In Corona SDK the background image always cover other images

Question

I'm currently making a tower defense game with Corona SDK. However, while I'm making the gaming scene, The background scene always cover the monster spawn, I've tried background:toBack() ,however it's doesn't work.Here is my code:

module(..., package.seeall)

function new()
    local localGroup = display.newGroup();

    local level=require(data.levelSelected);

    local currentDes = 1;

    monsters_list = display.newGroup()

    --The background
    local bg = display.newImage ("image/levels/1/bg.png");
    bg.x = _W/2;bg.y = _H/2;
    bg:toBack();

    --generate the monsters
    function spawn_monster(kind)
        local monster=require("monsters."..kind);
        newMonster=monster.new()
        --read the spawn(starting point) in level, and spawn the monster there
        newMonster.x=level.route[1][1];newMonster.y=level.route[1][2];
        monsters_list:insert(newMonster);
        localGroup:insert(monsters_list);
        return monsters_list;
    end

    function move(monster,x,y)
        -- Using pythagoras to calauate the moving distace, Hence calauate the time consumed according to speed
        transition.to(monster,{time=math.sqrt(math.abs(monster.x-x)^2+math.abs(monster.y-y)^2)/(monster.speed/30),x=x, y=y, onComplete=newDes})
    end

    function newDes()
        currentDes=currentDes+1;
    end

    --moake monster move according to the route
    function move_monster()
        for i=1,monsters_list.numChildren do
            move(monsters_list[i],200,200);
            print (currentDes);
        end 

    end

    function agent()
        spawn_monster("basic");
    end

    --Excute function above.
    timer2 = timer.performWithDelay(1000,agent,10);
    timer.performWithDelay(100,move_monster,-1);
        timer.performWithDelay(10,update,-1);
    move_monster();

    return localGroup;
end

and the monster just stuck at the spawn point and stay there.

but, When i comment these 3 lines of code:

--local bg = display.newImage ("image/levels/1/bg.png");
--bg.x = _W/2;bg.y = _H/2;
--bg:toBack();

The problem disappear

Any ideas??Thanks for helping

Answer 1

Since you are using director, you should insert all your display objects into the localGroup. You haven't inserted bg into localGroup.

SO director class inserts bg finally after inserting localGroup.

Modify your code as

    --The background

    local bg = display.newImage (localGroup,"image/levels/1/bg.png");
    bg.x = _W/2;bg.y = _H/2;
    bg:toBack();

or add the code

 localGroup:insert(bg)