Sonic CD decompilation

I got Collision Chaos to compile. Here's a first look:

 

Let's dive into Tidal Tempest!



Looks like there are some issues to fix.

 

No Quartz Quadrant yet, so here's a progress update on Tidal Tempest.

The cause of the missing background tiles is because after setting up a lookup table I forgot to call cg_change. However, the resulting changing background tiles are incorrect, and I haven't been able to figure out what I'm doing wrong. R32 also has changing background tiles, and they're wrong there, as well. I didn't notice before because the ones in R32 show high in the middle of the level.

There were no water currents due to forgetting to call watercoli. But the currents don't work properly. It seems as if Sonic is only in his floating state for one frame. I found one error in watercoli, but correcting it didn't fix the issue.

Next, I went through all the objects so they'd select the correct sprite ids depending on the loaded level. Tidal Tempest has a lot more objects, so this was a pain.

After fixing Sonic falling through the floor in R43D (error in collision map lookup table), I played through the level. The first phase of Robotnik's boss fight, his escape routes are wrong somehow. The second phase executes correctly.

No progress on the underwater palette always being applied.

 

This version is now officially better than Sonic Gems Collection's; the water palette has been implemented! The water currents aren't playing nice yet, however.



Technical explanation:

The game maintains a global variable called hintposi (or: horizontal interrupt position). It contains the scanline number where a H-interrupt occurs. In the original, a H-interrupt is used to change the current palette to the water palette while the screen is being drawn.

In the C version, we do the drawing ourselves, so we just need to know where to switch palettes, or if the entire screen is submerged (by reading waterflag). Switching palettes between colorwk and colorwk3 is as easy as incrementing every pixel by 64.

I'm not sure why Sonic Gems Collection didn't implement the water palette. At first I thought it was because they likely didn't have access to RDX's source code. But that doesn't make sense, because, if that's the case, they had to reverse engineer all the other drawing routines as well. Maybe they just forgot. It's only used in one round of the game, after all.

 

Ran out of ideas for the water currents, so I moved on to Quartz Quadrant, which I just got to compile. Here's a first look:



Cleaning this up should be faster, as this round has a normal amount of objects.

 

Yes, you will be able to choose the soundtrack.

 

I intend to correct what I can, but I'm not familiar with all the differences (yet).

 

Quartz Quadrant was already very tidy. I only had to clean up two objects!

On to Wacky Workbench!

 

Stardust Speedway is already here!

 

At last, here's Metallic Madness! Though it seems I can't proceed past a certain point. ...Wait, I just remembered that this round has its own version of SCRCHK. That's probably why. Enjoy the WIP!

 

Sonic was indeed being crushed by a piston.

I don't like the FM jump sound, either. But it should be possible to replace it. Sonic & Knuckles Collection's sound effects seem to have the same format.

I'm done with Metallic Madness!

Also, thanks to a fellow reverse engineer, I can now compile a PS2 ELF that's really close to the real thing, so I could verify my decompilation in even more detail. This made me correct some more errors, including those that caused the water currents in Tidal Tempest to misbehave. They now work perfectly!

Next I'll tackle either the title screen or the Special Stage. Both will involve the implementation of the Mega CD's scaling capabilities. Not sure how I will tackle that.

 

Not only were the different screens rewritten, they seem to call the graphics API (HMX?) directly instead of telling the control program what and where it wants something drawn. This might take some time to figure out.