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P0212, P0213
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rec98 th01 score pc98 performance unused cutscene debug

Wow, it's been 3 days and I'm already back with an unexpectedly long post about TH01's bonus point screens? 3 days used to take much longer in my previous projects…

Before I talk about graphics for the rest of this post, let's start with the exact calculations for both bonuses. Touhou Wiki already got these right, but it still makes sense to provide them here, in a format that allows you to cross-reference them with the source code more easily. For the card-flipping stage bonus:

Time min((Stage timer * 3), 6553)
Continuous min((Highest card combo * 100), 6553)
Bomb&Player min(((Lives * 200) + (Bombs * 100)), 6553)
STAGE min(((Stage number - 1) * 200), 6553)
BONUS Point Sum of all above values * 10

The boss stage bonus is calculated from the exact same metrics, despite half of them being labeled differently. The only actual differences are in the higher multipliers and in the cap for the stage number bonus. Why remove it if raising it high enough also effectively disables it? :tannedcirno:

Time min((Stage timer * 5), 6553)
Continuous min((Highest card combo * 200), 6553)
MIKOsan min(((Lives * 500) + (Bombs * 200)), 6553)
Clear min((Stage number * 1000), 65530)
TOTLE Sum of all above values * 10

The transition between the gameplay and TOTLE screens is one of the more impressive effects showcased in this game, especially due to how wavy it often tends to look. Aside from the palette interpolation (which is, by the way, the first time ZUN wrote a correct interpolation algorithm between two 4-bit palettes), the core of the effect is quite simple. With the TOTLE image blitted to VRAM page 1:

So it's really more like two interlaced shift effects with opposite directions, starting on different scanlines. No trigonometry involved at all.

Horizontally scrolling pixels on a single VRAM page remains one of the few 📝 appropriate uses of the EGC in a fullscreen 640×400 PC-98 game, regardless of the copied block size. The few inter-page copies in this effect are also reasonable: With 8 new lines starting on each effect frame, up to (8 × 20) = 160 lines are transferred at any given time, resulting in a maximum of (160 × 2 × 2) = 640 VRAM page switches per frame for the newly transferred pixels. Not that frame rate matters in this situation to begin with though, as the game is doing nothing else while playing this effect.
What does sort of matter: Why 32 pixels every 2 frames, instead of 16 pixels on every frame? There's no performance difference between doing one half of the work in one frame, or two halves of the work in two frames. It's not like the overhead of another loop has a serious impact here, especially with the PC-98 VRAM being said to have rather high latencies. 32 pixels over 2 frames is also harder to code, so ZUN must have done it on purpose. Guess he really wanted to go for that 📽 cinematic 30 FPS look 📽 here… :zunpet:

Removing the palette interpolation and transitioning from a black screen to CLEAR3.GRP makes it a lot clearer how the effect works.

Once all the metrics have been calculated, ZUN animates each value with a rather fancy left-to-right typing effect. As 16×16 images that use a single bright-red color, these numbers would be perfect candidates for gaiji… except that ZUN wanted to render them at the more natural Y positions of the labels inside CLEAR3.GRP that are far from aligned to the 8×16 text RAM grid. Not having been in the mood for hardcoding another set of monochrome sprites as C arrays that day, ZUN made the still reasonable choice of storing the image data for these numbers in the single-color .GRC form– yeah, no, of course he once again chose the .PTN hammer, and its 📝 16×16 "quarter" wrapper functions around nominal 32×32 sprites.

.PTN sprite for the TOTLE metric digits of 0, 1, 2, and 3.PTN sprite for the TOTLE metric digits of 4, 5, 6, and 7 .PTN sprite for the TOTLE metric digits of 8 and 9, filled with two blank quarters
The three 32×32 TOTLE metric digit sprites inside NUMB.PTN.

Why do I bring up such a detail? What's actually going on there is that ZUN loops through and blits each digit from 0 to 9, and then continues the loop with "digit" numbers from 10 to 19, stopping before the number whose ones digit equals the one that should stay on screen. No problem with that in theory, and the .PTN sprite selection is correct… but the .PTN quarter selection isn't, as ZUN wrote (digit % 4) instead of the correct ((digit % 10) % 4). :onricdennat: Since .PTN quarters are indexed in a row-major way, the 10-19 part of the loop thus ends up blitting 23016745(nothing):

This footage was slowed down to show one sprite blitting operation per frame. The actual game waits a hardcoded 4 milliseconds between each sprite, so even theoretically, you would only see roughly every 4th digit. And yes, we can also observe the empty quarter here, only blitted if one of the digits is a 9.

Seriously though? If the deadline is looming and you've got to rush some part of your game, a standalone screen that doesn't affect anything is the best place to pick. At 4 milliseconds per digit, the animation goes by so fast that this quirk might even add to its perceived fanciness. It's exactly the reason why I've always been rather careful with labeling such quirks as "bugs". And in the end, the code does perform one more blitting call after the loop to make sure that the correct digit remains on screen.

The remaining ¾ of the second push went towards transferring the final data definitions from ASM to C land. Most of the details there paint a rather depressing picture about ZUN's original code layout and the bloat that came with it, but it did end on a real highlight. There was some unused data between ZUN's non-master.lib VSync and text RAM code that I just moved away in September 2015 without taking a closer look at it. Those bytes kind of look like another hardcoded 1bpp image though… wait, what?!

An unused mouse cursor sprite found in all of TH01's binaries

Lovely! With no mouse-related code left in the game otherwise, this cursor sprite provides some great fuel for wild fan theories about TH01's development history:

  1. Could ZUN have 📝 stolen the basic PC-98 VSync or text RAM function code from a source that also implemented mouse support?
  2. Did he have a mouse-controlled level editor during development? It's highly likely that he had something, given all the 📝 bit twiddling seen in the STAGE?.DAT format.
  3. Or was this game actually meant to have mouse-controllable portions at some point during development? Even if it would have just been the menus.

… Actually, you know what, with all shared data moved to C land, I might as well finish FUUIN.EXE right now. The last secret hidden in its main() function: Just like GAME.BAT supports launching the game in a debug mode from the DOS command line, FUUIN.EXE can directly launch one of the game's endings. As long as the MDRV2 driver is installed, you can enter fuuin t1 for the 魔界/Makai Good Ending, or fuuin t for 地獄/Jigoku Good Ending.
Unfortunately, the command-line parameter can only control the route. Choosing between a Good or Bad Ending is still done exclusively through TH01's resident structure, and the continues_per_scene array in particular. But if you pre-allocate that structure somehow and set one of the members to a nonzero value, it would work. Trainers, anyone?

Alright, gotta get back to the code if I want to have any chance of finishing this game before the 15th… Next up: The final 17 functions in REIIDEN.EXE that tie everything together and add some more debug features on top.