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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20h

Seems like the devs did intend for HCZ to use the gradual H-BLANK function, as the color order table does get set. However, when I enabled it, it seemed fine for a bit, but then the game completely froze. I'll definitely be investigating more.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20h

Carnival Night Act 2 and Launch Base Act 2 make pretty prominent use of water, so I doubt that's the case. Hydrocity does have that 3D-like effect for the water surface, but so does Launch Base.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 22h

A technical dive (😉) into Sonic 3's water palette swapping!

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 22/08/2026

It doesn't apply to the USA version due to the demos being rerecorded, and thus the demo programs were built off of a newer version without the prototype logic.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 22/08/2026

Due to the demo programs in the Japanese and PAL versions (not USA) of Sonic CD being built off of a prototype, they retain the logic for enabling debug mode by pressing start on player 2's controller. You can do this so that you don't need to enter the code in the sound test.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 17/04/2026

In this case, implementing the depth check for when Sonic is moving horizontally, while fixing the issue I highlighted, would make this clip even easier. The proper fix for this would be to also check if the surface Sonic is colliding with is merely top solid, and only perform the check if it is.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 17/04/2026

However, in Lava Reef Zone, there's a set of fully solid stairs that Sonic can clip into in the same fashion, but because he'll get stopped by the next step, he'll start moving down, causing the depth check to take effect, allowing him to fall right through. (GIF courtesy of Flamewing)

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 17/04/2026

If you mean like this, then, no. The clip is possible, because floor collision is ignored when he's moving upwards, and the wall sensor is too high up in this instance to trigger a wall collision. He also isn't deep enough into the platform for the depth check to cause floor collision to be ignored.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 16/04/2026

This is what the missing check does, which is present when mostly falling downwards and less horizontally.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 16/04/2026

There are 4 collision modes while airborne, 1 for each cardinal direction. When he hits the rock, his Y speed is cancelled out while still moving right. Since he's technically colliding with the floor and the check isn't done, he clips on top of the surface, right into the rock.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 16/04/2026

There exists a collision bug where the check used to prevent Sonic from clipping on top of a surface he's too far below from isn't done if he's moving horizontally in the air. This can easily be seen if you put a solid object on a top solid surface and hit it from the bottom.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 02/03/2026

Here's a silly difference between the 510 and 0.28A (512) prototypes of Sonic CD. Special stage rings always appear in 510, but never in 0.28A. 0.28A added a ring count check, but only checked the top byte. As a result, they can only appear if the player has at least 12800 rings.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 27/02/2026

(You can find the hack here: sonicresearch.org/community/in...)

sonicresearch.org

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 27/02/2026

There's a ROM hack of Chaotix called Knuckles' Clackers by GenesisDoes that reimplements loops, and even uses the masking sprite... but it still illustrates the problematic nature of loops with the tethering mechanic.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 27/02/2026

Small fun fact, there's an unused sprite in the final build that would have acted as a sprite mask by drawing on top of the character sprites, but set to use a low priority color so that it would appear behind the loop, which is drawn by the Genesis.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 27/02/2026

Loops were planned on being implemented in Chaotix, even being present in prototypes... yeah, I can see why they were removed (footage provided by @mdtravis.bsky.social)

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 24/02/2026

Seems like you can mess around with the peelout to get the collision to register, though.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 24/02/2026

I do know you can glitch the physics in the 3D loop to get you to into that area... maybe it's just a failsafe in case that section goes wrong? Can't say for sure, though.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 24/02/2026

No, you can't time travel in this area. In the present, the camera will refuse to go back once you've gone past it, and if you are in the past or future, time travel gets disabled.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 24/02/2026

Curiously, there's a couple of hidden springs around here as well, but they cannot be touched, because they are completely obscured by the ramp.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 24/02/2026

There's a hidden tunnel that can be found right towards the end of the 3D loop section at the beginning of Palmtree Panic act 1 that I'm not sure is even accessible via normal means. Anyone have any ideas?

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Reposted by Under the Surface of Classic Sonic

Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20/02/2026

The Metallic Madness act 2 demo was, indeed, rerecorded as well. The USA demo struggles while the Japanese demo gets quite a bit further ahead. Left: Japan/Europe, Right: USA

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20/02/2026

The Metallic Madness act 2 demo was, indeed, rerecorded as well. The USA demo struggles while the Japanese demo gets quite a bit further ahead. Left: Japan/Europe, Right: USA

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Reposted by Under the Surface of Classic Sonic

Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20/02/2026

The Tidal Tempest act 3 demo was also rerecorded. What I find peculiar is how whoever recorded the USA demo slowly walked to the boss while the Japanese demo caught up. Were they trying to eyeball the Japanese demo? I dunno... Left: Japan/Europe, Right: USA

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20/02/2026

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20/02/2026

Note the different timings. With the special stages, the demo ends when the controller data reaches an entry with the start button pressed. The normal stage demos are fixed at 2048 frames.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20/02/2026

In the PAL version of Sonic CD, the physics of Sonic were modified to more closely resemble the NTSC physics while at a lower frame rate. As such, separate demo controller data had to be recorded for the special stage demos. Special Stage 1 Left: NTSC, Right: PAL

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 20/02/2026

For some reason, the demo for Palmtree Panic act 1 was rerecorded for the USA release of Sonic CD. Here's a comparison showing the differences. Left: Japan/Europe, Right: USA

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 18/02/2026

Thanks for the 200 followers :) Sorry that I've not been posting as much here. I admittedly don't really use Bluesky and I keep forgetting about this account. I should find the time to bring this account up to date.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 18/02/2026

This also doesn't apply to the special stages, but the special stages don't use separate files for the demos like the regular levels do, so there's not really anything to look into for that.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 18/02/2026

Note that you may find some parts of the level corrupted. This is because part of the level data is overwritten with the demo controller data, with the demo avoiding showing those corrupted parts.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 18/02/2026

A few ways to play and see it: 1. Modify the disc image to set the byte at 0x2A75353 from 1 to 0 (712 prototype only) 2. Modify RAM in an emulator to set address 0xFF1581 to 0 (works in final) 3. Use PAR code FF1581:00 or Game Genie code AAL9-8AEB in an emulator (works in final)

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 18/02/2026

Additional fun fact: the sprite here was repurposed for the Backup RAM manager.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 18/02/2026

This is because the demo files that get run during the title screen were built at an earlier time than the normal stage files found on the disc. This pattern of the demos being from earlier builds is also found in the final game.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 18/02/2026

Thanks to c4/seafour4 on the Hidden Palace Discord server, a new prototype signpost sprite was discovered in the Palmtree Panic act 1 demo (DEMO11A.MMD) in the 712 prototype of Sonic CD, which was previously only seen in a magazine.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 16/10/2025

I think what's interesting about the "Player shock jump up" line is the fact they deliberately used the unsigned byte representation of -2 (0xFE) when setting Mario's Y speed...

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 16/10/2025

Additionally, you can see the removed lines of code in the official All Stars source code!

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 16/10/2025

From @nfroggy99 on Twitter/X: "When you look at the All-Stars source code, the equivalent subroutine to BrickShatter (BBKINT) says "Player shock jump up", so you're right that this behavior was intended. I'm glad that they accidentally screwed it up because the All-Stars behavior feels worse."

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 15/10/2025

It should be noted that the temporary invisible solid block is what keeps the gap solid while the block bouncing animation plays, and is also what actually kills the enemies when you hit a block from under them.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 15/10/2025

More Mario content? Oh no! Anyways... so, you know how in Mario 1 on the NES, you bounce downwards when you destroy a brick, but hover upwards in the SNES version? Turns out the SNES behavior was possibly the intended behavior? Here's how it works.

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Under the Surface of Classic Sonic @classicsoniccode.bsky.social · 12/10/2025

This was fixed in Sonic 3 by also applying the modified ground speed cap logic to the air physics.

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