How to get collision coordinates onPostSolve(event) ?

RogerT · December 2012

Hi guys,
I was wondering if there's a way to get collision coordinates ( not center of either colliding bodies, but the place of actual impact ) from a onPostSolve(event) ?
I tried event.x and event.y , but they return a nil value.

Thanks.

atilim · December 2012

@RogerT - it's (getting contact info) not implemented yet.

Mells · March 2013

Hi,

I was wondering if this has been implemented or if an alternative to get collision coordinates exists?

ar2rsawseen · March 2013

@Mells, yes that have been implemented.
Going from the top of my head, but I think that every collision event has a contact property, which is a b2.Contact instance as in:

--define begin collision event handler function
function scene:onBeginContact(e)
    --b2.Contact object from the docs
    print(e.contact)
end

You can then use contact object to retrieve manifolds (contact points) or even reset collision friction and restitution.

Mells · March 2013

You can then use contact object to retrieve manifolds (contact points) or even reset collision friction and restitution.

@ar2rsawseen
Thank you
This is what I have so far (I had to dig to find relevant informations) :

        local manifold = event.contact:getManifold()
	for i, j in pairs(manifold.points) do
		for a, b in pairs(j) do
			print (a, b)
                        -- normalImpulse	number
                        -- localPoint	table
                        -- tangentImpulse	 number
                        for c, d in pairs(j.localPoint) do
			         print (c, d)
                                 -- y
                                 -- x
		end
	end

I am not sure that those x, y are the ones I was looking for?
I wanted to get the coordinates of the collision, but I got some coordinates that are local to...? I have no idea.

Is iterating through the table the more direct way to get those datas?
I feel that I'm missing a simple way but can't find anything relevant in the API.

Edit : I really can't see how I can find the collision's global coordinates.

ar2rsawseen · March 2013

@Mells for global coordinates you should use :getWorldManifold() and the points provided should be global to box2d world

I don't remember the exact structure now, but you can always print it out to see the structure of the table:

--recursive print of tables
function print_r (t, indent, done)
  done = done or {}
  indent = indent or ''
  local nextIndent -- Storage for next indentation value
  for key, value in pairs (t) do
    if type (value) == "table" and not done [value] then
      nextIndent = nextIndent or
          (indent .. string.rep(' ',string.len(tostring (key))+2))
          -- Shortcut conditional allocation
      done [value] = true
      print (indent .. "[" .. tostring (key) .. "] => Table {");
      print  (nextIndent .. "{");
      print_r (value, nextIndent .. string.rep(' ',2), done)
      print  (nextIndent .. "}");
    else
      print  (indent .. "[" .. tostring (key) .. "] => " .. tostring (value).."")
    end
  end
end
 
--use it on table
print_r(t)

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