Designing Games with Game Maker - YoYo Games

Generating Events
Miscellaneous Variables and Functions
Moving around
Obviously, an important aspect of games is the moving around of object instances. Each
instance has two built-in variables x and y that indicate the position of the instance. (To be
precise, they indicate the place where the origin of the sprite is placed. Position (0,0) is the
top-left corner of the room. You can change the position of the instance by changing its x and
y variables. If you want the object to make complicated motions this is the way to go. You
typically put this code in the step event for the object.
If the object moves with constant speed and direction, there is an easier way to do this. Each
object instance has a horizontal speed (hspeed) and a vertical speed (vspeed). Both are
indicated in pixels per step. A positive horizontal speed means a motion to the right, a
negative horizontal speed mean a motion to the left. Positive vertical speed is downwards and
negative vertical speed is upwards. So you have to set these variables only once (for example
in the creating event) to give the object instance a constant motion.
There is quite a different way for specifying motion, using a direction (in degrees 0-359), and
a speed (should be non-negative). You can set and read these variables to specify an arbitrary
motion. (Internally this is changed into values for hspeed and vspeed.) Also there is the
friction and the gravity and gravity direction. Finally, there is the function
motion_add(dir,speed) to add a motion to the current one.
To be complete, each instance has the following variables and functions dealing with its
position and motion:
x Its x-position.
y Its y-position.
xprevious Its previous x-position.
yprevious Its previous y-position.
xstart Its starting x-position in the room.
ystart Its starting y-position in the room.
hspeed Horizontal component of the speed.
vspeed Vertical component of the speed.
direction Its current direction (0-360, counter-clockwise, 0 = to the right).