hitbox

tiny helper functions for handling axis-aligned bounding boxes

If you remember that one article on collision detection written by the good folks over at Metanet Software, you may recall that we can model an axis-aligned bounding box by its central position and its halfsize vectors. If we were to translate this definition over to JavaScript, it might look something like this:

{
  halfsize: [ 8, 12 ],
  position: [ 128, 128 ]
}

Notice the use of array vectors, which enables us to further extend this definition into higher dimensions.

{
  halfsize: [ 10, 20, 30 ],
  position: [ 100, 200, 150 ]
}

hitbox provides flexible helper functions to enable easier reading and manipulation of these kinds of data structures in two and three dimensions. For example, the code snippet below employs generous use of these functions in order to simplify collision detection.

for (let block of world.blocks) {
  if (intersects2D(actor.hitbox, block.hitbox)) {
    if (dx < 0) {
      left(actor.hitbox, right(block.hitbox))
      actor.velocity[0] = 0
    } else if (dx > 0) {
      right(actor.hitbox, left(block.hitbox))
      actor.velocity[0] = 0
    }
    if (dy < 0) {
      top(actor.hitbox, bottom(block.hitbox))
      actor.velocity[1] = 0
    } else if (dy > 0) {
      bottom(actor.hitbox, top(block.hitbox))
      actor.velocity[1] = 0
    }
  }
}

usage

left(hitbox[, x])

Finds the x-coordinate of the left edge of the hitbox, or sets it if x is provided.

left(hitbox) === hitbox.position[0] - hitbox.halfsize[0]

right(hitbox[, x])

Finds the x-coordinate of the right edge of the hitbox, or sets it if x is provided.

right(hitbox) === hitbox.position[0] + hitbox.halfsize[0]

top(hitbox[, y])

Finds the y-coordinate of the top of the hitbox, or sets it if y is provided.

top(hitbox) === hitbox.position[1] - hitbox.halfsize[1]

bottom(hitbox[, y])

Finds the y-coordinate of the bottom of the hitbox, or sets it if y is provided.

bottom(hitbox) === hitbox.position[1] + hitbox.halfsize[1]

back(hitbox[, z])

Finds the z-coordinate of the back of the hitbox, or sets it if z is provided.

back(hitbox) === hitbox.position[2] - hitbox.halfsize[2]

front(hitbox[, z])

Finds the z-coordinate of the front of the hitbox, or sets it if z is provided.

front(hitbox) === hitbox.position[2] + hitbox.halfsize[2]

intersects2D(a, b)

Determines if hitbox a and hitbox b are intersecting in two dimensions.

if (intersects2D(actor.hitbox, enemy.hitbox)) {
  actor.health-- // ouch!
}

contains2D(a, b)

Determines if hitbox a contains hitbox b in two dimensions.

for (let actor of world.actors) {
  if (contains2D(viewport, actor.hitbox)) {
    draw(actor)
  }
}

intersects3D(a, b)

Determines if hitbox a and hitbox b are intersecting in three dimensions.

if (intersects3D(actor.hitbox, enemy.hitbox)) {
  actor.health-- // ouch but in 3D!
}

contains3D(a, b)

Determines if hitbox a contains hitbox b in three dimensions.

if (!contains3D(package, product) && !intersects3D(package, product)) {
  // oh no
}