我想做一个小的应用程序，让我可以移动三角形的顶部/底部左/右。
而当我按w -〉向上旋转并向上移动时，s =〉向下旋转并向下移动，d -〉向右旋转并向右移动，a -〉向左旋转并向左移动。
下面是我代码：

# include <GL/glut.h> // (or others, depending on the system in use)

float xpoint1 = 0.0f;
float xpoint2 = 50.0f;
float xpoint3 = 25.0f;
float ypoint1 = 0.0f, ypoint2 = 0.0f, ypoint3 = 20.0f;
double direction = 0.0;
void Keys(unsigned char key, int x, int y) {

    if (key == 'a') {
        if (xpoint1 != 0) {
            xpoint1 -= 1.0f;
            xpoint2 -= 1.0f;
            xpoint3 -= 1.0f;
            direction = 90.0;
        }

    }
    else if (key == 'd') {
        if (xpoint2 != 200) {
            xpoint1 += 1.0f;
            xpoint2 += 1.0f;
            xpoint3 += 1.0f;
            direction = 270.0;
        }
    }
    else if (key == 'w') {
        if (ypoint3 != 150) {
            ypoint1 += 1.0f;
            ypoint2 += 1.0f;
            ypoint3 += 1.0f;
            direction = 0.0;
        }
    }
    else if (key == 's') {
        if (ypoint3 != 0) {
            ypoint1 -= 1.0f;
            ypoint2 -= 1.0f;
            ypoint3 -= 1.0f;
            direction = 180.0;

        }
    }

    glutPostRedisplay();
}
void resizeChange(int w, int h) {

    glClearColor(1.0, 1.0, 1.0, 0.0); // Set display-window color to white.
    glMatrixMode(GL_PROJECTION); // Set projection parameters.
    glLoadIdentity();
    gluOrtho2D(0.0, 200.0, 0.0, 150.0);
    glViewport(0, 0, w, h);
    glMatrixMode(GL_MODELVIEW);
}
void lineSegment(void)
{
    glClear(GL_COLOR_BUFFER_BIT); // Clear display window.
    glColor3f(0.0, 0.4, 0.2); // Set line segment color to green.

    //gluLookAt(0.0, 0.0, 5.0,
    //  0.0, 0.0,0.0,
    //  0.0, 1.0, 0.0
    //);

    glLoadIdentity();

    glRotated(direction, 0.0, 0.0, 1.0);
        glBegin(GL_TRIANGLES);
            glColor3f(1.0, 0.0, 0.0);
            glVertex2f(xpoint1, ypoint1);
            glColor3f(0.0, 1.0, 0.0);
            glVertex2f(xpoint2, ypoint2);
            glColor3f(0.0, 0.0, 1.0);
            glVertex2f(xpoint3, ypoint3);
        glEnd();
        //degree += 1.0;
    glFlush(); // Process all OpenGL routines as quickly as possible.
        //glutSwapBuffers();
}
void main(int argc, char**argv)
{
    glutInit(&argc, argv); // Initialize GLUT.
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); // Set display mode.
    glutInitWindowPosition(50, 100); // Set top-left display-window position.
    glutInitWindowSize(600, 600); // Set display-window width and height.
    glutCreateWindow("An Example OpenGL Program"); // Create display window.
    glutDisplayFunc(lineSegment); // Send graphics to display window.
    glutReshapeFunc(resizeChange);
    //glutIdleFunc(lineSegment);
    glutKeyboardFunc(Keys);
    glutMainLoop(); // Display everything and wait.
}

问题是当我试图改变三角形的方向时，它消失了？！，有什么问题？！