Fix Sonic 2 Sega Screen

(Original guide by RobiWanKenobi)
(Guide updated by JGMR)

NOTE: The following code in this guide was directly based off of AURORA☆FIELDS' "AMPS in Sonic 2" hack, which can be found here.

First off, let's just say you've decided to change Sonic's sprites to something else, no harm, except this actually affects the Sega screen. The screen calls for the same art/mappings as the player object, which any changes would affect both aspects. Fortunately, the Sonic object in the Sega screen can use standalone art and mappings, so let's do that. It's effective for when you're porting sprites from Sonic 3 & Knuckles or doing custom sprites. This guide is designed to work in any version of the Sonic 2 split disassembly (as confirmed by the original author).

Beginning the process

Go into the art folder, then the uncompressed folder, and make a copy of Sonic's sprites. Do the same for Sonic's mappings and DPLCs in the mappings folder (sprite and spriteDPLC subfolders respectively). If you have replaced the files directly, grab the stock files from a fresh Sonic 2 split disassembly. All three duplicate files should be named as "SegaSonic.bin".

Next, in s2.asm, find this line of code for Sonic's art:

;---------------------------------------------------------------------------------------
; Uncompressed art
; Patterns for Sonic  ; ArtUnc_50000:
;---------------------------------------------------------------------------------------
	align $20
ArtUnc_Sonic:	BINCLUDE	"art/uncompressed/Sonic's art.bin"
;---------------------------------------------------------------------

We're going to add in the extra art file, so copy and paste the block of code with the only difference is that the duplicate will call for the Sega screen-specific art like so:

;---------------------------------------------------------------------
	align $20
ArtUnc_SegaSonic:	BINCLUDE	"art/uncompressed/SegaSonic.bin"
;---------------------------------------------------------------------------------------

Now we're going to add the extra mapping file. So let's scroll down to find this line of code:

;--------------------------------------------------------------------------------------
; Sprite Mappings
; Sonic			; MapUnc_6FBE0: SprTbl_Sonic:
;--------------------------------------------------------------------------------------

Add a new line underneath calling for the separate mapping file, like so:

;--------------------------------------------------------------------------------------
Mapunc_SegaSonic:	BINCLUDE	"mappings/sprite/SegaSonic.bin"
;--------------------------------------------------------------------------------------

Now we're going to expand upon the DPLC code, with this change it will allow us to have two distinct files in use. Don't worry, we'll resolve some other issues later. This is where the DPLC file is called:

;--------------------------------------------------------------------------------------
; Sprite Dynamic Pattern Reloading
; Sonic DPLCs   		; MapRUnc_714E0:
;--------------------------------------------------------------------------------------
; WARNING: the build script needs editing if you rename this label
;          or if you move Sonic's running frame to somewhere else than frame $2D
MapRUnc_Sonic:	BINCLUDE	"mappings/spriteDPLC/Sonic.bin"
;--------------------------------------------------------------------------------------

This header should instead call for the Sega screen file, so in the standard MapRUnc header, change "Sonic.bin" to "SegaSonic.bin" and add another code block to use the standard Sonic art for in-game stuff. For example, the gameplay sprite DPLC's will be referred to as Sonic3 in the header (and what we will use later in this guide as said example). The gameplay sprite DPLC file will not be renamed, obviously. You should have this after adding the new header:

;--------------------------------------------------------------------------------------
; Sprite Dynamic Pattern Reloading
; Sonic DPLCs on the Sega screen
;--------------------------------------------------------------------------------------
; WARNING: the build script needs editing if you rename this label
;          or if you move Sonic's running frame to somewhere else than frame $2D
;Sega Screen Sonic
MapRUnc_Sonic:	BINCLUDE	"mappings/spriteDPLC/SegaSonic.bin"
;--------------------------------------------------------------------------------------
; Sprite Dynamic Pattern Reloading
; Sonic DPLCs   		; MapRUnc_714E0:
;--------------------------------------------------------------------------------------
;Sonic's in-game sprites
MapRUnc_Sonic3:	BINCLUDE	"mappings/spriteDPLC/Sonic.bin"
;--------------------------------------------------------------------------------------

Changing the code to use the correct art and DPLC

Now we are going to apply these changes to the actual Sega screen and gameplay. If you've changed Sonic's art, it's going to be noticeable. Head over to LoadSonicDymPLC_Part2 in s2.asm and you will see this:

LoadSonicDynPLC_Part2:
	cmp.b	(Sonic_LastLoadedDPLC).w,d0
	beq.s	return_1B89A
	move.b	d0,(Sonic_LastLoadedDPLC).w
	lea	(MapRUnc_Sonic).l,a2
	add.w	d0,d0
	adda.w	(a2,d0.w),a2
	move.w	(a2)+,d5
	subq.w	#1,d5
	bmi.s	return_1B89A
	move.w	#tiles_to_bytes(ArtTile_ArtUnc_Sonic),d4

All we need to do is change the header being called for the DPLC used in gameplay sprites. The line you need to change is right underneath the move.b command for Sonic_LastLoadedDPLC. Change it to MapRUnc_Sonic3. Now scroll down to ObjB0_Init, and you should see this:

ObjB0_Init:
	bsr.w	LoadSubObject
	move.w	#$1E8,x_pixel(a0)
	move.w	#$F0,y_pixel(a0)
	move.w	#$B,objoff_2A(a0)
	move.w	#2,(SegaScr_VInt_Subrout).w
	bset	#0,render_flags(a0)
	bset	#0,status(a0)

	; Initialize streak horizontal offsets for Sonic going left.
	; 9 full lines (8 pixels) + 6 pixels, 2-byte interleaved entries for PNT A and PNT B
	lea	(Horiz_Scroll_Buf + 2 * 2 * (9 * 8 + 6)).w,a1
	lea	Streak_Horizontal_offsets(pc),a2
	moveq	#0,d0
	moveq	#$22,d6	; Number of streaks-1
-	move.b	(a2)+,d0
	add.w	d0,(a1)
	addq.w	#2 * 2 * 2,a1	; Advance to next streak 2 pixels down
	dbf	d6,-

	lea	off_3A294(pc),a1 ; pointers to mapping DPLC data
	lea	(ArtUnc_Sonic).l,a3
	lea	(Chunk_Table).l,a5
	moveq	#4-1,d5 ; there are 4 mapping frames to loop over

Much like before, all we need to do is change the header. This time, it's the uncompressed art file for the Sega screen in the third to last line. You should change said header to ArtUnc_SegaSonic. If this has been done correctly, the Sega screen will call for the correct file and gameplay sprites will be intact, as seen below:

That's it!

EXTRA: Steps for when using custom sprites but keeping default headers for the Sonic DPLC in gameplay

If you're using custom (or ported) sprites and kept the default header for the DPLC file used by the gameplay, here's what you should do:

Go to MapRUnc_Sonic:, and here's what you'll see:

;--------------------------------------------------------------------------------------
; Sprite Dynamic Pattern Reloading
; Sonic DPLCs   		; MapRUnc_714E0:
;--------------------------------------------------------------------------------------
; WARNING: the build script needs editing if you rename this label
;          or if you move Sonic's running frame to somewhere else than frame $2D
MapRUnc_Sonic:	BINCLUDE	"mappings/spriteDPLC/Sonic.bin"
;--------------------------------------------------------------------------------------

Change it to this, with one difference:

;--------------------------------------------------------------------------------------
; Sprite Dynamic Pattern Reloading
; Sonic DPLCs on the Sega screen
;--------------------------------------------------------------------------------------
; WARNING: the build script needs editing if you rename this label
;          or if you move Sonic's running frame to somewhere else than frame $2D
;Sega Screen Sonic
MapRUnc_SegaSonic:	BINCLUDE	"mappings/spriteDPLC/SegaSonic.bin"
;--------------------------------------------------------------------------------------
; Sprite Dynamic Pattern Reloading
; Sonic DPLCs   		; MapRUnc_714E0:
;--------------------------------------------------------------------------------------
;Sonic's in-game sprites
MapRUnc_Sonic:	BINCLUDE	"mappings/spriteDPLC/Sonic.bin"
;--------------------------------------------------------------------------------------

Note that we've used the new header "MapRUnc_SegaSonic" for the DPLC of Sonic that will appear only in the Sega screen, and "MapRUnc_Sonic" is the DPLC of Sonic that is used for the rest of the gameplay. Now we need to change bits in the code and build process to reflect the new header name. Go to off_3A294: and change instances of "MapRUnc_Sonic" to "MapRUnc_SegaSonic". Do the same thing with the line under EndOfRom: near the end of s2.asm.

Now in the build script, go to this line as shown below:

IF EXIST s2built.bin "win32/fixpointer" s2.h s2built.bin   off_3A294 MapRUnc_Sonic $2D 0 4   word_728C_user Obj5F_MapUnc_7240 2 2 1

Change the "MapRUnc_Sonic" to "MapRUnc_SegaSonic".

Once that's done, compile the code and it should work as intended!

|Fix Sonic 2 Sega Screen]]