Designing Games with Game Maker - YoYo Games

overwritten by the new path. (See a later chapter on how to create and
destroy paths.) The function will return whether a path was found. The
function will stop and report failure if no straight path exists between start and
goal. If it fails a path is still created that runs till the position where the
instance was blocked.
mp_linear_path_object(path,xg,yg,stepsize,obj) Same as the
function above but this time only instances of obj are considered as obstacles.
obj can be an object or an instance id.
mp_potential_path(path,xg,yg,stepsize,factor,checkall) This
function computes a path for the instance from its current position and
orientation to the position (xg,yg) using the indicated step size trying to avoid
collision with obstacles. It uses potential field steps, like in the function
mp_potential_step() and also the parameters that can be set with
mp_potential_settings(). The indicated path must already exist and will
be overwritten by the new path. (See a later chapter on how to create and
destroy paths.) The function will return whether a path was found. To avoid
the function continuing to compute forever you need to provide a length factor
larger than 1. The function will stop and report failure if it cannot find a path
shorter than this factor times the distance between start and goal. A factor of
4 is normally good enough but if you expect long detours you might make it
longer. If it fails a path is still created that runs in the direction of the goal but
it will not reach it.
mp_potential_path_object(path,xg,yg,stepsize,factor,obj)
Same as the function above but this time only instances of obj are considered
as obstacles. obj can be an object or an instance id.
The other functions use a much more complex mechanism using a grid-based approach
(sometimes called an A* algorithm). It will be more successful in finding paths (although it still
might fail) and will find shorter paths but it required more work on your side. The global idea
is as follows. First of all we put a grid on (the relevant part of) the room. You can choose to
use a fine grid (which will be slower) or a coarse grid. Next, for all relevant objects we
determine the grid cells they overlap (either using bounding boxes or precise checking) and
mark these cells as being forbidden. So a cell will be marked totally forbidden, even if it only
partially overlaps with an obstacle. Finally we specify a start and a goal position (which must
lie in free cells) and the function computes the shortest path (actually close to the shortest)