MRTK 2.X For Babylon.js

What is MRTK for Babylon.js

MRTK stands for the Mixed Reality Toolkit, a set of features, controls, and components designed to ease and accelerate development for VR and AR applications.

There currently only exists documentation for MRTK's Unity and Unreal implementations, and so when we talk about matching what MRTK offers, we are comparing to what these offer.

This page will go over each of the MRTK-relevant controls and features present in BabylonJS, showing how each can be used. Many of the controls can also be viewed on the 3D GUI page here, and if a section has a relevant page, that will also be linked.

GUI Controls

The following BabylonJS controls have roots as MRTK features. As they extend the Control3D class, they all support the basic capabilities that Control3D objects have.

More on the Control3D class, and GUI elements in general, can be found here.

The following is an example that showcases several of the controls listed, including TouchHolographicButtons , TouchMeshButtons, and the NearMenu. This scene is designed for use in XR, and supports use of WebXRNearInteraction:

Near Interaction Button Scene Demo

Holographic Button

The HolographicButton class is the standard, familiar button that can be placed and pressed in a 3D environment. It is based off of the MRTK ButtonHoloLens1, and supports a title, image, and tooltip.

Touch Holographic Button

The TouchHolographicButton is akin to MRTK's PressableButtonHoloLens2. It supports everything that the HolographicButton supports, but has more depth to it, allowing for visual feedback as it is being interacted with. As part of this added depth, it now responds to near interactions by having the button surface depress as the user's hand/motion controller presses it.

Touch Mesh Button

While the TouchMeshButton doesn't have a direct representation within MRTK, it is a button that offers custom visuals and support for near interactions. This allows for any object to react to near interaction, if desired, which aligns with MRTK's experimental Hand Physics Service.

Aside from the customization of its visuals, this button behaves in the same way as the TouchHolographicButton.

Holographic Slate

The HolographicSlate is a flat panel that can float in a 3D environment, used to display 2D content in XR experiences.

It consists of a titlebar and content pane. The titlebar has room for a custom title, plus a follow button and close button. The follow button will toggle whether the slate follows the user around or not, and the close button will destroy the slate. The content pane hosts an AdvancedDynamicTexture, which is explained in more detail here.

See it in action here:

Slate Scene w/ Label

The NearMenu is a HolographicMenu with an attached FollowBehavior that tries to stay tethered a set distance from the user at all times.

It is a menu designed for use in XR experiences, and by default stays about half an arm's length away. It also rotates to try and stay in the user's field of view.

The NearMenu also has a pin button, that when pressed, will freeze the menu in space. The menu can also be grabbed around the edges to move it (it comes with an attached SixDofDragBehavior), at which point it becomes pinned in order to maintain its new position.

The HandMenu is a HolographicMenu that is anchored to the user's hand using a HandConstraintBehavior.

Its default position lies on the outside of the user's left hand, such that when their palm is up, the menu is facing them and easily accessible by their right hand.

The menu can be customized to appear by default on the right hand or either hand, and can also be anchored to a different quadrant of the hand. Only one menu will be visible at a time though, even if both hands are up. More information on this can be found in the HandConstraintBehavior's section here.

Slider 3D

The Slider3D is a customizable slider with a 3D presence. The default range of the slider is from 0 to 1, though that can be customized, along with the step count and precision.

The Slider3D can be created with a backplate, which is determined on creation. After creation, the minimum and maximum values can be adjusted, as well as the step size.

See it in action here:

Slider Color Changer

Behaviors and Gizmos

Some behaviors and Gizmos are also shared with MRTK components. In MRTK terminology, these are usually referred to as Solvers, Bounds Control, or Object Manipulators. Behaviors and Gizmos are attached to Nodes, TransformNodes, and Meshes in order to provide additional functionality.

More information on Gizmos can be found here, while behaviors are covered in more detail here.

SixDof Drag Behavior

The SixDofDragBehavior, also known as MRTK's Object Manipulator component. By creating and attaching it to an object in the scene, users are able to grab, move, and rotate the object. Interacting with a ray-cast input allows the object to be moved in 3-dimensions, while interacting with near interaction adds the ability to rotate the object in-place.

const sixDofDragBehavior = new BABYLON.SixDofDragBehavior();

To attach a SixDofDragBehavior to a mesh in the scene, simply call the mesh's addBehavior function:

mesh.addBehavior(sixDofDragBehavior);

Follow Behavior

The FollowBehavior tries to maintain a set distance between the TransformNode it's attached to, and the followedCamera target (defaults to main camera).

There are many aspects to this behavior that can be customized. These are the core properties:

Property	Type	Description
Property followedCamera	Type Camera	Description The camera the behavior will try to follow
Property defaultDistance	Type float	Description The starting distance to the camera, in meters
Property minimumDistance	Type float	Description The minimum distance before the behavior starts to move the attached `TransformNode` further away
Property maximumDistance	Type float	Description The maximum distance before the behavior starts to move the attached `TransformNode` closer
Property verticalMaxDistance	Type float	Description The maximum height offset allowed between the camera's position and the `TransformNode`'s position

Hand Constraint Behavior

The HandConstraintBehavior is a behavior that constrains the attached mesh to a specific location on the user's hand. The visibility of the attached mesh can be toggled based on the rotation of the hand and how centered it is in the user's field of view, by setting the handConstraintVisibility property.

This behavior takes advantage of the WebXREyeTracking feature, allowing the handConstraintVisibility propery to check whether the user is actively looking at their hand/the mesh or not. As a fallback if no eye data is available, it uses the direction the device is facing instead.

The following are the core properties that can be set to customize the behavior:

Property	Type	Description
Property targetZone	Type HandConstraintSide	Description An enum that determines which side/quadrant of the hand the mesh is anchored to and offset from
Property zoneOrientationMode	Type HandConstraintOrientation	Description Determines whether the attached mesh's position should rotate with the hand
Property nodeOrientationMode	Type HandConstraintOrientation	Description Determines whether the attached mesh should rotate with the hand
Property handedness	Type XRHandedness	Description Should the attached mesh only stick to the left hand, or right hand? Defaults to none, allowing it to stick to either hand as they become available.

Bounding Box Gizmo

The BoundingBoxGizmo is also known as MRTK's Bounds Control component. This creates small objects that surround the attached mesh, which can be interacted with to rotate, scale, and translate the attached mesh.

Background Features

These are features that work in the background to enhance the capabilities available to a user.

General XR features are covered in detail here, while AR-specific features are covered in detail here.

Hand Tracking and Near Interaction

The Hand Tracking feature, WebXRHandTracking, is an enhancement to hand inputs for XR devices that support it.

This feature provides articulated joint tracking, hand meshes, and enables the use of touch interactions using near interaction.

The WebXRNearInteraction feature is enabled by default on the Babylon playground, and allows for touch interactions when using hands or motion controllers. Touch-supported objects will be able to dynamically react to 3D input, creating a more immersive experience than what basic buttons can provide.

If the WebXRHandTracking feature is not enabled, then the near interaction model used will be a visible orb that floats in front of the hand (or motion controller). This orb will only be visible while near a component that supports Near Interaction.

To provide support for Near Interaction to an arbitrary AbstractMesh, simply set the isNearPickable and/or isNearGrabbable properties. isNearPickable will enable near interaction in the form of touch input, whereas isNearPickable will allow the object to be grabbed. The main difference here is that touch input triggers a press on touch, but grabbing the object requires touch and a trigger of some kind (pulling the trigger on a controller, or performing a pinch gesture with a hand). Only one of these options should be enabled for a given object, to prevent unintended interactions.

Eye Tracking

Disclaimer: The Eye tracking feature is only supported with BabylonNative for now.

The Eye Tracking feature, WebXREyeTracking, is an opt-in feature for supported XR devices that captures the direction the user's eyes are facing, and passes that along for developers to use.

To use, simply enable the Eye Tracking feature when setting up a project in BabylonNative.

You then have the option to either listen to an observable, or directly query the component for the latest data. Eye tracking data is always provided as a Ray.