Add a new battle transition

This tutorial is for how to add a new battle transition, like the Poké Ball that covers the screen for a trainer battle. As an example, we'll add an "R" for battles with Team Rocket.

Contents

1. Add a new transition
2. Create a custom palette for that transition

1. Add a new transition

First, edit engine/battle/battle_transition.asm:

 StartTrainerBattle_LoadPokeBallGraphics:
 	...
 
 	call .loadpokeballgfx
 	...
 
 .loadpokeballgfx
+	ld de, TeamRocketTransition
 	ld a, [wOtherTrainerClass]
+	cp GRUNTM
+	ret z
+	cp GRUNTF
+	ret z
+	cp EXECUTIVEM
+	ret z
+	cp EXECUTIVEF
+	ret z
+	cp SCIENTIST
+	ret z
 	ld de, PokeBallTransition
 	ret

 PokeBallTransition:
 ; 16x16 overlay of a Poke Ball
 pusho
 opt b.X ; . = 0, X = 1
 	bigdw %......XXXX......
 	bigdw %....XXXXXXXX....
 	bigdw %..XXXX....XXXX..
 	bigdw %..XX........XX..
 	bigdw %.XX..........XX.
 	bigdw %.XX...XXXX...XX.
 	bigdw %XX...XX..XX...XX
 	bigdw %XXXXXX....XXXXXX
 	bigdw %XXXXXX....XXXXXX
 	bigdw %XX...XX..XX...XX
 	bigdw %.XX...XXXX...XX.
 	bigdw %.XX..........XX.
 	bigdw %..XX........XX..
 	bigdw %..XXXX....XXXX..
 	bigdw %....XXXXXXXX....
 	bigdw %......XXXX......
 popo

+TeamRocketTransition:
+pusho
+opt b.X ; . = 0, X = 1
+	bigdw %XXXXXXXXXXXX....
+	bigdw %XXXXXXXXXXXXXX..
+	bigdw %XXXXXXXXXXXXXXX.
+	bigdw %XXXXXXXXXXXXXXX.
+	bigdw %XXXXX.....XXXXXX
+	bigdw %XXXXX......XXXXX
+	bigdw %XXXXX.....XXXXXX
+	bigdw %XXXXXXXXXXXXXXX.
+	bigdw %XXXXXXXXXXXXXXX.
+	bigdw %XXXXXXXXXXXXXX..
+	bigdw %XXXXXXXXXXXXX...
+	bigdw %XXXXX....XXXXX..
+	bigdw %XXXXX....XXXXX..
+	bigdw %XXXXX.....XXXXX.
+	bigdw %XXXXX......XXXXX
+	bigdw %XXXXX......XXXXX
+popo

Older versions of pokecrystal define the PokeBallTransition pattern with literal binary numbers:

PokeBallTransition:
	db %00000011, %11000000
	db %00001111, %11110000
	db %00111100, %00111100
	db %00110000, %00001100
	db %01100000, %00000110
	db %01100011, %11000110
	db %11000110, %01100011
	db %11111100, %00111111
	db %11111100, %00111111
	db %11000110, %01100011
	db %01100011, %11000110
	db %01100000, %00000110
	db %00110000, %00001100
	db %00111100, %00111100
	db %00001111, %11110000
	db %00000011, %11000000

Either way works, but the new method shows the 16x16 tile pattern more clearly.

Notice that StartTrainerBattle_LoadPokeBallGraphics.loadpokeballgfx already has the line ld a, [wOtherTrainerClass] but doesn't do anything with it. Maybe trainer-based patterns like this were a scrapped feature.

Anyway, that's all it takes to change the overlay pattern:

2. Create a custom palette for that transition

We can create a custom palette for any transition. Let's try creating a file called rocket_battle.pal in gfx/overworld.

	RGB 31, 08, 08
	RGB 31, 07, 06
	RGB 31, 02, 03
	RGB 07, 07, 07

Next, edit engine/battle/battle_transition.asm again:

.cgb
+	ld hl, .rocketpals
+	ld a, [wOtherTrainerClass]
+	cp GRUNTM
+	jr z, .load_rocket_pals
+	cp GRUNTF
+	jr z, .load_rocket_pals
+	cp EXECUTIVEM
+	jr z, .load_rocket_pals
+	cp EXECUTIVEF
+	jr z, .load_rocket_pals
+	cp SCIENTIST
+	jr z, .load_rocket_pals
	ld hl, .pals
+.load_rocket_pals
	ld a, [wTimeOfDayPal]
	maskbits NUM_DAYTIMES
	cp DARKNESS_F
	jr nz, .not_dark
	ld hl, .darkpals
.not_dark
	...

.pals:
INCLUDE "gfx/overworld/trainer_battle.pal"

.darkpals:
INCLUDE "gfx/overworld/trainer_battle_dark.pal"

+.rocketpals:
+INCLUDE "gfx/overworld/rocket_battle.pal"

And that's it! Now the overlay colors for the pattern have been changed!

You can create more palettes to handle different trainer classes.

Changing the transition animations (the ones that fade, wipe, distort, or otherwise erase the screen to begin a battle) is more complicated.

TODO: new transition animation