Random Movement

Besides chasing and evading we can also have our enemies move in random motion. To
do this we need to add some more properties to the enemy class. We want to change
directions every so often (each enemy should have a different timer it uses to determine
when to switch directions). The direction should be random for each enemy (each enemy
will need a random velocity). The properties we need to add to our Enemy class are as
follows:
public float ChangeDirectionTimer;
public Vector3 RandomVelocity;
public int RandomSeconds;
We will create a method called MoveRandomly that will have the logic to move our
enemies in a random motion:
private void MoveRandomly(Enemy enemy, GameTime gameTime)
{
344 CHAPTER 16 AI Algorithms
if (enemy.ChangeDirectionTimer == 0)
enemy.ChangeDirectionTimer =
(float)gameTime.TotalGameTime.TotalMilliseconds;
//has the appropriate amount of time passed?
if (gameTime.TotalGameTime.TotalMilliseconds >
enemy.ChangeDirectionTimer + enemy.RandomSeconds * 1000)
{
enemy.RandomVelocity = Vector3.Zero;
enemy.RandomVelocity.X = rand.Next(-1, 2);
enemy.RandomVelocity.Y = rand.Next(-1, 2);
//restrict to 2D?
if (!restrictToXY)
enemy.RandomVelocity.Z = rand.Next(-1, 2);
enemy.ChangeDirectionTimer = 0;
}
enemy.Velocity = enemy.RandomVelocity;
enemy.Velocity *= moveUnit;
enemy.Color = Color.Orange;
}
Sometimes random movement can appear intelligent. Our method is utilizing the game
time to keep track of how long the enemy should keep moving. It is very similar to the
code we used for our fading method. This method checks to see if enough time has
passed to change directions. Once enough time has elapsed, the enemy velocity vector is
populated by random numbers ranging from -1 to 1. We need to add the following
random number variable to our code:
Random rand = new Random();
Random numbers are very important in AI work. We have taken care to not just have our
enemies be jittery by changing directions every frame. As a result we can have enemies
that look like they are searching or patrolling. None of the code we have seen has been
rocket science. All the algorithms are pretty simple but can provide great results.
We can replace our line in Update with:
MoveRandomly(enemy, gameTime);
Before we can run the code to see the results of our randomly moving enemies, we need
to initialize our properties. We do this inside of the enemy for loop in the Initialize
method:
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16
enemies[i].ChangeDirectionTimer = 0;
enemies[i].RandomVelocity = Vector3.Left;
enemies[i].RandomSeconds = (i + 2) / 2;
As we run the code we can see that each enemy is just moving around randomly. They
are not moving in synchronization and they definitely appear random.