"Table-top" Interactions in Virtual Reality

UI and UX conventions are very different in VR than they are in traditional games. As I write this, the Oculus Rift and HTC Vive are still in the pre-order stage, and so a polished VR experience is not something that is widely available. The possibilities are wide open, and game developers are experimenting like crazy to find the best ways to let players interact in this new medium.

Traditionally, tower defense games provide a top-down view of the battlefield where players can design the layout of their turrets and walls which route the enemy to destruction.

XLR first-person point of view

In XLR we allow players to build their defenses from a first person point of view, but we also thought it would feel great to be able to manage everything from a top-down vantage point. Building a game table where the player could see all the pieces in play seemed like an obvious solution. Imagine a futuristic VR version of a war room plotting table1. How cool would that be?

Plotting Table

So we built our plotting table and began to test it out. Having the opportunity to interact with it first-hand, we immediately saw that we had a scale problem. Unless we were going to have a very small battlefield, it was going to be difficult to present the whole thing at a reasonable scale on a table. We're fortunate enough to be developing for the HTC Vive, which offers room-scale VR. That is, you can physically move around in the VR environment with your IRL legs and body. But we still have to deal with a limited-size play area. Room-scale VR is only as big as the player's real world room, after all2.

Room Scale VR

The whole point of an overview is that you can see it all at once, so we didn't want to make a game table so large that the player would have to teleport to different positions around it. The overview table had to be a reasonable size. If we zoomed the battlefield out far enough to see the whole thing on the table, it was too small to work with. If we zoomed in to a comfortable level, there was always this feeling of missing out on a big portion of the battlefield.

XLR Plotting Table

Our initial solution was to provide a dongle of sorts that would float in 3D space above the game table. The player could grab the dongle and move it horizontally to pan the battlefield map, and vertically to zoom in or out. It was a neat toy to play with, and it might have been passable for the initial release. But "passable" wasn't what we were going for. It really bugged us that it didn't feel right, so we kept iterating.

Omni Tool

The tool we use for placing and upgrading defenses in the game is held in your hand, and it shoots out a targeting laser to the position on the battlefield that you want to interact with. There are a number of reasons we chose this type of tool over direct manipulation with virtual hands for our game. But the combination of this tool and the traditional game table we had set up meant that you always had to bend over and hold your arm up so that your elbow was close to the same level as your head in order to really get the most out of it. It was physically uncomfortable to do for any non-trivial amount of time.


This was to be a core mechanic of XLR, and we wanted to get it right. So Vox and I buckled ourselves in and embarked on an evening of experimentation. I think it was about one o'clock in the morning that Vox threw a sphere into the game world in place of the game table. I had previously tried a sphere, but I did it in a different way and had dismissed the idea. My sphere was about the diameter of a tire, and it was something to be used from an external point of view. Saruman's Palantír Vox's sphere was something to be interacted with from within, and was large enough to walk into. I was skeptical when I saw him drop in the sphere. But my jaw dropped as soon as I walked into it. We were really on to something here! It feels so great to step into this portal and interact with an overview of the battlefield.

It's very exciting to have the opportunity to be one of the first developers on the mainstream virtual reality scene. Experimentation and discoveries like this one are what make it so fulfilling. It's difficult to get a feel for what room scale VR is really like without actually experiencing it. We think people are going to go nuts for it, and we can't wait to be able share XLR with the world.