Oh look, it's another rather short and straightforward boss with a rather
small number of bugs and quirks. Yup, contrary to the character's
popularity, Mima's premiere is really not all that special in terms of code,
and continues the trend established with
📝 Kikuri and
📝 SinGyoku. I've already covered
📝 the initial sprite-related bugs last November,
so this post focuses on the main code of the fight itself. The overview:
The TH01 Mima fight consists of 3 phases, with phases 1 and 3 each
corresponding to one half of the 12-HP bar.
📝 Just like with SinGyoku, the distinction
between the red-white and red parts is purely visual once again, and doesn't
reflect anything about the boss script. As usual, all of the phases have to
be completed in order.
Phases 1 and 3 cycle through 4 danmaku patterns each, for a total of 8.
The cycles always start on a fixed pattern.
3 of the patterns in each phase feature rotating white squares, thus
introducing a new sprite in need of being unblitted.
Phase 1 additionally features the "hop pattern" as the last one in its
cycle. This is the only pattern where Mima leaves the seal in the center of
the playfield to hop from one edge of the playfield towards the other, while
also moving slightly higher up on the Y axis, and staying on the final
position for the next pattern cycle. For the first time, Mima selects a
random starting edge, which is then alternated on successive cycles.
Since the square entities are local to the respective pattern function,
Phase 1 can only end once the current pattern is done, even if Mima's HP are
already below 6. This makes Mima susceptible to the
📝 test/debug mode HP bar heap corruption bug.
Phase 2 simply consists of a spread-in teleport back to Mima's initial
position in the center of the playfield. This would only have been strictly
necessary if phase 1 ended on the hop pattern, but is done regardless of the
previous pattern, and does provide a nice visual separation between the two
main phases.
That's it – nothing special in Phase 3.
And there aren't even any weird hitboxes this time. What is maybe
special about Mima, however, is how there's something to cover about all of
her patterns. Since this is TH01, it's won't surprise anyone that the
rotating square patterns are one giant copy-pasta of unblitting, updating,
and rendering code. At least ZUN placed the core polar→Cartesian
transformation in a separate function for creating regular polygons
with an arbitrary number of sides, which might hint toward some more varied
shapes having been planned at one point?
5 of the 6 patterns even follow the exact same steps during square update
frames:
Calculate square corner coordinates
Unblit the square
Update the square angle and radius
Use the square corner coordinates for spawning pellets or missiles
Recalculate square corner coordinates
Render the square
Notice something? Bullets are spawned before the corner coordinates
are updated. That's why their initial positions seem to be a bit off – they
are spawned exactly in the corners of the square, it's just that it's
the square from 8 frames ago.
Once ZUN reached the final laser pattern though, he must have noticed that
there's something wrong there… or maybe he just wanted to fire those
lasers independently from the square unblit/update/render timer for a
change. Spending an additional 16 bytes of the data segment for conveniently
remembering the square corner coordinates across frames was definitely a
decent investment.
When Mima isn't shooting bullets from the corners of a square or hopping
across the playfield, she's raising flame pillars from the bottom of the playfield within very specifically calculated
random ranges… which are then rendered at byte-aligned VRAM positions, while
collision detection still uses their actual pixel position. Since I don't
want to sound like a broken record all too much, I'll just direct you to
📝 Kikuri, where we've seen the exact same issue with the teardrop ripple sprites.
The conclusions are identical as well.
However, I'd say that the saddest part about this pattern is how choppy it
is, with the circle/pillar entities updating and rendering at a meager 7
FPS. Why go that low on purpose when you can just make the game render ✨
smoothly ✨ instead?
The reason quickly becomes obvious: With TH01's lack of optimization, going
for the full 56.4 FPS would have significantly slowed down the game on its
intended 33 MHz CPUs, requiring more than cheap surface-level ASM
optimization for a stable frame rate. That might very well have been ZUN's
reason for only ever rendering one circle per frame to VRAM, and designing
the pattern with these time offsets in mind. It's always been typical for
PC-98 developers to target the lowest-spec models that could possibly still
run a game, and implementing dynamic frame rates into such an engine-less
game is nothing I would wish on anybody. And it's not like TH01 is
particularly unique in its choppiness anyway; low frame rates are actually a
rather typical part of the PC-98 game aesthetic.
The final piece of weirdness in this fight can be found in phase 1's hop
pattern, and specifically its palette manipulation. Just from looking at the
pattern code itself, each of the 4 hops is supposed to darken the hardware
palette by subtracting #444 from every color. At the last hop,
every color should have therefore been reduced to a pitch-black
#000, leaving the player completely blind to the movement of
the chasing pellets for 30 frames and making the pattern quite ghostly
indeed. However, that's not what we see in the actual game:
Looking at the frame counter, it appears that something outside the
pattern resets the palette every 40 frames. The only known constant with a
value of 40 would be the invincibility frames after hitting a boss with the
Orb, but we're not hitting Mima here…
But as it turns out, that's exactly where the palette reset comes from: The
hop animation darkens the hardware palette directly, while the
📝 infamous 12-parameter boss collision handler function
unconditionally resets the hardware palette to the "default boss palette"
every 40 frames, regardless of whether the boss was hit or not. I'd classify
this as a bug: That function has no business doing periodic hardware palette
resets outside the invincibility flash effect, and it completely defies
common sense that it does.
That explains one unexpected palette change, but could this function
possibly also explain the other infamous one, namely, the temporary green
discoloration in the Konngara fight? That glitch comes down to how the game
actually uses two global "default" palettes: a default boss
palette for undoing the invincibility flash effect, and a default
stage palette for returning the colors back to normal at the end of
the bomb animation or when leaving the Pause menu. And sure enough, the
stage palette is the one with the green color, while the boss
palette contains the intended colors used throughout the fight. Sending the
latter palette to the graphics chip every 40 frames is what corrects
the discoloration, which would otherwise be permanent.
The green color comes from BOSS7_D1.GRP, the scrolling
background of the entrance animation. That's what turns this into a clear
bug: The stage palette is only set a single time in the entire fight,
at the beginning of the entrance animation, to the palette of this image.
Apart from consistency reasons, it doesn't even make sense to set the stage
palette there, as you can't enter the Pause menu or bomb during a blocking
animation function.
And just 3 lines of code later, ZUN loads BOSS8_A1.GRP, the
main background image of the fight. Moving the stage palette assignment
there would have easily prevented the discoloration.
But yeah, as you can tell, palette manipulation is complete jank in this
game. Why differentiate between a stage and a boss palette to begin with?
The blocking Pause menu function could have easily copied the original
palette to a local variable before darkening it, and then restored it after
closing the menu. It's not so easy for bombs as the intended palette could
change between the start and end of the animation, but the code could have
still been simplified a lot if there was just one global "default palette"
variable instead of two. Heck, even the other bosses who manipulate their
palettes correctly only do so because they manually synchronize the two
after every change. The proper defense against bugs that result from wild
mutation of global state is to get rid of global state, and not to put up
safety nets hidden in the middle of existing effect code.
In any case, that's Mima done! 7th PC-98 Touhou boss fully
decompiled, 24 bosses remaining, and 59 functions left in all of TH01.
In other thrilling news, my call for secondary funding priorities in new
TH01 contributions has given us three different priorities so far. This
raises an interesting question though: Which of these contributions should I
now put towards TH01 immediately, and which ones should I leave in the
backlog for the time being? Since I've never liked deciding on priorities,
let's turn this into a popularity contest instead: The contributions with
the least popular secondary priorities will go towards TH01 first, giving
the most popular priorities a higher chance to still be left over after TH01
is done. As of this delivery, we'd have the following popularity order:
TH05 (1.67 pushes), from T0182
Seihou (1 push), from T0184
TH03 (0.67 pushes), from T0146
Which means that T0146 will be consumed for TH01 next, followed by T0184 and
then T0182. I only assign transactions immediately before a delivery though,
so you all still have the chance to change up these priorities before the
next one.
Next up: The final boss of TH01 decompilation, YuugenMagan… if the current
or newly incoming TH01 funds happen to be enough to cover the entire fight.
If they don't turn out to be, I will have to pass the time with some Seihou
work instead, missing the TH01 anniversary deadline as a result.Edit (2022-07-18): Thanks to Yanga for
securing the funding for YuugenMagan after all! That fight will feature
slightly more than half of all remaining code in TH01's
REIIDEN.EXE and the single biggest function in all of PC-98
Touhou, let's go!
With Elis, we've not only reached the midway point in TH01's boss code, but
also a bunch of other milestones: Both REIIDEN.EXE and TH01 as
a whole have crossed the 75% RE mark, and overall position independence has
also finally cracked 80%!
And it got done in 4 pushes again? Yup, we're back to
📝 Konngara levels of redundancy and
copy-pasta. This time, it didn't even stop at the big copy-pasted code
blocks for the rift sprite and 256-pixel circle animations, with the words
"redundant" and "unnecessary" ending up a total of 18 times in my source
code comments.
But damn is this fight broken. As usual with TH01 bosses, let's start with a
high-level overview:
The Elis fight consists of 5 phases (excluding the entrance animation),
which must be completed in order.
In all odd-numbered phases, Elis uses a random one-shot danmaku pattern
from an exclusive per-phase pool before teleporting to a random
position.
There are 3 exclusive girl-form patterns per phase, plus 4
additional bat-form patterns in phase 5, for a total of 13.
Due to a quirk in the selection algorithm in phases 1 and 3, there
is a 25% chance of Elis skipping an attack cycle and just teleporting
again.
In contrast to Konngara, Elis can freely select the same pattern
multiple times in a row. There's nothing in the code to prevent that
from happening.
This pattern+teleport cycle is repeated until Elis' HP reach a certain
threshold value. The odd-numbered phases correspond to the white (phase 1),
red-white (phase 3), and red (phase 5) sections of the health bar. However,
the next phase can only start at the end of each cycle, after a
teleport.
Phase 2 simply teleports Elis back to her starting screen position of
(320, 144) and then advances to phase 3.
Phase 4 does the same as phase 2, but adds the initial bat form
transformation before advancing to phase 5.
Phase 5 replaces the teleport with a transformation to the bat form.
Rather than teleporting instantly to the target position, the bat gradually
flies there, firing a randomly selected looping pattern from the 4-pattern
bat pool on the way, before transforming back to the girl form.
This puts the earliest possible end of the fight at the first frame of phase
5. However, nothing prevents Elis' HP from reaching 0 before that point. You
can nicely see this in 📝 debug mode: Wait
until the HP bar has filled up to avoid heap corruption, hold ↵ Return
to reduce her HP to 0, and watch how Elis still goes through a total of
two patterns* and four
teleport animations before accepting defeat.
But wait, heap corruption? Yup, there's a bug in the HP bar that already
affected Konngara as well, and it isn't even just about the graphical
glitches generated by negative HP:
The initial fill-up animation is drawn to both VRAM pages at a rate of 1
HP per frame… by passing the current frame number as the
current_hp number.
The target_hp is indicated by simply passing the current
HP…
… which, however, can be reduced in debug mode at an equal rate of up to
1 HP per frame.
The completion condition only checks if
((target_hp - 1) == current_hp). With the
right timing, both numbers can therefore run past each other.
In that case, the function is repeatedly called on every frame, backing
up the original VRAM contents for the current HP point before blitting
it…
… until frame ((96 / 2) + 1), where the
.PTN slot pointer overflows the heap buffer and overwrites whatever comes
after. 📝 Sounds familiar, right?
Since Elis starts with 14 HP, which is an even number, this corruption is
trivial to cause: Simply hold ↵ Return from the beginning of the
fight, and the completion condition will never be true, as the
HP and frame numbers run past the off-by-one meeting point.
Regular gameplay, however, entirely prevents this due to the fixed start
positions of Reimu and the Orb, the Orb's fixed initial trajectory, and the
50 frames of delay until a bomb deals damage to a boss. These aspects make
it impossible to hit Elis within the first 14 frames of phase 1, and ensure
that her HP bar is always filled up completely. So ultimately, this bug ends
up comparable in seriousness to the
📝 recursion / stack overflow bug in the memory info screen.
These wavy teleport animations point to a quite frustrating architectural
issue in this fight. It's not even the fact that unblitting the yellow star
sprites rips temporary holes into Elis' sprite; that's almost expected from
TH01 at this point. Instead, it's all because of this unused frame of the
animation:
With this sprite still being part of BOSS5.BOS, Girl-Elis has a
total of 9 animation frames, 1 more than the
📝 8 per-entity sprites allowed by ZUN's architecture.
The quick and easy solution would have been to simply bump the sprite array
size by 1, but… nah, this would have added another 20 bytes to all 6 of the
.BOS image slots. Instead, ZUN wrote the manual
position synchronization code I mentioned in that 2020 blog post.
Ironically, he then copy-pasted this snippet of code often enough that it
ended up taking up more than 120 bytes in the Elis fight alone – with, you
guessed it, some of those copies being redundant. Not to mention that just
going from 8 to 9 sprites would have allowed ZUN to go down from 6 .BOS
image slots to 3. That would have actually saved 420 bytes in
addition to the manual synchronization trouble. Looking forward to SinGyoku,
that's going to be fun again…
As for the fight itself, it doesn't take long until we reach its most janky
danmaku pattern, right in phase 1:
For whatever reason, the lower-right quarter of the circle isn't
animated? This animation works by only drawing the new dots added with every
subsequent animation frame, expressed as a tiny arc of a dotted circle. This
arc starts at the animation's current 8-bit angle and ends on the sum of
that angle and a hardcoded constant. In every other (copy-pasted, and
correct) instance of this animation, ZUN uses 0x02 as the
constant, but this one uses… 0.05 for the lower-right quarter?
As in, a 64-bit double constant that truncates to 0 when added
to an 8-bit integer, thus leading to the start and end angles being
identical and the game not drawing anything.
On Easy and Normal, the pattern then spawns 32 bullets along the outline
of the circle, no problem there. On Lunatic though, every one of these
bullets is instead turned into a narrow-angled 5-spread, resulting in 160
pellets… in a game with a pellet cap of 100.
Now, if Elis teleported herself to a position near the top of the playfield,
most of the capped pellets would have been clipped at that top edge anyway,
since the bullets are spawned in clockwise order starting at Elis' right
side with an angle of 0x00. On lower positions though, you can
definitely see a difference if the cap were high enough to allow all coded
pellets to actually be spawned.
The Hard version gets dangerously close to the cap by spawning a total of 96
pellets. Since this is the only pattern in phase 1 that fires pellets
though, you are guaranteed to see all of the unclipped ones.
The pellets also aren't spawned exactly on the telegraphed circle, but 4 pixels to the left.
Then again, it might very well be that all of this was intended, or, most
likely, just left in the game as a happy accident. The latter interpretation
would explain why ZUN didn't just delete the rendering calls for the
lower-right quarter of the circle, because seriously, how would you not spot
that? The phase 3 patterns continue with more minor graphical glitches that
aren't even worth talking about anymore.
And then Elis transforms into her bat form at the beginning of Phase 5,
which displays some rather unique hitboxes. The one against the Orb is fine,
but the one against player shots…
… uses the bat's X coordinate for both X and Y dimensions.
In regular gameplay, it's not too bad as most
of the bat patterns fire aimed pellets which typically don't allow you to
move below her sprite to begin with. But if you ever tried destroying these
pellets while standing near the middle of the playfield, now you know why
that didn't work. This video also nicely points out how the bat, like any
boss sprite, is only ever blitted at positions on the 8×1-pixel VRAM byte
grid, while collision detection uses the actual pixel position.
The bat form patterns are all relatively simple, with little variation
depending on the difficulty level, except for the "slow pellet spreads"
pattern. This one is almost easiest to dodge on Lunatic, where the 5-spreads
are not only always fired downwards, but also at the hardcoded narrow delta
angle, leaving plenty of room for the player to move out of the way:
Finally, we've got another potential timesave in the girl form's "safety
circle" pattern:
After the circle spawned completely, you lose a life by moving outside it,
but doing that immediately advances the pattern past the circle part. This
part takes 200 frames, but the defeat animation only takes 82 frames, so
you can save up to 118 frames there.
Final funny tidbit: As with all dynamic entities, this circle is only
blitted to VRAM page 0 to allow easy unblitting. However, it's also kind of
static, and there needs to be some way to keep the Orb, the player shots,
and the pellets from ripping holes into it. So, ZUN just re-blits the circle
every… 4 frames?! 🤪 The same is true for the Star of David and its
surrounding circle, but there you at least get a flash animation to justify
it. All the overlap is actually quite a good reason for not even attempting
to 📝 mess with the hardware color palette instead.
Reproducing the crash was the whole challenge here. Even after moving Elis
and Reimu to the exact positions seen in Pearl's video and setting Elis' HP
to 0 on the exact same frame, everything ran fine for me. It's definitely no
division by 0 this time, the function perfectly guards against that
possibility. The line specified in the function's parameters is always
clipped to the VRAM region as well, so we can also rule out illegal memory
accesses here…
… or can we? Stepping through it all reminded me of how this function brings
unblitting sloppiness to the next level: For each VRAM byte touched, ZUN
actually unblits the 4 surrounding bytes, adding one byte to the left
and two bytes to the right, and using a single 32-bit read and write per
bitplane. So what happens if the function tries to unblit the topmost byte
of VRAM, covering the pixel positions from (0, 0) to (7, 0)
inclusive? The VRAM offset of 0x0000 is decremented to
0xFFFF to cover the one byte to the left, 4 bytes are written
to this address, the CPU's internal offset overflows… and as it turns out,
that is illegal even in Real Mode as of the 80286, and will raise a General Protection
Fault. Which is… ignored by DOSBox-X,
every Neko Project II version in common use, the CSCP
emulators, SL9821, and T98-Next. Only Anex86 accurately emulates the
behavior of real hardware here.
OK, but no laser fired by Elis ever reaches the top-left corner of the
screen. How can such a fault even happen in practice? That's where the
broken laser reset+unblit function comes in: Not only does it just flat out pass the wrong
parameters to the line unblitting function – describing the line
already traveled by the laser and stopping where the laser begins –
but it also passes them
wrongly, in the form of raw 32-bit fixed-point Q24.8 values, with no
conversion other than a truncation to the signed 16-bit pixels expected by
the function. What then follows is an attempt at interpolation and clipping
to find a line segment between those garbage coordinates that actually falls
within the boundaries of VRAM:
right/bottom correspond to a laser's origin position, and
left/top to the leftmost pixel of its moved-out top line. The
bug therefore only occurs with lasers that stopped growing and have started
moving.
Moreover, it will only happen if either (left % 256) or
(right % 256) is ≤ 127 and the other one of the two is ≥ 128.
The typecast to signed 16-bit integers then turns the former into a large
positive value and the latter into a large negative value, triggering the
function's clipping code.
The function then follows Bresenham's
algorithm: left is ensured to be smaller than right
by swapping the two values if necessary. If that happened, top
and bottom are also swapped, regardless of their value – the
algorithm does not care about their order.
The slope in the X dimension is calculated using an integer division of
((bottom - top) /
(right - left)). Both subtractions are done on signed
16-bit integers, and overflow accordingly.
(-left × slope_x) is added to top,
and left is set to 0.
If both top and bottom are < 0 or
≥ 640, there's nothing to be unblitted. Otherwise, the final
coordinates are clipped to the VRAM range of [(0, 0),
(639, 399)].
If the function got this far, the line to be unblitted is now very
likely to reach from
the top-left to the bottom-right corner, starting out at
(0, 0) right away, or
from the bottom-left corner to the top-right corner. In this case,
you'd expect unblitting to end at (639, 0), but thanks to an
off-by-one error,
it actually ends at (640, -1), which is equivalent to
(0, 0). Why add clipping to VRAM offset calculations when
everything else is clipped already, right?
tl;dr: TH01 has a high chance of freezing at a boss defeat sequence if there
are diagonally moving lasers on screen, and if your PC-98 system
raises a General Protection Fault on a 4-byte write to offset
0xFFFF, and if you don't run a TSR with an INT
0Dh handler that might handle this fault differently.
The easiest fix option would be to just remove the attempted laser
unblitting entirely, but that would also have an impact on this game's…
distinctive visual glitches, in addition to touching a whole lot of
code bytes. If I ever get funded to work on a hypothetical TH01 Anniversary
Edition that completely rearchitects the game to fix all these glitches, it
would be appropriate there, but not for something that purports to be the
original game.
(Sidenote to further hype up this Anniversary Edition idea for PC-98
hardware owners: With the amount of performance left on the table at every
corner of this game, I'm pretty confident that we can get it to work
decently on PC-98 models with just an 80286 CPU.)
Since we're in critical infrastructure territory once again, I went for the
most conservative fix with the least impact on the binary: Simply changing
any VRAM offsets >= 0xFFFD to 0x0000 to avoid
the GPF, and leaving all other bugs in place. Sure, it's rather lazy and
"incorrect"; the function still unblits a 32-pixel block there, but adding a
special case for blitting 24 pixels would add way too much code. And
seriously, it's not like anything happens in the 8 pixels between
(24, 0) and (31, 0) inclusive during gameplay to begin with.
To balance out the additional per-row if() branch, I inlined
the VRAM page change I/O, saving two function calls and one memory write per
unblitted row.
That means it's time for a new community_choice_fixes
build, containing the new definitive bugfixed versions of these games:
2022-05-31-community-choice-fixes.zip
Check the th01_critical_fixes
branch for the modified TH01 code. It also contains a fix for the HP bar
heap corruption in test or debug mode – simply changing the ==
comparison to <= is enough to avoid it, and negative HP will
still create aesthetic glitch art.
Once again, I then was left with ½ of a push, which I finally filled with
some FUUIN.EXE code, specifically the verdict screen. The most
interesting part here is the player title calculation, which is quite
sneaky: There are only 6 skill levels, but three groups of
titles for each level, and the title you'll see is picked from a random
group. It looks like this is the first time anyone has documented the
calculation?
As for the levels, ZUN definitely didn't expect players to do particularly
well. With a 1cc being the standard goal for completing a Touhou game, it's
especially funny how TH01 expects you to continue a lot: The code has
branches for up to 21 continues, and the on-screen table explicitly leaves
room for 3 digits worth of continues per 5-stage scene. Heck, these
counts are even stored in 32-bit long variables.
Next up: 📝 Finally finishing the long
overdue Touhou Patch Center MediaWiki update work, while continuing with
Kikuri in the meantime. Originally I wasn't sure about what to do between
Elis and Seihou,
but with Ember2528's surprise
contribution last week, y'all have
demonstrated more than enough interest in the idea of getting TH01 done
sooner rather than later. And I agree – after all, we've got the 25th
anniversary of its first public release coming up on August 15, and I might
still manage to completely decompile this game by that point…