Designing Games with Game Maker - YoYo Games

mp_grid_add_instances(id,obj,prec) Marks all cells that intersect an
instance of the indicated object as being forbidden. You can also use an
individual instance by making obj the id of the instance. Also you can use the
keyword all to indicate all instances of all objects. prec indicates whether
precise collision checking must be used (will only work if precise checking is
enabled for the sprite used by the instance).
mp_grid_path(id,path,xstart,ystart,xgoal,ygoal,allowdiag)
Computes a path through the grid. path must indicate an existing path that
will be replaced by the computer path. xstart and ystart indicate the start of
the path and xgoal and ygoal the goal. allowdiag indicates whether diagonal
moves are allowed instead of just horizontal or vertical. The function returns
whether it succeeded in finding a path. (Note that the path is independent of
the current instance; It is a path through the g rid, not a path for a specific
instance.)
mp_grid_draw(id) This function draws the grid with green cells being free
and red cells being forbidden. This function is slow and only provided as a
debug tool.
Collision checking
When planning motions or deciding on certain actions, it is often important to see whether
there are collisions with other objects at certain places. The following routines can be used for
this. All these have three arguments in common: The argument obj can be an object, the
keyword all, or the id of an instance. The argument prec indicates whether the check should
be precise or only based on the bounding box of the instance. Precise checking is only done
when the sprite for the instance has the precise collision checking set. The argument notme
can be set to true to indicate that the calling instance should not be checked. All these
functions return either the id of one of the instances that collide, or they return a negative
value when there is no collision.
collision_point(x,y,obj,prec,notme) This function tests whether at
point (x,y) there is a collision with entities of object obj.
collision_rectangle(x1,y1,x2,y2,obj,prec,notme) This function
tests whether there is a collision between the (filled) rectangle with the
indicated opposite corners and entities of object obj. For example, you can use
this to test whether an area is free of obstacles.