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Box2dEasy - no collision / isSensor question

NinjadoodleNinjadoodle Member
edited October 2017 in General questions
Hi guys

I'm making some shards of a bottle breaking and I don't want them to collide, astray get created in the same spot. I'm using Box2dEasy, but can't figure out how to disable collision.

I've tried setting isSensor = true, but it doesn't seem to have an effect.

Any help would be awesome :)

Thanks in advance!
local shard1 = Prop.new(self.pack, "shard.png", target.sprite:getX(), target.sprite:getY() - 96, 0.5, 0.5, mg)
world:createRectangle(shard1, {type = "dynamic", isSensor = true})
shard1:set("linearVelocityX", 5 * math.cos(math.random(360)))
shard1:set("linearVelocityY", 5 * math.sin(math.random(360)))
local shard2 = Prop.new(self.pack, "shard.png", target.sprite:getX(), target.sprite:getY() - 96, 0.5, 0.5, mg)
world:createRectangle(shard2, {type = "dynamic", isSensor = true})
shard2:set("linearVelocityX", 5 * math.cos(math.random(360)))
shard2:set("linearVelocityY", 5 * math.sin(math.random(360)))


  • antixantix Member
    edited October 2017 Accepted Answer
    Check out collision filtering. If you make these objects in a group that doesn't collide with any others that should get you the desired result. If you want to read more about this checkout the iForce tutorial... http://www.iforce2d.net/b2dtut/collision-filtering

    and here's a wee example I made over breakfast :)
    -- Simple box2d objects collision filter example
    -- objects in this example collide with only the walls, not other objects
    require ("box2d")
    local random, sin, cos = math.random, math.sin, math.cos
    local w = application:getContentWidth()
    local h = application:getContentHeight()
    -- our collision masks
    BALL_MASK = 1
    WALL_MASK = 2
    -- init box2d
    local world = b2.World.new(0, 0, true)
    -- set box2d debug
    local debugDraw = b2.DebugDraw.new()
    -- create walls
    local walls = world:createBody{type = b2.STATIC_BODY}
    local chain = b2.ChainShape.new()
    chain:createLoop(1, 1, w, 1, w, h, 1, h)
    walls.fixture = walls:createFixture{shape = chain, density = 1.0, friction = 0, restitution = 1}
    walls.fixture:setFilterData({categoryBits = WALL_MASK, maskBits = BALL_MASK, groupIndex = 0}) -- determines what collides with this object
    MAX_BALLS = 50
    local heading = 0
    -- create balls
    for i = 1, MAX_BALLS do
      local x, y = w * 0.5, h * 0.5
      local ball = world:createBody{type = b2.DYNAMIC_BODY} -- create the box2d thing
      local circle = b2.CircleShape.new(0, 0, 10)
      ball.fixture = ball:createFixture{shape = circle, density = 0.1, friction = 0, restitution = 0}
      ball.fixture.userData = TYPE_BALL
      ball.fixture:setFilterData({categoryBits = BALL_MASK, maskBits = WALL_MASK, groupIndex = 0}) -- determines what the ball collides with
      ball:setPosition(x, y)
      local location = {x = x + sin(heading), y = y + cos(heading)}  -- set ball moving at random speed in random direction
      local mag = random() * 2 + 2.34 -- velocity
      local vx = (x - location.x) * mag
      local vy = (y - location.y) * mag
      print(xVect, yVect, x, y, location.x, location.y)
      ball:applyLinearImpulse(vx, vy, location.x, location.y) -- go ball go!
      heading = heading + (360 / MAX_BALLS)
    -- run this every frame
    local function onEnterFrame(event)
      world:step(1/60, 8, 3) -- run box2d simulation
    stage:addEventListener(Event.ENTER_FRAME, onEnterFrame)
  • antixantix Member
    edited October 2017
    Then again, if you don't require any collision at all then maybe you could just use the new Particle system built into Gideros?
  • NinjadoodleNinjadoodle Member
    edited October 2017
    Hi @antix

    Thanks for the reply, I pretty much got it going after a bit of hair pulling, but I'm having an issue.

    I can't seem to add a physics sprite into a group and have it effected by the physics world (probably doing something wrong here).

    Instead I have to add it to the scene e.g:self:addChild(sprite)

    What I really need to do is to add it to a group I've created called 'bg' - bg:addChild(sprite)

    Everything is working fine when I don't try and add it to the group.

    This probably has something to do with me not understanding how the physics world etc. works.

    Any ideas would be awesome!

    Thanks again!
  • Got it! It was an error I've made in the onEnterFrame function :)

    Thanks heaps for your help!

    Likes: antix

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  • YanYan Member
    Is better to use FilterCollisions, because sensors, are handled any way, but when you use filters, bodies are ignoring totally
  • Hi @Yan

    Thank for the tip, I'll keep that in mind!

    In this case I'm only using 4 small physics objects that get destroyed after half a second, so it should not cause any performance issues :)
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