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.

Simply edit [engine/battle/battle_transition.asm](../blob/master/engine/battle/battle_transition.asm):

```diff
 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:

![Screenshot](screenshots/rocket-battle-transition.png)

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