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NewtonVR: Physics-based interaction on the Vive (Part 2 + Github)

Tomorrow Today Labs is working on an unannounced VR game for the HTC Vive in Unity and we've spent a lot of design and development time trying to find a method of interacting with objects that feels good to us. Using a mouse to move a box on a screen is a pretty straight forward process. You've only got two axes of input to worry about. But we're in VR now, we've got all three positional axes, plus rotation. This requires a new approach to object interaction.

There are so many amazing experiences yet to be created for VR and we'd like to help accelerate their development by releasing our interaction system for other developers to use - free of charge (MIT license).

Newton VR

Our system allows players to pick up, drop, throw, and use held objects. Items never change their parent during this process and are never set to kinematic. This means that items won't pass through other items (rigidbodies), or the environment (non-rigidbodies). Held items interact with other rigidbodies naturally - taking mass into account.

For example, if you have two boxes of the same mass they can push each other equally, but a balloon, with considerably less mass, can't push a box. For more information on this style of mass based interaction see this post by Nick Abel.

By default, items are configured to be picked up at any point. But, if you have something you want to hold at a certain point, items can be setup to rotate and position themselves to match a predefined orientation. This lets you pick up a box from any corner as well as pick up a gun and have it orient to the grip. Again, we let physics do the heavy lifting here, so items don't clip through walls or the ground while reorienting.

We've created a few physical UI elements to help with basic configuration and menu type scenarios. We also give you the option to dynamically let the controllers turn into physical objects on a button press. This lets you interact with the world as if your controllers were physical objects. Which means that in this mode they are no longer a one to one representation of your real world controllers. I know this may sound sketchy, but in practice it's awesome.

Grip buttons

A hotly debated issue is whether or not to use the grip buttons to pick things up. We feel like the benefit gained by using the grip buttons outweighs the trouble users can have with them. One of the benefits of releasing the code with this system is that if you disagree you're welcome to change the mappings. But, if you use the system with the defaults, then pressing grip button(s) will let you pick something up and releasing it will drop (or throw) the item.

Using the grip buttons to hold an item frees up other buttons on the Vive controller for items that are designed to be used while held (for example holding a gun and then pressing the trigger button to fire). If your controller is not hovering over an interactable object, and you hit the grip button, your controller becomes a physical object that you can use to interact with the world. This mode can be used to press buttons on a control panel or push objects out of your way.

Implementation

Clone or download our repo here: https://github.com/TomorrowTodayLabs/NewtonVR/

We've included SteamVR so the project compiles and will try and keep the version updated. The meat of the project is in the NewtonVR folder. I recommend you clone the repo locally and create a symbolic link to your project so you can get updates and merge changes cleanly.

You can get the desktop github client here: http://desktop.github.com. On windows, open a command line as administrator and use the following command to create a link: mklink /D c:\git\MyProject\Assets\NewtonVR c:\git\NewtonVR\Assets\NewtonVR The first parameter is the location you want to put NewtonVR and the second parameter is the location of your local NewtonVR repo. This is not required, just recommended.

After you've got the project you can check out our example scene in NewtonVR/Example/NVRExampleScene. We've got everything scaled up by a factor of 10 because PhysX seems to work more reliably with larger colliders. The scene includes one of each of everything:

NVRInteractableItem

There's some stacked boxes which have NVRInteractableItem components on them. There's a tiny box on top that you can use to try and push over the stack of boxes to see the mass based system in action. In the drawer there's a gun that has a configured NVRInteractableItem.InteractionPoint set to the handle. When you pick it up the system tries to rotate and position the gun in your hand, and keep it at that orientation.

NVRInteractableRotator

The door is an example of an object with a hinge that has a static position but that you want to rotate by dragging a specific point. We've got the interaction script on just the door knobs, and NVRInteractableRotator.Rigidbody is set to the door's rigidbody. You could also just stick the actual script on the whole door if that makes more sense for your application. To get the currently selected angle (from a zeroed rotation) you can access NVRInteractableRotator.CurrentAngle.

NVRLetterSpinner

There's a letter selection spinner that inherits from NVRInteractableRotator. You can grab and spin it to select a letter. This isn't necessarily the best text input method for VR, but it is a fun one. You can get the currently selected letter by calling NVRLetterSpinner.GetLetter().

NVRSlider

There's a slider example that lerps the color of a sphere between black and yellow. To get the slider's value you can check NVRSlider.CurrentValue. To setup this slider outside of the example you need to set the transforms NVRSlider.StartPoint to the slider's starting location, and NVRSlider.EndPoint to the slider's ending location. Like a lot of these UI Elements we've got a Configurable Joint attached to it to handle the limits and lock position / rotation.

NVRInteractableItem

The interactable item class can also be used to create dial or knob type elements. There's an example of this that reports the current angle of the knob. You can get the current rotation from simply accessing the local euler angles NVRInteractableItem.transform.localEulerAngles.y.

NVRButton

To interact with a button you can either enable NVRPlayer.PhysicalHands and then press the grip buttons to turn your controllers physical, or put pressure on it with another object. The button in the example scene here has a script on it called NVRExampleSpawner which will spawn a cube when the button registers as pressed. Button presses are based off NVRButton.DistanceToEngage. If you move a button far enough from it's initial location then NVRButton.ButtonDown will trigger for a single frame. NVRButton.ButtonIsPushed will be true for as long as the button is down. Then, when the button moves back into its initial position, NVRButton.ButtonUp and NVRButton.ButtonWasPushed will trigger for that frame.

NVRSwitch

Like with NVRButton, NVRSwitch requires either physical hands, or another physical object to interact with it. The switch example in the scene controls a spot light next to it. On Awake() it will set it's rotation to match the value of NVRSwitch.CurrentState.

NVRExampleGun

There's a gun in the drawer that is a nice example of how to use pickup points with NVRInteractableItem as well as how to get input from that component. You can pick up the gun with the grip buttons and shoot with the trigger.

Basic Integration

To integrate NewtonVR into a project you can use our included player prefab in NewtonVR\NVRCameraRig. This is a copy of the SteamVR camerarig prefab with the NewtonVR scripts added. Specifically, there's a NVRPlayer component on the root, a NVRHead component on the head, and NVRHand components on both hands. Alternatively, you can just add those components to your player. Take note though, if you're not using the standard controllers in your project then the physical hand option will not work correctly.

When you've got an item you'd like to pick up, simply drop a NVRInteractableItem component on it. You'll need to give it a Rigidbody (and ideally set the mass) if you haven't already. If the item has a specific point that you'd like to pick it up at you can create a new GameObject, parent it to your item, and position it in the location and at the rotation that you'd like the controller to be. Then set NVRInteractableItem.InteractionPoint to that new gameobject.

Closing

We hope that this system can help you make an awesome VR experience! If you do, be sure to let us know. Anybody is free to use it for basically any purpose: game jams, commercial games, educational apps, etc. Check the license for more info.

We are actively using this system in our game and plan to update it as development continues. If you have questions or comments about this system you can contact us by leaving a message below, on twitter at @TTLabsVR, or creating issues on the github.

NewtonVRDevelopment: Keith Bradner, Nick AbelUX: Adrienne Hunter

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