📝 Posted:

2023-03-14

🏷 Tags:

rec98 th01 anniversary-edition gameplay bullet boss sariel glitch

Turns out I was not quite done with the TH01 Anniversary Edition yet. You might have noticed some white streaks at the beginning of Sariel's second form, which are in fact a bug that I accidentally added to the initial release.
These can be traced back to a quirk I wasn't aware of, and hadn't documented so far. When defeating Sariel's first form during a pattern that spawns pellets, it's likely for the second form to start with additional pellets that resemble the previous pattern, but come out of seemingly nowhere. This shouldn't really happen if you look at the code: Nothing outside the typical pattern code spawns new pellets, and all existing ones are reset before the form transition…

Except if they're currently showing the 10-frame delay cloud animation , activated for all pellets during the symmetrical radial 2-ring pattern in Phase 2 and left activated for the rest of the fight. These pellets will continue their animation after the transition to the second form, and turn into regular pellets you have to dodge once their animation completed.

By itself, this is just one more quirk to keep in mind during refactoring. It only turned into a bug in the Anniversary Edition because the game tracks the number of living pellets in a separate counter variable. After resetting all pellets, this counter is simply set to 0, regardless of any delay cloud pellets that may still be alive, and it's merely incremented or decremented when pellets are spawned or leave the playfield.
In the original game, this counter is only used as an optimization to skip spawning new pellets once the cap is reached. But with batched EGC-accelerated unblitting, it also makes sense to skip the rather costly setup and shutdown of the EGC if no pellets are active anyway. Except if the counter you use to check for that case can be 0 even if there are pellets alive, which consequently don't get unblitted…
There is an optimal fix though: Instead of unconditionally resetting the living pellet counter to 0, we decrement it for every pellet that does get reset. This preserves the quirk and gives us a consistently correct counter, allowing us to still skip every unnecessary loop over the pellet array.

Ultimately, this was a harmless bug that didn't affect gameplay, but it's still something that players would have probably reported a few more times. So here's a free bugfix:

TH01 Anniversary Edition, version P0234-1 2023-03-14-th01-anniv.zip

Thanks to mu021 for reporting this issue and providing helpful videos to identify the cause!