Sonic CD decompilation

The original source code assumes that an int is 4 bytes for accessing the "scratch RAM" part of each sprite_status struct, so compiling it as 64-bit will make it malfunction. I do plan to rectify this in my version down the line, but the priority is getting everything to work.

Thanks for the report! I've corrected it.

 

The time warp sequence has been implemented!

I thought it'd be done quick, but instead of calling SetGrid to place tiles on planes, it relies on the control program loading a binary file that contains the layout information. The function was already decompiled, but without symbols I hadn't been able to name much at the time. But now I could.

Then I struggled with the palette, as the fading wasn't working, and the sprites were white. Until it dawned on me that for fading to work, I had to account for it in the calling loop, just like for stages. Once that was fixed, everything looked correct.

 

You can get the exact same sprite, tile, and level data from the PC version.

For an alternative when it comes to background music, I can add support for using CD audio, but only for the SDL1 version. Other solutions would be more involved, like ripping the BGM from the CD and converting them to MP3 or OGG Vorbis or something, and adding support for that format.

 

The current timed delay implementation is something I quickly cobbled together. I intend to replace it with the implementation I used in Rockman Copy System.

Thanks for the patch. Not being familiar with SDL2, I didn't know vsync was an option.

You'd recommend to move the timer-related code from timer.c to graphics.c?

 

Apologies for the wait. I've added the vsync code!

 

I noticed you used vsync both as a boolean and as a frame multiplier, so I split it into vsync and frame_multiplier. vsync will be either 0 or 1.

 

I want to compile this for the PlayStation Portable, but the PSPSDK maintainers have really dropped the ball when it comes to Windows. Basically, there is no Windows version: "On Windows the PSPDEV toolchain is run on Ubuntu running on Microsoft’s WSL.".

Back in 2020 they tried to make it a bit easier by allowing you to build a Docker image with the toolchain. However, this seems to have been abandoned in 2022, and I suppose that's why building it no longer works (I tried).

10 years ago I used MINPSPW (Minimalist PSP homebrew SDK for Windows). Luckily I kept a backup of that. After struggling to write a working makefile, I'm stuck with tons of linker errors. Here are some of them:

Code (Text):

1. error: stray '\6' in program
2. error: stray '\22' in program

There are also tons of warnings about ignored null characters.

I searched, but have yet to find out what the problem is.

 

The Special Stage is a tough cookie.

The Mega Drive has two tile planes, and using tile priority you can fake having three or four. During regular gameplay, Sonic CD uses plane B for the background, and uses tile priority to use plane A as two foreground planes.

For the PC version, they translated the plane concept to a "grid", and treat the tiles of plane A's high and low priority as their own grids. To set a tile, SetGrid is called, which makes sure that a tile can't be in the same place on the grid for plane A high priority as on the grid for plane A low priority.

Whoever ported the Special Stage mode must have been mad, because the result uses up to eight tile planes, depending on the Special Stage.

It uses SetGrid like the regular stages to set tiles, but that function has specific code for it, which places all tiles on plane 0 (plane A high priority). The other planes are static and populated by what I've called "plane layouts" that are loaded from files, which I already had to implement for the time warp sequence.

There's a table with initialisation information for all eight planes. It defines the width and height, in tiles, of each, as well as a third parameter. As it's never bigger than a Mega Drive's I ignored it. However, that third parameter is important: it defines where the plane starts vertically on the screen. So now I have to figure out how to hack that into my engine.

I can't get the stamp map (the Special Stage's floor) to render correctly. First, the numbers I'd get were way too high, causing it to try to retrieve pixels from outside the map. After limiting it to 4096, all I see is a sea of bumpers. I then stepped through the function that sets the stamp map. Found out I don't advance the data pointer. No change after fixing that; it's still reading zeroes. Then I found out that Sonic CD doesn't actually check if it opening the map file was successful. After placing the files there, at last Sonic isn't considered to be running to be running in water anymore, and I see... weird shapes. Sometimes they resemble the actual stage, but most of the time they don't.

Here's a short video of the work in progress:

 

I don't use any fancy tools to debug this. I just try to understand how it's supposed to work (which also involves cross-referencing decompiled code), and check things over. At the moment, the floor looks close to the original. The grass tile is light green instead of dark green, and I don't know why. I'm currently comparing the data, and have fixed a bunch of sprite errors, but it looks like the palettes match.

As for makefiles, I've got it figured out for now. But thanks for the offer.

So, should I boot into GNU/Linux to compile a PSP build?

 

Welcome to my Let's Play of the Special Stage:



I wanted to showcase the secret 8th special stage as well, but I pasted one too few special_stage calls in main(). Oops.

There are a couple graphics glitches left, but, you know, something had to be off so it wouldn't appear to just be a video of the official release, right?

• Starting from the second Time Stone, its palette is incorrect.
• Areas where I think there's supposed to be water have bumpers instead.