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SCHG How-to

Use correct height when roll jumping

From Sonic Retro

Revision as of 19:17, 15 May 2011 by MoDule (talk | contribs)

(Original guide by MoDule)

In every main series 16-bit Sonic game, there is a small bug in Sonic's jumping code that assigns him the wrong height and width when jumping while rolling on the ground. Most of the time this goes unnoticed, however occasionally when landing it will cause Sonic to be moved up too far, back into the air and make him temporarily unable to jump.

Fixing the bug

Sonic 2, SVN disassembly

Locate the Sonic_Jump routine and comment out two lines as follows: <asm>; loc_1AA38: Sonic_Jump: move.b (Ctrl_1_Press_Logical).w,d0 andi.b #button_B_mask|button_C_mask|button_A_mask,d0 ; is A, B or C pressed? beq.w return_1AAE6 ; if not, return

[...]

;move.b #$13,y_radius(a0) ; set to standing height ;<-- the offending lines ;move.b #9,x_radius(a0) ; and width ;<-- btst #2,status(a0) ; is Sonic rolling? bne.s Sonic_RollJump ; if yes, branch move.b #$E,y_radius(a0) ; set to rolling height move.b #7,x_radius(a0) ; and width move.b #2,anim(a0) ; use "jumping" animation bset #2,status(a0) addq.w #5,y_pos(a0)

return_1AAE6: rts

---------------------------------------------------------------------------
loc_1AAE8

Sonic_RollJump: bset #4,status(a0) ; set the rolling+jumping flag rts

End of function Sonic_Jump

</asm>

Things to keep in mind

This bug will be present in any other player objects as well, that means it needs to be fixed for Tails and Knuckles if they are present. Another small thing to note is that this reduces Sonic's hitbox when roll jumping. This shouldn't be much of a problem, though.

The bug explained

The normal procedure for when Sonic jumps is to change him into a ball, reduce his size and due to the height difference move him down a bit, as seen here: <asm> move.b #$E,y_radius(a0) ; set to rolling height move.b #7,x_radius(a0) ; and width move.b #2,anim(a0) ; use "jumping" animation bset #2,status(a0) addq.w #5,y_pos(a0) ;<-- move to ground </asm> The problem lies with the four lines above this code: <asm> move.b #$13,y_radius(a0) ; set to standing height move.b #9,x_radius(a0) ; and width btst #2,status(a0) ; is Sonic rolling? bne.s Sonic_RollJump ; if yes, branch </asm> The last two lines make sense. If Sonic is already rolling his size and position should be correct, so the code underneath can be skipped. The other two lines would appear to reset Sonic to his normal size, something that doesn't make sense in this context. When jumping normally these two lines have no effect whatsoever, as they are undone shortly afterward. Unfortunately, the code that fixes this problem is skipped when Sonic is already rolling and he ends up with the wrong height. Now when he lands on the ground this happens: <asm>; loc_1B0AC: Sonic_ResetOnFloor_Part2: [...]

btst #2,status(a0) ; is Sonic rolling? beq.s Sonic_ResetOnFloor_Part3 ; if not, branch bclr #2,status(a0) move.b #$13,y_radius(a0) ; reset Sonic's size to standing move.b #9,x_radius(a0) move.b #AniIDSonAni_Walk,anim(a0) ; use running/walking/standing animation subq.w #5,y_pos(a0) ;<-- move Sonic up 5 pixels so the increased height doesn't push him into the ground </asm> What this code does, is reset Sonic to his normal standing height and move him up by 5 pixels when he is in a spinning state. This all makes sense under the assumption that Sonic is actually using his rolling height. If he isn't, he'll end up 5 pixels in the air, which is exactly what happens during roll jumping.