The unique algorithm behind rewind in Braid | Hacker News Reader

The unique algorithm behind rewind in Braid

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I couldn’t tease out from his tweets if he meant that every object in the game was comprised of particles like some kind of VOXEL game or whatnot. If the whole rewind system was based on particles?

«Waaaay back in 2010 I gave a talk at the GDC detailing exactly how rewind was implemented in Braid, including e.g. how does it reproduce all the particles perfectly without using tons of data.«

But I was able to find his original blog post, which show the particle system code and crucially shows the animation of what parts of the visuals the particle system (with the random number seeding etc). was actually for:

http://number-none.com/blow/blog/programming/2016/07/07/brai...

(The particle system - recreate particles from scratch every time, and just go one step less, N-1, when rewinding - reminds me of Redux’ time travel algorithm, as demoed by Dan Abramov.)

I also found the original GDC talk Blow referenced, where he details the rewind system more fully, with illustrations. It becomes more clear that the world state is comprised of regular game objects in addition to particles. His overall approach was:

«Record full world state for every frame, always. Do not drop frames. Compress data somehow.»

(Reminds me of React’s «rerender whole UI, but virtual DOM diff afterwards», approach.)

Skip to 3:00 for the various rewind approaches he considered but dropped (including the StarCraft replays like approach of capturing actions and playing them back): https://www.gdcvault.com/play/1012210/The-Implementation-of-....

Game Helpin' Squad: Time Travel Understander:

https://www.youtube.com/watch?v=1fABGyVzVwI

That’s very funny satire!

Released over a year after Braid.

To save you a click: “ However, the particles in Braid don't affect gameplay in any way; they're just cosmetic. ” so he used deterministic functions.

So pretty much exactly what you would expect someone to do.

But he got to get his clicks somehow!

I preferred his own explanation, to be perfectly honest.

What did you find unique or surprising about it?

Not trying to start an argument, just good to know.

Not the OP but I would guess it's not that it was unique or surprising, but that it was didactically/pedagogically nicely explained. Experienced devs might find it superfluous since they've already guessed the solution from as soon as the problem is stated, but this is something I could share with a CS 100 student and they'd get a lot out of it.

My initial thought was recording the position in each frame.

Obviously after reading it, it seems obvious. And if I were implementing it, I am pretty sure I would have came to the same solution.

But it was not immediately obvious to me from the title.

Yeah it's a bit obvious, isn't it? It's the first thing I thought before I clicked through.

Well Braid came out in like 2008, and it was not obvious to me as a noob game dev when I first read it.

... It does seem real obvious _now_

That was also my first guess, and it makes me quite annoyed at his claim that "no other game that he is aware of does it this way", because I'm fairly certain that there are plenty of situations where closed-form particles are also the more performant option.

As the same time, to be fair, plenty of people do not realize closed form formulas are the best approach here. I'd say that requires a level of intuition for the physics and maths involved that is not strictly necessary to make functioning particles the "regular" way.

Anyway, to switch the topic to something more fun: do you think one could make Age of Empires 1 and 2 perfectly rewindable?

[0] https://www.gamedeveloper.com/programming/1500-archers-on-a-...

Right. I also doubt this approach is that unique since closed form computation is also a desirable property for simulating the particles entirely on the GPU.

Most important part is: its a great game. Strongly recommended.

It is. It's sort of a wallowing game, but I replayed it, and finally beat it, after my recent divorce and it really hit me.

The trouble is, I found the gameplay difficult. I had to lean on walkthroughs a lot.

Some games are hard and punishing. Celeste is hard and forgiving, and I love it. Braid is a puzzle game, so you can get stuck for ages with no clue what you're even doing wrong, and the forgiveness is worthless then. At least with Celeste I know which way I'm headed - Usually up.

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