Sprite Scroll Fix

Submission Details

Name:	Sprite Scroll Fix
Author:	HammerBrother
Added:	2016-05-23 06:31:25 PM
Version History:	View
Tool:	Asar
Requires Free Space:	Yes
Bug Fix:	Yes
Featured:	No
Description:	This patch fixes a bug where if you only clear out $7E1411 and/or $7E1412 and try to manually scroll the screen by incrementing or decrementing $7E1462 and/or $7E1464, sprites may fail to spawn or spawn on the side where the screen moves away from depending on how Mario is positioned to scroll the screen.
Tags:	bugfix glitch lorom needs remoderation sa-1 scroll
Comments:	7 (jump to comments)

Download 3.84 KiB | 722 downloads

Screenshots

Comments (7)

Showing comments from:

simon.caio 2022-06-28 07:41:17 AM Link

this patch changes me the spawning timings...

HammerBrother Author 2019-04-13 01:34:19 AM Link

This is how SMW handles spawning sprites:
Code: Show/Hide

Code

CODE_00F71A:        A5 94         LDA RAM_MarioXPos         ;\$00 and A = Mario's x position on-screen
CODE_00F71C:        38            SEC                       ;|
CODE_00F71D:        E5 1A         SBC RAM_ScreenBndryXLo    ;|
CODE_00F71F:        85 00         STA $00                   ;/
CODE_00F721:        CD 2A 14      CMP.W $142A               ;\If mario is right side of the static camera region (right side of the area
CODE_00F724:        10 02         BPL CODE_00F728           ;/that doesn't scroll the screen horizontally), leave Y = #$02
CODE_00F726:        A0 00         LDY.B #$00                ;>Otherwise, rewrite Y = #$00
CODE_00F728:        84 55         STY $55                   ;\Set scrolling direction
CODE_00F72A:        84 56         STY $56                   ;/
CODE_00F72C:        38            SEC                       ;\Find distance between mario and the center of the static cam region
CODE_00F72D:        F9 2C 14      SBC.W $142C,Y             ;|
CODE_00F730:        F0 28         BEQ CODE_00F75A           ;/>if zero, skip
CODE_00F732:        85 02         STA $02                   ;>Store distance into $02

This checks if Mario's X on-screen position is to the right or left of the “center” of the “static camera region” (which is $142A (2 bytes)), to determine the direction to spawn sprites.

And to load sprites onto the screen:
Check_55: Show/Hide

Code

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;This subroutine handles spawning sprites onto the screen when the player were to
;scroll it to where the sprite would be in. It is also used by generators to spawn
;sprites continuously.
;
;This runs every even frames (when $7E0013's bit 0 is clear) during a level.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
LoadSprFromLevel:   A5 13         LDA RAM_FrameCounter      ; \ Return every other frame 
CODE_02A7FE:        29 01         AND.B #$01                ;  | 
CODE_02A800:        D0 49         BNE Return02A84B          ; / 
CODE_02A802:        A4 55         LDY $55                   ; >Screen scroll direction (note: Depends if mario's screen x pos is less than/greater-equal to $142A) by Y.
CODE_02A804:        A5 5B         LDA RAM_IsVerticalLvl     ; \ Branch if horizontal level

This patch modifies:

Code

$00F728 to $00F72B - 4 bytes
$00F80C to $00F80F - 4 bytes
$00F713 to $00F717 - 5 bytes
$00F7F4 to $00F7F8 - 5 bytes

To detect a change of X position to determine direction. Thank goodness this works with the extended levels on LM v3+

HammerBrother Author 2018-07-11 02:52:48 AM Link

Sprites loading uses $55 to determine which side to spawn.

mjbg 2017-09-01 12:27:51 PM Link

i think it really fixed one problem i was having with a level
thank you!

HammerBrother Author 2016-06-10 01:57:51 AM Link

This is a type of bug that can only happen if you are in ASM hacking. That is why it is hard to find that bug.

I found this bug during the early development of the Mega Man scroll gates with sprites disappearing.

Von Fahrenheit 2016-05-23 06:34:22 PM Link

Moderator's note: A very... conditionally useful patch. It solves a bug I honestly didn't know existed, but if sprites despawning due to camera trickery is a problem in your hack, then this patch is for you!

Also big props to GreenHammerBro for taking the time to come up with fixes for all the little bugs in SMW.

DiscoTheBat From older version: Sprite scroll fix 2016-04-01 04:17:30 AM Link

This is some good fixing for the SMW engine. A fix is always welcome.