Designing Games with Game Maker - YoYo Games

Next you specify the end behavior, that is, what should happen when the end of the path is
reached. You can choose to stop the motion and end the path. You can also restart the path
from the beginning, that is, the instance jumps back to the position where the path was
started and executes the path again. A third option is to restart from the current position, that
is, the instance follows the path again but now with this new starting position (this is the same
when the path is closed). Finally you can choose to reverse the motion, making the instance
go back and forth along the path. Note that also at the end of the path an event happens; see
below.
Finally you can indicate whether the path must be absolute or relative. An absolute path is
executed at the place where it is defined. The instance is placed at the start position and
moved from there (end position when speed is negative). This is, for example, useful when
you have a race track on which you have defined the path. When you choose relative the
instances starts executing the path from its current position. This is useful when an instance
should make a local motion. For example, space ships in a space invader game can make a
particular turn from their current position.
When you want to place the instance at a different point along its path you can use the action
to set the path position. A path position always lies between 0 and 1, 0 indicating the start
position and 1 the end position on the path. Note that in each step the direction variable is
automatically set to the correct direction along the path. You can use this variable to choose
the correct orientation for the sprite.
When using scripts or pieces of code you have more control over the way the path is
executed. There is a function to start a path for an instance. The variable path_position
indicates the current position on the path (between 0 and 1 as indicated above). The variable
path_speed indicates the speed along the path. A variable path_scale can be used to scale
the path. A value of 1 is the original size. A larger value indicates that the path is made larger;
a smaller value makes it smaller. The variable path_orientation indicates the orientation
in which the path is executed (in degrees counter-clockwise). This enables you to execute the
path in a different orientation (e.g. moving up and down rather than left and right). There is
also a variable to control the end behavior. Finally there are lots of functions to ask for
properties of paths (e.g. the x and y coordinate at a certain positions) and there are functions
to create paths. There are even functions that create collision free paths for an instance to
reach a certain goal. See the later sections on GML for details on this.