the paper folding/unfolding effect in twitter for iPad

Question

Twitter for iPad implements a fancy "pinch to expand paper fold" effect. A short video clip here. http://www.youtube.com/watch?v=B0TuPsNJ-XY

Can this be done with CATransform3D without OpenGL? A working example would be thankful.

Update: I was interested in the approach or implementation to this animation effect. That's why I offered bounty on this question - srikar

Answer 1

Here's a really simple example using a gesture recognizer and CATransform3D to get you started. Simply pinch to rotate the gray view.

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // ...

    CGRect rect = self.window.bounds;
    view = [[UIView alloc] initWithFrame:CGRectMake(rect.size.width/4, rect.size.height/4,
                                                         rect.size.width/2, rect.size.height/2)];
    view.backgroundColor = [UIColor lightGrayColor];
    [self.window addSubview:view];

    CATransform3D transform = CATransform3DIdentity;
    transform.m34 = -1/500.0; // this allows perspective
    self.window.layer.sublayerTransform = transform;

    UIPinchGestureRecognizer *rec = [[UIPinchGestureRecognizer alloc] initWithTarget:self
                                                                              action:@selector(pinch:)];
    [self.window addGestureRecognizer:rec];
    [rec release];

    return YES;
}

- (void)pinch:(UIPinchGestureRecognizer *)rec
{
    CATransform3D t = CATransform3DIdentity;
    t = CATransform3DTranslate(t, 0, -self.view.bounds.size.height/2, 0);
    t = CATransform3DRotate(t, rec.scale * M_PI, 1, 0, 0);
    t = CATransform3DTranslate(t, 0, -self.view.bounds.size.height/2, 0);
    self.view.layer.transform = t;
}