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Showing all posts tagged shinki-

📝 Posted:
🚚 Summary of:
P0190, P0191, P0192
5734815...293e16a, 293e16a...71cb7b5, 71cb7b5...e1f3f9f
💰 Funded by:
nrook, -Tom-, [Anonymous]
🏷 Tags:
rec98+ th02+ th03+ th04+ th05+ gameplay+ boss+ shinki- danmaku-pattern+ midboss+ master.lib+ file-format+ tcc+ pc98+ micro-optimization+

The important things first:

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:

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" pattern is 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:

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:

Raw file layout of TH05's ST05.BB2, demonstrating master.lib's supposed BFNT width limit of 64 pixels
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. :onricdennat: Even in TH05, these boss and midboss update functions are still very imperative:

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.
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

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:

📝 Posted:
🚚 Summary of:
P0184, P0185
f9d983e...f918298, f918298...a21ab3d
💰 Funded by:
-Tom-, Blue Bolt, [Anonymous]
🏷 Tags:
rec98+ th04+ th05+ gameplay+ louise+ shinki- bullet+ danmaku-pattern+ uth05win+ blitting+

Two years after 📝 the first look at TH04's and TH05's bullets, we finally get to finish their logic code by looking at the special motion types. Bullets as a whole still aren't completely finished as the rendering code is still waiting to be RE'd, but now we've got everything about them that's required for decompiling the midboss and boss fights of these games.

Just like the motion types of TH01's pellets, the ones we've got here really are special enough to warrant an enum, despite all the overlap in the "slow down and turn" and "bounce at certain edges of the playfield" types. Sure, including them in the bitfield I proposed two years ago would have allowed greater variety, but it wouldn't have saved any memory. On the contrary: These types use a single global state variable for the maximum turn count and delta speed, which a proper customizable architecture would have to integrate into the bullet structure. Maybe it is possible to stuff everything into the same amount of bytes, but not without first completely rearchitecting the bullet structure and removing every single piece of redundancy in there. Simply extending the system by adding a new enum value for a new motion type would be way more straightforward for modders.

Speaking about memory, TH05 already extends the bullet structure by 6 bytes for the "exact linear movement" type exclusive to that game. This type is particularly interesting for all the prospective PC-98 game developers out there, as it nicely points out the precision limits of Q12.4 subpixels.
Regular bullet movement works by adding a Q12.4 velocity to a Q12.4 position every frame, with the velocity typically being calculated only once on spawn time from an 8-bit angle and a Q12.4 speed. Quantization errors from this initial calculation can quickly compound over all the frames a bullet spends moving across the playfield. If a bullet is only supposed to move on a straight line though, there is a more precise way of calculating its position: By storing the origin point, movement angle, and total distance traveled, you can perform a full polar→Cartesian transformation every frame. Out of the 10 danmaku patterns in TH05 that use this motion type, the difference to regular bullet movement can be best seen in Louise's final pattern:

Louise's final pattern in its original form, demonstrating exact linear bullet movement. Note how each bullet spawns slightly behind the delay cloud: ZUN simply forgot to shift the fixed origin point along with it.
The same pattern with standard bullet movement, corrupting its intended appearance. No delay cloud-related oversights here though, at least.

Not far away from the regular bullet code, we've also got the movement function for the infamous curve / "cheeto" bullets. I would have almost called them "cheetos" in the code as well, which surely fits more nicely into 8.3 filenames than "curve bullets" does, but eh, trademarks…

As for hitboxes, we got a 16×16 one on the head node, and a 12×12 one on the 16 trail nodes. The latter simply store the position of the head node during the last 16 frames, Snake style. But what you're all here for is probably the turning and homing algorithm, right? Boiled down to its essence, it works like this:

// [head] points to the controlled "head" part of a curve bullet entity.
// Angles are stored with 8 bits representing a full circle, providing free
// normalization on arithmetic overflow.
// The directions are ordered as you would expect:
// • 0x00: right	(sin(0x00) =  0, cos(0x00) = +1)
// • 0x40: down 	(sin(0x40) = +1, cos(0x40) =  0)
// • 0x80: left 	(sin(0x80) =  0, cos(0x80) = -1)
// • 0xC0: up   	(sin(0xC0) = -1, cos(0xC0) =  0)
uint8_t angle_delta = (head->angle - player_angle_from(
	head->pos.cur.x, head->pos.cur.y

// Stop turning if the player is 1/128ths of a circle away from this bullet
const uint8_t SNAP = 0x02;

// Else, turn either clockwise or counterclockwise by 1/256th of a circle,
// depending on what would reach the player the fastest.
if((angle_delta > SNAP) && (angle_delta < static_cast(-SNAP))) {
	angle_delta = (angle_delta >= 0x80) ? -0x01 : +0x01;
head_p->angle -= angle_delta;

5 lines of code, and not all too difficult to follow once you are familiar with 8-bit angles… unlike what ZUN actually wrote. Which is 26 lines, and includes an unused "friction" variable that is never set to any value that makes a difference in the formula. :zunpet:. uth05win correctly saw through that all and simplified this code to something equivalent to my explanation. Redoing that work certainly wasted a bit of my time, and means that I now definitely need to spend another push on RE'ing all the shared boss functions before I can start with Shinki.

So while a curve bullet's speed does get faster over time, its angular velocity is always limited to 1/256th of a circle per frame. This reveals the optimal strategy for dodging them: Maximize this delta angle by staying as close to 180° away from their current direction as possible, and let their acceleration do the rest.

At least that's the theory for dodging a single one. As a danmaku designer, you can now of course place other bullets at these technically optimal places to prevent a curve bullet pattern from being cheesed like that. I certainly didn't record the video above in a single take either… :tannedcirno:

After another bunch of boring entity spawn and update functions, the playfield shaking feature turned out as the most notable (and tricky) one to round out these two pushes. It's actually implemented quite well in how it simply "un-shakes" the screen by just marking every stage tile to be redrawn. In the context of all the other tile invalidation that can take place during a frame, that's definitely more performant than 📝 doing another EGC-accelerated memmove() . Due to these two games being double-buffered via page flipping, this invalidation only really needs to happen for the frame after the next one though. The immediately next frame will show the regular, un-shaken playfield on the other VRAM page first, except during the multi-frame shake animation when defeating a midboss, where it will also appear shifted in a different direction… 😵 Yeah, no wonder why ZUN just always invalidates all stage tiles for the next two frames after every shaking animation, which is guaranteed to handle both sporadic single-frame shakes and continuous ones. So close to good-code here.

Finally, this delivery was delayed a bit because -Tom- requested his round-up amount to be limited to the cap in the future. Since that makes it kind of hard to explain on a static page how much money he will exactly provide, I now properly modeled these discounts in the website code. The exact round-up amount is now included in both the pre-purchase breakdown, as well as the cap bar on the main page.
With that in place, the system is now also set up for round-up offers from other patrons. If you'd also like to support certain goals in this way, with any amount of money, now's the time for getting in touch with me about that. Known contributors only, though! 😛

Next up: The final bunch of shared boring boss functions. Which certainly will give me a break from all the maintenance and research work, and speed up delivery progress again… right?

📝 Posted:
🚚 Summary of:
💰 Funded by:
Ember2528, -Tom-
🏷 Tags:
rec98+ th04+ th05+ file-format+ pc98+ player+ bomb+ boss+ shinki- ex-alice+ animation+ waste+

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. :tannedcirno: 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? :thonk: 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… :onricdennat:

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. :zunpet:

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.

📝 Posted:
🚚 Summary of:
💰 Funded by:
Ember2528, -Tom-
🏷 Tags:
rec98+ th05+ tcc+ animation+ boss+ shinki- micro-optimization+ waste+ uth05win+

Y'know, I kinda prefer the pending crowdfunded workload to stay more near the middle of the cap, rather than being sold out all the time. So to reach this point more quickly, let's do the most relaxing thing that can be easily done in TH05 right now: The boss backgrounds, starting with Shinki's, 📝 now that we've got the time to look at it in detail.

… Oh come on, more things that are borderline undecompilable, and require new workarounds to be developed? Yup, Borland C++ always optimizes any comparison of a register with a literal 0 to OR reg, reg, no matter how many calculations and inlined function calls you replace the 0 with. Shinki's background particle rendering function contains a CMP AX, 0 instruction though… so yeah, 📝 yet another piece of custom ASM that's worse than what Turbo C++ 4.0J would have generated if ZUN had just written readable C. This was probably motivated by ZUN insisting that his modified master.lib function for blitting particles takes its X and Y parameters as registers. If he had just used the __fastcall convention, he also would have got the sprite ID passed as a register. 🤷
So, we really don't want to be forced into inline assembly just because of the third comparison in the otherwise perfectly decompilable four-comparison if() expression that prevents invisible particles from being drawn. The workaround: Comparing to a pointer instead, which only the linker gets to resolve to the actual value of 0. :tannedcirno: This way, the compiler has to make room for any 16-bit literal, and can't optimize anything.

And then we go straight from micro-optimization to waste, with all the duplication in the code that animates all those particles together with the zooming and spinning lines. This push decompiled 1.31% of all code in TH05, and thanks to alignment, we're still missing Shinki's high-level background rendering function that calls all the subfunctions I decompiled here.
With all the manipulated state involved here, it's not at all trivial to see how this code produces what you see in-game. Like:

  1. If all lines have the same Y velocity, how do the other three lines in background type B get pushed down into this vertical formation while the top one stays still? (Answer: This velocity is only applied to the top line, the other lines are only pushed based on some delta.)
  2. How can this delta be calculated based on the distance of the top line with its supposed target point around Shinki's wings? (Answer: The velocity is never set to 0, so the top line overshoots this target point in every frame. After calculating the delta, the top line itself is pushed down as well, canceling out the movement. :zunpet:)
  3. Why don't they get pushed down infinitely, but stop eventually? (Answer: We only see four lines out of 20, at indices #0, #6, #12, and #18. In each frame, lines [0..17] are copied to lines [1..18], before anything gets moved. The invisible lines are pushed down based on the delta as well, which defines a distance between the visible lines of (velocity * array gap). And since the velocity is capped at -14 pixels per frame, this also means a maximum distance of 84 pixels between the midpoints of each line.)
  4. And why are the lines moving back up when switching to background type C, before moving down? (Answer: Because type C increases the velocity rather than decreasing it. Therefore, it relies on the previous velocity state from type B to show a gapless animation.)

So yeah, it's a nice-looking effect, just very hard to understand. 😵

With the amount of effort I'm putting into this project, I typically gravitate towards more descriptive function names. Here, however, uth05win's simple and seemingly tiny-brained "background type A/B/C/D" was quite a smart choice. It clearly defines the sequence in which these animations are intended to be shown, and as we've seen with point 4 from the list above, that does indeed matter.

Next up: At least EX-Alice's background animations, and probably also the high-level parts of the background rendering for all the other TH05 bosses.

📝 Posted:
🚚 Summary of:
💰 Funded by:
[Anonymous], Blue Bolt
🏷 Tags:
rec98+ th02+ th03+ th04+ th05+ animation+ tcc+ shinki- ex-alice+

… and just as I explained 📝 in the last post how decompilation is typically more sensible and efficient than ASM-level reverse-engineering, we have this push demonstrating a counter-example. The reason why the background particles and lines in the Shinki and EX-Alice battles contributed so much to position dependence was simply because they're accessed in a relatively large amount of functions, one for each different animation. Too many to spend the remaining precious crowdfunded time on reverse-engineering or even decompiling them all, especially now that everyone anticipates 100% PI for TH05's MAIN.EXE.

Therefore, I only decompiled the two functions of the line structure that also demonstrate best how it works, which in turn also helped with RE. Sadly, this revealed that we actually can't 📝 overload operator =() to get that nice assignment syntax for 12.4 fixed-point values, because one of those new functions relies on Turbo C++'s built-in optimizations for trivially copyable structures. Still, impressive that this abstraction caused no other issues for almost one year.

As for the structures themselves… nope, nothing to criticize this time! Sure, one good particle system would have been awesome, instead of having separate structures for the Stage 2 "starfield" particles and the one used in Shinki's battle, with hardcoded animations for both. But given the game's short development time, that was quite an acceptable compromise, I'd say.
And as for the lines, there just has to be a reason why the game reserves 20 lines per set, but only renders lines #0, #6, #12, and #18. We'll probably see once we get to look at those animation functions more closely.

This was quite a 📝 TH03-style RE push, which yielded way more PI% than RE%. But now that that's done, I can finally not get distracted by all that stuff when looking at the list of remaining memory references. Next up: The last few missing structures in TH05's MAIN.EXE!

📝 Posted:
🚚 Summary of:
P0078, P0079
f4eb7a8...9e52cb1, 9e52cb1...cd48aa3
💰 Funded by:
iruleatgames, -Tom-
🏷 Tags:
rec98+ th02+ th04+ th05+ gameplay+ animation+ bullet+ boss+ alice+ mai+ yuki+ yumeko+ shinki- ex-alice+ uth05win+

To finish this TH05 stretch, we've got a feature that's exclusive to TH05 for once! As the final memory management innovation in PC-98 Touhou, TH05 provides a single static (64 * 26)-byte array for storing up to 64 entities of a custom type, specific to a stage or boss portion. The game uses this array for

  1. the Stage 2 star particles,
  2. Alice's puppets,
  3. the tip of curve ("jello") bullets,
  4. Mai's snowballs and Yuki's fireballs,
  5. Yumeko's swords,
  6. and Shinki's 32×32 bullets,

which makes sense, given that only one of those will be active at any given time.

On the surface, they all appear to share the same 26-byte structure, with consistently sized fields, merely using its 5 generic fields for different purposes. Looking closer though, there actually are differences in the signedness of certain fields across the six types. uth05win chose to declare them as entirely separate structures, and given all the semantic differences (pixels vs. subpixels, regular vs. tiny master.lib sprites, …), it made sense to do the same in ReC98. It quickly turned out to be the only solution to meet my own standards of code readability.

Which blew this one up to two pushes once again… But now, modders can trivially resize any of those structures without affecting the other types within the original (64 * 26)-byte boundary, even without full position independence. While you'd still have to reduce the type-specific number of distinct entities if you made any structure larger, you could also have more entities with fewer structure members.

As for the types themselves, they're full of redundancy once again – as you might have already expected from seeing #4, #5, and #6 listed as unrelated to each other. Those could have indeed been merged into a single 32×32 bullet type, supporting all the unique properties of #4 (destructible, with optional revenge bullets), #5 (optional number of twirl animation frames before they begin to move) and #6 (delay clouds). The *_add(), *_update(), and *_render() functions of #5 and #6 could even already be completely reverse-engineered from just applying the structure onto the ASM, with the ones of #3 and #4 only needing one more RE push.

But perhaps the most interesting discovery here is in the curve bullets: TH05 only renders every second one of the 17 nodes in a curve bullet, yet hit-tests every single one of them. In practice, this is an acceptable optimization though – you only start to notice jagged edges and gaps between the fragments once their speed exceeds roughly 11 pixels per second:

And that brings us to the last 20% of TH05 position independence! But first, we'll have more cheap and fast TH01 progress.