# Question about Physics Collision Detection

Does anyone know if it is possible to detect what side of a physics object has been collided with? For example if the bottom of a square has been hit and then it does something, but NOT do anything when it is hit from any other side?

Thanks for any help in advance,
Jonathon

If you have a look at one (I can’t remember which) of the physics demos, it actually circles the vertices which collide.

@JonoGaming00 This kind of interested me, so I thought I’d create a small example. The red circles show the contact points. This should work with any type body, but I was lazy and only used circles. The function that determines the contact points for collisions is the bod:collide function. Everything else is just standard code. I tried to make it as simple as I could so it would be easy to understand.

``````function setup()
b1=bod()
c=physics.body(CIRCLE,70)
c.x=370
c.y=600
c.restitution=.5
c.gravityScale=.1
c=physics.body(CIRCLE,50)
c.x=190
c.y=280
c.restitution=.8
c.type=STATIC
c=physics.body(CIRCLE,50)
c.x=410
c.y=370
c.type=STATIC
c=physics.body(CIRCLE,20)
c.x=320
c.y=254
c.type=STATIC
end

function draw()
background(0)
b1:draw()
end

function collide(c)
b1:collide(c)
end

bod=class()

function bod:init()
self.bodies={}
self.contacts={}
end

table.insert(self.bodies,b)
end

function bod:draw()
stroke(255)
strokeWidth(2)
noFill()
-- draw circles
for a,b in pairs(self.bodies) do
end
noStroke()
fill(255,0,0)
-- draw contact points
for c,d in pairs(self.contacts) do
for e,f in pairs(d.points) do
ellipse(f.x,f.y,20)
end
end
end

function bod:collide(c)
if c.state==BEGAN then
self.contacts[c.id]=c
elseif c.state==ENDED then
self.contacts[c.id]=nil
end
end
``````

@Ignatz I couldn’t find the demo I think you mean, but I have found they all do what @dave1707 has done with his code.

@dave1707 I think this what I am looking for. However how do I use this to determine what side has been collided with?

@JonoGaming00 In your first post you’re talking about a square. To determine which side of a square was hit, use the x,y values of the squares corners and the x,y values from my program. That can tell you which side was hit.

When I have time, I’ll setup a square in my program and see what really happens.

@dave1707 I see what you mean. I’ll give that a shot!