Game Physics - Learning C# by Programming Games - page 348

the GameObjectGrid class. This behavior has nothing to do with the grid of game
objects, but it is particular to our platform game. Therefore, we will define a new
class called TileField that inherits from the GameObjectGrid class. We add a single
method to that class called GetTileType , which returns the type of the tile, given its x
and y position in the grid. The nice thing about this method is that we allow these
indices to fall outside of the valid indices in the grid. For example, it would be
perfectly fine to ask for the tile type of the tile at position ( − 2 , 500 ) . By using an

if -instruction in this method, we check if the x index is out of range. If so, we return
a normal (wall) tile type:

if (x < 0 || x >= Columns)
return TileType.Normal;

If the y index is out of range, we return a 'background' tile type, so that the character
can jump through the top of the screen, or fall into a hole:

if (y<0||y>=Rows)
return TileType.Background;

If both of the if instructions' conditions are false , this means that the type of an
actual tile in the grid is requested, so we retrieve that tile and return its tile type:

Tile current = this .Objects[x, y] as Tile;
return current.TileType;

The complete class can be found in Listing 27.1 .

27.3 Setting the Player at the Right Position

When we load the level tiles from the text file, we use the character '1' to indicate
on which tile the player is starting. Based on the location of that tile, we have to
create the Player object and set it at the right position. For this, we add a method
LoadStartTile to the Level class in the LevelLoading.cs file. In this method, we first
retrieve the tile field, and then we calculate the starting position of the player. Since
the origin of the player is the bottom-center point of the sprite, we can calculate this
position as follows:

TileField tiles = this .Find("tiles") as TileField;
Vector2 startPosition = new Vector2((( float )x + 0.5f) ∗ tiles.CellWidth,

∗ tiles.CellHeight); (y + 1)

Note that we use the width and height of the tiles and multiply them with the x and
y indices of where the player should be located. We multiply with x + 0.5f so that the
player is placed in the middle of the tile position and we multiply with y+1 to place
the player on the bottom of the tile. We can then create the Player object and add it
to the game world.