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Issues detecting direction of collision — Gideros Forum
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    Issues detecting direction of collision

    ClaudioBAClaudioBA Member
    edited February 2013 in Game & application design
    Hello, first time using Gideros! (and building apps for mobile devices)
    im trying to get the collision and the direction of it but it doesnt work...
    this is part of my code:

    function GCchar:onBeginCollision(event)
    local bodyA, bodyB = event.fixtureA:getBody(), event.fixtureB:getBody();

    if self:collision(bodyA.xSprite) == "B" then
    print("Down")
    end

    end

    function Sprite:collision(sprite2)

    -- self bottom < other sprite top
    if self:getY() + self:getHeight() < sprite2:getY() then
    return "T";
    end
    -- self top > other sprite bottom
    if self:getY() > sprite2:getY() + sprite2:getHeight() then
    return "B";
    end
    -- self left > other sprite right
    if self:getX() > sprite2:getX() + sprite2:getWidth() then
    return "R";
    end
    -- self right < other sprite left
    if self:getX() + self:getWidth() < sprite2:getX() then
    return "L";
    end

    return "N";
    end

    is this approach correct???
    anyone has done this??

    Thanks!
    Tagged:
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    Comments

    • atilimatilim Maintainer
      Hi @ClaudioBA, welcome here :)

      You can use event.contact information like
      local worldManifold = event.contact:getWorldManifold()
print(worldManifold.normal.x, worldManifold.normal.y)
      http://en.wikipedia.org/wiki/Cytorus
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    • ClaudioBAClaudioBA Member
      edited February 2013
      Thanks for the fast response!

      There is a way to detect the bodies making contact?
      for example, my character is walking against a wall (made by a pile of squares), and i get 2 lectures.... i supose one of the lectures is the square of the wall and the other is the square under the wall (on the floor)???


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    • atilimatilim Maintainer
      edited February 2013 Accepted Answer
      After creating your bodies, set a field like "type" (or any other name as you want):
      local body1 = world:createBody(....)
body1.type = "wall"
 
local body2 = world:createBody(...)
body2.type = "character"
      And then you can check
      local bodyA = event.fixtureA:getBody()
 
if bodyA.type == "wall" then
    -- this is wall
else if bodyA.type == "character"
    -- oh no, it's character
end
      http://en.wikipedia.org/wiki/Cytorus
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    • ClaudioBAClaudioBA Member
      Thanks, i'll try that!!!

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    • ClaudioBAClaudioBA Member
      Another question related to contact....
      if i have my character against a wall at his right, how can i get the body at his right if they dont collide at the touch event?
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    • ar2rsawseenar2rsawseen Maintainer
      edited February 2013
      @ClaudioBA did not really understand the problem, but if the bodies are not colliding, but are near, you could use b2.World:queryAABB(lowerx, lowery, upperx, uppery) to get the fixture and then the body, where lowerx, lowery, upperx, uppery is bounding box where to check for box2d bodies
      http://appcodingeasy.com/Gideros-Mobile
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    • ClaudioBAClaudioBA Member
      edited February 2013
      sorry my English is not very good...
      i was trying to get the body colliding with the character, but after the onBeginCollision event take place....

      EDIT: i'm doing it right now, but doing something like this:

      charH is char Height

      if ( bodyBY < bodyAY+charH/2 and bodyBY+32> bodyAY+charH/2) then

      is pretty ugly and im sure there is a better way to acomplish this

      Thanks!
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    • ar2rsawseenar2rsawseen Maintainer
      edited February 2013
      if onBeginCollision event is raised, then it means that bodies are already colliding, so I can't really understand where the problem is :)

      You can check this tutorial: http://appcodingeasy.com/Gideros-Mobile/Animating-Box2d-objects

      where collision detection happens here:
      --define begin collision event handler function
function scene:onBeginContact(e)
    --getting contact bodies
    local fixtureA = e.fixtureA
    local fixtureB = e.fixtureB
    local bodyA = fixtureA:getBody()
    local bodyB = fixtureB:getBody()
 
    --check if first colliding body is a ball
    if bodyB.type and bodyB.type == "ball" then
        --switch animation using our created function
        bodyB.object:switchAnimation()
    end
end
      http://appcodingeasy.com/Gideros-Mobile
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    • ClaudioBAClaudioBA Member
      i mean i need to check contact after another action happens, not collition, so i need to identify the bodies in actual contact with it
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    • ar2rsawseenar2rsawseen Maintainer
      oh ok I think I get it now.
      Well there are two ways, you can implement two methods for Event.BEGIN_CONTACT and Event.END_CONTACT

      inside first one you can set some value self.isColliding = true and on second event set it to false. Then any time you can check if self.isColliding then.

      Second not tested but should work, is to retrieve contact object from contact event and store it for later use
      --define begin collision event handler function
function scene:onBeginContact(e)
   self.contact = e.contact
end
 
function scene:isColliding()
    return self.contact:isTouching()
end
      http://appcodingeasy.com/Gideros-Mobile
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