Unique Portals?

local Portals = RegisterMod("uniqueportals", 1)
local game = Game()

local Mod = {}

function Mod:onPortalSpawn(entity)



    local level = game:GetLevel()

    local room = game:GetRoom()

    local backdrop = room:GetBackdropType()

    local sprite = entity:GetSprite()

    local data = entity:GetData()


        -- check if the enemy already has the different sprite

        if data.Changed == nil then

            data.Changed = true



            -- check the current backdrop

            if backdrop == 1 then  -- Basement

                -- replace the sprite/spritesheet

                sprite:ReplaceSpritesheet(0, "gfx/monsters/custom/monster_portal_basement.png")

                sprite:LoadGraphics()



            elseif backdrop == 2 then  -- Cellar

                -- replace the sprite/spritesheet

                sprite:ReplaceSpritesheet(0, "gfx/monsters/custom/monster_portal_cellar.png")

                sprite:LoadGraphics()



            elseif backdrop == 3 then  -- Burning Basement

                -- replace the sprite/spritesheet

                sprite:ReplaceSpritesheet(0, "gfx/monsters/custom/monster_portal_burning.png")

                sprite:LoadGraphics()

           -- and so on...

           end
    end
end

Portals:AddCallback(ModCallbacks.MC_POST_NPC_INIT, Mod.onPortalSpawn, EntityType.ENTITY_PORTAL)