Zool: Taking It to the N Dimension - The Game Maker's Companion: Game Development - page 64

Now when you run your game, you should have a much better view of Zool as he moves
around the level (albeit in mid-air and at incredible speeds). He should also stop when he collides
with walls (but pass through ramps). If you have problems, then you will find a version of the
game at this point in the file zool1.gmk in the Chapter03/Games directory on the CD.

Crash Landing a Ninja
Clearly, walking on thin air is not right, so we'll start by putting some limits on Zool's speed and
then we'll get his feet on the ground. We know from the previous chapter that it is necessary to
limit the speed of moving objects to prevent them from moving so fast that they pass through
other objects (one of the problems of discrete time sampling). We'll do this in the Step event
because this takes place immediately after the keyboard events that change Zool's speed. We'll
put this Step event in obj_zool (the parent Zool object) because then the event will be inherited
by all Zool objects and that will save us repeating the same actions in each object.

Limiting Zool's Speed
1.

Define two new constants using the Define Constants option on the Resources menu
(or two new variables in the Create event of obj_zool , if you are using Game Maker
Lite). Set both MAX_HSPEED and MAX_FALL_SPEED to 14 .

Add a Step, Step event to obj_zool and include a Set Variable action. Set the Variable
vspeed and type min(MAX_FALL_SPEED, vspeed) into Value . Here, min is a Game Maker
function that returns the minimum value of its arguments (the values separated by
commas). So while vspeed is smaller than MAX_FALL_SPEED , the min function will return
the value of vspeed and the action will set vspeed to the value of vspeed (which changes
nothing). Yet, when vspeed is greater than MAX_FALL_SPEED , the min function will return
MAX_FALL_SPEED and the action will set vspeed to MAX_FALL_SPEED . In this way, the min
function limits Zool's falling speed to a maximum of 14 pixels per step.

2.

Include another Set Variable action. This time, set the Variable hspeed and type
median(-MAX_HSPEED, hspeed, MAX_HSPEED) into Value . Here, median is a different
function that computes the middle value of its arguments when they are arranged in
order of size. So while hspeed is less than MAX_SPEED and more than -MAX_HSPEED , the
middle value will be hspeed . However, if hspeed is greater than MAX_HSPEED , then the
middle value becomes MAX_HSPEED and if hspeed is smaller than -MAX_HSPEED , then the
middle value becomes -MAX_HSPEED . In this way, we can keep hspeed between -14 and
+14 pixels per step just using a single action.

3.

Caution We'll need to remember that if we create a Step event for any of the other Zool objects (which
have obj_zool as a parent), then the new Step event will replace this inherited Step event. That means
our speed clamping actions won't be called for that object and Zool will potentially move too fast again.

■

If you play the game now, Zool is limited to a sensible running speed—also notice how he
slides to a halt when you release a key when he is moving fast. This is a result of the friction
gradually bringing him to a halt when you are no longer applying a force to move him. Next, we'll
start to implement behaviors for obj_zool_air and add a bit of gravity into the equation.
For our purposes, we will consider a “normal” gravity setting to be a downwards force of 2
pixels per step. Logically, obj_zool_air should always be subject to this same gravitational pull,