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Implementing "Rewind Time", like in the Prince of Persia - Sands of time series

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Hi,

I am not sure if you guys are familiar with the PoP Sands of time series, I was trying to look for a video on youtube demonstrating this feature, surprisingly I haven't found one, I'll try to describe it however. In these games, you have the ability, to hold down a button, and rewind the last 5-10 seconds of time that has just passed, affecting you, the objects and enemies, and all this is going smoothly in real time. It would be similar as if you are watching a video, then you rewind 5 seconds of it, then go again. Only in the game if you made a mistake, fell off a platform now you have the chance to get another shot at it.

I found this feature improving the game experience a lot, such that after any small mistake you don't die, face a loading screen, have the whole thing interrupted, then start from a checkpoint, but at the same time the usage of this ability was limited, so it didn't make the game too easy either.


I am now thinking of implementing this in my 2d scrolling space shooter engine, but it makes me wonder how am I supposed to do this..
The only way I can imagine it, is that the past 5 seconds from current time, so the past 300frames at 60FPS is "recorded", such that for the player and every object, at every iteration of the game loop "saves" all their important variables, for example x,y position, velocity, current action etc into a corresponding past_x[300], past_y[300], past_velocity[300] etc.. array, and when rewind is flagged, at each game loop iteration, the current x,y, velocity, etc is updated from the past_x, past_y etc.

Now, considering enemies, bullets, particles, that is a lot of information to store, and would take up quite some memory.


Do you guys think this is the way I should try to implement this as I described above, or there is some better/more efficient way that I am overlooking?

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Basically, yes, that is what you need to do. Another fun game to do it was Braid.


Rather than recording the position of every particle, store the events behind everything.

Recording events at timestamps requires less space. You know an event at a time "bullet 7 fired from (x,y) @30:14.03", another event "bullet 7 hit target at (x,y0) @30:29.54". Then if your clock is at 30:16.43, you can figure out where the bullet was at that time.

Also, consider organizing your events into a spatial tree with one dimension as time so your list of items doesn't overwhelm you when rolling back and forth over time.

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[indent=1]What happens to your physics or particle pathing if you use negative time as your timestep?


[indent=1]if for instance particles animated exactly backwards for path and effects using a negative timestep, you wouldn't have to record anything but their "fizz out" locations to spawn backwards animating particles as it rewinds the scene.

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I've experimented a bit with this some time ago for a very stupid platformer. I could more or less make it work for most "well known" objects but in general, allowing a generic object to go back in time imposed quite some limitations I could not really live with.
Simply mandating timestep to be always positive nonzero (time monotonically increasing) simplifies things a lot. When I abandoned that assumption I had to rewrite some gameplay code with some propagations in logic as well.
I later experimented with "stateless" systems, where objects would collect events in queues. Instead of going back and forth in time, each object would be fetched a time gap from t[sub]0[/sub] (start of the event queue). This initially worked as expected but at this point I ran out of time for further experiments.

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Recording events is the idea, but events are not only when things are created (ie a bullet fired), but also when their velocity changes. If you record their changing velocity for a certain amount of time, you can reverse the velocities and work backwards. So, you store all these events in a queue, but clear out old events that happen longer than 5 seconds ago (or whatever your max rewind time is).

For example, an enemy spawns at position 10, 10 and begins chasing the player to the right (record timestamp, new location (10, 10), and velocity (10, 0)). 1.5 seconds later, he jumps up, following the player onto a platform (record timestamp, location and new velocity (10, -10)). When he lands on the platform 0.5 seconds later, his y velocity goes to 0 (record timestamp, location, and new velocity (10, 0)). 2 seconds later, the player kills the enemy (record timestamp and location of enemy dying).

1 second later, the player falls off the screen, but hits the "rewind button." Record the time he hit the rewind button, and work through the queue in reverse by subtracting the timestamp when rewind was hit by the time elapsed since then. 1 second later, you'll hit the enemy dies event, and location, but you'll actually spawn the enemy, and set his velocity to reverse what it was when he died (-10, 0).

And so on. If you are doing a platformer, I'd suggest working gravity into the events, since, when a player or enemy is jumping/falling, the velocity will be changing every frame, and you don't want to store all those changes.

Anyway, just a thought.

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Thank You for all your comments and help!
I would use this on a scrolling shooter, with no physics involved. I would plan on making 3 seconds of max rewind time at 60FPS -> 180frames

My concerns so far:
Randomly generated stuff:
most enemies are randomly generated, and let's say they have a chance of dropping something when they die too.
My solution to this would be, that everything that is randomly generated has to be determined/generated BEFORE the rewind time range, so before 180frames. So a randomly appearing enemy at the current frame must have been generated 180 frames ago, that way the rewind won't screw with anything that was supposed to happen in the future (counted from the -3 sec rewind time smile.png)
I am still wondering if this would be a good method, or it won't work well.

Other than that I could divide everything else into 2 categories:
1. Stuff that has a constant speed or constant acceleration, not AI driven.
These are primitive bullets that just go straight, stars, scrolling background and background objects and particles.
To record the state of every frame for each of these would consume too much resources, and would be unnecessary, and the above suggested timestamp method would be enough to calculate these objects position at any time having known the velocity (or acceleration if applicable).

2. Anything more complex than above
More intelligent projectiles and AI driven enemies, that would be really hard (or not quite possible) to calculate their behavior at any given timestamp.
For these I really would just "record" their related variables/states for the last 180 frames in a stack, and during rewind I'd keep popping the topmost one into the corresponding current variables.

Do you guys think this is a good plan, or would you make some adjustments, or do it in an entirely different way?

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Recording events is the idea, but events are not only when things are created (ie a bullet fired), but also when their velocity changes.
Yes, let me elaborate. Main problem is that each object must have some sort of log to post its interesting state changes and variable changes. In general this can be done with some way for each object to add events to a queue.
I experimented with both a "central event repository" (try to centralize management) and a distributed queue system (try to simplify). I am inclined to the second, although the amount of redundant code was a source of concern.


Randomly generated stuff:
most enemies are randomly generated, and let's say they have a chance of dropping something when they die too.
My solution to this would be, that everything that is randomly generated has to be determined/generated BEFORE the rewind time range...
I'd generate the loot when the enemy is spawned. I'd eventually give it its own random seed and generator.

Do you guys think this is a good plan, or would you make some adjustments, or do it in an entirely different way?
I think it's a good start.

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Personally, I would attempt to create an object that hold everything on the screen on each frame. So you could have a screen object. The screen object has a list of objects and there current state that is currently on the screen. You then create an array of these, for the first 300 frames, you save the state of each screen.

After your at 300 frames, you pop off the first frame, and add the next so frame 301 becomes becomes 300 and all other screens move back in the array(probably faster just to copy it into a new array). In this way, after the first 300 your taking just a single "screen shot" of the screen and getting rid of the first one.

Your also not worried about projectile calculations or anything like that since it should just be a matter of running backwards through the frames to restore the state of all objects on the screen 300 frames ago.

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I just thought of a terrific short cut for this. Just capture a screenshot of every frame and store them in an array, do the pop off of the first and add onto the back operation describe above. Then just run through the array backwards displaying each screen for a rewind and restore the players state at the end of it. Your grabbing 1 screen shot per frame, no huge object array, no calcuations, no hassles.

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If you're curious enough to spend $3.95, you can get Jonathan Blow's GDC 2010 talk on the Implementation of Rewind in Braid. There's an interesting thread about it here, as well.

If I remember what Jonathan Blow said about it, he took a full world snapshot every some-odd frames (or seconds). That was to make sure a user couldn't abuse the physics engine through floating point errors. Then, for the in between frames, I believe he recorded the deltas for each object, but I believe he limited the deltas to just a few bits in order to save storage space (with the limitation that there was an upper limit of how fast an object could move, otherwise the object's delta couldn't be saved in just a few bits). Braid needed to do this though because you can rewind the entire game, practically. If you're only rewinding a few seconds, you don't have to worry as much about compressing the deltas. Unfortunately, I can't remember any other details. (for the curious, this is similar to how a lot of video codecs work, like H.264).

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