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SCHG How-to:Fix Scattered Rings Underwater Physics

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m (→Sonic 3&K Fix: Header change)
m (Added a fix for SVN users)
Line 1: Line 1:
−
''(Guide for all 3 games written by [[redhotsonic]].)''
+
''(Original Guide for all 3 games written by [[redhotsonic]].)''
  
 
Quote from ''redhotsonic'', - "Ah, the scattered rings when you get hurt. It can cause slowdown to your game, and even mess up the underwater palette. Now, when Sonic is underwater, his movement and his gravity is changed so he moves slower to act like he's in water. But when he is hurt under water, isn't it funny that the scattered rings still act the same as if you weren't in water?"
 
Quote from ''redhotsonic'', - "Ah, the scattered rings when you get hurt. It can cause slowdown to your game, and even mess up the underwater palette. Now, when Sonic is underwater, his movement and his gravity is changed so he moves slower to act like he's in water. But when he is hurt under water, isn't it funny that the scattered rings still act the same as if you weren't in water?"
Line 119: Line 119:
  
 
==Sonic 2 fix==
 
==Sonic 2 fix==
−
''This guide uses the Xenowhirl disassembly.''
+
''This guide uses the Xenowhirl disassembly. See below for the SVN fix.''
  
 
First, go to "''loc_120BA:''" (part of the scattered rings object code) and find this bit of coding:
 
First, go to "''loc_120BA:''" (part of the scattered rings object code) and find this bit of coding:
Line 244: Line 244:
 
This ensures when underwater, the gravity is not as great. Otherwise, the rings will gain speed and then it won't give the desired underwater effect.
 
This ensures when underwater, the gravity is not as great. Otherwise, the rings will gain speed and then it won't give the desired underwater effect.
  
 +
===Sonic 2 SVN fix===
 +
(SVN fix by [[KingofHarts]])
 +
 +
There is ONE small hiccup when trying to apply this to the SVN disassembly of Sonic 2. If you go ahead and apply the above changes to the corresponding code, and build & run, you will find that there is a SERIOUS error with your ring loss. This is due to the way that the SVN disassembly is labeled, compared to the original Xenowhirl disassembly, and is a REALLY EASY thing to fix. I'll take a second to show you what you need to do. After you have finished applying the steps above, please go back to the first section you edited. It should now look something like this, perhaps minus the extra comments I made to guide me.
 +
<asm>
 +
; loc_120B2:
 +
- bsr.w SingleObjLoad
 +
bne.w +++
 +
; loc_120BA:
 +
+
 +
_move.b #ObjID_LostRings,id(a1) ; load obj37
 +
addq.b #2,routine(a1)
 +
move.b #8,y_radius(a1)
 +
move.b #8,x_radius(a1)
 +
move.w x_pos(a0),x_pos(a1)
 +
move.w y_pos(a0),y_pos(a1)
 +
move.l #Obj25_MapUnc_12382,mappings(a1)
 +
move.w #make_art_tile(ArtTile_ArtNem_Ring,1,0),art_tile(a1)
 +
bsr.w Adjust2PArtPointer2
 +
move.b #$84,render_flags(a1)
 +
move.b #3,priority(a1)
 +
move.b #$47,collision_flags(a1)
 +
move.b #8,width_pixels(a1)
 +
move.b #-1,(Ring_spill_anim_counter).w
 +
tst.w d4
 +
bmi.s + ; <- This line is incorrectly branching.
 +
move.w d4,d0
 +
bsr.w JmpTo4_CalcSine
 +
move.w d4,d2
 +
lsr.w #8,d2
 +
; RHS FIX
 +
        tst.b  (Water_flag).w          ; Does the level have water?
 +
        beq.s  +                      ; If not, branch and skip underwater checks
 +
        move.w  (Water_Level_2).w,d6    ; Move water level to d6
 +
        cmp.w  y_pos(a0),d6            ; Is the ring object underneath the water level?
 +
        bgt.s  +                      ; If not, branch and skip underwater commands
 +
        asr.w  #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
 +
        asr.w  #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
 +
+
 +
; RHS FIX
 +
        asl.w d2,d0
 +
asl.w d2,d1
 +
move.w d0,d2
 +
move.w d1,d3
 +
addi.b #$10,d4
 +
bcc.s + ; <- This line is branching correctly, BUT we will need to change it.
 +
subi.w #$80,d4
 +
bcc.s + ; <- This line is branching correctly, BUT we will need to change it.
 +
move.w #$288,d4
 +
;loc_12132
 +
+
 +
move.w d2,x_vel(a1)
 +
move.w d3,y_vel(a1)
 +
...
 +
</asm>
 +
 +
Now, as you can see, the SVN has relabeled a LOT of these sections of code with "+". This is all well and good, BUT it causes problems for us when applying this fix. Note the first line I marked. It is supposed to branch to the former "loc_12132". Instead it branches to "+". But there is a "+" section BEFORE that, causing the issue. My fix is to relabel "loc_12132" back to its former label, and then I want you to edit all 3 lines I marked, and have them branch to this label. That's it! Everything SHOULD work out fine now for your SVN disassembly.
  
 
==Sonic 3&K fix==
 
==Sonic 3&K fix==

Revision as of 01:34, 7 August 2012

(Original Guide for all 3 games written by redhotsonic.)

Quote from redhotsonic, - "Ah, the scattered rings when you get hurt. It can cause slowdown to your game, and even mess up the underwater palette. Now, when Sonic is underwater, his movement and his gravity is changed so he moves slower to act like he's in water. But when he is hurt under water, isn't it funny that the scattered rings still act the same as if you weren't in water?"

This is something that occurs in all 3 Sonic the Hedgehog Mega Drive games (and by 3, that IS counting S3&K as one game, though S&K has no water anyway, so it wouldn't be included regardless... but I digress). But is it really a bug? or is it merely a design choice? In either case, it's something that could stand to be fixed, and this guide tells you how to fix the ring physics in all of the games. First off... let's start with the original...


Contents

Sonic 1 fix

This guide uses the SVN disassembly.

First, In your disassembly, go to "_incObj\25 & 37 Rings.asm" Go to @makerings: (part of the scattered rings object code) and find this bit of coding: 

 
     bsr.w   CalcSine
     move.w  d4,d2
     lsr.w   #8,d2
 

Just underneath, paste this: 

 
                tst.b   ($FFFFF64C).w           ; Does the level have water?
                beq.s   @skiphalvingvel         ; If not, branch and skip underwater checks
                move.w  ($FFFFF646).w,d6        ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   @skiphalvingvel         ; If not, branch and skip underwater commands
                asr.w   #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
                asr.w   #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
@skiphalvingvel:
 

'You should end up with something looking like this:' 

 
@makerings:
                move.b  #id_RingLoss,0(a1) ; load bouncing ring object
                addq.b  #2,obRoutine(a1)
                move.b  #8,obHeight(a1)
                move.b  #8,obWidth(a1)
                move.w  obX(a0),obX(a1)
                move.w  obY(a0),obY(a1)
                move.l  #Map_Ring,obMap(a1)
                move.w  #$27B2,obGfx(a1)
                move.b  #4,obRender(a1)
                move.b  #3,obPriority(a1)
                move.b  #$47,obColType(a1)
                move.b  #8,obActWid(a1)
                move.b  #-1,(v_ani3_time).w
                tst.w   d4
                bmi.s   @loc_9D62
                move.w  d4,d0
                bsr.w   CalcSine
                move.w  d4,d2
                lsr.w   #8,d2
                tst.b   ($FFFFF64C).w           ; Does the level have water?
                beq.s   @skiphalvingvel         ; If not, branch and skip underwater checks
                move.w  ($FFFFF646).w,d6        ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   @skiphalvingvel         ; If not, branch and skip underwater commands
                asr.w   #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
                asr.w   #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
@skiphalvingvel:
                asl.w   d2,d0
                asl.w   d2,d1
                move.w  d0,d2
                move.w  d1,d3
                addi.b  #$10,d4
                bcc.s   @loc_9D62
                subi.w  #$80,d4
                bcc.s   @loc_9D62
                move.w  #$288,d4
 

This ensures that when underwater, the rings x_vel and y_vel are halved so they appear to be moving slower.

Next, go to label "RLoss_Bounce:" and under 

 
                move.b  (v_ani3_frame).w,obFrame(a0)
                bsr.w   SpeedToPos
                addi.w  #$18,obVelY(a0)
 

place this: 

 
                tst.b   ($FFFFF64C).w           ; Does the level have water?
                beq.s   @skipbounceslow         ; If not, branch and skip underwater checks
                move.w  ($FFFFF646).w,d6        ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   @skipbounceslow         ; If not, branch and skip underwater commands
                subi.w  #$E,obVelY(a0)          ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect
@skipbounceslow:
 

You can change the value of the gravity if you like, but I found $E to suit. Anyway, you should have something looking like this: 

 
RLoss_Bounce:   ; Routine 2
                move.b  (v_ani3_frame).w,obFrame(a0)
                bsr.w   SpeedToPos
                addi.w  #$18,obVelY(a0)
                tst.b   ($FFFFF64C).w           ; Does the level have water?
                beq.s   @skipbounceslow         ; If not, branch and skip underwater checks
                move.w  ($FFFFF646).w,d6        ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   @skipbounceslow         ; If not, branch and skip underwater commands
                subi.w  #$E,obVelY(a0)          ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect
@skipbounceslow:
                bmi.s   @chkdel
                move.b  (v_vbla_byte).w,d0
                add.b   d7,d0
                andi.b  #3,d0
                bne.s   @chkdel
                jsr     ObjFloorDist
                tst.w   d1
                bpl.s   @chkdel
                add.w   d1,obY(a0)
                move.w  obVelY(a0),d0
                asr.w   #2,d0
                sub.w   d0,obVelY(a0)
                neg.w   obVelY(a0)
 

This ensures when underwater, the gravity is not as great. Otherwise, the rings will gain speed and then it won't give the desired underwater effect.


Sonic 2 fix

This guide uses the Xenowhirl disassembly. See below for the SVN fix.

First, go to "loc_120BA:" (part of the scattered rings object code) and find this bit of coding: 

 
  bsr.w JmpTo4_CalcSine
  move.w d4,d2
  lsr.w #8,d2
 


Just underneath, paste this: 

 
        tst.b   (Water_flag).w          ; Does the level have water?
        beq.s   +                       ; If not, branch and skip underwater checks
        move.w  (Water_Level_2).w,d6    ; Move water level to d6
        cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
        bgt.s   +                       ; If not, branch and skip underwater commands
        asr.w   #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
        asr.w   #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
+
 


So, you should end up with something looking like this: 

 
loc_120BA:
        _move.b #$37,0(a1) ; load obj37
        addq.b  #2,routine(a1)
        move.b  #8,y_radius(a1)
        move.b  #8,x_radius(a1)
        move.w  x_pos(a0),x_pos(a1)
        move.w  y_pos(a0),y_pos(a1)
        move.l  #Obj25_MapUnc_12382,mappings(a1)
        move.w  #$26BC,art_tile(a1)
        bsr.w   Adjust2PArtPointer2
        move.b  #$84,render_flags(a1)
        move.b  #3,priority(a1)
        move.b  #$47,collision_flags(a1)
        move.b  #8,width_pixels(a1)
        move.b  #-1,(Ring_spill_anim_counter).w
        tst.w   d4
        bmi.s   loc_12132
        move.w  d4,d0
        bsr.w   JmpTo4_CalcSine
        move.w  d4,d2
        lsr.w   #8,d2
        tst.b   (Water_flag).w          ; Does the level have water?
        beq.s   +                       ; If not, branch and skip underwater checks
        move.w  (Water_Level_2).w,d6    ; Move water level to d6
        cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
        bgt.s   +                       ; If not, branch and skip underwater commands
        asr.w   #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
        asr.w   #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
+
        asl.w   d2,d0
        asl.w   d2,d1
        move.w  d0,d2
        move.w  d1,d3
        addi.b  #$10,d4
        bcc.s   loc_12132
        subi.w  #$80,d4
        bcc.s   loc_12132
        move.w  #$288,d4
 

This ensures that when underwater, the rings x_vel and y_vel are halved so they appear to be moving slower.


Next, go to label "Obj_37_sub_2:" and under 

 
        move.b  (Ring_spill_anim_frame).w,mapping_frame(a0)
        bsr.w   ObjectMove
        addi.w  #$18,y_vel(a0)
 


Put this: 

 
        tst.b   (Water_flag).w          ; Does the level have water?
        beq.s   +                       ; If not, branch and skip underwater checks
        move.w  (Water_Level_2).w,d6    ; Move water level to d6
        cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
        bgt.s   +                       ; If not, branch and skip underwater commands
        subi.w  #$E,y_vel(a0)           ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect for the rings
+
 

You can change the value of the gravity if you like, but I found $E to suit. Anyway, you should have something looking like this: 

 
Obj_37_sub_2:
        move.b  (Ring_spill_anim_frame).w,mapping_frame(a0)
        bsr.w   ObjectMove
        addi.w  #$18,y_vel(a0)
        tst.b   (Water_flag).w          ; Does the level have water?
        beq.s   +                       ; If not, branch and skip underwater checks
        move.w  (Water_Level_2).w,d6    ; Move water level to d6
        cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
        bgt.s   +                       ; If not, branch and skip underwater commands
        subi.w  #$E,y_vel(a0)           ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect for the rings
+
        bmi.s   loc_121B8
        move.b  ($FFFFFE0F).w,d0
        add.b   d7,d0
        andi.b  #7,d0
        bne.s   loc_121B8
        tst.b   render_flags(a0)
        bpl.s   loc_121D0
        jsr     (RingCheckFloorDist).l
        tst.w   d1
        bpl.s   loc_121B8
        add.w   d1,y_pos(a0)
        move.w  y_vel(a0),d0
        asr.w   #2,d0
        sub.w   d0,y_vel(a0)
        neg.w   y_vel(a0)
 

This ensures when underwater, the gravity is not as great. Otherwise, the rings will gain speed and then it won't give the desired underwater effect.

Sonic 2 SVN fix

(SVN fix by KingofHarts)

There is ONE small hiccup when trying to apply this to the SVN disassembly of Sonic 2. If you go ahead and apply the above changes to the corresponding code, and build & run, you will find that there is a SERIOUS error with your ring loss. This is due to the way that the SVN disassembly is labeled, compared to the original Xenowhirl disassembly, and is a REALLY EASY thing to fix. I'll take a second to show you what you need to do. After you have finished applying the steps above, please go back to the first section you edited. It should now look something like this, perhaps minus the extra comments I made to guide me. 

 
; loc_120B2:
-	bsr.w	SingleObjLoad
	bne.w	+++
; loc_120BA:
+
	_move.b	#ObjID_LostRings,id(a1) ; load obj37
	addq.b	#2,routine(a1)
	move.b	#8,y_radius(a1)
	move.b	#8,x_radius(a1)
	move.w	x_pos(a0),x_pos(a1)
	move.w	y_pos(a0),y_pos(a1)
	move.l	#Obj25_MapUnc_12382,mappings(a1)
	move.w	#make_art_tile(ArtTile_ArtNem_Ring,1,0),art_tile(a1)
	bsr.w	Adjust2PArtPointer2
	move.b	#$84,render_flags(a1)
	move.b	#3,priority(a1)
	move.b	#$47,collision_flags(a1)
	move.b	#8,width_pixels(a1)
	move.b	#-1,(Ring_spill_anim_counter).w
	tst.w	d4
	bmi.s	+ ; <- This line is incorrectly branching.
	move.w	d4,d0
	bsr.w	JmpTo4_CalcSine
	move.w	d4,d2
	lsr.w	#8,d2
	; RHS FIX
        tst.b   (Water_flag).w          ; Does the level have water?
        beq.s   +                       ; If not, branch and skip underwater checks
        move.w  (Water_Level_2).w,d6    ; Move water level to d6
        cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
        bgt.s   +                       ; If not, branch and skip underwater commands
        asr.w   #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
        asr.w   #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
+
	; RHS FIX
        asl.w	d2,d0
	asl.w	d2,d1
	move.w	d0,d2
	move.w	d1,d3
	addi.b	#$10,d4
	bcc.s	+ ; <- This line is branching correctly, BUT we will need to change it.
	subi.w	#$80,d4
	bcc.s	+ ; <- This line is branching correctly, BUT we will need to change it.
	move.w	#$288,d4
;loc_12132
+
	move.w	d2,x_vel(a1)
	move.w	d3,y_vel(a1)
...
 

Now, as you can see, the SVN has relabeled a LOT of these sections of code with "+". This is all well and good, BUT it causes problems for us when applying this fix. Note the first line I marked. It is supposed to branch to the former "loc_12132". Instead it branches to "+". But there is a "+" section BEFORE that, causing the issue. My fix is to relabel "loc_12132" back to its former label, and then I want you to edit all 3 lines I marked, and have them branch to this label. That's it! Everything SHOULD work out fine now for your SVN disassembly.

Sonic 3&K fix

This guide uses the SVN disassembly.

Go to "off_1A658:" (the routines of the scattered rings). You'll see that it's first job is to go to "loc_1A67A". That needs changing to "loc_1A68C". So, from this: 

 
off_1A658:      dc.w loc_1A67A-off_1A658
 
                dc.w loc_1A75C-off_1A658
                dc.w loc_1A7C2-off_1A658
                dc.w loc_1A7D6-off_1A658
                dc.w loc_1A7E4-off_1A658
 


And replace with this: 

 
off_1A658:      dc.w loc_1A68C-off_1A658
 
                dc.w loc_1A75C-off_1A658
                dc.w loc_1A7C2-off_1A658
                dc.w loc_1A7D6-off_1A658
                dc.w loc_1A7E4-off_1A658
 


This will skip "loc_1A67A" commands. This is because this location pushes the scattered rings object (or the reverse scattered rings object) to d6. For our water code, we need d6, because all other data registers are being used. It doesn't do this in Sonic 1 or 2 because there's no such thing as "reverse gravity". The scattered rings object needs to be moved to a1, but we can't do it here, because shortly after, a1 is being used for the amount of rings.

Next, go to "off_1A670:" This is extremely similar to what we just done, but for reverse gravity. so no point explaining what it's doing here as I just did that. Anyway, you should see: 

 
off_1A670:      dc.w loc_1A67A-off_1A670
 
                dc.w loc_1A7E8-off_1A670
                dc.w loc_1A7C2-off_1A670
                dc.w loc_1A7D6-off_1A670
                dc.w loc_1A7E4-off_1A670
 


And replace with: 

 
off_1A670:      dc.w loc_1A68C-off_1A670
 
                dc.w loc_1A7E8-off_1A670
                dc.w loc_1A7C2-off_1A670
                dc.w loc_1A7D6-off_1A670
                dc.w loc_1A7E4-off_1A670
 


Again, this will skip "loc_1A67A" commands. Explained why above.

Next, go to "loc_1A67A:" and you should see this. 

 
loc_1A67A:
 
                move.l  #Obj_Bouncing_Ring,d6
                tst.b   (Reverse_gravity_flag).w
                beq.s   loc_1A68C
                move.l  #Obj_Bouncing_Ring_Reverse_Gravity,d6
 


Notice "Bouncing_ring" (and the reverse one) are being pushed to d6. We need d6, so, delete it. It's no longer needed. Trust me. We have now free'd d6. Although we've lost pushing the scattered rings into place. Do not worry, that's next.


Next, go to "loc_1A6B6:". The very first command should be "addq.b #2,5(a1)" Just before that, add this: 

 
                move.l  #Obj_Bouncing_Ring,(a1)
                tst.b   (Reverse_gravity_flag).w
                beq.s   loc_1A6B6CONTINUE
                move.l  #Obj_Bouncing_Ring_Reverse_Gravity,(a1)
loc_1A6B6CONTINUE:
 

Yes, you might recognize this. It's part of that code you just this second deleted. We're pushing the scattered rings back into place. But instead of putting it to d6, we're putting it to a1. a1 is now available as it's not being used again for a while.

So you have something looking at this: 

 
loc_1A6B6:
                move.l  #Obj_Bouncing_Ring,(a1)
                tst.b   (Reverse_gravity_flag).w
                beq.s   loc_1A6B6CONTINUE
                move.l  #Obj_Bouncing_Ring_Reverse_Gravity,(a1)
loc_1A6B6CONTINUE:
                addq.b  #2,5(a1)
                move.b  #8,$1E(a1)
                move.b  #8,$1F(a1)
                move.w  $10(a0),$10(a1)
                move.w  $14(a0),$14(a1)
                move.l  #Map_Ring,$C(a1)
                move.w  #$A6BC,$A(a1)
                move.b  #$84,4(a1)
                move.w  #$180,8(a1)
                move.b  #$47,$28(a1)
                move.b  #8,7(a1)
                move.b  #-1,(Ring_spill_anim_counter).w
                tst.w   d4
                bmi.s   loc_1A728
                move.w  d4,d0
                jsr     (GetSineCosine).l
                move.w  d4,d2
                lsr.w   #8,d2
                asl.w   d2,d0
                asl.w   d2,d1
                move.w  d0,d2
                move.w  d1,d3
                addi.b  #$10,d4
                bcc.s   loc_1A728
                subi.w  #$80,d4
                bcc.s   loc_1A728
                move.w  #$288,d4
 


Still looking at "loc_1A6B6:" look and find this bit of coding: 

 
                jsr     (GetSineCosine).l
                move.w  d4,d2
                lsr.w   #8,d2
 


Just underneath, paste this: 

 
                tst.b   (Water_flag).w          ; Does the level have water?
                beq.s   skiphalvingvel          ; If not, branch and skip underwater checks
                move.w  (Water_level).w,d6      ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   skiphalvingvel          ; If not, branch and skip underwater commands
                asr.w   #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
                asr.w   #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
skiphalvingvel:
 


So, you should end up with something looking like this: 

 
loc_1A6B6:
                move.l  #Obj_Bouncing_Ring,(a1)
                tst.b   (Reverse_gravity_flag).w
                beq.s   loc_1A6B6CONTINUE
                move.l  #Obj_Bouncing_Ring_Reverse_Gravity,(a1)
loc_1A6B6CONTINUE:
                addq.b  #2,5(a1)
                move.b  #8,$1E(a1)
                move.b  #8,$1F(a1)
                move.w  $10(a0),$10(a1)
                move.w  $14(a0),$14(a1)
                move.l  #Map_Ring,$C(a1)
                move.w  #$A6BC,$A(a1)
                move.b  #$84,4(a1)
                move.w  #$180,8(a1)
                move.b  #$47,$28(a1)
                move.b  #8,7(a1)
                move.b  #-1,(Ring_spill_anim_counter).w
                tst.w   d4
                bmi.s   loc_1A728
                move.w  d4,d0
                jsr     (GetSineCosine).l
                move.w  d4,d2
                lsr.w   #8,d2
                tst.b   (Water_flag).w          ; Does the level have water?
                beq.s   skiphalvingvel          ; If not, branch and skip underwater checks
                move.w  (Water_level).w,d6      ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   skiphalvingvel          ; If not, branch and skip underwater commands
                asr.w   #$1,d0                  ; Half d0.  Makes the ring's x_vel bounce to the left/right slower
                asr.w   #$1,d1                  ; Half d1.  Makes the ring's y_vel bounce up/down slower
skiphalvingvel:
                asl.w   d2,d0
                asl.w   d2,d1
                move.w  d0,d2
                move.w  d1,d3
                addi.b  #$10,d4
                bcc.s   loc_1A728
                subi.w  #$80,d4
                bcc.s   loc_1A728
                move.w  #$288,d4
 


This ensures that when underwater, the rings x_vel and y_vel are halved so they appear to be moving slower.


Next, go to label "loc_1A75C:" and under 

 
                move.b  (Ring_spill_anim_frame).w,$22(a0)
                bsr.w   MoveSprite2
                addi.w  #$18,$1A(a0)
 


Put this: 

 
                tst.b   (Water_flag).w          ; Does the level have water?
                beq.s   skiphalvinggrav         ; If not, branch and skip underwater checks
                move.w  (Water_level).w,d6      ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   skiphalvinggrav         ; If not, branch and skip underwater commands
                subi.w  #$E,$1A(a0)             ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect
skiphalvinggrav:
 


You can change the value of the gravity if you like, but I found $E to suit. Anyway, you should have something looking like this: 

 
loc_1A75C:
                move.b  (Ring_spill_anim_frame).w,$22(a0)
                bsr.w   MoveSprite2
                addi.w  #$18,$1A(a0)
                tst.b   (Water_flag).w          ; Does the level have water?
                beq.s   skiphalvinggrav         ; If not, branch and skip underwater checks
                move.w  (Water_level).w,d6      ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   skiphalvinggrav         ; If not, branch and skip underwater commands
                subi.w  #$E,$1A(a0)             ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect
skiphalvinggrav:
                bmi.s   loc_1A7B0
                move.b  (V_int_run_count+3).w,d0
                add.b   d7,d0
                andi.b  #7,d0
                bne.s   loc_1A7B0
                tst.b   4(a0)
                bpl.s   loc_1A79C
                jsr     (sub_F994).l
                tst.w   d1
                bpl.s   loc_1A79C
                add.w   d1,$14(a0)
                move.w  $1A(a0),d0
                asr.w   #2,d0
                sub.w   d0,$1A(a0)
                neg.w   $1A(a0)
 


This ensures when underwater, the gravity is not as great. Otherwise, the rings will gain speed and then it won't give the desired underwater effect.


This last bit is optional. You only need to do this if you're planning to have reverse gravity underwater. You'd be a bit mental if you do have this, but you never know =P

Go to label "loc_1A7E8:" and under 

 
                move.b  (Ring_spill_anim_frame).w,$22(a0)
                bsr.w   MoveSprite_TestGravity2
                addi.w  #$18,$1A(a0)
 


Put this: 

 
                tst.b   (Water_flag).w          ; Does the level have water?
                beq.s   skiphalvingrevgrav              ; If not, branch and skip underwater checks
                move.w  (Water_level).w,d6      ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   skiphalvingrevgrav              ; If not, branch and skip underwater commands
                subi.w  #$E,$1A(a0)             ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect
skiphalvingrevgrav:
 


Again, you can change the value of the reverse gravity if you like. So, you should have something looking like this: 

 
loc_1A7E8:
 
                move.b  (Ring_spill_anim_frame).w,$22(a0)
                bsr.w   MoveSprite_TestGravity2
                addi.w  #$18,$1A(a0)
                tst.b   (Water_flag).w          ; Does the level have water?
                beq.s   skiphalvingrevgrav      ; If not, branch and skip underwater checks
                move.w  (Water_level).w,d6      ; Move water level to d6
                cmp.w   y_pos(a0),d6            ; Is the ring object underneath the water level?
                bgt.s   skiphalvingrevgrav      ; If not, branch and skip underwater commands
                subi.w  #$E,$1A(a0)             ; Reduce gravity by $E ($18-$E=$A), giving the underwater effect
skiphalvingrevgrav:
                bmi.s   loc_1A83C
                move.b  (V_int_run_count+3).w,d0
                add.b   d7,d0
                andi.b  #7,d0
                bne.s   loc_1A83C
                tst.b   4(a0)
                bpl.s   loc_1A828
                jsr     (sub_FCA0).l
                tst.w   d1
                bpl.s   loc_1A828
                sub.w   d1,$14(a0)
                move.w  $1A(a0),d0
                asr.w   #2,d0
                sub.w   d0,$1A(a0)
                neg.w   $1A(a0)
 


This ensures when underwater, the gravity is not as great when it's in reverse gravity mode.


SCHG How-To Guide: Sonic the Hedgehog (16-bit)
Fixing Bugs
Fix Demo Playback | Fix a Race Condition with Pattern Load Cues | Fix the SEGA Sound | Display the Press Start Button Text | Fix the Level Select Menu | Fix the Hidden Points Bug | Fix Accidental Deletion of Scattered Rings | Fix the Walk-Jump Bug | Correct Drowning Bugs | Fix the death boundary bug | Fix Ring Timers
Design Choices and Features
Change Spike Behavior | Fix Special Stage Jumping Physics | Improve the fade in\fade out progression routines in Sonic 1 | Remove the Speed Cap | Add Spin Dash ( Part 1 / Part 2 / Part 3 / Part 4) | Port the REV01 Background Effects | Retain Rings Between Acts | Fix scattered rings' underwater physics
Sound Features
Expand Music Index From $94 to $9F | Extend Music Slots | Play Different Songs Per Act | Expand Music Index to Start at $00 | Port Sonic 2 Final Sound Driver | Port Sonic 3's Sound Driver
Extending the Game
Load chunks from ROM | Add Extra Characters | Make an Alternative Title Screen | Use Dynamic Tilesets | Make GHZ Load Alternate Art | Add a New Zone | Set Up the Goggle Monitor | Add New Moves | Add a Dynamic Collision System | Extend Sprite Mappings and Art Limit | Add Eggman Monitor | Sonic 2 (Simon Wai Prototype) Level Select | Enigma Credits
Split Disassembly Guides
Set up a Split Disassembly | Basic Level Editing | Basic Art Editing | Basic ASM Editing (Spin Dash)
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 monitor collision bug | Fix Rexon crash | 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 Ring Timers
Design Choices and Features
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 | Create Insta-kill and High Jump Monitors | Create Clone and Special Stage Monitors | Improve the fade in\fade out progression routines in Sonic 2 | Fix scattered rings' underwater physics | Insert LZ water ripple effect | Restore lost CPZ boss feature | Prevent SCZ Tornado spindash death | Free up 2 universal SSTs | Improve ObjectMove subroutines
Extending the Game
Port S1 Sound Driver | Port S3K Rings Manager | Port S3K Object Manager | Port S3K Priority Manager | Extend the Level Index Past $10 | Extend Water Tables | Expand the Music Index to Start at $00 Instead of $80 (Sonic 2 Clone Driver Version) | Add Extra Characters
Miscellaneous
Edit Level Order with ASM‎ | Alter Ring Requirements in Special Stages | Speed up ring loss process
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