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📝 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 up to 8 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:
P0120, P0121
453dd3c...3c008b6, 3c008b6...5c42fcd
💰 Funded by:
🏷 Tags:
rec98+ th01+ pc98- blitting+ waste- jank+ boss+ mima-th01+

Back to TH01, and its boss sprite format… with a separate class for storing animations that only differs minutely from the 📝 regular boss entity class I covered last time? Decompiling this class was almost free, and the main reason why the first of these pushes ended up looking pretty huge.

Next up were the remaining shape drawing functions from the code segment that started with the .GRC functions. P0105 already started these with the (surprisingly sanely implemented) 8×8 diamond, star, and… uh, snowflake (?) sprites , prominently seen in the Konngara, Elis, and Sariel fights, respectively. Now, we've also got:

  • ellipse arcs with a customizable angle distance between the individual dots – mostly just used for drawing full circles, though
  • line loops – which are only used for the rotating white squares around Mima, meaning that the white star in the YuugenMagan fight got a completely redundant reimplementation
  • and the surprisingly weirdest one, drawing the red invincibility sprites.
The weirdness becomes obvious with just a single screenshot:

First, we've got the obvious issue of the sprites not being clipped at the right edge of VRAM, with the rightmost pixels in each row of the sprite extending to the beginning of the next row. Well, that's just what you get if you insist on writing unique low-level blitting code for the majority of the individual sprites in the game… 🤷
More importantly though, the sprite sheet looks like this: So how do we even get these fully filled red diamonds?

Well, turns out that the sprites are never consistently unblitted during their 8 frames of animation. There is a function that looks like it unblits the sprite… except that it starts with by enabling the GRCG and… reading from the first bitplane on the background page? If this was the EGC, such a read would fill some internal registers with the contents of all 4 bitplanes, which can then subsequently be blitted to all 4 bitplanes of any VRAM page with a single memory write. But with the GRCG in RMW mode, reads do nothing special, and simply copy the memory contents of one bitplane to the read destination. Maybe ZUN thought that setting the RMW color to red also sets some internal 4-plane mask register to match that color? :zunpet:
Instead, the rather random pixels read from the first bitplane are then used as a mask for a second blit of the same red sprite. Effectively, this only really "unblits" the invincibility pixels that are drawn on top of Reimu's sprite. Since Reimu is drawn first, the invincibility sprites are overwritten anyway. But due to the palette color layout of Reimu's sprite, its pixels end up fully masking away any invincibility sprite pixels in that second blit, leaving VRAM untouched as a result. Anywhere else though, this animation quickly turns into the union of all animation frames.

Then again, if that 16-dot-aligned rectangular unblitting function is all you know about the EGC, and you can't be bothered to write a perfect unblitter for 8×8 sprites, it becomes obvious why you wouldn't want to use it:

Because Reimu would barely be visible under all that flicker. In comparison, those fully filled diamonds actually look pretty good.

After all that, the remaining time wouldn't have been enough for the next few essential classes, so I closed out the push with three more VRAM effects instead:

  • Single-bitplane pixel inversion inside a 32×32 square – the main effect behind the discoloration seen in the bomb animation, as well as the exploding squares at the end of Kikuri's and Sariel's entrance animation
  • EGC-accelerated VRAM row copies – the second half of smooth and fully hardware-accelerated scrolling for backgrounds that are twice the size of VRAM
  • And finally, the VRAM page content transition function using meshed 8×8 squares, used for the blocky transition to Sariel's first and second phases. Which is quite ridiculous in just how needlessly bloated it is. I'm positive that this sort of thing could have also been accelerated using the PC-98's EGC… although simply writing better C would have already gone a long way. The function also comes with three unused mesh patterns.

And with that, ReC98, as a whole, is not only ⅓ done, but I've also fully caught up with the feature backlog for the first time in the history of this crowdfunding! Time to go into maintenance mode then, while we wait for the next pushes to be funded. Got a huge backlog of tiny maintenance issues to address at a leisurely pace, and of course there's also the 📝 16-bit build system waiting to be finished.