cocos2d-x背景循环移动问题，求解

Cocos2d-x
#1

function MyScene:moveY()
local function setPos()
local y = self.bg:getPositionY()
local size = self.bg:getContentSize()
self.bg:setPositionY(y - 2*size.height)

    local move1 = cc.MoveBy:create(10,cc.p(0, 2*size.height))
    local fun = cc.Place:create(cc.p(display.cx, - size.height/2))
    local seq1 = cc.Sequence:create(move1, fun)
    local req = cc.RepeatForever:create(seq1)
    
    self.bg:stopAllActions()
    self.bg:runAction(req)

end

local fun = cc.CallFunc:create(setPos)
local size = self.bg:getContentSize()
local move = cc.MoveBy:create(5,cc.p(0, size.height))
local seq = cc.Sequence:create(move, fun)
self.bg:runAction(seq)


local move2 = cc.MoveBy:create(10,cc.p(0, 2*size.height))
local fun = cc.Place:create(cc.p(display.cx,- size.height/2))
local seq2 = cc.Sequence:create(move2, fun)
local req1 = cc.RepeatForever:create(seq2)

self.bg1:stopAllActions()
self.bg1:runAction(req1)

这是一段背景循环滚动代码，但是背景是向上循环移动的，那我想让它往下循环移动，我应该怎么改呢？求大神拯救我