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Improve the Loading of Battle Terrain

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Josh edited this page Mar 4, 2022 · 2 revisions

WARNING: a bit of a long tutorial.

When making a ROM hack, it's natural to want to add new battle terrain for battles. Emerald unfortunately handles loading these through big switch statements in functions. FRLG on the other hand, has a few extra functions that handle the loading of these terrain by giving them the required battle terrain ID for example BATTLE_TERRAIN_PLAIN at the cost of needing every single battle terrain in sBattleTerrainTable. These changes are done in battle_bg.c.

This will walk you through porting this behaviour over to Emerald. Firstly, we're going to look at some things from pokefirered.

// Maps map scene values to battle terrain IDs.
// Maps with any of the MAP_BATTLE_SCENE_* values seen here will use these
// regardless of whether the player is on water or in tall grass etc
static const struct {
    u8 mapScene;
    u8 battleTerrain;
} sMapBattleSceneMapping[] = {
    {MAP_BATTLE_SCENE_GYM,      BATTLE_TERRAIN_GYM},
    {MAP_BATTLE_SCENE_INDOOR_1, BATTLE_TERRAIN_INDOOR_1},
    {MAP_BATTLE_SCENE_INDOOR_2, BATTLE_TERRAIN_INDOOR_2},
    {MAP_BATTLE_SCENE_LORELEI,  BATTLE_TERRAIN_LORELEI},
    {MAP_BATTLE_SCENE_BRUNO,    BATTLE_TERRAIN_BRUNO},
    {MAP_BATTLE_SCENE_AGATHA,   BATTLE_TERRAIN_AGATHA},
    {MAP_BATTLE_SCENE_LANCE,    BATTLE_TERRAIN_LANCE},
    {MAP_BATTLE_SCENE_LINK,     BATTLE_TERRAIN_LINK}
};
// If current map scene equals any of the values in sMapBattleSceneMapping,
// use its battle terrain value. Otherwise, use the default.
static u8 GetBattleTerrainByMapScene(u8 mapBattleScene)
{
    int i;
    for (i = 0; i < NELEMS(sMapBattleSceneMapping); i++)
    {
        if (mapBattleScene == sMapBattleSceneMapping[i].mapScene)
            return sMapBattleSceneMapping[i].battleTerrain;
    }
    return BATTLE_TERRAIN_PLAIN;
}
// Loads the initial battle terrain.
static void LoadBattleTerrainGfx(u16 terrain)
{
    if (terrain >= NELEMS(sBattleTerrainTable))
        terrain = BATTLE_TERRAIN_PLAIN;  // If higher than the number of entries in sBattleTerrainTable, use the default.
    // Copy to bg3
    LZDecompressVram(sBattleTerrainTable[terrain].tileset, (void *)BG_CHAR_ADDR(2));
    LZDecompressVram(sBattleTerrainTable[terrain].tilemap, (void *)BG_SCREEN_ADDR(26));
    LoadCompressedPalette(sBattleTerrainTable[terrain].palette, 0x20, 0x60);
}
// Loads the entry associated with the battle terrain.
// This can be the grass moving on the screen at the start of a wild encounter in tall grass.
static void LoadBattleTerrainEntryGfx(u16 terrain)
{
    if (terrain >= NELEMS(sBattleTerrainTable))
        terrain = BATTLE_TERRAIN_PLAIN;
    // Copy to bg1
    LZDecompressVram(sBattleTerrainTable[terrain].entryTileset, (void *)BG_CHAR_ADDR(1));
    LZDecompressVram(sBattleTerrainTable[terrain].entryTilemap, (void *)BG_SCREEN_ADDR(28));
}
// Gets the battle terrain value if conditions are met.
// This could be a specific trainer class, battle type or wild Pokémon.
// If no conditions are met or if the map scene isn't MAP_BATTLE_SCENE_NORMAL,
// use the battle terrain mapped to map scenes.
static u8 GetBattleTerrainOverride(void)
{
    u8 battleScene;
    if (gBattleTypeFlags & (BATTLE_TYPE_TRAINER_TOWER | BATTLE_TYPE_LINK | BATTLE_TYPE_BATTLE_TOWER | BATTLE_TYPE_EREADER_TRAINER))
    {
        return BATTLE_TERRAIN_LINK;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_POKEDUDE)
    {
        gBattleTerrain = BATTLE_TERRAIN_GRASS;
        return BATTLE_TERRAIN_GRASS;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
    {
        if (gTrainers[gTrainerBattleOpponent_A].trainerClass == CLASS_LEADER_2)
        {
            return BATTLE_TERRAIN_LEADER;
        }
        else if (gTrainers[gTrainerBattleOpponent_A].trainerClass == CLASS_CHAMPION_2)
        {
            return BATTLE_TERRAIN_CHAMPION;
        }
    }
    battleScene = GetCurrentMapBattleScene();
    if (battleScene == MAP_BATTLE_SCENE_NORMAL)
    {
        return gBattleTerrain;
    }
    return GetBattleTerrainByMapScene(battleScene);
}

We will basically be taking advantage of these functions to get the battle terrain we are wanting to use. We will need to add some more BATTLE_TERRAIN_* values. In include/constants/battle.h, We should see these:

// Battle terrain defines for gBattleTerrain.
#define BATTLE_TERRAIN_GRASS        0
#define BATTLE_TERRAIN_LONG_GRASS   1
#define BATTLE_TERRAIN_SAND         2
#define BATTLE_TERRAIN_UNDERWATER   3
#define BATTLE_TERRAIN_WATER        4
#define BATTLE_TERRAIN_POND         5
#define BATTLE_TERRAIN_MOUNTAIN     6
#define BATTLE_TERRAIN_CAVE         7
#define BATTLE_TERRAIN_BUILDING     8
#define BATTLE_TERRAIN_PLAIN        9

It's going to need to look like this:

// Battle terrain defines for gBattleTerrain.
#define BATTLE_TERRAIN_GRASS        0
#define BATTLE_TERRAIN_LONG_GRASS   1
#define BATTLE_TERRAIN_SAND         2
#define BATTLE_TERRAIN_UNDERWATER   3
#define BATTLE_TERRAIN_WATER        4
#define BATTLE_TERRAIN_POND         5
#define BATTLE_TERRAIN_MOUNTAIN     6
#define BATTLE_TERRAIN_CAVE         7
#define BATTLE_TERRAIN_BUILDING     8
#define BATTLE_TERRAIN_PLAIN        9
#define BATTLE_TERRAIN_FRONTIER     10
#define BATTLE_TERRAIN_GYM          11
#define BATTLE_TERRAIN_LEADER       12
#define BATTLE_TERRAIN_MAGMA        13
#define BATTLE_TERRAIN_AQUA         14
#define BATTLE_TERRAIN_SIDNEY       15
#define BATTLE_TERRAIN_PHOEBE       16
#define BATTLE_TERRAIN_GLACIA       17
#define BATTLE_TERRAIN_DRAKE        18
#define BATTLE_TERRAIN_CHAMPION     19
#define BATTLE_TERRAIN_GROUDON      20
#define BATTLE_TERRAIN_KYOGRE       21
#define BATTLE_TERRAIN_RAYQUAZA     22

We then need to update sBattleTerrainTable with our new BATTLE_TERRAIN_* values. They need to be in order of these values. So if you change BATTLE_TERRAIN_PLAIN to be for example 20, you will need to move it further down the terrain table. Changes needed are large so I'll show individually what's needed.

    [BATTLE_TERRAIN_FRONTIER] =
    {
        .tileset = gBattleTerrainTiles_Building,
        .tilemap = gBattleTerrainTilemap_Building,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_Frontier,
    },
    [BATTLE_TERRAIN_GYM] =
    {
        .tileset = gBattleTerrainTiles_Building,
        .tilemap = gBattleTerrainTilemap_Building,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_BuildingGym,
    },
    [BATTLE_TERRAIN_LEADER] =
    {
        .tileset = gBattleTerrainTiles_Building,
        .tilemap = gBattleTerrainTilemap_Building,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_BuildingLeader,
    },
    [BATTLE_TERRAIN_MAGMA] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumMagma,
    },
    [BATTLE_TERRAIN_AQUA] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumAqua,
    },
    [BATTLE_TERRAIN_SIDNEY] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumSidney,
    },
    [BATTLE_TERRAIN_PHOEBE] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumPhoebe,
    },
    [BATTLE_TERRAIN_GLACIA] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumGlacia,
    },
    [BATTLE_TERRAIN_DRAKE] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumDrake,
    },
    [BATTLE_TERRAIN_CHAMPION] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumWallace,
    },
    [BATTLE_TERRAIN_GROUDON] =
    {
        .tileset = gBattleTerrainTiles_Cave,
        .tilemap = gBattleTerrainTilemap_Cave,
        .entryTileset = gBattleTerrainAnimTiles_Cave,
        .entryTilemap = gBattleTerrainAnimTilemap_Cave,
        .palette = gBattleTerrainPalette_Groudon,
    },
    [BATTLE_TERRAIN_KYOGRE] =
    {
        .tileset = gBattleTerrainTiles_Water,
        .tilemap = gBattleTerrainTilemap_Water,
        .entryTileset = gBattleTerrainAnimTiles_Underwater,
        .entryTilemap = gBattleTerrainAnimTilemap_Underwater,
        .palette = gBattleTerrainPalette_Kyogre,
    },
    [BATTLE_TERRAIN_RAYQUAZA] =
    {
        .tileset = gBattleTerrainTiles_Rayquaza,
        .tilemap = gBattleTerrainTilemap_Rayquaza,
        .entryTileset = gBattleTerrainAnimTiles_Rayquaza,
        .entryTilemap = gBattleTerrainAnimTilemap_Rayquaza,
        .palette = gBattleTerrainPalette_Rayquaza,
    },

The tedious bit is done. Now we need to start adding in the functions mentioned above but making changes to them so they work for Emerald's scenarios.

Our sMapBattleSceneMapping should look like this. I recommend putting this just below the terrain table but it's a matter of preference.

static const struct {
    u8 mapScene;
    u8 battleTerrain;
} sMapBattleSceneMapping[] = {
    {MAP_BATTLE_SCENE_GYM,      BATTLE_TERRAIN_GYM},
    {MAP_BATTLE_SCENE_MAGMA,    BATTLE_TERRAIN_MAGMA},
    {MAP_BATTLE_SCENE_AQUA,     BATTLE_TERRAIN_AQUA},
    {MAP_BATTLE_SCENE_SIDNEY,   BATTLE_TERRAIN_SIDNEY},
    {MAP_BATTLE_SCENE_PHOEBE,   BATTLE_TERRAIN_PHOEBE},
    {MAP_BATTLE_SCENE_GLACIA,   BATTLE_TERRAIN_GLACIA},
    {MAP_BATTLE_SCENE_DRAKE,    BATTLE_TERRAIN_DRAKE},
    {MAP_BATTLE_SCENE_FRONTIER, BATTLE_TERRAIN_FRONTIER}
};

GetBattleTerrainByMapScene,LoadBattleTerrainGfx and LoadBattleTerrainEntryGfx mentioned above can be used as is. I recommend putting these below sMapBattleSceneMapping, but again, up to preference.

Now we need to add GetBattleTerrainOverride with our changes, this should look like this:

static u8 GetBattleTerrainOverride(void)
{
    u8 battleScene;
    if (gBattleTypeFlags & (BATTLE_TYPE_FRONTIER | BATTLE_TYPE_LINK | BATTLE_TYPE_RECORDED_LINK | BATTLE_TYPE_EREADER_TRAINER))
    {
        return BATTLE_TERRAIN_FRONTIER;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_GROUDON)
    {
        return BATTLE_TERRAIN_GROUDON;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_KYOGRE)
    {
        return BATTLE_TERRAIN_KYOGRE;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_RAYQUAZA)
    {
        return BATTLE_TERRAIN_RAYQUAZA;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
    {
        if (gTrainers[gTrainerBattleOpponent_A].trainerClass == TRAINER_CLASS_LEADER)
        {
            return BATTLE_TERRAIN_LEADER;
        }
        else if (gTrainers[gTrainerBattleOpponent_A].trainerClass == TRAINER_CLASS_CHAMPION)
        {
            return BATTLE_TERRAIN_CHAMPION;
        }
    }
    battleScene = GetCurrentMapBattleScene();
    if (battleScene == MAP_BATTLE_SCENE_NORMAL)
    {
        return gBattleTerrain;
    }
    return GetBattleTerrainByMapScene(battleScene);
}

I recommend putting this just before LoadChosenBattleElement but up to preference.

Don't forget to extern it under static void CB2_UnusedBattleInit(void); like so:

static void CB2_UnusedBattleInit(void);
static u8 GetBattleTerrainOverride(void);

We then need to replace the entirety of DrawMainBattleBackground with:

void DrawMainBattleBackground(void)
{
    LoadBattleTerrainGfx(GetBattleTerrainOverride());
}

We need to look for DrawBattleEntryBackground and replace that with:

void DrawBattleEntryBackground(void)
{
    if (gBattleTypeFlags & BATTLE_TYPE_LINK)
    {
        LZDecompressVram(gBattleVSFrame_Gfx, (void*)(BG_CHAR_ADDR(1)));
        LZDecompressVram(gVsLettersGfx, (void*)OBJ_VRAM0);
        LoadCompressedPalette(gBattleVSFrame_Pal, 0x60, 0x20);
        SetBgAttribute(1, BG_ATTR_SCREENSIZE, 1);
        SetGpuReg(REG_OFFSET_BG1CNT, 0x5C04);
        CopyToBgTilemapBuffer(1, gBattleVSFrame_Tilemap, 0, 0);
        CopyToBgTilemapBuffer(2, gBattleVSFrame_Tilemap, 0, 0);
        CopyBgTilemapBufferToVram(1);
        CopyBgTilemapBufferToVram(2);
        SetGpuReg(REG_OFFSET_WININ, WININ_WIN0_BG1 | WININ_WIN0_BG2 | WININ_WIN0_OBJ | WININ_WIN0_CLR);
        SetGpuReg(REG_OFFSET_WINOUT, WINOUT_WIN01_BG1 | WINOUT_WIN01_BG2 | WINOUT_WIN01_OBJ | WINOUT_WIN01_CLR);
        gBattle_BG1_Y = 0xFF5C;
        gBattle_BG2_Y = 0xFF5C;
        LoadCompressedSpriteSheetUsingHeap(&sVsLettersSpriteSheet);
    }
    else if (gBattleTypeFlags & (BATTLE_TYPE_FRONTIER | BATTLE_TYPE_LINK | BATTLE_TYPE_RECORDED_LINK | BATTLE_TYPE_EREADER_TRAINER))
    {
        if (!(gBattleTypeFlags & BATTLE_TYPE_INGAME_PARTNER) || gPartnerTrainerId == TRAINER_STEVEN_PARTNER)
        {
            LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
        }
        else
        {
            // Set up bg for the multi battle intro where both teams slide in facing the screen.
            // Note Steven's multi battle (which has a dedicated back pic) is excluded above.
            SetBgAttribute(1, BG_ATTR_CHARBASEINDEX, 2);
            SetBgAttribute(2, BG_ATTR_CHARBASEINDEX, 2);
            CopyToBgTilemapBuffer(1, gMultiBattleIntroBg_Opponent_Tilemap, 0, 0);
            CopyToBgTilemapBuffer(2, gMultiBattleIntroBg_Player_Tilemap, 0, 0);
            CopyBgTilemapBufferToVram(1);
            CopyBgTilemapBufferToVram(2);
        }
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_GROUDON)
    {
        LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_CAVE);
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_KYOGRE)
    {
        LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_UNDERWATER);
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_RAYQUAZA)
    {
        LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_RAYQUAZA);
    }
    else
    {
        if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
        {
            u8 trainerClass = gTrainers[gTrainerBattleOpponent_A].trainerClass;
            if (trainerClass == TRAINER_CLASS_LEADER)
            {
                LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
                return;
            }
            else if (trainerClass == TRAINER_CLASS_CHAMPION)
            {
                LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
                return;
            }
        }

        if (GetCurrentMapBattleScene() == MAP_BATTLE_SCENE_NORMAL)
        {
            LoadBattleTerrainEntryGfx(gBattleTerrain);
        }
        else
        {
            LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
        }
    }
}

And then finally, we replace LoadChosenBattleElement with:

bool8 LoadChosenBattleElement(u8 caseId)
{
    bool8 ret = FALSE;

    switch (caseId)
    {
    case 0:
        LZDecompressVram(gBattleTextboxTiles, (void*)(BG_CHAR_ADDR(0)));
        break;
    case 1:
        CopyToBgTilemapBuffer(0, gBattleTextboxTilemap, 0, 0);
        CopyBgTilemapBufferToVram(0);
        break;
    case 2:
        LoadCompressedPalette(gBattleTextboxPalette, 0, 0x40);
        break;
    case 3:
        LZDecompressVram(sBattleTerrainTable[GetBattleTerrainOverride()].tileset, (void *)BG_CHAR_ADDR(2));
        break;
    case 4:
        LZDecompressVram(sBattleTerrainTable[GetBattleTerrainOverride()].tilemap, (void *)BG_SCREEN_ADDR(26));
        break;
    case 5:
        LoadCompressedPalette(sBattleTerrainTable[GetBattleTerrainOverride()].palette, 0x20, 0x60);
        break;
    case 6:
        LoadBattleMenuWindowGfx();
        break;
    default:
        ret = TRUE;
        break;
    }

    return ret;
}

And with that, if I didn't miss anything out, battle terrain should be functionally equivalent to before without the need for big switch statements which can be a pain to work with and just a waste, in my opinion.

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