
# Getting Started - Avoiding a Car Crash

## Avoiding a Car Crash

The simplest way of seeing if two meshes are in contact is to use the _intersectsMesh_ method, as in

```javascript
mesh1.intersectsMesh(mesh2);
```

which returns true if the bounding box of mesh1 overlaps the bounding box of mesh2. Each mesh has a built-in bounding box that lies close to the surface of the mesh and is used when checking for intersections.

![dudebox](/img/getstarted/dudebox.webp) ![carbox](/img/getstarted/carbox.webp)

Since the character's walk and the car's journey are not phased together, there will be a time when they are in the same place. However, it is not possible to predict when the character, taking a long walk around the village, and the car, on its short journey, might intersect. In order to demonstrate the _intersectsMesh_ method, we will make the character walk backwards and forwards across the stopping place of the car.

In our case, we want the character to stop moving if the car is in a "hit" zone and the character is not. It would, after all, be dangerous for the character to stop if they were both in the danger zone. Also, because of the way _Dude_ is constructed, we need to use one of its children to check intersection. Basically, _Dude_ is just a holder node for the head, torso, legs, and arms, and the bounding box around it is too small to be effective in this case.

<Playground id="#KBS9I5#83" title="Basic Collision Detection" description="Detect when a car collides with a box and stop the character's animation." image="/img/playgroundsAndNMEs/gettingStartedCollisions1.webp"/>

Now make the hit box invisible.

<Playground id="#KBS9I5#84" title="Basic Collision Detection Invisible Box" description="Detect when a car collides with an invisible box and stop the character's animation." image="/img/playgroundsAndNMEs/gettingStartedCollisions2.webp"/>

This repeated animation looks a bit out of place. Even if we revert to the character walking around the village, the repeated motion of the car not only looks a bit silly, it is also annoying. Let's improve the environment a little so that the car appears to be driving through the village. We will place the village in a valley with distant hills created from a height map and add a road for the car to travel along. While we are doing that, let's also add a sky and some distant trees.



## A Bump in the Road

Or rather, a way of avoiding one. Just a quick step this time. Our character is moving around the village, and we want him to avoid being hit by a passing car. We set up a danger zone that stops the character in his tracks when the car enters it. Unless, of course, he is already in the danger zone, in which case he had better keep moving.




# Getting Started - Working With Code

## A Walk Around The Village

There is a useful property of a mesh, *movePOV*, which allows us to move a mesh relative to its point of view. Generally, a newly created mesh is considered to face the negative z direction, and this is the direction of its point of view. To move a mesh forward 6 units in the direction of its point of view, you use

```javascript
mesh.movePOV(0, 0, -6)
```
The parameters are, in order, the distance to move to the right, up, and forward. In the mesh's local space, these are generally the negative x axis, the positive y axis, and the negative z axis.

In Babylon.js you can write code that will be executed before the rendering of the next frame using

```javascript
scene.onBeforeRenderObservable.add(() => {
    //code to execute
});
```

In this way, the properties of objects can be changed frame by frame as the scene renders.

Let us take the simple case of a sphere moving around the edges of a triangle. We want the sphere to appear to slide along one side, turn to slide along the next, then turn and slide along the last side before repeating.

This is also an opportunity to introduce two types of mesh you can create: a sphere and a series of lines. Take a sphere sliding around an isosceles right-angled triangle as an example.

```javascript
const sphere = BABYLON.MeshBuilder.CreateSphere("sphere", {diameter: 0.25});

//end points for the line sequence in an array
//y component can be non zero
const points = [];
points.push(new BABYLON.Vector3(2, 0, 2));
points.push(new BABYLON.Vector3(2, 0, -2));
points.push(new BABYLON.Vector3(-2, 0, -2));
points.push(points[0]); //close the triangle;

BABYLON.MeshBuilder.CreateLines("triangle", {points: points})
```

Also you can see another method, *rotate*, of rotating. This method rotates the mesh about the given axis by the given angle in radians. It adds to the current rotation.
```javascript
mesh.rotate(axis, angle, BABYLON.Space.LOCAL);
```

To produce the animation, the sphere moves a distance of 0.05 before each render frame. When the distance it has travelled is greater than 4, the sphere makes a turn. When it is greater than 8, it turns again, and when it is greater than the perimeter, it resets and starts again.

We set up a track array of objects with the properties turn and distance. After travelling the given total distance, the sphere rotates by the given turn value.

```javascript
const slide = function (turn, dist) { //after covering dist apply turn
    this.turn = turn;
    this.dist = dist;
}
const track = [];
track.push(new slide(Math.PI / 2, 4));  //first side length 4
track.push(new slide(3 * Math.PI / 4, 8)); //at finish of second side distance covered is 4 + 4
track.push(new slide(3 * Math.PI / 4, 8 + 4 * Math.sqrt(2))); //all three sides cover the distance 4 + 4 + 4 * sqrt(2)
```

Whenever the required distance is reached, a turn is made and the array index pointer, p, is increased by 1. The modulo operator *%* is used to reset the pointer to zero at the end of the array.

```javascript
if (distance > track[p].dist) {        
    sphere.rotate(BABYLON.Axis.Y, track[p].turn, BABYLON.Space.LOCAL);
    p +=1;
    p %= track.length;
}
```

To prevent floating-point errors from accumulating, whenever the index pointer is reset to 0, the position and rotation of the sphere are also reset.

```javascript
if (p === 0) {
    distance = 0;
    sphere.position = new BABYLON.Vector3(2, 0, 2); //reset to initial conditions
    sphere.rotation = BABYLON.Vector3.Zero();//prevents error accumulation 
}
```

<Playground id="#N9IZ8M#1" title="Animating To a Path" description="Simple example of animating an object along a path." image="/img/playgroundsAndNMEs/gettingStartedVillageWalk1.webp"/>

This is a little trickier, and by using a bit of trial and error for the turns and distances, we can achieve a more complicated walk for the character around the village. One reason for using degrees and converting them to radians for the *rotate* method is that it is easier to adjust by adding one or two degrees.

Since the character, dude, imported from the *.babylon* file has had its rotation set using a *rotationQuaternion* rather than rotation, we use the rotate method to reset the character's orientation.

```javascript
dude.position = new BABYLON.Vector3(-6, 0, 0);
dude.rotate(BABYLON.Axis.Y, BABYLON.Tools.ToRadians(-95), BABYLON.Space.LOCAL);
const startRotation = dude.rotationQuaternion.clone(); //use clone so that variables are independent not linked copies
```

```javascript
if (p === 0) {
    distance = 0;
    dude.position = new BABYLON.Vector3(-6, 0, 0);
    dude.rotationQuaternion = startRotation.clone();
}
```

<Playground id="#KBS9I5#81" title="Character Walking Through Town" description="Animate a character walking through the village." image="/img/playgroundsAndNMEs/gettingStartedVillageWalk2.webp"/>

We now have two things moving around the village: a car and a character. How can we avoid them colliding?


Getting Started - Chapter 4 - Avoiding Collisions

A Bump in the Road

Coming next

Getting Started - Chapter 4 - Avoiding a Car Crash

Learn how to avoid collisions between your characters and cars in the village.