Negative Spell Bonus

Specifications

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

What happens

It is possible to achieve a negative Spell Card Bonus (SCB) on Yuugi's first spell-card (枷符「咎人の外さぬ枷」) on the difficulties Hard/Lunatic. Capturing the spell-card with a negative value decreases the current score. The SCB displayed will not be very comprehensible:

If the spell is captured with an SCB of exactly 0, no score is obtained and no score value is displayed:

How it happens

This is done by timing down Yuugi's first spell-card (枷符「咎人の外さぬ枷」) on either Hard or Lunatic difficulty. To be more precise, the player must time down this spell-card to a time below 04.68s for the SCB to become negative. This bug does not appear anywhere else, or when doing this spell-card on Easy/Normal mode.

Why it happens

How SCB is calculated

The rate at which the SCB decreases is determined by the ECL instruction spell3. The timeRate parameter is the only parameter that influences the SCB. Calculating the SCB at a given point is done with the following formula:

int compute_output_score(max_score, time_rate, current_time) {
    if current_time < 300 // scb decreases only after first 5 seconds (300 frames)
        return max_score
    int score_div_10 = floor(max_score / 10)
    int score_loss = truncate((max_score - score_div_10) / (time_rate - 300))
    int score = max_score - (score_loss * (current_time - 300))
    score = score - (score % 10)
    return score
}

• max_score is the maximum score obtainable from capturing the spell-card. This value is hard-coded.
• time_rate is the second value passed in the ECL instruction spell3.
• current_time is a number the user passes to calculate what the SCB is at that given time.

Examples:

• compute_output_score(6000000, 2100, 1000) outputs an SCB of 3900000;
• compute_output_score(6000000, 2100, 2099) outputs 603000;
• compute_output_score(6000000, 300, 123456789) throws a division by 0 error and would crash the game, but this has not appeared in any ZUNcode (yet).

In the example of Yuugi's first spell-card, the ECL instruction spell3 has a timeRate of 2100. The max_score is 6000000, which is hard-coded for this stage on Lunatic. For this spell-card to obtain a negative spell-card the output of compute_output_score would have to be below 0, or in other words we have to find a value t such that compute_output_score(6000000, 2100, t) < 0. For this to be true, t must be 2301 or greater. But how would this be possible?

The ECL instruction setInterrupt

The ECL instruction setInterrupt has a parameter duration. This parameter determines how long a pattern lasts for. For example, if duration is 3600 then the attack lasts for 3600 frames or 60 seconds.

The duration parameter of setInterrupt should always be equal or less than the value timeRate of spell3. This is true for every spell-card in Touhou except for one spell-card, Yuugi's first spell-card.

On Yuugi's first spell-card, the duration parameter of setInterrupt is 2580 (= 43.00s). In contrast, the timeRate parameter of spell3 on Hard/Lunatic is 2100. This means that it is possible to get a t value greater than the time_rate value. This happens because ZUN forgot to change the duration parameter to 2100 for Hard/Lunatic.

Below is the ECL code that is responsible for this bug. setInterrupt's duration parameter is the same for the spell on Easy/Normal and Hard/Lunatic despite the different timeRate value.

setInterrupt(0, 0, 2580, "Boss2");
if (EASY or NORMAL) {
    spell(30, 2580, 500000, "怪輪「地獄の苦輪」");
} else if (HARD or LUNATIC) {
    spell3(32, 2100, 500000, "枷符「咎人の外さぬ枷」");
} 

This issue could easily be fixed by changing the timeRate value from 2100 to 2580.

setInterrupt(0, 0, 2580, "Boss2");
if (EASY or NORMAL) {
    spell(30, 2580, 500000, "怪輪「地獄の苦輪」");
} else if (HARD or LUNATIC) {
    spell3(32, 2580, 500000, "枷符「咎人の外さぬ枷」");
} 

Alternatively, the spell-card duration could be reduced to 2100 frames on Hard/Lunatic:

if (EASY or NORMAL) {
    setInterrupt(0, 0, 2580, "Boss2");
    spell(30, 2580, 500000, "怪輪「地獄の苦輪」");
} else if (HARD or LUNATIC) {
    setInterrupt(0, 0, 2100, "Boss2");
    spell3(32, 2100, 500000, "枷符「咎人の外さぬ枷」");
} 

Conclusion

Links

Replays

• Replay Nylilsa by "Nylilsa". Hard, ReimuA, 1.00a. 「February 15, 2023」. Download link
• Replay Nylilsa by "Nylilsa". Lunatic, MarisaB, 1.00a. 「September 13, 2022」. Download link

Videos

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