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How Void Daggers replays your whole run

A flipbook, not a film

Beat your record in Void Daggers and the game keeps the run. You can watch it back later, from your own eyes, at triple speed if you are impatient. The file it keeps is not a video, and it is not a recording of which keys you pressed. It is something in between, and the reason it ended up that way is the interesting part.

Three ways to save a run

There are broadly three options, and every game that does replays picks one.

You can record video. Straightforward, and enormous. Every frame is a full screen of pixels, and a five minute run is hundreds of megabytes before compression. You also lose the ability to move the camera or change speed, because a video is just pictures.

You can record the inputs. This is the elegant one. Store which keys were held on which frame, then feed those same inputs back into the same game and let it play itself. The file is tiny, a few kilobytes for an entire run. This is how speedrun verification works in a lot of games.

Or you can record positions: where everything was, several times a second, and rebuild the scene from that. Bigger than inputs, far smaller than video.

Why the elegant option did not work

Input recording only works if the game is deterministic, which means identical inputs always produce identical results. Press the same keys on the same frames, and the tenth enemy spawns in exactly the same spot, with exactly the same wobble on its approach, both times.

Void Daggers is not deterministic, and it is not close. All over the code the game asks for a random number: where an enemy appears, which direction a swarm splits, how much spread a shot has, how the debris flies. Every one of those comes back different on every run. Feed the same inputs back through and you are watching a completely different fight within seconds.

Making it deterministic afterwards would mean replacing every one of those random calls with a seeded generator, the same way Moon Miner derives its Moon from a single number. That is doable. It is also invasive, and it stays fragile forever, because from then on any change anywhere in the game can silently break every replay ever recorded. One frame of drift compounds into nonsense.

So Void Daggers records positions instead.

What a snapshot actually holds

Twenty times a second the game writes down one line about you: the time on the run clock, where you were, which way you were looking, which weapon you had, whether the trigger was down, and whether your feet were off the ground.

Ten times a second it writes down the rest of the arena: every living enemy, every boss and where it is pointing, every spawner, every floating token, and the shrinking edge of the floor.

Some things happen at an instant rather than continuously, and those get a list of their own, each stored with the exact time it occurred: an enemy died here, you picked up an upgrade, a boss spawned, you dashed, you jumped.

Filling in the gaps

Ten snapshots a second would look terrible played straight, like a slideshow. So playback interpolates, which simply means it makes up the frames in between by sliding smoothly from one recorded position toward the next.

One rule keeps this honest: the snapshots are the truth and the smooth motion is decoration. If the slide drifts slightly away from reality, the next snapshot arrives a fraction of a second later and puts everything back exactly where it belongs. Errors never pile up, because they are overwritten before they can grow. Compare that with input replay, where a tiny error at second two is still there at second two hundred, and considerably larger.

Where it ends up

All of that is written out as plain structured text, then compressed, which works very well on long lists of repetitive numbers. The result goes into a small database that browsers provide for exactly this kind of thing, so it survives closing the tab.

The game records during every attempt, but only a personal best gets kept. On Steam the same compressed file is attached to your leaderboard entry, which is why you can watch other people's runs instead of only your own.