问在OpenGL中如何创建移动的球体？

EN

#include<iostream>
#include<stdlib.h>

#ifdef __APPLE__
#include<openGL/openGL.h>
#include<GLUT/glut.h>
#else 
#include<GL/glut.h>
#endif

using namespace std;

float ballX = -0.3f;
float ballY = 0.0f;
float ballZ = -1.0f;
/*
float ballX2 = 0.3f;
float ballY2 = 0.0f;
float ballZ2 = -1.0f;
*/
static int flag = 1;

void drawBall(void) {

    glColor3f(0.0, 1.0, 0.0); //set ball colour
    glTranslatef(ballX, ballY, ballZ); //moving it toward the screen a bit on creation
    glutSolidSphere(0.1, 10, 10); //create ball.

}

void keyPress(int key, int x, int y)
{
    if (key == GLUT_KEY_RIGHT)
        ballX -= 0.05f;
    if (key == GLUT_KEY_LEFT)
        ballX += 0.05f;
    if (key == GLUT_KEY_UP) {
        ballY += 0.05f;

    }


    glutPostRedisplay();
}

void initRendering()
{
    glEnable(GL_DEPTH_TEST);
}

//Called when the window is resized
void handleResize(int w, int h) {
    //Tell OpenGL how to convert from coordinates to pixel values
    glViewport(0, 0, w, h);

    glMatrixMode(GL_PROJECTION); //Switch to setting the camera perspective

                                 //Set the camera perspective
    glLoadIdentity(); //Reset the camera
    gluPerspective(45.0,                  //The camera angle
        (double)w / (double)h, //The width-to-height ratio
        1.0,                   //The near z clipping coordinate
        200.0);                //The far z clipping coordinate
}

void drawScene()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);

    glLoadIdentity();
    drawBall();
    //drawBall2();

    glutSwapBuffers();
}

//float _angle = 30.0f;
void update(int value) {
    if (flag)
    {
        ballX += 0.01f;
        if (ballX>0.3)
        {
            flag = 0;

        }
    }
    if (!flag)
    {
        ballX -= 0.01f;
        if (ballX<-0.3)
        {
            flag = 1;

        }
    }

    glutPostRedisplay(); //Tell GLUT that the display has changed

                         //Tell GLUT to call update again in 25 milliseconds
    glutTimerFunc(25, update, 0);
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);

    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);

    glutInitWindowSize(720, 720);

    glutCreateWindow("Moving Circle");

    initRendering();

    glutDisplayFunc(drawScene);

    glutSpecialFunc(keyPress);
    glutReshapeFunc(handleResize);
    //autorot();

    glutTimerFunc(25, update, 0);

    glutMainLoop();

    return(0);
}