Creators – Week 10

This week we continued the second week of our Gem Search project. We completed the ItemScatterer component by:

  • Completing the Distribute() function to scatter our items, temporarily lamp posts, across the environment
  • Writing a FindNearest() function to keep track of which item that we’ve spawned is closest to the player at all times

We then finally tested it by adding an ActivateNearest() function to light up the nearest lamp post and give us a visual validation that things are working.


Our function to distribute the items comes in has five main pieces to it:

  1. A calculation to workout the top-left-back corner of the space covered by the ItemScatterer
  2. A pair of loops which move us over the space in the X and Z directions
  3. A calculation to workout the current position that we’re at on the top of the ItemScatterer
  4. A probability check to determine if this is a location we should be placing an item
  5. A Physics.Raycast() call which allows us to determine the exact height of the ground at this position

The Corner Calculation

The top-left-back corner, which we’re calling start, is calculated using the position of the ItemScatterer and the size that’s been specified:

        Vector3 start = new Vector3(transform.position.x - size.x / 2.0f,
                                    transform.position.y + size.y / 2.0f,
                                    transform.position.z - size.z / 2.0f);

The transform.position of ItemScatterer is going to be in the centre of its box. To get to the left and the back we take off half the box’s size in the X and Z directions respectively. To get to the top of the box, we add half the box’s size in the Y direction.

The Loops

The pair of loops look like this:

for (float xOffset = 0.0f; xOffset < size.x; xOffset += spacing)
    for (float zOffset = 0.0f; zOffset < size.z; zOffset += spacing)

The first loop, or outer loop, takes a variable called xOffset, initially zero, and keeps increasing it by spacing (default of 3) until it’s bigger than size.x. The represents us moving from the left to the right of the ItemScatterer.

The second loop, or inner loop, takes a variable called zOffset, initially zero, and keeps increasing it by spacing (default of 3) until it’s bigger than size.z. The represents us moving from the back to the front of the ItemScatterer.

The other important thing to note is that because the second loop is inside the first, for every step we make in X, we cover all positions in Z. Here’s what this looks like:

Calculating the Current Position

The calculation if the current position is relatively straightforward, the offsets in the X and Z directions are our loop variables and already calculate and we’re not changing the value of Y at all:

            Vector3 thisPos = start + new Vector3(xOffset, 0.0f, zOffset);

Probability Check

We already have our probability property for us to specify the chances of a location having an item. We compare this to Random.value, which is a random value between 0 and 1. If Random.Value is less than probability, then we’ll place something:

 // Check probability
 if (Random.value < probability)
    // Place an item

Physics Raycast

We have already placed our rocks and terrain into a layer named “Ground”. We only want to place our items on this layer. We can make sure that we only check this layer by using a LayerMask in our call to Physics.Raycast(). We add a new pubic string property to our class called groundLayerName and give it the default “Ground”:

public string groundLayerName = "Ground";

In our Distribute() function then, at the top before the loops, we can create the LayerMask:

LayerMask lm = LayerMask.GetMask(groundLayerName);

We also need to create a RayCastHit variable. This is somewhere that Physics.RayCast() can fill in details about what it hit. The complete code looks like this:

 // Check for ground
 RaycastHit hit = new RaycastHit();
 if (Physics.Raycast(thisPos, Vector3.down, out hit, size.y, lm))
     Instantiate(item, hit.point, Quaternion.identity, transform);

Physics.Raycast() returns a bool value (either true or false) and the if statement determines whether or not to Instantiate an item depending on whether or not ground was hit. The version of Physics.Raycast() we’re using takes the following inputs:

  1. The position to start the search from: thisPos which we calculated previously.
  2. The direction to search in: Vector3.down.
  3. The RaycastHit object that we want filled in: hit. Note the special out instruction that shows we’re going to getting values back from this – it’s not an input.
  4. The distance to search: size.y – no point searching further than this
  5. The LayerMask to use: lm which we calculated previously

The Instantiate is very standard, but we’ve also supplied our own transform as the parent transform. This means when the items are created, they are beneath ItemScatterer in the Heirarchy.

Find Nearest

For a human to scan a classroom and determine which desk is closest is something we do very quickly and intuitively, but a computer program generally has to take a more methodical approach. We look at each desk in turn and calculate how far away it is from us. If this is the first desk, or it’s closer than the previous desk we thought was the closest, we remember this desk and distance is is from is. We keep doing this until there are no more desks to compare against.

Here’s what the code to do this looks like:

   public Transform player;Q 
   public GameObject nearestItem;

    private void FindNearest()
        GameObject foundNearest = null;
        float nearestDist = 0.0f;

        for (int childIndex = 0; childIndex < transform.childCount; childIndex++)
            Transform thisChildTransform = transform.GetChild(childIndex);
            float thisDistToPlayer = (player.position - thisChildTransform.position).magnitude;

            if (foundNearest == null || thisDistToPlayer < nearestDist)
                foundNearest = thisChildTransform.gameObject;
                nearestDist = thisDistToPlayer;

        nearestItem = foundNearest;

We have two new public properties, one to hold the Transform of the player so we can know their position, and the other which will get assigned the nearestItem to the player every time it’s calculated. We call this FindNearest() function from Update() so that it gets run every frame.

Since we Instantiated our items as children of ItemScatterer, we can use transform.childCount and transform.GetChild(n) to know how many children we have and to get each one in turn, respectively.

Subtracting one Vector3 from another Vector3 returns a third Vector3 containing the displacement between them. Getting the magnitude of this returns the actual distance.

Finally our check:

if (foundNearest == null || thisDistToPlayer < nearestDist)

Says if we’ve never assigned a value to foundNearest, which will true the first time, OR (that is what the double-bars || means) the distance we just calculated is smaller than the previously smallest distance we knew about, then make this item the nearest item and remember its distance.

Code Download

The code for this week’s project is on our GitHub, as always. 

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