Bullet Displacement Bug

---

Specifications

---

• Versions: 1.00 - 1.01 - 1.02 - 1.02a - 1.02b - 1.02c - 1.02d - 1.02e - 1.02f - 1.02h
• Difficulty: Easy - Normal - Hard - Lunatic - Extra
• Mode: Main game - Practice mode
• Shottype: ReimuA - ReimuB - MarisaA - MarisaB

What happens

---

The bullet displacement bug is a bug where a boss's bullets are fired from a wrong position relative to the boss. This bug is encountered on various boss attacks, which are as follows:

• Rumia's mid-spell
• Meiling's mid-non-spell
• Patchouli's "Bury in Lake" spell-card
• Patchouli's 2nd non-spell
• Sakuya's 1st non-spell
• Sakuya's 2nd non-spell
• Remilia's 4th non-spell
• All of Flandre's non-spells
• Flandre's "Four of a Kind" spell-card

How it happens

---

This bug happens occasionally on the attacks mentioned in the previous sections. This bug is not consistent to replicate, as it depends on boss movement (which is RNG-based) and sometimes it depends on rank.

Why it happens

---

In order to understand how this bug takes place, we must understand how the game processes its events. Every frame, the game updates events in a specific order:

1. Clamping boss position to its movement limits
2. Process scripted boss behavior
   a. Apply interpolation-based movement updates
   b. Handle bullet firing
3. Clamping boss position to its movement limits

Boss movement limit

---

To prevent a boss from going off-screen, the boss is given a movement limit. This limit is a rectangle that represents the space the boss can exist in. The limit's (x,y) position as well as their (width, height) are usually set at the start of a new pattern.

Enemy movement interpolation

---

Movement interpolation smoothly moves an enemy from one position to another position over a fixed duration. Different interpolation modes can be used to create movement patterns such as constant speed or smooth deceleration.

At the start of the interpolated movement, the enemy's final position is calculated. This will not be used until the very end of the interpolated movement. Each frame, the enemy's velocity is updated according to its interpolation mode (e.g. a constant decrease each frame for constant deceleration). The velocity is then used to update the enemy's position. On the final frame of the interpolation, the game directly sets the enemy position to the interpolation target position instead of continuing to derive it from velocity. This is done to ensure that the enemy ends up in the correct final position.

For example, say an enemy is at the coordinates (2, 18) and has an initial velocity of 7. The enemy is linearly decelerating to a speed of 0 across 5 frames, only moving in the positive x-direction. Given this information, the final position is computed as (2 + (5 * 7 / 2), 18) = (19.5, 18). The velocity decreases by 7/5 = 1.4 at each frame.

Frame	Speed	Position
0	7	(2, 18)
1	5.6	(9, 18)
2	4.2	(14.6, 18)
3	2.8	(18.8, 18)
4	1.4	(21.6, 18)
5	0	(19.5, 18)

On the final frame, the position is set to the pre-calculated (19.5, 18). If it were not set to this pre-calculated position, then the final position would have been (23.0, 18).

Bullet firing logic

---

On some patterns, boss bullet firing is controlled by a timer-based system. Each frame, the firing timer is incremented, and once the timer reaches the configured firing interval, the boss fires its pattern and the shooting timer is reset.

When spawning bullets, the game uses the boss's current position combined with a given offset. As a result, bullets always spawn relative to the boss position when the firing logic executes.

Where the bug occurs

---

The movement limit clamps (steps 1 and 3) prevent the boss from visibly leaving its allowed movement area. However, a problem occurs because of interpolation-based movement updates happen between these two clamp operations.

On the final frame of an interpolation movement, the game directly sets the boss position to the interpolation target position. If this target position lies outside the movement limits, the boss temporarily exists outside its allowed area (step 2a). This target position does not take into account the movement limit, meaning that this area can lie outside of the movement limit.

Immediately afterwards (step 2b), the bullet firing logic executes. Since bullets spawn using the boss's current position, any bullets fired during this frame will use the boss's temporary out-of-bounds position as its spawn location. Note that it only affects bullets fired from a timer-based system, bullets fired through other means are unaffected by it.

Of course, for this to happen, the shooting timer (step 2b) has to coincide with the final frame of the interpolation (step 2a).

After the bullets have been spawned, the game performs the second movement limit clamp (step 3). This moves the boss back within bounds before rendering occurs. As such, the boss itself never visibly appears outside the movement area, but bullets can still appear displaced from the boss.

---

Links

---

Replays

---

• Replay kana0603 by "kana0603". Extra, MarisaB, 1.02h. 「April 11, 2026」. Download link (Note: Bug happens twice on Flandre's 7th non-spell)
• Replay A by "kana0603". Extra, MarisaB, 1.02h. 「April 5, 2026」. Download link (Note: Bug happens twice on Flandre's 7th non-spell)

Videos

---

This section has no content yet. Would you like to add to this section? Contact me if you are interested!