMatrix();
            glMatrixMode(GL_PROJECTION);
            glPushMatrix();
                glLoadIdentity();
                // Arbitrary ortho
                glOrtho(0, 1000, 0, 1000, 0, 1000);
                glCallList(1);
            glPopMatrix();
            glMatrixMode(GL_MODELVIEW);
        glPopMatrix();
    glPopMatrix(); 

    glDisable(GL_COLOR_MATERIAL);
    glEnable(GL_LIGHTING);
    glFlush();
    glutSwapBuffers();
}

void reshape(int w, int h)
{
    glViewport(0, 0, (GLsizei)w, (GLsizei)h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(120.0, (GLfloat)w / (GLfloat)h, 1.0, 20.0); 
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(0.0, 0.0, -11.0); // Find location. 

}
void keyboard(unsigned char key, int x, int y)
{
    switch (key) {
        // Triton + Proteus Orbit. 
    case 'o':
        glutIdleFunc(orbit);
        break;
    case 'p':
        glutIdleFunc(backorbit);
        break;
        // Camera Rotations. 
    case 'd':
        glRotatef(4, 1.0, 0.0, 0.0);
        glutPostRedisplay();
        break;
    case 'a':
        glRotatef(-4, 1.0, 0.0, 0.0);
        glutPostRedisplay();
        break;
    case 'w':
        angle -= 4.0;
        glRotatef(-4.0, 0.0, 1.0, 0.0);
        glutPostRedisplay();
        break;
    case 's':
        glRotatef(4.0, 0.0, 1.0, 0.0);
        glutPostRedisplay();
        break;
        // Stop Orbit.
    case 't':
        glutIdleFunc(NULL);
        break;
        // Reset to Origin (IP)
    case '1':
        angle = 0;
        glutPostRedisplay();
        break;
    case ',':
        glTranslatef(-0.3, 0.0, 0.0);
        glutPostRedisplay();
        break;
    case '.':
        glTranslatef(0.3, 0.0, 0.0);
        glutPostRedisplay();
        break;
        // Exit
    case 27:
        exit(0);
        break;
    default:
        break;
    }
}
int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_DEPTH | GLUT_RGB | GLUT_MULTISAMPLE);
    glutInitWindowSize(1000, 1000);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("Solar System");
    init();
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutKeyboardFunc(keyboard);
    glutMainLoop();
    return 0;
}

Answer 1

你必须将“指南针”绘制到 View 的中心 (0, 0, 0)，然后你必须应用 View 矩阵，最后你必须将“指南针”平移到它的最终位置。
当前模型 View 矩阵可以通过glGetFloatv( GL_MODELVIEW_MATRIX, view_mat )获取:

glPushMatrix();
GLfloat view_mat[16];
glGetFloatv( GL_MODELVIEW_MATRIX, view_mat );
glLoadIdentity();
glTranslatef( -15.0, 14.5, 0.0 );
glMultMatrixf( view_mat );

    glBegin(GL_LINES);
        glColor3f(1.0, 1.0, 1.0);
        // Y axis
        glVertex3f(0.0, -2.5, 0.0);
        glVertex3f(0.0,  2.5, 0.0);
        glColor3f(1.0, 0.0, 0.0);
        // X axis
        glVertex3f(-2.5, 0.0, 0.0);
        glVertex3f(2.5, 0.0, 0.0);
        glColor3f(0.0, 0.0, 1.0);
        // Z axis
        glVertex3f(-0.0, 0.0, 1.0);
        glVertex3f(-0.0, 0.0, -1.5);
    glEnd();

glPopMatrix();

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但请注意，通过 glBegin/glEnd 序列绘制和使用固定功能管道矩阵堆栈已被弃用多年。了解 Fixed Function Pipeline并查看 Vertex Specification和 Shader最先进的渲染方式。