Oh, it's 2024 already and I didn't even have a delivery for December or January? Yeah… I can only repeat what I said at the end of November, although the finish line is actually in sight now. With 10 pushes across 4 repositories and a blog post that has already reached a word count of 9,240, the Shuusou Gyoku SC-88Pro BGM release is going to break 📝 both the push record set by TH01 Sariel two years ago, and 📝 the blog post length record set by the last Shuusou Gyoku delivery. Until that's done though, let's clear some more PC-98 Touhou pushes out of the backlog, and continue the preparation work for the non-ASCII translation project starting later this year.
But first, we got another free bugfix according to my policy! 📝 Back in April 2022 when I researched the Divide Error crash that can occur in TH04's Stage 4 Marisa fight, I proposed and implemented four possible workarounds and let the community pick one of them for the generally recommended small bugfix mod. I still pushed the others onto individual branches in case the gameplay community ever wants to look more closely into them and maybe pick a different one… except that I accidentally pushed the wrong code for the warp workaround, probably because I got confused with the second warp variant I developed later on.
Fortunately, I still had the intended code for both variants lying around, and used the occasion to merge the current master branch into all of these mod branches. Thanks to wyatt8740 for spotting and reporting this oversight!
As the final piece of code shared in largely identical form between 4 of the 5 games, the Music Rooms were the biggest remaining piece of low-hanging fruit that guaranteed big finalization% gains for comparatively little effort. They seemed to be especially easy because I already decompiled TH02's Music Room together with the rest of that game's OP.EXE back in early 2015, when this project focused on just raw decompilation with little to no research. 9 years of increased standards later though, it turns out that I missed a lot of details, and ended up renaming most variables and functions. Combined with larger-than-expected changes in later games and the usual quality level of ZUN's menu code, this ended up taking noticeably longer than the single push I expected.
The undoubtedly most interesting part about this screen is the animation in the background, with the spinning and falling polygons cutting into a single-color background to reveal a spacey image below. However, the only background image loaded in the Music Room is OP3.PI (TH02/TH03) or MUSIC3.PI (TH04/TH05), which looks like this in a .PI viewer or when converted into another image format with the usual tools:
Let's call this "the blank image".
That is definitely the color that appears on top of the polygons, but where is the spacey background? If there is no other .PI file where it could come from, it has to be somewhere in that same file, right?
And indeed: This effect is another bitplane/color palette trick, exactly like the 📝 three falling stars in the background of TH04's Stage 5. If we set every bit on the first bitplane and thus change any of the resulting even hardware palette color indices to odd ones, we reveal a full second 8-color sub-image hiding in the same .PI file:
The spacey sub-image. Never before seen!1!! …OK, touhou-memories beat me by a month. Let's add each image's full 16-color palette to deliver some additional value.
On a high level, the first bitplane therefore acts as a stencil buffer that selects between the blank and spacey sub-image for every pixel. The important part here, however, is that the first bitplane of the blank sub-images does not consist entirely of 0 bits, but does have 1 bits at the pixels that represent the caption that's supposed to be overlaid on top of the animation. Since there now are some pixels that should always be taken from the spacey sub-image regardless of whether they're covered by a polygon, the game can no longer just clear the first bitplane at the start of every frame. Instead, it has to keep a separate copy of the first bitplane's original state (called nopoly_B in the code), captured right after it blitted the .PI image to VRAM. Turns out that this copy also comes in quite handy with the text, but more on that later.
Then, the game simply draws polygons onto only the reblitted first bitplane to conditionally set the respective bits. ZUN used master.lib's grcg_polygon_c() function for this, which means that we can entirely thank the uncredited master.lib developers for this iconic animation – if they hadn't included such a function, the Music Rooms would most certainly look completely different.
This is where we get to complete the series on the PC-98 GRCG chip with the last remaining four bits of its mode register. So far, we only needed the highest bit (0x80) to either activate or deactivate it, and the bit below (0x40) to choose between the 📝 RMW and 📝 TCR/📝 TDW modes. But you can also use the lowest four bits to restrict the GRCG's operations to any subset of the four bitplanes, leaving the other ones untouched:
// Enable the GRCG (0x80) in regular RMW mode (0x40). All bitplanes are
// enabled and written according to the contents of the tile register.
outportb(0x7C, 0xC0);
// The same, but limiting writes to the first bitplane by disabling the
// second (0x02), third (0x04), and fourth (0x08) one, as done in the
// PC-98 Touhou Music Rooms.
outportb(0x7C, 0xCE);
// Regular GRCG blitting code to any VRAM segment…
pokeb(0xA8000, offset, …);
// We're done, turn off the GRCG.
outportb(0x7C, 0x00);
This could be used for some unusual effects when writing to two or three of the four planes, but it seems rather pointless for this specific case at first. If we only want to write to a single plane, why not just do so directly, without the GRCG? Using that chip only involves more hardware and is therefore slower by definition, and the blitting code would be the same, right?
This is another one of these questions that would be interesting to benchmark one day, but in this case, the reason is purely practical: All of master.lib's polygon drawing functions expect the GRCG to be running in RMW mode. They write their pixels as bitmasks where 1 and 0 represent pixels that should or should not change, and leave it to the GRCG to combine these masks with its tile register and OR the result into the bitplanes instead of doing so themselves. Since GRCG writes are done via MOV instructions, not using the GRCG would turn these bitmasks into actual dot patterns, overwriting any previous contents of each VRAM byte that gets modified.
Technically, you'd only have to replace a few MOV instructions with OR to build a non-GRCG version of such a function, but why would you do that if you haven't measured polygon drawing to be an actual bottleneck.
An example with three polygons drawn from top to bottom. Without the GRCG, edges of later polygons overwrite any previously drawn pixels within the same VRAM byte. Note how treating bitmasks as dot patterns corrupts even those areas where the background image had nonzero bits in its first bitplane.
As far as complexity is concerned though, the worst part is the implicit logic that allows all this text to show up on top of the polygons in the first place. If every single piece of text is only rendered a single time, how can it appear on top of the polygons if those are drawn every frame?
Depending on the game (because of course it's game-specific), the answer involves either the individual bits of the text color index or the actual contents of the palette:
Colors 0 or 1 can't be used, because those don't include any of the bits that can stay constant between frames.
If the lowest bit of a palette color index has no effect on the displayed color, text drawn in either of the two colors won't be visually affected by the polygon animation and will always appear on top. TH04 and TH05 rely on this property with their colors 2/3, 4/5, and 6/7 being identical, but this would work in TH02 and TH03 as well.
But this doesn't apply to TH02 and TH03's palettes, so how do they do it? The secret: They simply include all text pixels in nopoly_B. This allows text to use any color with an odd palette index – the lowest bit then won't be affected by the polygons ORed into the first bitplane, and the other bitplanes remain unchanged.
TH04 is a curious case. Ostensibly, it seems to remove support for odd text colors, probably because the new 10-frame fade-in animation on the comment text would require at least the comment area in VRAM to be captured into nopoly_B on every one of the 10 frames. However, the initial pixels of the tracklist are still included in nopoly_B, which would allow those to still use any odd color in this game. ZUN only removed those from nopoly_B in TH05, where it had to be changed because that game lets you scroll and browse through multiple tracklists.
The contents of nopoly_B with each game's first track selected.
Finally, here's a list of all the smaller details that turn the Music Rooms into such a mess:
Due to the polygon animation, the Music Room is one of the few double-buffered menus in PC-98 Touhou, rendering to both VRAM pages on alternate frames instead of using the other page to store a background image. Unfortunately though, this doesn't actually translate to tearing-free rendering because ZUN's initial implementation for TH02 mixed up the order of the required operations. You're supposed to first wait for the GDC's VSync interrupt and then, within the display's vertical blanking interval, write to the relevant I/O ports to flip the accessed and shown pages. Doing it the other way around and flipping as soon as you're finished with the last draw call of a frame means that you'll very likely hit a point where the (real or emulated) electron beam is still traveling across the screen. This ensures that there will be a tearing line somewhere on the screen on all but the fastest PC-98 models that can render an entire frame of the Music Room completely within the vertical blanking interval, causing the very issue that double-buffering was supposed to prevent.
ZUN only fixed this landmine in TH05.
The polygons have a fixed vertex count and radius depending on their index, everything else is randomized. They are also never reinitialized while OP.EXE is running – if you leave the Music Room and reenter it, they will continue animating from the same position.
TH02 and TH04 don't handle it at all, causing held keys to be processed again after about a second.
TH03 and TH05 correctly work around the quirk, at the usual cost of a 614.4 µs delay per frame. Except that the delay is actually twice as long in frames in which a previously held key is released, because this code is a mess.
But even in 2024, DOSBox-X is the only emulator that actually replicates this detail of real hardware. On anything else, keyboard input will behave as ZUN intended it to. At least I've now mentioned this once for every game, and can just link back to this blog post for the other menus we still have to go through, in case their game-specific behavior matches this one.
TH02 is the only game that
separately lists the stage and boss themes of the main game, rather than following the in-game order of appearance,
continues playing the selected track when leaving the Music Room,
always loads both MIDI and PMD versions, regardless of the currently selected mode, and
does not stop the currently playing track before loading the new one into the PMD and MMD drivers.
The combination of 2) and 3) allows you to leave the Music Room and change the music mode in the Option menu to listen to the same track in the other version, without the game changing back to the title screen theme. 4), however, might cause the PMD and MMD drivers to play garbage for a short while if the music data is loaded from a slow storage device that takes longer than a single period of the OPN timer to fill the driver's song buffer. Probably not worth mentioning anymore though, now that people no longer try fitting PC-98 Touhou games on floppy disks.
Exactly 40 (TH02/TH03) / 38 (TH04/TH05) visible bytes per line,
padded with 2 bytes that can hold a CR/LF newline sequence for easier editing.
Every track starts with a title line that mostly just duplicates the names from the hardcoded tracklist,
followed by a fixed 19 (TH02/TH03/TH04) / 9 (TH05) comment lines.
In TH04 and TH05, lines can start with a semicolon (;) to prevent them from being rendered. This is purely a performance hint, and is visually equivalent to filling the line with spaces.
All in all, the quality of the code is even slightly below the already poor standard for PC-98 Touhou: More VRAM page copies than necessary, conditional logic that is nested way too deeply, a distinct avoidance of state in favor of loops within loops, and – of course – a couple of gotos to jump around as needed.
In TH05, this gets so bad with the scrolling and game-changing tracklist that it all gives birth to a wonderfully obscure inconsistency: When pressing both ⬆️/⬇️ and ⬅️/➡️ at the same time, the game first processes the vertical input and then the horizontal one in the next frame, making it appear as if the latter takes precedence. Except when the cursor is highlighting the first (⬆️ ) or 12th (⬇️ ) element of the list, and said list element is not the first track (⬆️ ) or the quit option (⬇️ ), in which case the horizontal input is ignored.
And that's all the Music Rooms! The OP.EXE binaries of TH04 and especially TH05 are now very close to being 100% RE'd, with only the respective High Score menus and TH04's title animation still missing. As for actual completion though, the finalization% metric is more relevant as it also includes the ZUN Soft logo, which I RE'd on paper but haven't decompiled. I'm 📝 still hoping that this will be the final piece of code I decompile for these two games, and that no one pays to get it done earlier…
For the rest of the second push, there was a specific goal I wanted to reach for the remaining anything budget, which was blocked by a few functions at the beginning of TH04's and TH05's MAINE.EXE. In another anticlimactic development, this involved yet another way too early decompilation of a main() function…
Generally, this main() function just calls the top-level functions of all other ending-related screens in sequence, but it also handles the TH04-exclusive congratulating All Clear images within itself. After a 1CC, these are an additional reward on top of the Good Ending, showing the player character wearing a different outfit depending on the selected difficulty. On Easy Mode, however, the Good Ending is unattainable because the game always ends after Stage 5 with a Bad Ending, but ZUN still chose to show the EASY ALL CLEAR!! image in this case, regardless of how many continues you used.
While this might seem inconsistent with the other difficulties, it is consistent within Easy Mode itself, as the enforced Bad Ending after Stage 5 also doesn't distinguish between the number of continues. Also, Try to Normal Rank!! could very well be ZUN's roundabout way of implying "because this is how you avoid the Bad Ending".
With that out of the way, I was finally able to separate the VRAM text renderer of TH04 and TH05 into its own assembly unit, 📝 finishing the technical debt repayment project that I couldn't complete in 2021 due to assembly-time code segment label arithmetic in the data segment. This now allows me to translate this undecompilable self-modifying mess of ASM into C++ for the non-ASCII translation project, and thus unify the text renderers of all games and enhance them with support for Unicode characters loaded from a bitmap font. As the final finalized function in the SHARED segment, it also allowed me to remove 143 lines of particularly ugly segmentation workarounds 🙌
The remaining 1/6th of the second push provided the perfect occasion for some light TH02 PI work. The global boss position and damage variables represented some equally low-hanging fruit, being easily identified global variables that aren't part of a larger structure in this game. In an interesting twist, TH02 is the only game that uses an increasing damage value to track boss health rather than decreasing HP, and also doesn't internally distinguish between bosses and midbosses as far as these variables are concerned. Obviously, there's quite a bit of state left to be RE'd, not least because Marisa is doing her own thing with a bunch of redundant copies of her position, but that was too complex to figure out right now.
Also doing their own thing are the Five Magic Stones, which need five positions rather than a single one. Since they don't move, the game doesn't have to keep 📝 separate position variables for both VRAM pages, and can handle their positions in a much simpler way that made for a nice final commit.
And for the first time in a long while, I quite like what ZUN did there!
Not only are their positions stored in an array that is indexed with a consistent ID for every stone, but these IDs also follow the order you fight the stones in: The two inner ones use 0 and 1, the two outer ones use 2 and 3, and the one in the center uses 4. This might look like an odd choice at first because it doesn't match their horizontal order on the playfield. But then you notice that ZUN uses this property in the respective phase control functions to iterate over only the subrange of active stones, and you realize how brilliant it actually is.
This seems like a really basic thing to get excited about, especially since the rest of their data layout sure isn't perfect. Splitting each piece of state and even the individual X and Y coordinates into separate 5-element arrays is still counter-productive because the game ends up paying more memory and CPU cycles to recalculate the element offsets over and over again than this would have ever saved in cache misses on a 486. But that's a minor issue that could be fixed with a few regex replacements, not a misdesigned architecture that would require a full rewrite to clean it up. Compared to the hardcoded and bloated mess that was 📝 YuugenMagan's five eyes, this is definitely an improvement worthy of the good-code tag. The first actual one in two years, and a welcome change after the Music Room!
These three pieces of data alone yielded a whopping 5% of overall TH02 PI in just 1/6th of a push, bringing that game comfortably over the 60% PI mark. MAINE.EXE is guaranteed to reach 100% PI before I start working on the non-ASCII translations, but at this rate, it might even be realistic to go for 100% PI on MAIN.EXE as well? Or at least technical position independence, without the false positives.
Next up: Shuusou Gyoku SC-88Pro BGM. It's going to be wild.
TH05 has passed the 50% RE mark, with both MAIN.EXE and the
game as a whole! With that, we've also reached what -Tom-
wanted out of the project, so he's suspending his discount offer for a
bit.
Curve bullets are now officially called cheetos! 76.7% of
fans prefer this term, and it fits into the 8.3 DOS filename scheme much
better than homing lasers (as they're called in
OMAKE.TXT) or Taito
lasers (which would indeed have made sense as well).
…oh, and I managed to decompile Shinki within 2 pushes after all. That
left enough budget to also add the Stage 1 midboss on top.
So, Shinki! As far as final boss code is concerned, she's surprisingly
economical, with 📝 her background animations
making up more than ⅓ of her entire code. Going straight from TH01's
📝 final📝 bosses
to TH05's final boss definitely showed how much ZUN had streamlined
danmaku pattern code by the end of PC-98 Touhou. Don't get me wrong, there
is still room for improvement: TH05 not only
📝 reuses the same 16 bytes of generic boss state we saw in TH04 last month,
but also uses them 4× as often, and even for midbosses. Most importantly
though, defining danmaku patterns using a single global instance of the
group template structure is just bad no matter how you look at it:
The script code ends up rather bloated, with a single MOV
instruction for setting one of the fields taking up 5 bytes. By comparison,
the entire structure for regular bullets is 14 bytes large, while the
template structure for Shinki's 32×32 ball bullets could have easily been
reduced to 8 bytes.
Since it's also one piece of global state, you can easily forget to set
one of the required fields for a group type. The resulting danmaku group
then reuses these values from the last time they were set… which might have
been as far back as another boss fight from a previous stage.
And of course, I wouldn't point this out if it
didn't actually happen in Shinki's pattern code. Twice.
Declaring a separate structure instance with the static data for every
pattern would be both safer and more space-efficient, and there's
more than enough space left for that in the game's data segment.
But all in all, the pattern functions are short, sweet, and easy to follow.
The "devil"
patternis significantly more complex than the others, but still
far from TH01's final bosses at their worst. I especially like the clear
architectural separation between "one-shot pattern" functions that return
true once they're done, and "looping pattern" functions that
run as long as they're being called from a boss's main function. Not many
all too interesting things in these pattern functions for the most part,
except for two pieces of evidence that Shinki was coded after Yumeko:
The gather animation function in the first two phases contains a bullet
group configuration that looks like it's part of an unused danmaku
pattern. It quickly turns out to just be copy-pasted from a similar function
in Yumeko's fight though, where it is turned into actual
bullets.
As one of the two places where ZUN forgot to set a template field, the
lasers at the end of the white wing preparation pattern reuse the 6-pixel
width of Yumeko's final laser pattern. This actually has an effect on
gameplay: Since these lasers are active for the first 8 frames after
Shinki's wings appear on screen, the player can get hit by them in the last
2 frames after they grew to their final width.
Of course, there are more than enough safespots between the lasers.
Speaking about that wing sprite: If you look at ST05.BB2 (or
any other file with a large sprite, for that matter), you notice a rather
weird file layout:
A large sprite split into multiple smaller ones with a width of
64 pixels each? What's this, hardware sprite limitations? On my
PC-98?!
And it's not a limitation of the sprite width field in the BFNT+ header
either. Instead, it's master.lib's BFNT functions which are limited to
sprite widths up to 64 pixels… or at least that's what
MASTER.MAN claims. Whatever the restriction was, it seems to be
completely nonexistent as of master.lib version 0.23, and none of the
master.lib functions used by the games have any issues with larger
sprites.
Since ZUN stuck to the supposed 64-pixel width limit though, it's now the
game that expects Shinki's winged form to consist of 4 physical
sprites, not just 1. Any conversion from another, more logical sprite sheet
layout back into BFNT+ must therefore replicate the original number of
sprites. Otherwise, the sequential IDs ("patnums") assigned to every newly
loaded sprite no longer match ZUN's hardcoded IDs, causing the game to
crash. This is exactly what used to happen with -Tom-'s
MysticTK automation scripts,
which combined these exact sprites into a single large one. This issue has
now been fixed – just in case there are some underground modders out there
who used these scripts and wonder why their game crashed as soon as the
Shinki fight started.
And then the code quality takes a nosedive with Shinki's main function.
Even in TH05, these boss and midboss update
functions are still very imperative:
The origin point of all bullet types used by a boss must be manually set
to the current boss/midboss position; there is no concept of a bullet type
tracking a certain entity.
The same is true for the target point of a player's homing shots…
… and updating the HP bar. At least the initial fill animation is
abstracted away rather decently.
Incrementing the phase frame variable also must be done manually. TH05
even "innovates" here by giving the boss update function exclusive ownership
of that variable, in contrast to TH04 where that ownership is given out to
the player shot collision detection (?!) and boss defeat helper
functions.
Speaking about collision detection: That is done by calling different
functions depending on whether the boss is supposed to be invincible or
not.
Timeout conditions? No standard way either, and all done with manual
if statements. In combination with the regular phase end
condition of lowering (mid)boss HP to a certain value, this leads to quite a
convoluted control flow.
The manual calls to the score bonus functions for cleared phases at least provide some sense of orientation.
One potentially nice aspect of all this imperative freedom is that
phases can end outside of HP boundaries… by manually incrementing the
phase variable and resetting the phase frame variable to 0.
The biggest WTF in there, however, goes to using one of the 16 state bytes
as a "relative phase" variable for differentiating between boss phases that
share the same branch within the switch(boss.phase)
statement. While it's commendable that ZUN tried to reduce code duplication
for once, he could have just branched depending on the actual
boss.phase variable? The same state byte is then reused in the
"devil" pattern to track the activity state of the big jerky lasers in the
second half of the pattern. If you somehow managed to end the phase after
the first few bullets of the pattern, but before these lasers are up,
Shinki's update function would think that you're still in the phase
before the "devil" pattern. The main function then sequence-breaks
right to the defeat phase, skipping the final pattern with the burning Makai
background. Luckily, the HP boundaries are far away enough to make this
impossible in practice.
The takeaway here: If you want to use the state bytes for your custom
boss script mods, alias them to your own 16-byte structure, and limit each
of the bytes to a clearly defined meaning across your entire boss script.
One final discovery that doesn't seem to be documented anywhere yet: Shinki
actually has a hidden bomb shield during her two purple-wing phases.
uth05win got this part slightly wrong though: It's not a complete
shield, and hitting Shinki will still deal 1 point of chip damage per
frame. For comparison, the first phase lasts for 3,000 HP, and the "devil"
pattern phase lasts for 5,800 HP.
And there we go, 3rd PC-98 Touhou boss
script* decompiled, 28 to go! 🎉 In case you were expecting a fix for
the Shinki death glitch: That one
is more appropriately fixed as part of the Mai & Yuki script. It also
requires new code, should ideally look a bit prettier than just removing
cheetos between one frame and the next, and I'd still like it to fit within
the original position-dependent code layout… Let's do that some other
time.
Not much to say about the Stage 1 midboss, or midbosses in general even,
except that their update functions have to imperatively handle even more
subsystems, due to the relative lack of helper functions.
The remaining ¾ of the third push went to a bunch of smaller RE and
finalization work that would have hardly got any attention otherwise, to
help secure that 50% RE mark. The nicest piece of code in there shows off
what looks like the optimal way of setting up the
📝 GRCG tile register for monochrome blitting
in a variable color:
mov ah, palette_index ; Any other non-AL 8-bit register works too.
; (x86 only supports AL as the source operand for OUTs.)
rept 4 ; For all 4 bitplanes…
shr ah, 1 ; Shift the next color bit into the x86 carry flag
sbb al, al ; Extend the carry flag to a full byte
; (CF=0 → 0x00, CF=1 → 0xFF)
out 7Eh, al ; Write AL to the GRCG tile register
endm
Thanks to Turbo C++'s inlining capabilities, the loop body even decompiles
into a surprisingly nice one-liner. What a beautiful micro-optimization, at
a place where micro-optimization doesn't hurt and is almost expected.
Unfortunately, the micro-optimizations went all downhill from there,
becoming increasingly dumb and undecompilable. Was it really necessary to
save 4 x86 instructions in the highly unlikely case of a new spark sprite
being spawned outside the playfield? That one 2D polar→Cartesian
conversion function then pointed out Turbo C++ 4.0J's woefully limited
support for 32-bit micro-optimizations. The code generation for 32-bit
📝 pseudo-registers is so bad that they almost
aren't worth using for arithmetic operations, and the inline assembler just
flat out doesn't support anything 32-bit. No use in decompiling a function
that you'd have to entirely spell out in machine code, especially if the
same function already exists in multiple other, more idiomatic C++
variations.
Rounding out the third push, we got the TH04/TH05 DEMO?.REC
replay file reading code, which should finally prove that nothing about the
game's original replay system could serve as even just the foundation for
community-usable replays. Just in case anyone was still thinking that.
Next up: Back to TH01, with the Elis fight! Got a bit of room left in the
cap again, and there are a lot of things that would make a lot of
sense now:
TH04 would really enjoy a large number of dedicated pushes to catch up
with TH05. This would greatly support the finalization of both games.
Continuing with TH05's bosses and midbosses has shown to be good value
for your money. Shinki would have taken even less than 2 pushes if she
hadn't been the first boss I looked at.
Oh, and I also added Seihou as a selectable goal, for the two people out
there who genuinely like it. If I ever want to quit my day job, I need to
branch out into safer territory that isn't threatened by takedowns, after
all.
Didn't quite get to cover background rendering for TH05's Stage 1-5
bosses in this one, as I had to reverse-engineer two more fundamental parts
involved in boss background rendering before.
First, we got the those blocky transitions from stage tiles to bomb and
boss backgrounds, loaded from BB*.BB and ST*.BB,
respectively. These files store 16 frames of animation, with every bit
corresponding to a 16×16 tile on the playfield. With 384×368 pixels to be
covered, that would require 69 bytes per frame. But since that's a very odd
number to work with in micro-optimized ASM, ZUN instead stores 512×512
pixels worth of bits, ending up with a frame size of 128 bytes, and a
per-frame waste of 59 bytes. At least it was
possible to decompile the core blitting function as __fastcall
for once.
But wait, TH05 comes with, and loads, a bomb .BB file for every character,
not just for the Reimu and Yuuka bomb transitions you see in-game… 🤔
Restoring those unused stage tile → bomb image transition
animations for Mima and Marisa isn't that trivial without having decompiled
their actual bomb animation functions before, so stay tuned!
Interestingly though, the code leaves out what would look like the most
obvious optimization: All stage tiles are unconditionally redrawn
each frame before they're erased again with the 16×16 blocks, no matter if
they weren't covered by such a block in the previous frame, or are
going to be covered by such a block in this frame. The same is true
for the static bomb and boss background images, where ZUN simply didn't
write a .CDG blitting function that takes the dirty tile array into
account. If VRAM writes on PC-98 really were as slow as the games'
README.TXT files claim them to be, shouldn't all the
optimization work have gone towards minimizing them?
Oh well, it's not like I have any idea what I'm talking about here. I'd
better stop talking about anything relating to VRAM performance on PC-98…
Second, it finally was time to solve the long-standing confusion about all
those callbacks that are supposed to render the playfield background. Given
the aforementioned static bomb background images, ZUN chose to make this
needlessly complicated. And so, we have two callback function
pointers: One during bomb animations, one outside of bomb
animations, and each boss update function is responsible for keeping the
former in sync with the latter.
Other than that, this was one of the smoothest pushes we've had in a while;
the hardest parts of boss background rendering all were part of
📝 the last push. Once you figured out that
ZUN does indeed dynamically change hardware color #0 based on the current
boss phase, the remaining one function for Shinki, and all of EX-Alice's
background rendering becomes very straightforward and understandable.
Meanwhile, -Tom- told me about his plans to publicly
release 📝 his TH05 scripting toolkit once
TH05's MAIN.EXE would hit around 50% RE! That pretty much
defines what the next bunch of generic TH05 pushes will go towards:
bullets, shared boss code, and one
full, concrete boss script to demonstrate how it's all combined. Next up,
therefore: TH04's bullet firing code…? Yes, TH04's. I want to see what I'm
doing before I tackle the undecompilable mess that is TH05's bullet firing
code, and you all probably want readable code for that feature as
well. Turns out it's also the perfect place for Blue Bolt's
pending contributions.
And indeed, I got to end my vacation with a lot of image format and
blitting code, covering the final two formats, .GRC and .BOS. .GRC was
nothing noteworthy – one function for loading, one function for
byte-aligned blitting, and one function for freeing memory. That's it –
not even a unblitting function for this one. .BOS, on the other hand…
…has no generic (read: single/sane) implementation, and is only
implemented as methods of some boss entity class. And then again for
Sariel's dress and wand animations, and then again for Reimu's
animations, both of which weren't even part of these 4 pushes. Looking
forward to decompiling essentially the same algorithms all over again… And
that's how TH01 became the largest and most bloated PC-98 Touhou game. So
yeah, still not done with image formats, even at 44% RE.
This means I also had to reverse-engineer that "boss entity" class… yeah,
what else to call something a boss can have multiple of, that may or may
not be part of a larger boss sprite, may or may not be animated, and that
may or may not have an orb hitbox?
All bosses except for Kikuri share the same 5 global instances of this
class. Since renaming all these variables in ASM land is tedious anyway, I
went the extra mile and directly defined separate, meaningful names for
the entities of all bosses. These also now document the natural order in
which the bosses will ultimately be decompiled. So, unless a backer
requests anything else, this order will be:
Konngara
Sariel
Elis
Kikuri
SinGyoku
(code for regular card-flipping stages)
Mima
YuugenMagan
As everyone kind of expects from TH01 by now, this class reveals yet
another… um, unique and quirky piece of code architecture. In
addition to the position and hitbox members you'd expect from a class like
this, the game also stores the .BOS metadata – width, height, animation
frame count, and 📝 bitplane pointer slot
number – inside the same class. But if each of those still corresponds to
one individual on-screen sprite, how can YuugenMagan have 5 eye sprites,
or Kikuri have more than one soul and tear sprite? By duplicating that
metadata, of course! And copying it from one entity to another
At this point, I feel like I even have to congratulate the game for not
actually loading YuugenMagan's eye sprites 5 times. But then again, 53,760
bytes of waste would have definitely been noticeable in the DOS days.
Makes much more sense to waste that amount of space on an unused C++
exception handler, and a bunch of redundant, unoptimized blitting
functions
(Thinking about it, YuugenMagan fits this entire system perfectly. And
together with its position in the game's code – last to be decompiled
means first on the linker command line – we might speculate that
YuugenMagan was the first boss to be programmed for TH01?)
So if a boss wants to use sprites with different sizes, there's no way
around using another entity. And that's why Girl-Elis and Bat-Elis are two
distinct entities internally, and have to manually sync their position.
Except that there's also a third one for Attacking-Girl-Elis,
because Girl-Elis has 9 frames of animation in total, and the global .BOS
bitplane pointers are divided into 4 slots of only 8 images each.
Same for SinGyoku, who is split into a sphere entity, a
person entity, and a… white flash entity for all three forms,
all at the same resolution. Or Konngara's facial expressions, which also
require two entities just for themselves.
And once you decompile all this code, you notice just how much of it the
game didn't even use. 13 of the 50 bytes of the boss entity class are
outright unused, and 10 bytes are used for a movement clamping and lock
system that would have been nice if ZUN also used it outside of
Kikuri's soul sprites. Instead, all other bosses ignore this system
completely, and just
party on
the X/Y coordinates of the boss entities directly.
As for the rendering functions, 5 out of 10 are unused. And while those
definitely make up less than half of the code, I still must have
spent at least 1 of those 4 pushes on effectively unused functionality.
Only one of these functions lends itself to some speculation. For Elis'
entrance animation, the class provides functions for wavy blitting and
unblitting, which use a separate X coordinate for every line of the
sprite. But there's also an unused and sort of broken one for unblitting
two overlapping wavy sprites, located at the same Y coordinate. This might
indicate that Elis could originally split herself into two sprites,
similar to TH04 Stage 6 Yuuka? Or it might just have been some other kind
of animation effect, who knows.
After over 3 months of TH01 progress though, it's finally time to look at
other games, to cover the rest of the crowdfunding backlog. Next up: Going
back to TH05, and getting rid of those last PI false positives. And since
I can potentially spend the next 7 weeks on almost full-time ReC98 work,
I've also re-opened the store until October!
Sadly, we've already reached the end of fast triple-speed TH01 progress
with 📝 the last push, which decompiled the
last segment shared by all three of TH01's executables. There's still a
bit of double-speed progress left though, with a small number of code
segments that are shared between just two of the three executables.
At the end of the first one of these, we've got all the code for the .GRZ
format – which is yet another run-length encoded image format, but this
time storing up to 16 full 640×400 16-color images with an alpha bit. This
one is exclusively used to wastefully store Konngara's sword slash and
kuji-in kill
animations. Due to… suboptimal code organization, the code for the format
is also present in OP.EXE, despite not being used there. But
hey, that brings TH01 to over 20% in RE!
Decoupling the RLE command stream from the pixel data sounds like a nice
idea at first, allowing the format to efficiently encode a variety of
animation frames displayed all over the screen… if ZUN actually made
use of it. The RLE stream also has quite some ridiculous overhead,
starting with 1 byte to store the 1-bit command (putting a single 8×1
pixel block, or entering a run of N such blocks). Run commands then store
another 1-byte run length, which has to be followed by another
command byte to identify the run as putting N blocks, or skipping N blocks.
And the pixel data is just a sequence of these blocks for all 4 bitplanes,
in uncompressed form…
Also, have some rips of all the images this format is used for:
To make these, I just wrote a small viewer, calling the same decompiled
TH01 code: 2020-03-07-grzview.zip
Obviously, this means that it not only must to be run on a PC-98, but also
discards the alpha information.
If any backers are really interested in having a proper converter
to and from PNG, I can implement that in an upcoming push… although that
would be the perfect thing for outside contributors to do.
Next up, we got some code for the PI format… oh, wait, the actual files
are called "GRP" in TH01.
Sometimes, "strategically picking things to reverse-engineer" unfortunately also means "having to move seemingly random and utterly uninteresting stuff, which will only make sense later, out of the way". Really, this was so boring. Gonna get a lot more exciting in the next ones though.
being spawned outside the playfield? That one 2D polar→Cartesian
conversion function then pointed out Turbo C++ 4.0J's woefully limited
support for 32-bit micro-optimizations. The code generation for 32-bit
