Frame Perfect Attack Skip

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 player must trigger this glitch on a non-spell for it to have any effect. This glitch can be done on all non-spells as well as some spells. Usually, the subsequent spell is skipped but in few cases the effect is different:

If done on Cirno's second non-spell, all her subsequent spells are skipped and the fight ends.
If done on Meiling's third non-spell, all her subsequent spells are skipped and the fight ends.
If done on Remilia's fourth non-spell, the subsequent spell is skipped, but the final spell-card is not skipped. Instead, the player faces the final spell-card "Red Magic", regardless of difficulty. This means it is possible to face Red Magic on Hard and Lunatic mode.

On top of this, it is also possible to do execute this glitch on the spells Meiling's third spell and Cirno's second spell.

How it happens

The player must end the attack exactly during the last frame of the attack, meaning it is a frame-perfect skip. This only works on attacks where the attack is a non-spell or Meiling's third spell or Cirno's second spell.

Why it happens

During a bossfight the player can go from one boss attack to another boss attack through different ways. Those different ways are by attacking or by timing out the pattern. In either case, the boss calls for a callback function.

Callback functions

A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

Almost all bosses in EoSD use three different types of callback functions that are all relevant to this glitch:

1. The life_callback_sub(func), which is triggered when the boss reaches a pre-determined HP threshold,
2. The timer_callback_sub(func), which is triggered when the timer reaches 00s.
3. The death_callback_sub(func), which is triggered when the boss reaches an HP threshold of 0 HP.

All three callback functions take a subroutine func as an argument. Once the condition of the callback function is met, then the game jumps to the function func.

For example, if Remilia's first non-spell is timed out, the timer_callback_sub is called. If enough damage has been dealt to Remilia's first non-spell, the life_callback_sub is called. If I capture Remilia's first spell-card, her death_callback_sub is called since she reaches 0 HP.

Example

Below is a real example of the code of Rumia's first non-spell and Rumia's first spell-card. Any code that doesn't contain the three above mentioned callback functions is removed for simplicity.

• sub11 - Rumia's 1st non-spell
• sub16 - Rumia's 2nd non-spell
• sub22 - Rumia's 1st spell-card

void sub11(int IPAR, float FPAR) {
    [...]
    death_callback_sub(sub16);
    {"NHL"}:  life_callback_threshold(900);
    {"NHL"}:  life_callback_sub(sub22);
    timer_callback_threshold(2100);
    timer_callback_sub(sub22);
    [...]
}

void sub22(int IPAR, float FPAR) {
    timer_callback_threshold(1500);
}

We start off on Rumia's first non-spell (sub11).

Suppose you timeout Rumia's first non-spell. The following happens:

• If 2100 frames have passed since the start of the non-spell, then Rumia "starts at" sub22, which is her first spell-card. The line timer_callback_sub(sub22) is triggered. This applies to all difficulties, even though this is a bug.

Suppose you shoot down Rumia's first non-spell. The following happens:

• On Easy, if Rumia's HP is 0 HP, then Rumia "starts at" sub16, which is her second non-spell. The line death_callback_sub(sub16) is called.
• On Normal/Hard/Lunatic, if Rumia's HP is 900 or less, then Rumia "starts at" sub22, which is her first spell-card. The line life_callback_sub(sub22) is called.

Now let us see what the code does on Rumia's first spell-card.

Suppose you timeout Rumia's first spell-card. The following happens:

• If 1500 frames have passed since the start of the non-spell, then Rumia "starts at" sub16, which is her second non-spell. The line timer_callback_sub(sub16) is called*.

Suppose you shoot down Rumia's first non-spell. The following happens:

• If Rumia's HP is 0 HP, then Rumia "starts at" sub16, which is her second non-spell. The line death_callback_sub(sub16) is called.

Explanation

Though, in the codeblock of void sub22, there are no callback functions to be found, so how does that explain the previous paragraphs ? Since the callback functions aren't overwritten, the game inherits the callback functions from the previous function. In this case, the callback function death_callback_sub(sub16) is also applied to the subroutine void sub22.

However, the line timer_callback_sub(sub22) is NOT applied to the subroutine void sub22. Instead, the line timer_callback_sub(sub16) is applied to the subroutine void sub22.

For an unknown reason in this game, if the function life_callback_sub OR the function timer_callback_sub is called, the argument of the function timer_callback_sub is automatically set to be the argument of the function death_callback_sub.

Order of code execution

Let's look back at the example on Rumia's first non-spell.

Suppose we were to shoot down Rumia on her first non-spell on Lunatic mode. The following sequence of events happen:

1. The player shoots down her first non-spell until her HP is at 900 HP.
2. The function life_callback_sub(sub22) is called.
3. Because the function life_callback_sub(sub22) is called, the function timer_callback_sub(sub22) sets its new argument to be the argument of death_callback_sub(sub16). This means the new timer callback is timer_callback_sub(sub16).
4. The first spell-card (sub22) starts.

Suppose we were to timeout Rumia's first non-spell. The following sequence of events happen:

1. The player times out Rumia's first non-spell.
2. The function timer_callback_sub(sub22) is called.
3. Because the function timer_callback_sub(sub22) is called, the function timer_callback_sub(sub22) sets its new argument to be the argument of death_callback_sub(sub16). This means the new timer callback is timer_callback_sub(sub16).
4. The first spell-card (sub22) starts.

Now let's set what happens when the player both shoots down and times out Rumia's first non-spell.

1. The player shoots down and times out Rumia's first non-spell on the same frame.
2. Because the function life_callback_sub(sub22) is called, the function timer_callback_sub(sub22) sets its new argument to be the argument of death_callback_sub(sub16). This means the new timer callback is timer_callback_sub(sub16).
3. The new function timer_callback_sub(sub16) is called.
4. The second non-spell (sub16) starts.

Exceptions

There are however a couple of weird edge-cases where this does not work. Below are explained why certain skips do not work or lead to different results.

Meiling & Cirno

In the case of Meiling's third spell and Cirno's second spell, the death_callback_sub is set to a subroutine after the spells that tells the boss to explode. This means that these spells use the same set-up as what the non-spells use, so the skip is possible on these spells.

Patchouli

Patchouli's third, fourth and fifth spell-cards cannot be skipped. This is because the argument of death_callback_sub is the same as the life_callback_sub in her subroutines. While the set-up is the same, the last three spell-cards are setup differently than the other bosses of this game.

Remilia

Remilia's fourth spell-card can be skipped, but doing the skip guarantees that the player faces the final spell-card "Red Magic" regardless of difficulty. This is because on the fourth-non-spell the argument of death_callback_sub is sub43, which is the spell "Red Magic". The reason why you normally do not face "Red Magic" on Hard/Lunatic is because on Remilia's fourth spell-card on Hard/Lunatic the death_callback_sub is overwritten and is given the argument sub44, which is the spell-card "Scarlet Gensokyo".

Conclusion

Links

Replays

• Replay kana0603 by "kana0603". Lunatic, ReimuB, 1.02h. 「March 16, 2021」. Download link (Note: Rumia's 1st spell-card skip)
• Replay kana0603 by "kana0603". Lunatic, ReimuA, 1.02h. 「October 11, 2021」. Download link (Note: Rumia's 1st spell-card skip)
• Replay kana0603 by "kana0603". Lunatic, ReimuB, 1.02h. 「March 18, 2021」. Download link (Note: Rumia's 1st spell-card skip)
• Replay A by "kana0603". Lunatic, ReimuB, 1.02h. 「March 2, 2023」. Download link (Note: Rumia's 1st spell-card skip)
• Replay s3 by "kana0603". Lunatic, ReimuB, 1.02h. 「March 27, 2020」. Download link (Note: Meiling's 1st spell-card skip)
• Replay skip by "kana0603". Lunatic, ReimuB, 1.02h. 「September 15, 2022」. Download link (Note: Meiling's 1st spell-card skip)
• Replay kana0603 by "kana0603". Lunatic, ReimuA, 1.02h. 「October 9, 2021」. Download link (Note: Meiling's 4th spell-card skip)
• Replay 2noncncl by "kana0603". Lunatic, MarisaB, 1.02h. 「May 24, 2018」. Download link (Note: Patchouli's 2nd spell-card skip)
• Replay kana0603 by "kana0603". Lunatic, ReimuB, 1.02h. 「March 17, 2021」. Download link (Note: Patchouli's 2nd spell-card skip)

Videos