To be clear “The” Portal Hopper is our Hopper which is also sometimes referred to as the Standard Hopper. However, a Hopper is an app which supports a simple protocol which ties all worlds and locations in a multiverse together.
In order to deserve the name “Hopper”, it must be able to
– interpret a VRML file which describes at least which Hopper is required to enter a world and the link to this world
– be able to “hop” through the portal, meaning it can either load the next world or jumpstart another hopper who can handle this world
– run an installer for other hopper-aware apps
Theoretically, a game which is based on a completely different physics/graphics/game engine can be a Hopper if it complies to above rules.
If you are a developer and want to make your code hopper-aware, please refer to the Fundamentals for Developers section, for instance Technology Stack of VRWeb, and our forum.