title: Can't Move Group backwards! Corona SDK
body: <p>i'm new here, i'm trying to do a videogame for android with corona sdk, but i'm stuck with the creation of random particle on the background,
  here is the code:</p>
  
  <pre><code>local menu = composer.newScene()
  
  function menu:create( event )
    local bg = self.view
    local particlesGroup = self.view
    local sceneGroup = self.view
  
  
    blackLayer = display.newImageRect(  "black_layer.png",  1280, 300 )
    blackLayer.y = display.contentCenterY + 150
    blackLayer.x = display.contentCenterX
    blackLayer.alpha = 0.6
  
  
  
    background = display.newImageRect( "background.png",  1280 , 720 )
    background.x = display.contentCenterX
    background.y = display.contentCenterY
  
  
  
  
    playBtn = widget.newButton( {
      id="playBtn",
      x=display.contentCenterX+350,
      y=blackLayer.y,
      labelColor = { default={0.922, 0.938, 0.941}, over={0.9, 0.3, 0.234, 0.6}},
      label = "Play Now",
      defaultFile = "redBtn.png",
      overFile = "whiteBtn.png",
      width = 200,
      height = 200,
      fontSize = 30,
      font = "Fonts/Raleway-Bold.ttf",
      --onRelease = composer.gotoScene( "game")
    } )
  
  
    settingsBtn = widget.newButton( {
        id="playBtn",
        x=display.contentCenterX,
        y=blackLayer.y-50,
        font = "Fonts/Raleway-Bold.ttf",
        label = "Settings",
        defaultFile = "rectWhiteBtn.png",
        overFile = "rectRedBtn.png",
        width =250,
        height = 100,
        fontSize = 30,
        labelColor = { default={0.9, 0.3, 0.234} , over={0.922, 0.938, 0.941, 0.6} },
        onRelease = gotoSettings
      } )
  
  
    highScoresBtn = widget.newButton( {
          id="highScoresBtn",
          x=display.contentCenterX,
          y=settingsBtn.y + 100,
          font = "Fonts/Raleway-Bold.ttf",
          label = "High Scores",
          defaultFile = "rectWhiteBtn.png",
          overFile = "rectRedBtn.png",
          width =250,
          height = 100,
          fontSize = 30,
          labelColor = { default={0.9, 0.3, 0.234} , over={0.922, 0.938, 0.941, 0.6} },
          --onRelease = composer.gotoScene("highScores")
        } )
  
        bg:insert(background)
        sceneGroup:insert(blackLayer)
        sceneGroup:insert(playBtn)
        sceneGroup:insert(settingsBtn)
        sceneGroup:insert(highScoresBtn)
  
        function generateParticles()
          local xy=math.random(30,70)
          local newParticle = display.newImageRect( particlesGroup,  objectSheet , math.random(13), xy, xy )
          newParticle.x = (math.random(1280)*math.random(88932))%1280
          newParticle.y = (math.random(720)*math.random(13546))%720
          newParticle.alpha = 0
          transition.to( newParticle, {alpha=1, time=150, onComplete=dimParticle, onCompleteParams=newParticle})
        end
  
        function generateParticlesSmall()
          local xy=math.random(5,15)
          local newParticle = display.newImageRect( particlesGroup,  objectSheet , 1, xy, xy )
          newParticle.x = (math.random(1280)*math.random(88932))%1280
          newParticle.y = (math.random(720)*math.random(13546))%720
          newParticle.alpha = 0
          transition.to( newParticle, {alpha=1, time=150, onComplete=dimParticle, onCompleteParams=newParticle})
        end
  
  end
  
  
  function menu:show( event )
    local bg = self.view
    local particlesGroup = self.view
    local sceneGroup = self.view
    local phase = event.phase
  
  
  
      if ( phase == "will" ) then
  
  
        bigPTimer1=timer.performWithDelay( 1, generateParticles, 0 )
        -- Start the timer wich call 'generateParticles' for each ms
  
  
        smallPTimer=timer.performWithDelay( 1, generateParticlesSmall, 0 )
        -- Start the timer wich call 'generateParticlesSmall' for each ms
        particlesGroup:toBack()
  
      elseif ( phase == "did" ) then
  
  
          if event.phase=="began"then
          end
          if event.phase=="ended"then
          end
  
  
      end
  end
  
  
  function menu:hide( event )
    local bg = self.view
    local particlesGroup = self.view
    local sceneGroup = self.view
      local phase = event.phase
  
      if ( phase == "will" ) then
          -- Code here runs when the scene is still off screen (but is about to come on screen)
  
  
  
      elseif ( phase == "did" ) then
          -- Code here runs when the scene is entirely on screen
  
      end
  end
  
  function menu:destroy( event )
    local bg = self.view
    local particlesGroup = self.view
    local sceneGroup = self.view
  end
  
  --Funzioni
  
  local function gotoGame()
      composer.gotoScene( "game" )
  end
  
  local function gotoHighScores()
      composer.gotoScene( "highscores" )
  end
  
  function gotoSettings()
    reproduceEffet1()
    timer.cancel(bigPTimer1)
    timer.cancel(smallPTimer)
    composer.gotoScene( "settings", options)
  end
  
  --funzioni
  
  local function removetotally( particle )
    display.remove( particle )
  end
  
  function dimParticle( particle )
      transition.to( particle, {alpha=0, time=500, onComplete = removetotally, onCompleteParams = particle})
  end
  
  --
  
  menu:addEventListener( "create", menu )
  menu:addEventListener( "show", menu )
  menu:addEventListener( "hide", menu )
  menu:addEventListener( "destroy", menu )
  
  
  --
  
  return menu
  </code></pre>
  
  <p>All functions are working perfectly, but the problem is that I can't move back the group of particles, between bg and sceneGroup.</p>
  
  <p>Thank for listening me.</p>