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SCHG How-to

Collide with water after being hurt

From Sonic Retro

(Original guide by MoDule)

In the main series 16-bit Sonic games, Sonic behaves strangely when getting hurt and then falling into water. He will retain his above water gravity even under water until he lands.

This guide will show how to have Sonic interact with water even when falling after getting hurt.

Applying the change

Sonic 2 SVN

First, locate Obj01_Hurt_Normal, which should look like this:

; loc_1B13A:
Obj01_Hurt_Normal:
	tst.b	routine_secondary(a0)
	bmi.w	Sonic_HurtInstantRecover
	jsr	(ObjectMove).l
	addi.w	#$30,y_vel(a0)
	btst	#6,status(a0)
	beq.s	+
	subi.w	#$20,y_vel(a0)
+
	cmpi.w	#-$100,(Camera_Min_Y_pos).w
	bne.s	+
	andi.w	#$7FF,y_pos(a0)
+
	bsr.w	Sonic_HurtStop
	bsr.w	Sonic_LevelBound
	bsr.w	Sonic_RecordPos
	bsr.w	Sonic_Animate
	bsr.w	LoadSonicDynPLC
	jmp	(DisplaySprite).l

Somewhere before the last line, add a branch to Sonic_Water, like so:

	bsr.w	Sonic_HurtStop
	bsr.w	Sonic_LevelBound
	bsr.w	Sonic_RecordPos
	bsr.w	Sonic_Water	; <--
	bsr.w	Sonic_Animate
	bsr.w	LoadSonicDynPLC
	jmp	(DisplaySprite).l

With this, Sonic should now behave as expected when falling into water, but still feels strange exiting it.He bounces awfully low, if he exits the water after getting hurt. To fix this, locate Obj01_OutWater and comment out a few line as shown here:

; loc_1A1FE:
Obj01_OutWater:
	bclr	#6,status(a0) ; unset underwater flag
	beq.s	return_1A18C ; if already above water, branch

	[...]

;	cmpi.b	#4,routine(a0)	; <-- is Sonic falling back from getting hurt?
;	beq.s	+		; <-- if yes, branch
	asl	y_vel(a0)
;+				; <--
	tst.w	y_vel(a0)
	beq.w	return_1A18C
	move.w	#$100,(Sonic_Dust+anim).w	; splash animation
	movea.l	a0,a1
	bsr.w	ResumeMusic
	cmpi.w	#-$1000,y_vel(a0)
	bgt.s	+
	move.w	#-$1000,y_vel(a0)	; limit upward y velocity exiting the water
+
	move.w	#SndID_Splash,d0	; splash sound
	jmp	(PlaySound).l
; End of subroutine Sonic_Water

With these lines commented out, Sonic will always get the speed boost when exiting water.

Things to keep in mind

This change in behavior needs to be applied to every character object in the game, not just Sonic.

Explanation

The reason Sonic doesn't interact with water when getting hurt is because the water handling routine is only called when he is in his normal state. The code for his hurt state lacks a call to this routine. Adding a branch to the water handling routine makes him interact with water almost as one would expect. The lines that are commented out in the second part of the guide are code that checks if Sonic is in his Hurt state, something that was originally impossible, and skips the speed boost he would usually get from exiting the water. With the lower y-velocity he gets from being hurt under water, the missing boost and higher gravity above water explains why he falls back into the water almost immediately.

SCHG How-To Guide: Sonic the Hedgehog 2 (16-bit)
Fixing Bugs
Fix Demo Playback | Fix a Race Condition with Pattern Load Cues | Fix Super Sonic Bugs | Use Correct Height When Roll Jumping | Fix Jump Height Bug When Exiting Water | Fix Spin Dash Code and Add Spin Dash Speeds | Fix Screen Boundary Spin Dash Bug | Correct Drowning Bugs | Fix Camera Y Position for Tails | Fix Tails Subanimation Error | Fix Tails' Respawn Speeds | Fix Accidental Deletion of Scattered Rings | Fix Ring Timers | Fix Rexon Crash | Fix Monitor Collision Bug | Fix EHZ Deformation Bug | Correct CPZ Boss Attack Behavior | Fix Bug in ARZ Boss Arrow's Platform Behavior | Fix ARZ Boss Walking on Air Glitch | Fix ARZ Boss Sprite Behavior | Fix Multiple CNZ Boss Bugs | Fix HTZ Background Scrolling Mountains | Fix OOZ Launcher Speed Up Glitch | Fix DEZ Giant Mech Collision Glitch | Fix Boss Deconstruction Behavior | Fix Speed Bugs
Design Choices
Remove the Air Speed Cap | Disable Floor Collision While Dying | Modify Super Sonic Transformation Methods & Behavior | Enable/Disable Tails in Certain Levels | Collide with Water After Being Hurt | Retain Rings When Returning at a Star Post | Improve the Fade In\Fade Out Progression Routines | Fix Scattered Rings' Underwater Physics | Insert LZ Water Ripple Effect | Restore Lost CPZ Boss Feature | Prevent SCZ Tornado Spin Dash Death | Improve ObjectMove Subroutines | Port S3K Rings Manager | Port S3K Object Manager | Port S3K Priority Manager | Edit Level Order with ASM‎ | Alter Ring Requirements in Special Stages | Make Special Stage Characters Use Normal DPLCs | Speed Up Ring Loss Process
Adding Features
Create Insta-kill and High Jump Monitors | Create Clone and Special Stage Monitors | Port Knuckles
Sound Features
Port Sonic 1 Sound Driver | Port Sonic 2 Clone Driver | Port Sonic 3 Sound Driver | Expand the Music Index to Start at $00 (Sonic 2 Clone Driver Version)
Extending the Game
Extend the Level Index Past $10 | Extend the Level Select | Extend Water Tables | Add Extra Characters | Free Up 2 Universal SSTs