Stage 6 Desync

Specifications

• Versions: 1.00a
• Difficulty: Easy - Normal - Hard - Lunatic - Extra
• Mode: Main game - Practice mode
• Shottype: ReimuA - ReimuB - ReimuC - MarisaA - MarisaB - MarisaC

What happens

All full-game replays are able to desync on Stage 6 during Utsuho under two unique circumstances.

How it happens

For the desync to happen, the player must do one of the following:

• Start the replay from Stage 5. Right when the stage is about to end, fast-forward by using Ctrl.
• Start the replay from Stage 6.

The desync always happens on Utsuho, usually during her first non-spell or first spell-card. The desync in both cases is differently.

Why it happens

The desync is caused because there is a desync in the RNG. A desync in the RNG means that the RNG value is different than what it is supposed to be. This means that after an RNG desync, if the RNG value is called for, the game desyncs.

There are two different versions desyncs: one starting from Stage 5 and where the game is fast-forwarded during the stage transition, and one starting from Stage 6. The two desyncs seem like they might be the same, but they are different. Nonetheless, their cause is similar.

In summary, there are three different outcomes when playing back Stage 6:

• Method 1: You fast-forward during the Stage 5 stage transition. This method desyncs the replay.
• Method 2: You do not fast-forward during the Stage 5 stage transition. This method does not desync the replay.
• Method 3: You start the replay directly from Stage 6. This method desyncs the replay.

RNG difference

From replay analysis, we can take a look at the number of RNG calls the replay makes.

We take any replay that enters Utsuho on 1cc pace, and we watch Stage 6 with Method 2. Then, we look at its number of RNG calls during the 7200th frame of Stage 6 (which is always during Utsuho's dialogue). Let's suppose that Method 2 yields n number of RNG calls.
Using the same replay, we watch Stage 6 using Method 1. Its number of RNG calls is equal to n-5.
Using the same replay, we watch Stage 6 using Method 3. Its number of RNG calls is equal to n+1.

This means that Method 1 makes 5 RNG calls too few compared to the non-desynced Method 2. This also means that Method 3 makes 1 RNG call too many compared to the non-desynced Method 2. If too many or too few RNG calls are made relative to the non-desynced Method 2, it means that anything that the RNG value is different and therefore anything that uses the RNG value does it so wrongly.

Desync cause

Upon analysis, it turns out the stage background is loaded at a different time depending on how the stage is entered from. Usually, the stage background is independent of the playing area, but in few cases the two are intertwined. The stage background in question is the file stage06.std.

When entering Stage 6 using Method 1, the stage background is loaded 5 frames later compared to Method 2.
When entering Stage 6 using Method 3, the stage background is loaded 1 frame earlier compared to Method 2.

In the file stage06.std, data about the Stage 6 background is noted. In it, there is an instruction that applies some stage background effects which will be called distortion, (thanks ExpHP for documentation).
In particular, distortion(2) is the instruction that calls for RNG once per frame (Note: no other stages use this instruction hence it not happening on other stages). This STD instruction is only used right when the stage's background switches to the sun right before Utsuho spawns, meaning the desync only happens on Utsuho and never before Utsuho's appearance.

In terms of why the stage background is loaded differently depending on how it is viewed, we do not know. Please refer to ZUN for such questions.

Conclusion

Links

Replays

Any replay that reaches Utsuho suffices, so none are provided.

Videos

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