Laser Midpoint Bug

Specifications

Versions: 1.00 - 1.00a - 1.00b
Difficulty: Easy - Normal - Hard - Lunatic - Extra - Phantasm
Mode: Main game - Practice mode
Shottype: ReimuA - ReimuB - MarisaA - MarisaB - SakuyaA - SakuyaB

What happens

When a straight laser spawns, there is a small period of time before the laser fully appears and disappears. During these times, the laser only shows a thin warning line that warns the player about a spawning/despawning laser. This warning line usually does not have a hitbox except in one spot, which is in the middle of the laser.

An in-game screenshot of ReimuB dying to Alice's attack. — Figure 1: ReimuB is being hit by a laser as the laser is despawning, which should not happen.

How it happens

This bug can be replicated by being at the midpoint of the laser while the laser is spawning or despawning naturally: if a laser despawns by getting cancelled, then this midpoint bug will not occur. Since it is the midpoint, it is rather small. On top of this, the hitbox only stays for a few frames. In PCB, this bug can happen on almost any attack that features a laser. Below is a table listing every section of the game with a laser, and whether it's possible to get hit on the spawning phase, or on the despawning phase.

Pattern	Spawning	Despawning
Alice's mid-spell-card	Yes (20f)	Yes (16f)
Merlin's 2nd non-spell	Yes (34f)	No*
Merlin's 3rd non-spell	Yes (10f)	Yes (20f)
Prismrivers' 3rd spell-card	Yes (14f)	Yes (16f)
Prismrivers' 4th spell-card	Yes (104f)	No*
Yuyuko's 1st spell-card	No**	Yes (20f)
Yuyuko's 7th spell-card	Yes (30f)	Yes (36f)
Ran's 3rd spell-card	Yes (10f)	Yes (30f)
Ran's 6th spell-card (blue)	No, No, Yes (30f)	Yes (16f), Yes (16f), Yes (32f)
Ran's 6th spell-card (red)	No	Yes (32f)
Ran's 9th spell-card	No	No*
Yukari's 3rd spell-card	Yes (10f)	Yes (30f)
Yukari's 4th spell-card	No	Yes (30f)
Yukari's 5th spell-card	No	Yes (30f)
Yukari's 6th spell-card (blue)	No, No, Yes (30f)	Yes (16f), Yes (16f), Yes (32f)
Yukari's 6th spell-card (red)	No	Yes (32f)
Yukari's 9th spell-card	No	No*

*While the despawning conditions are met, the lasers take longer to despawn than what the attack lasts for, rendering it not possible.
**While the spawning conditions are met, it cannot actually happen because the laser midpoint is offscreen for the entire duration of the spawning phase.

Ran and Yukari's 6th spell-card has three different values. This is because the lasers there are different from one another.

Why it happens

The tiny hitbox at the midpoint has a width equal to the full width of the laser, and a height equal to the visual width of the laser. Presumably, ZUN intended for lasers hitbox's length to match the laser length and the width to match the visual width of the laser. This way, the laser's hitbox would get wider as the laser sprite thickens right before fully materializing, and shrink as the sprite thins when it begins to despawn. However, it seems like ZUN accidentally mixed up the fields for hitbox width and height.

Two lasers, one not fixed and the other fixed. — Figure 2: This image compares the current hitbox with the possible intended fix. (1) Current laser. (2) Fixed laser. While this does make more sense, it also means that there is a smaller hitbox in the fixed laser now.

But why does this sometimes happen with some lasers? To understand the cause of this bug, we must first understand how lasers work in this game.

Laser States

A laser progresses through five visual states over its lifetime. They are explained through the following figure:

Four lasers, each representing a different state from left to right. — Figure 3: This image shows all laser states labelled 1 (left) through 5 (right). (1) Spawning laser without hitbox. (2) Spawning laser with hitbox. (3) Active laser. (4) Despawning laser with hitbox. (5) Despawning laser without hitbox.

In the code, a laser can take many parameters, but we will be focussing only on the following parameters:

hitbox_start_time: time until laser's partial hitbox materializes (state 1 -> state 2)
start_time: time until laser fully materializes (state 1 -> state 3)
duration: time the laser is active for (state 3 -> state 4)
hitbox_end_delay: time the laser's partial hitbox is active for (state 4 -> state 5)
despawn_duration: time until the laser fully despawns (state 4 -> despawn)

A rough pseudocode layout is shown below describing the logic of the states and parameters:

laser_state = SPAWNING
timer = 0

while (true) {
    if (laser_state == SPAWNING) {
        if (timer >= hitbox_start_time) {
            activate_laser_midpoint_hitbox()
        }   

        if (timer >= start_time) {
            laser_state = ACTIVE
            timer = 0
        }
    }
    if (laser_state == ACTIVE) {
        if (timer < duration) {
            activate_laser_hitbox()
        } else {
            laser_state = DESPAWNING
            timer = 0
        }   
    }
    if (laser_state == DESPAWNING) {
        if (timer < hitbox_end_delay) {
            activate_laser_midpoint_hitbox()
        }
        if (timer >= despawn_duration) {
            despawn_laser()
        }
    }
    timer++
}

Hitbox while spawning (states 1, 2)

if (laser_state == SPAWNING) {
    if (timer >= hitbox_start_time) {
        activate_laser_midpoint_hitbox()
    }   

    if (timer >= start_time) {
        laser_state = ACTIVE
        timer = 0
    }
}

For the laser to only have the hitbox in its midpoint, it means that time >= hitbox_start_time must be true and time >= start_time must be false, or to put it simply, hitbox_start_time < start_time must be true. The duration the midpoint will have a hitbox is equal to start_time - hitbox_start_time.

Hitbox while despawning (states 4, 5)

if (laser_state == DESPAWNING) {
    if (timer < hitbox_end_delay) {
        activate_laser_midpoint_hitbox()
    }
    if (timer >= despawn_duration) {
        despawn_laser()
    }
}

For the laser to only have the hitbox in its midpoint, it means that time < hitbox_end_delay must be true and time >= despawn_duration must be false, or to put it simply, hitbox_end_delay > 0 must be true. Every laser in PCB has hitbox_end_delay > 0, so it means every laser has a hitbox in its midpoint while despawning. The duration the midpoint will have a hitbox is equal to min(hitbox_end_delay, despawn_duration).