mvbg

VM_GET_TILE_XY

VM_GET_TILE_XY is a GBVM instruction used to retrieve the tile ID of a background tile at a specified X and Y position on the game map.

Purpose: Game Boy games often use tile-based maps for their environments. Each tile has a unique ID that corresponds to a specific graphic. VM_GET_TILE_XY is essential for:

• Reading map data: Determining the type of terrain, obstacle, or interactive element at a given location.
• Collision detection: Checking if a character is about to move onto an impassable tile.
• Event triggers: Activating events when a player steps on a specific tile (e.g., a trap, a warp zone, a hidden switch).
• Dynamic environment interaction: Changing character properties (e.g., movement speed) based on the tile they are standing on (e.g., water, ice).

This instruction allows your script to query the game world’s background layer and get information about its composition at a granular level.

Syntax:

VM_GET_TILE_XY TILE_IDX, X_IDX, Y_IDX

• TILE_IDX: The target variable (a GBVM variable) that will receive the retrieved tile ID. This variable will hold the numerical identifier of the tile at the specified coordinates.
• X_IDX: The X-coordinate (horizontal position) of the background tile you want to query. This is typically in tile units (e.g., 0-31 for a 32x32 tile map).
• Y_IDX: The Y-coordinate (vertical position) of the background tile you want to query. This is typically in tile units.

Usage Example: Checking if Player is Standing on a Water Tile

Imagine your game has water tiles (e.g., tile ID 5) that slow down the player. You can use VM_GET_TILE_XY to check the tile beneath the player and adjust their speed accordingly.

; In your player movement script or update loop:

; Assume PLAYER_ACTOR_ID is the actor ID for the player
PLAYER_ACTOR_ID:
  .R_INT8 0 ; Example Actor ID

; Variables to store player's tile coordinates and the tile ID
VAR_PLAYER_TILE_X:
  .R_INT8 0
VAR_PLAYER_TILE_Y:
  .R_INT8 0
VAR_CURRENT_TILE_ID:
  .R_INT8 0

; Define the tile ID for water
TILE_ID_WATER:
  .R_INT8 5 ; Example: Tile ID 5 represents water

; Define player speeds
SPEED_NORMAL:
  .R_INT8 1
SPEED_SLOW:
  .R_INT8 0 ; Slower speed (e.g., half of normal)

; Get player's current pixel position
VM_ACTOR_GET_POS PLAYER_ACTOR_ID, VAR_PLAYER_PIXEL_X, VAR_PLAYER_PIXEL_Y

; Convert pixel coordinates to tile coordinates (assuming 8x8 pixel tiles)
; Divide by 8 (or shift right by 3) to get tile X and Y
VM_RPN
  .R_REF_MEM .R_INT16, VAR_PLAYER_PIXEL_X
  .R_INT16 3 ; Shift right by 3 for division by 8
  .R_OPERATOR >>
  .R_INT8
VM_STOP
VM_SET_INT8 VAR_PLAYER_TILE_X, 0

VM_RPN
  .R_REF_MEM .R_INT16, VAR_PLAYER_PIXEL_Y
  .R_INT16 3
  .R_OPERATOR >>
  .R_INT8
VM_STOP
VM_SET_INT8 VAR_PLAYER_TILE_Y, 0

; Get the tile ID at the player's current tile position
VM_GET_TILE_XY VAR_CURRENT_TILE_ID, VAR_PLAYER_TILE_X, VAR_PLAYER_TILE_Y

; Check if the current tile is a water tile
VM_IF_CONST .EQ, VAR_CURRENT_TILE_ID, TILE_ID_WATER, PLAYER_IN_WATER, 0
  ; If not water, set normal speed
  VM_ACTOR_SET_MOVE_SPEED PLAYER_ACTOR_ID, SPEED_NORMAL
  VM_JUMP END_TILE_CHECK

PLAYER_IN_WATER:
  ; If in water, set slow speed
  VM_ACTOR_SET_MOVE_SPEED PLAYER_ACTOR_ID, SPEED_SLOW

END_TILE_CHECK:
  ; ... continue player movement logic ...

In this example, VM_GET_TILE_XY VAR_CURRENT_TILE_ID, VAR_PLAYER_TILE_X, VAR_PLAYER_TILE_Y retrieves the ID of the background tile the player is currently standing on. This ID is then compared to TILE_ID_WATER. If they match, the player’s movement speed is reduced, creating a gameplay effect based on the terrain.

Analogy to other programming languages/game engines: This is analogous to querying a tilemap or a 2D array representing your game world:

• Unity (Tilemap): TileBase tile = tilemap.GetTile(new Vector3Int(x, y, 0));
• Godot (TileMap): int tile_id = get_cell(x, y);
• Python (2D list): tile_id = game_map[y][x]

It provides a way to read the static background data of your game world, which is crucial for environmental interactions and map-based logic.