3D Game Programming

avatar.position.set(-50, 50, 0);

scene.add(avatar);

avatar.setAngularFactor(newTHREE.Vector3( 0, 0, 0 ));// no rotation

avatar.setLinearFactor(newTHREE.Vector3( 1, 1, 0 )); // only move on X/Y axes

avatar.setLinearVelocity(newTHREE.Vector3(0, 150, 0));

avatar.addEventListener('collision',function(object) {

if(object.is_fruit) {

scoreboard.addPoints(10);

avatar.setLinearVelocity(newTHREE.Vector3(0, 50, 0));

scene.remove(object);

}

if(object == ground) {

game_over = true;

scoreboard.message("Game Over!");

}

});

returnavatar;

}

We’re familiar with JavaScript event listeners from Chapter 4, Project: Moving

Avatars, on page 35, where we used them to listen for keys being pressed on

the keyboard. Here we’re listening for something different: the purple fruit

monster colliding with something.

If the avatar collides with fruit, we add 10 points to the score, give the avatar

a little bump up, and remove the fruit from the screen (because the purple

fruit monster ate the fruit). If the avatar collides with the ground, then the

game is over and the purple fruit monster can eat no more.

There’s a lot going on in that function. The most important is the collision

event listener. This is where all the action takes place.

Several other new things in that function are worth mentioning. First is a

THREE.Vector3(). If you have seen the movie Despicable Me, then you know that

a vector is an arrow with direction and magnitude (Oh, yeah!). That means a

vector includes two pieces of information: the direction in which it points and

how strongly it points in that direction. You would use a vector to describe

how hard you would need to jump to reach a ledge that is up and to the left.

Vectors are very important in physics, which is why they’re used with Physijs

things. As the comments suggest, setting the “angular factor” to a vector of

all zeros prevents our avatar from rotating. Setting the “linear factor” is a way

to prevent motion up, down, left, right, forward, or backward. Since we want

our avatar to move only up, down, left, and right (but not into or out of the

screen), we set the X and Y components of the vector to 1 (allow motion) and

the Z component to 0 (don’t allow motion).

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Outline the Game • 137

Prepared exclusively for Michael Powell