If the Battle scene (in Porymap) is MAP_BATTLE_SCENE_NORMAL
, then the game will use a default terrain based on various attributes. For example, being in tall grass sets the battle terrain to a tall grass background. Editing these requires adding new code at src/battle_setup.c#L637.
The default battle terrains are defined at src/battle_bg.c#L601 and the pointers to the graphics are defined at src/data/graphics/battle_terrrain.h#L60. You can replace the filepaths with the files of your choosing to replace the default battle terrains. Note that the pointers are scattered about the file so you'll need to track down all the pointers within [src/data/graphics/battle_terrrain.h#L60](https://github.com/rh-hideout/pokeemerald-expans