Hello everyone!
I am sort of confused how to visualize this falling ball.
[source lang="cpp"]#ifdef __APPLE__
#include <GLUT/glut.h>
#else
#include <GL/glut.h>
#endif
#include <stdlib.h>
#include <iostream>
#include <btBulletDynamicsCommon.h>
static int slices = 16;
static int stacks = 16;
time_t last_update;
static float timestep = 1/60;
double fallY;
/* GLUT callback Handlers */
static void resize(int width, int height)
{
const float ar = (float) width / (float) height;
glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-ar, ar, -1.0, 1.0, 2.0, 100.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity() ;
}
static void display(void)
{
const double t = glutGet(GLUT_ELAPSED_TIME) / 1000.0;
const double a = t*90.0;
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3d(0.8,0.77,0.7);
glPushMatrix();
glTranslated(-2.4,1.2,-30);
glRotated(60,1,0,0);
time_t current_time = glutGet(GLUT_ELAPSED_TIME);
time_t since_update = current_time - last_update;
//if enough time has passed since the last redraw calculate physics
if(since_update>timestep)
{
dynamicsWorld->stepSimulation(timestep,10);
last_update = current_time;
}
btTransform trans;
fallRigidBody->getMotionState()->getWorldTransform(trans);
fallY = trans.getOrigin().getY();
glTranslated(0,0,fallY);
glutSolidSphere(1,slices,stacks);
glPopMatrix();
glutSwapBuffers();
}

static void key(unsigned char key, int x, int y)
{
switch (key)
{
case 27 :
exit(0);
break;
}
glutPostRedisplay();
}
static void idle(void)
{
glutPostRedisplay();
}
const GLfloat light_ambient[] = { 0.0f, 0.0f, 0.0f, 1.0f };
const GLfloat light_diffuse[] = { 1.0f, 1.0f, 1.0f, 1.0f };
const GLfloat light_specular[] = { 1.0f, 1.0f, 1.0f, 1.0f };
const GLfloat light_position[] = { 2.0f, 5.0f, 5.0f, 0.0f };
const GLfloat mat_ambient[] = { 0.7f, 0.7f, 0.7f, 1.0f };
const GLfloat mat_diffuse[] = { 0.8f, 0.8f, 0.8f, 1.0f };
const GLfloat mat_specular[] = { 1.0f, 1.0f, 1.0f, 1.0f };
const GLfloat high_shininess[] = { 100.0f };
/* Program entry point */
int main(int argc, char *argv[])
{
glutInit(&argc, argv);
glutInitWindowSize(640,480);
glutInitWindowPosition(10,10);
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
glutCreateWindow("GLUT Shapes");
glutReshapeFunc(resize);
glutDisplayFunc(display);
glutKeyboardFunc(key);
glutIdleFunc(idle);
glClearColor(1,1,1,1);
glEnable(GL_CULL_FACE);
glCullFace(GL_BACK);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);
glEnable(GL_LIGHT0);
glEnable(GL_NORMALIZE);
glEnable(GL_COLOR_MATERIAL);
glEnable(GL_LIGHTING);
glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);
glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);
glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
glLightfv(GL_LIGHT0, GL_POSITION, light_position);
glMaterialfv(GL_FRONT, GL_AMBIENT, mat_ambient);
glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse);
glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
glMaterialfv(GL_FRONT, GL_SHININESS, high_shininess);

btBroadphaseInterface* broadphase = new btDbvtBroadphase();
btDefaultCollisionConfiguration* collisionConfiguration = new btDefaultCollisionConfiguration();
btCollisionDispatcher* dispatcher = new btCollisionDispatcher(collisionConfiguration);
btSequentialImpulseConstraintSolver* solver = new btSequentialImpulseConstraintSolver;
btDiscreteDynamicsWorld* dynamicsWorld = new btDiscreteDynamicsWorld(dispatcher,broadphase,solver,collisionConfiguration);
dynamicsWorld->setGravity(btVector3(0,-10,0));

btCollisionShape* groundShape = new btStaticPlaneShape(btVector3(0,1,0),1);
btCollisionShape* fallShape = new btSphereShape(1);

btDefaultMotionState* groundMotionState = new btDefaultMotionState(btTransform(btQuaternion(0,0,0,1),btVector3(0,-1,0)));
btRigidBody::btRigidBodyConstructionInfo
groundRigidBodyCI(0,groundMotionState,groundShape,btVector3(0,0,0));
btRigidBody* groundRigidBody = new btRigidBody(groundRigidBodyCI);
dynamicsWorld->addRigidBody(groundRigidBody);

btDefaultMotionState* fallMotionState =
new btDefaultMotionState(btTransform(btQuaternion(0,0,0,1),btVector3(0,50,0)));
btScalar mass = 1;
btVector3 fallInertia(0,0,0);
fallShape->calculateLocalInertia(mass,fallInertia);
btRigidBody::btRigidBodyConstructionInfo fallRigidBodyCI(mass,fallMotionState,fallShape,fallInertia);
btRigidBody* fallRigidBody = new btRigidBody(fallRigidBodyCI);
dynamicsWorld->addRigidBody(fallRigidBody);

glutMainLoop();
return EXIT_SUCCESS;
}
[/source]
When I compile the code I got the following errors:
COMPUTER\C++\CppMyProjects\HW\main.cpp||In function 'void display()':|
COMPUTER\C++\CppMyProjects\HW\main.cpp|49|error: 'dynamicsWorld' was not declared in this scope|
COMPUTER\C++\CppMyProjects\HW\main.cpp|54|error: 'fallRigidBody' was not declared in this scope|
||=== Build finished: 2 errors, 0 warnings ===|

What should I do?
Please help!
Thank you so much!

You need to have the variables used in display to actually be global variables and declared before the display function.

Can any give some modified code?
Many thanks!!!

Can any give some modified code?
Many thanks!!!

Really?
If you don't know the difference between a local and a global variable, it's probably a good time to learn now.
http://lmgtfy.com/?q=global+vs+local+variable+c%2B%2B

Based on the errors, I'm guessing you either have a typo or haven't defined those functions.