Get the project @

I was searching for some techniques to do flip screen transitions as in Flipboard iphone app and I found this awesome project, EPGLTransitionView.

EPGLTransitionView is an excellent project to do some cool UIViewController transitions. It basically works by taking screenshots of the two views you would like to switch and manipulating the screenshots with OpenGL commands for cool transition effects. Official URL for the project is

The project comes with 3 basic transitions:

fall turn flip

Reverse Animation:

The class Demo2Transition in the ‘Demo Project’ provided is exactly what I wanted. But it lacked certain features like turning in reversed direction and turning from bottom up.

So I downloaded the project from github, tinkered with the Demo2Transition class and in a while found a way to reverse the animation. Actually reversing was not a big deal, the animation is controlled by a control variable f, which determines the turned position of view at any given time. I just reversed the initial and final values and it worked flawlessly.

Up Down Transition:

I am not much into OpenGL before, so pulling this’up down turn transition’ took me some time with lots of trial and errors. I did this by changing the order of vertices and texture co-ordinates in which the image is read.

Now both of reversal and up down transition and working great except for 1 frame flicker in ‘reverse animation’. Working on fixing it.

My fork:

I forked the project and committed my changes to the forked repo @

I also sent a pull request to the original repo, lets see if its accepted. I done a few open source projects before, but this is the first time that I contribute to some one else’s project.


I changed the ‘init’ method in the original project into the following:

– (id)initWithView1:(UIView*)view1 andView2:(UIView*)view2 delegate:(id)_delegate fwdDirection:(BOOL)fwd

where `view1` is the UIView which is currently being active and `view2` is the UIView which need to be shown, `delegate` is the transition class that conforms to `EPGLTransitionViewDelegate` protocol, and `fwd` is a flag that determines the direction of transition.

The EPGL TransitionView can reverse the transition only if the Transition class supports it by implementing the `isVertical` property. If it isn’t then, the value of `fwd` doesn’t have any effect.

A sample use case is given below:

UIViewController *controller = [[[UIViewController alloc] init] autorelease];
controller.delegate = self;
controller.view.frame = CGRectMake(0, 20, 320, 460);
NSObject<EPGLTransitionViewDelegate> *transition;
EPGLTransitionView *glview;

transition = [[[Demo2Transition alloc] init] autorelease];
[transition setIsVertical:YES];
glview = [[EPGLTransitionView alloc]

[glview startTransition];
[self presentModalViewController:controller animated:NO];

Happy flipping.