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.
Only one newly ordered push since I've reopened the store? Great, that's
all the justification I needed for the extended maintenance delay that was
part of these two pushes 😛
Having to write comments to explain whether coordinates are relative to
the top-left corner of the screen or the top-left corner of the playfield
has finally become old. So, I introduced
distinct
types for all the coordinate systems we typically encounter, applying
them to all code decompiled so far. Note how the planar nature of PC-98
VRAM meant that X and Y coordinates also had to be different from each
other. On the X side, there's mainly the distinction between the
[0; 640] screen space and the corresponding [0; 80] VRAM byte
space. On the Y side, we also have the [0; 400] screen space, but
the visible area of VRAM might be limited to [0; 200] when running in
the PC-98's line-doubled 640×200 mode. A VRAM Y coordinate also always
implies an added offset for vertical scrolling.
During all of the code reconstruction, these types can only have a
documenting purpose. Turning them into anything more than just
typedefs to int, in order to define conversion
operators between them, simply won't recompile into identical binaries.
Modding and porting projects, however, now have a nice foundation for
doing just that, and can entirely lift coordinate system transformations
into the type system, without having to proofread all the meaningless
int declarations themselves.
So, what was left in terms of memory references? EX-Alice's fire waves
were our final unknown entity that can collide with the player. Decently
implemented, with little to say about them.
That left the bomb animation structures as the one big remaining PI
blocker. They started out nice and simple in TH04, with a small 6-byte
star animation structure used for both Reimu and Marisa. TH05, however,
gave each character her own animation… and what the hell is going
on with Reimu's blue stars there? Nope, not going to figure this out on
ASM level.
A decompilation first required some more bomb-related variables to be
named though. Since this was part of a generic RE push, it made sense to
do this in all 5 games… which then led to nice PI gains in anything
but TH05. Most notably, we now got the
"pulling all items to player" flag in TH04 and TH05, which is
actually separate from bombing. The obvious cheat mod is left as an
exercise to the reader.
So, TH05 bomb animations. Just like the
📝 custom entity types of this game, all 4
characters share the same memory, with the superficially same 10-byte
structure.
But let's just look at the very first field. Seen from a low level, it's a
simple struct { int x, y; } pos, storing the current position
of the character-specific bomb animation entity. But all 4 characters use
this field differently:
For Reimu's blue stars, it's the top-left position of each star, in the
12.4 fixed-point format. But unlike the vast majority of these values in
TH04 and TH05, it's relative to the top-left corner of the
screen, not the playfield. Much better represented as
struct { Subpixel screen_x, screen_y; } topleft.
For Marisa's lasers, it's the center of each circle, as a regular 12.4
fixed-point coordinate, relative to the top-left corner of the playfield.
Much better represented as
struct { Subpixel x, y; } center.
For Mima's shrinking circles, it's the center of each circle in regular
pixel coordinates. Much better represented as
struct { screen_x_t x; screen_y_t y; } center.
For Yuuka's spinning heart, it's the top-left corner in regular pixel
coordinates. Much better represented as
struct { screen_x_t x; screen_y_t y; } topleft.
And yes, singular. The game is actually smart enough to only store a single
heart, and then create the rest of the circle on the fly. (If it were even
smarter, it wouldn't even use this structure member, but oh well.)
Therefore, I decompiled it as 4 separate structures once again, bundled
into an union of arrays.
As for Reimu… yup, that's some pointer arithmetic straight out of
Jigoku* for setting and updating the positions of the falling star
trails. While that certainly required several
comments to wrap my head around the current array positions, the one "bug"
in all this arithmetic luckily has no effect on the game.
There is a small glitch with the growing circles, though. They are
spawned at the end of the loop, with their position taken from the star
pointer… but after that pointer has already been incremented. On
the last loop iteration, this leads to an out-of-bounds structure access,
with the position taken from some unknown EX-Alice data, which is 0 during
most of the game. If you look at the animation, you can easily spot these
bugged circles, consistently growing from the top-left corner (0, 0)
of the playfield:
After all that, there was barely enough remaining time to filter out and
label the final few memory references. But now, TH05's
MAIN.EXE is technically position-independent! 🎉
-Tom- is going to work on a pretty extensive demo of this
unprecedented level of efficient Touhou game modding. For a more impactful
effect of both the 100% PI mark and that demo, I'll be delaying the push
covering the remaining false positives in that binary until that demo is
done. I've accumulated a pretty huge backlog of minor maintenance issues
by now…
Next up though: The first part of the long-awaited build system
improvements. I've finally come up with a way of sanely accelerating the
32-bit build part on most setups you could possibly want to build ReC98
on, without making the building experience worse for the other few setups.
… 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!
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
the Stage 2 star particles,
Alice's puppets,
the tip of curve ("jello") bullets,
Mai's snowballs and Yuki's fireballs,
Yumeko's swords,
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.
At least it was
possible to decompile the core blitting function as 
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…
