CODEWITHSUNDEEP
luacoronasdkcollision-detection
tommo
tommo

Reputation: 1350

Detecting which side of a square collides with collision detection

I am using Corona SDK to detect collisions. I have no problem detecting whether the collision was to the left of the square or to the right using the following code:

if (event.other.x < displayObj.x)

the collision code:

function box:collision(event)
  if event.phase == "ended" then 
    if event.other.id and event.other.id == "c" then 
      --need to detect top collision 
      test = 1 
    end

How do I detect whether the top of the square has collided with something?

Upvotes: 0

Views: 291

Answers (1)

Jose Llausas
Jose Llausas

Reputation: 3406

You can use linear algebra! Vectors and the dot product are very useful for figuring out what you want. You can find more indepth information if you google: Half-space test dot product

Assuming you have a box named A and collision point B, with positions (A.x,A.y) and (B.x,B.y) and the forward direction of the box determined by (A.dirX, A.dirY) you can determine if point B is in front or behind box A's direction like this:

  1. Get a vector from position A to B and normalize it:

    vectorToBNormalized = normalize((B.x,B.y)-(A.x,A.y))

  2. Use the dot product of vectorToBNormalized and A's forward vector to determine if B is in front of behind.

    result = dot(vectorToBNormalized, normalize(A.dirX, A.dirY))

If the result is less than 0 then: B is behind A; If the result is greater than 0 then: B is in front of A.

To find out if the box B is left or right of box A, repeat the process but using a right-facing vector instead of a forward vector for box A. For example, if up is defined by (0, 1), then right would be (1, 0).

This way you can determine if the point of collision is behind/infront, or left/right.

Upvotes: 1

Related Questions