Many of you would be familiar with the concept, having played or read about Prince of Persia: Sands of Time. I've been tring to find information on how to implement a time rewind/reverse function into a real-time 3D game similar to that found in this game. The closest articles i can find are ones on game input recording and playback, and a couple of paragraphs from the article "Mapping Time In Video Games". Such an undertaking in a game of today's standards would require the ability to rewind a physics engine, as well as the animations of the game characters and the game-state itself. So how would one go about doing this? If you have any advice, or can point me in the direction of any articles, that would be great.

I think PoP succeeded at this because they used a simulation of rather limited complexity. It may even have been possible for them to just store away the full physics state of each frame and interpolate when playing it backwards, and warping objects to their new position, overriding the simulation stepping of the physics engine.

Or they had their own physics simulation that supported reverse stepping by ensuring each operation had a reverse. This is probably complex and there's no (as far as I know) general way to treat all operations by simply reversing operations and using a normal physics engine.

Imagine for example someone jumping from a platform out into empty space. You can reverse the forward velocity and gravity acceleration at the time you switch to rewinding, but the jump action just disappears in the reversal. Moving forward knocking a box into movement forward with simultaneous angular rotation would be quite complex too. If you rewind back to the point of collision (should be easy), how handle the "collision reaction" you get at the point of collision? It should in this case be that the player moves back and the box comes to rest. Even with the knowledge that the player moved forward before that, at the point of rewinding, the timing must be perfect and rounding errors must not happen, to ensure that the box doesn't collision react to the moving away of the player.

I'd say storing away the old state is your best bet, if you can afford the memory consumption. I'd say you can, if you only intend to rewind as little as they did in PoP - you have character position, velocity, active animation and animation key frame for each character that is within sight (all other characters are standing still, you could assume), making for a total of maybe 100-2000 bytes per frame. That's at most 1.2MB for 10 seconds of rewinding capability.

Quote:Original post by Sargeras
Many of you would be familiar with the concept, having played or read about Prince of Persia: Sands of Time.

I've been tring to find information on how to implement a time rewind/reverse function into a real-time 3D game similar to that found in this game.

The closest articles i can find are ones on game input recording and playback, and a couple of paragraphs from the article "Mapping Time In Video Games".

Such an undertaking in a game of today's standards would require the ability to rewind a physics engine, as well as the animations of the game characters and the game-state itself.

So how would one go about doing this?

If you have any advice, or can point me in the direction of any articles, that would be great.

You're going to need an undo stack of sorts, containing a set of events - everything from player input to object collisions to game-state - which lists both before-and-after states, and also update routines for each object that are bidirectional.

edit: in terms of floating-point errors, you can use these event listings to re-calibrate - so the falling box might not rewind to exactly where it was, but when it hits the event, that then re-positions it in its precise original location before whatever caused it to fall happened.

I have experience with replay (i.e. playback), but not rewinding simulation. I haven't thought much about it, but I'd make a way to set the game state based on replay buffer information, and when reversing time simply playing back the replay buffer in reverse, when exiting the replay buffer use the frame to set the proper game state information and resume the game.

This is easier to say than to do though. You'll need to be able to ensure you can track which animation sequences are playing and have the animation system be able to start a sequence from a specified frame. Also, the more complicated your game state, the harder this is to maintain, and dead enemies/allies are another thing to deal with (not to mention dealing with particles, and rigid bodies affected by physics).