Shadow library. Given a display group, this will ensure that non-physics-based shadows will be updated to track their parent, whether the parent is physics-based or not. The test sample demonstrates using physics to provide a shadow, as well.

_shadowlib.lua

local lib = {}

function lib.newShadowLayer( shadowgroup )
	local function refresh()
		if (shadowgroup and shadowgroup.numChildren) then
			for i=1, shadowgroup.numChildren do
				local shadow = shadowgroup[i]
				local object = shadowgroup[i].shadowparent
				
				if (not shadow.isBodyActive) then
					shadow.x, shadow.y, shadow.rotation = object.x, object.y, object.rotation
					shadow.rotation = shadow.rotation
				end
			end
		end
	end
	
	function shadowgroup:enterFrame()
		timer.performWithDelay( 1, refresh )
	end
	
	Runtime:addEventListener( "enterFrame", shadowgroup )
	
	function shadowgroup:finalize()
		Runtime:removeEventListener( "enterFrame", shadowgroup )
		shadowgroup = nil
	end
	
	shadowgroup:addEventListener( "finalize" )
	
	return shadowgroup
end

return lib

main.lua

local function shadowLibTest()
	local physics = require("physics")
	physics.start()
	physics.setGravity(0,0)
--	physics.setDrawMode("hybrid")

	local shadowlib = require("shadowlib")
	
	local shadows, objects = display.newGroup(), display.newGroup()
	
	shadows.y = 7
	
	local shadA = display.newRoundedRect( shadows , 200,200 , 200,100 , 25 )
	local rectA = display.newRoundedRect( objects , 200,200 , 200,100 , 25 )
	shadA.shadowparent = rectA
	
	local shadB = display.newRoundedRect( shadows , 200,500 , 200,100 , 25 )
	local rectB = display.newRoundedRect( objects , 200,500 , 200,100 , 25 )
	
	shadA.fill = {.7,.7,.7}
	shadB.fill = {.7,.7,.7}
	
	physics.addBody( rectB, "dynamic" )
	physics.addBody( shadB, "dynamic" )
	
	physics.newJoint( "weld", rectB, shadB, rectB.x, rectB.y )
	
	transition.to( rectA, { time=5000, iterations=0, rotation=rectA.rotation+360 } )
	rectB.angularVelocity = 200
	
	shadowlib.newShadowLayer( shadows )
	
	shadowlib.add( rectA, shadA )
	shadowlib.add( rectB, shadB )
end
shadowLibTest()