1. Introduction

1.1. Terminology

This document uses the acronym XR throughout to refer to the spectrum of hardware, applications, and techniques used for Virtual Reality, Augmented Reality, and other related technologies. Examples include, but are not limited to:

• Head-mounted displays, whether they are opaque, transparent, or utilize video passthrough

• Mobile devices with positional tracking

• Fixed displays with head tracking capabilities

The important commonality between them being that they offer some degree of spatial tracking with which to simulate a view of virtual content.

Terms like "XR Device", "XR Application", etc. are generally understood to apply to any of the above. Portions of this document that only apply to a subset of these devices will indicate so as appropriate.

The terms 3DoF and 6DoF are used throughout this document to describe the tracking capabilities of XR devices.

• A 3DoF device, short for "Three Degrees of Freedom", is one that can only track rotational movement. This is common in devices which rely exclusively on accelerometer and gyroscope readings to provide tracking. 3DoF devices do not respond translational movements from the user, though they may employ algorithms to estimate translational changes based on modeling of the neck or arms.

• A 6DoF device, short for "Six Degrees of Freedom", is one that can track both rotation and translation, enabling for precise 1:1 tracking in space. This typically requires some level of understanding of the user’s environment. That environmental understanding may be achieved via inside-out tracking, where sensors on the tracked device itself (such as cameras or depth sensors) are used to determine the device’s position, or outside-in tracking, where external devices placed in the user’s environment (like a camera or light emitting device) provides a stable point of reference against which the XR device can determine it’s position.

1.2. Application flow

2. Model

2.1. XR device

An XR device is a physical unit of hardware that can present imagery to the user. On desktop clients, this is usually a headset peripheral. On mobile clients, it may represent the mobile device itself in conjunction with a viewer harness. It may also represent devices without stereo-presentation capabilities but with more advanced tracking.

An XR device has a list of supported modes (a list of strings) that contains the enumeration values of XRSessionMode that the XR device supports.

The user-agent MUST have an inline XR Device , which is an XR Device that MUST contain "inline" in its list of supported modes. The Inline XR Device will report as much pose information of the physical device the user agent is rendering to as possible. This device MAY be the same as the immersive XR device if one is present, but doesn’t have to be.

Note: On phones, the inline XR Device will report gyroscopic pose information of the phone itself. On desktops and laptops without gyroscopes, the inline XR Device will not be able to report a pose. In case the user agent is already running on an XR device, the inline XR device will be the same device, and may support multiple views.

3. Initialization

3.1. navigator.xr

 partial   interface   Navigator  {
  [ SecureContext ,  SameObject ]  readonly   attribute   XR   xr ;
};

The xr attribute’s getter MUST return the XR object that is associated with the context object.

3.2. XR

[ SecureContext ,  Exposed = Window ]  interface    XR   :  EventTarget  {
  // Methods
   Promise < boolean >  isSessionSupported ( XRSessionMode    mode  );
  [ NewObject ]  Promise < XRSession >  requestSession ( XRSessionMode    mode  ,  optional   XRSessionInit    options   = {});
  // Events
   attribute   EventHandler   ondevicechange ;
};

The user agent MUST create an XR object when a Navigator object is created and associate it with that object.

An XR object is the entry point to the API, used to query for XR features available to the user agent and initiate communication with XR hardware via the creation of XRSessions.

An XR object has a list of immersive XR devices (a list of XR device), which MUST be initially an empty list.

An XR object has an immersive XR device (null or XR device) which is initially null and represents the active XR device from the list of immersive XR devices.

The user agent MUST be able to enumerate immersive XR devices attached to the system, at which time each available device is placed in the list of immersive XR devices. Subsequent algorithms requesting enumeration MUST reuse the cached list of immersive XR devices. Enumerating the devices should not initialize device tracking. After the first enumeration the user agent MUST begin monitoring device connection and disconnection, adding connected devices to the list of immersive XR devices and removing disconnected devices.

Note: The user agent is allowed to use any criteria it wishes to select an immersive XR device when the list of immersive XR devices contains multiple devices. For example, the user agent may always select the first item in the list, or provide settings UI that allows users to manage device priority. Ideally the algorithm used to select the default device is stable and will result in the same device being selected across multiple browsing sessions.

The ondevicechange attribute is an Event handler IDL attribute for the devicechange event type.

Calling isSessionSupported() MUST NOT trigger device-selection UI as this would cause many sites to display XR-specific dialogs early in the document lifecycle without user activation. Additionally, calling isSessionSupported() MUST NOT interfere with any running XR applications on the system, and MUST NOT cause XR-related applications to launch such as system trays or storefronts.

navigator . xr . isSessionSupported (  'immersive-vr'  ). then (( supported )   =>   {
   if   ( supported )   {
     // 'immersive-vr' sessions are supported.
     // Page should advertise support to the user.
   }   else   {
     // 'immersive-vr' sessions are not supported.
   }
 }

The XR object has a pending immersive session boolean, which MUST be initially false, an active immersive session , which MUST be initially null, and a list of inline sessions , which MUST be initially empty.

 let  xrSession ;
navigator . xr . requestSession (  "immersive-vr"  ). then (( session )   =>   {
  xrSession  =  session ;
 });

3.3. XRSessionMode

The XRSessionMode enum defines the modes that an XRSession can operate in.

 enum    XRSessionMode   {
   "inline" ,
   "immersive-vr" 
};

• A session mode of inline indicates that the session’s output will be shown as an element in the HTML document. inline session content MUST be displayed in mono (i.e., with a single view). It MAY allow for viewer tracking. User agents MUST allow inline sessions to be created.

• A session mode of immersive-vr indicates that the session’s output will be given exclusive access to the immersive XR device display and that content is not intended to be integrated with the user’s environment.

In this document, the term inline session is synonymous with an inline session and the term immersive session is synonymous with an immersive-vr session.

Immersive sessions MUST provide some level of viewer tracking, and content MUST be shown at the proper scale relative to the user and/or the surrounding environment. Additionally, Immersive sessions MUST be given exclusive access to the immersive XR device, meaning that while the immersive session is "visible" the HTML document is not shown on the immersive XR device's display, nor does content from any other source have exclusive access. Exclusive access does not prevent the user agent from overlaying its own UI, however this UI SHOULD be minimal.

Note: Future specifications or modules may expand the definition of immersive session include additional session modes.

Note: Examples of ways exclusive access may be presented include stereo content displayed on a virtual reality headset.

Note: As an example of overlaid UI, the user-agent or operating system in an immersive session may show notifications over the rendered content.

3.4. Feature Dependencies

Some features of an XRSession may not be universally available for a number of reasons, among which is the fact not all XR devices can support the full set of features. Another consideration is that some features expose sensitive information which may require a clear signal of user intent before functioning.

Since it is a poor user experience to initialize the underlying XR platform and create an XRSession only to immediately notify the user that the applications cannot function correctly, developers can indicate required features by passing an XRSessionInit dictionary to requestSession(). This will block the creation of the XRSession if any of the required features are unavailable due to device limitations or in the absence of a clear signal of user intent to expose sensitive information related to the feature.

Additionally, developers are encouraged to design experiences which progressively enhance their functionality when run one more capable devices. Optional features which the experience does not require but will take advantage of when available must also be indicated in an XRSessionInit dictionary to ensure that user intent can be determined before enabling the feature if necessary.

 dictionary    XRSessionInit   {
   sequence < any >  requiredFeatures ;
   sequence < any >  optionalFeatures ;
};

The requiredFeatures array contains any Required features for the experience. If any value in the list is not a recognized feature descriptor the XRSession will not be created. If any feature listed in the requiredFeatures array is not supported by the XR Device or, if necessary, has not received a clear signal of user intent the XRSession will not be created.

The optionalFeatures array contains any Optional features for the experience. If any value in the list is not a recognized feature descriptor it will be ignored. Features listed in the optionalFeatures array will be enabled if supported by the XR Device and, if necessary, given a clear signal of user intent, but will not block creation of the XRSession if absent.

Values given in the feature lists are considered a valid feature descriptor if the value is one of the following:

• Any XRReferenceSpaceType enum value

Future iterations of this specification and additional modules may expand the list of accepted feature descriptors.

Note: Features are accepted as an array of any values to ensure forwards compatibility. It allows unrecognized optional values to be properly ignored as new feature descriptor types are added.

Depending on the XRSessionMode requested, certain feature descriptors are added to the requiredFeatures or optionalFeatures lists by default. The following table describes the default features associated with each session type and feature list:

The combined list of feature descriptors given by the requiredFeatures and optionalFeatures are collectively considered the requested features for an XRSession.

Some feature descriptors, when present in the requested features list, are subject to feature policy and/or requirements that user intent to use the feature is well understood, via either explicit consent or implicit consent. The following table describes the feature requirements that must be satisfied prior to being enabled:

Note: "local" is always included in the requested features of immersive sessions as a default feature, and as such immersive sessions always need to obtain explicit consent or implicit consent.

Requested features can only be enabled for a session if the XR Device is capable of supporting the feature, which means that the feature is known to be supported by the XR Device in some configurations, even if the current configuration has not yet been verified as supporting the feature. The user agent MAY apply more rigorous constraints if desired in order to yield a more consistent user experience.

Note: For example, several VR devices support either configuring a safe boundary for the user to move around within or skipping boundary configuration and operating in a mode where the user is expected to stand in place. Such a device can be considered to be capable of supporting "bounded-floor" XRReferenceSpaces even if they are currently not configured with safety boundaries, because it’s expected that the user could configure the device appropriately if the experience required it. This is to allow user agents to avoid fully initializing the XR Device or waiting for the user’s environment to be recognized prior to resolving the requested features if desired. If, however, the user agent knows that the boundary state at the time the session is requested without additional initialization it may choose to reject the "bounded-floor" feature if the safety boundary not already configured.

4. Session

4.1. XRSession

Any interaction with XR hardware is done via an XRSession object, which can only be retrieved by calling requestSession() on the XR object. Once a session has been successfully acquired, it can be used to poll the viewer pose, query information about the user’s environment, and present imagery to the user.

The user agent, when possible, SHOULD NOT initialize device tracking or rendering capabilities until an XRSession has been acquired. This is to prevent unwanted side effects of engaging the XR systems when they’re not actively being used, such as increased battery usage or related utility applications from appearing when first navigating to a page that only wants to test for the presence of XR hardware in order to advertise XR features. Not all XR platforms offer ways to detect the hardware’s presence without initializing tracking, however, so this is only a strong recommendation.

 enum    XRVisibilityState   {
   "visible" ,
   "visible-blurred" ,
   "hidden" ,
};
[ SecureContext ,  Exposed = Window ]  interface    XRSession   :  EventTarget  {
  // Attributes
   readonly   attribute   XRVisibilityState   visibilityState ;
  [ SameObject ]  readonly   attribute   XRRenderState   renderState ;
  [ SameObject ]  readonly   attribute   XRInputSourceArray   inputSources ;
  // Methods
   void   updateRenderState ( optional   XRRenderStateInit    state   = {});
  [ NewObject ]  Promise < XRReferenceSpace >  requestReferenceSpace ( XRReferenceSpaceType    type  );
   long   requestAnimationFrame ( XRFrameRequestCallback    callback  );
   void   cancelAnimationFrame ( long    handle  );
   Promise < void >  end ();
  // Events
   attribute   EventHandler   onend ;
   attribute   EventHandler   onselect ;
   attribute   EventHandler   oninputsourceschange ;
   attribute   EventHandler   onselectstart ;
   attribute   EventHandler   onselectend ;
   attribute   EventHandler   onvisibilitychange ;
};

Each XRSession has a mode , which is one of the values of XRSessionMode.

A number of different circumstances may shut down the session , which is permanent and irreversible. Once a session has been shut down the only way to access the XR device's tracking or rendering capabilities again is to request a new session. Each XRSession has an ended boolean, initially set to false, that indicates if it has been shut down.

Each XRSession has a list of enabled features , which is a list of feature descriptors which MUST be initially an empty list

Each XRSession has an active render state which is a new XRRenderState, and a pending render state , which is an XRRenderState which is initially null.

The renderState attribute returns the XRSession's active render state.

Each XRSession has a minimum inline field of view and a maximum inline field of view , defined in radians. The values MUST be determined by the user agent and MUST fall in the range of 0 to PI.

Each XRSession has a minimum near clip plane and a maximum far clip plane , defined in meters. The values MUST be determined by the user agent and MUST be non-negative. The minimum near clip plane SHOULD be less than 0.1. The maximum far clip plane SHOULD be greater than 1000.0 (and MAY be infinite).

Each XRSession has a list of active XR input sources (a list of XRInputSource) which MUST be initially an empty list.

Each XRSession has an XR device , which is an XR device set at initialization.

The inputSources attribute returns the XRSession's list of active XR input sources.

The user agent MUST monitor any XR input sources associated with the XR Device, including detecting when XR input sources are added, removed, or changed.

Each XRSession has a visibility state value, which is an enum which MUST be set to whichever of the following values best matches the state of session.

• A state of visible indicates that imagery rendered by the XRSession can be seen by the user and requestAnimationFrame() callbacks are processed at the XR device's native refresh rate. Input is processed by the XRSession normally.

• A state of visible-blurred indicates that imagery rendered by the XRSession may be seen by the user, but is not the primary focus. requestAnimationFrame() callbacks MAY be throttled. Input is not processed by the XRSession.

• A state of hidden indicates that imagery rendered by the XRSession cannot be seen by the user. requestAnimationFrame() callbacks will not be processed until the visibility state changes. Input is not processed by the XRSession.

The visibilityState attribute returns the XRSession's visibility state. The onvisibilitychange attribute is an Event handler IDL attribute for the visibilitychange event type.

The visibility state MAY be changed by the user agent at any time other than during the processing of an XR animation frame, and the user agent SHOULD monitor the XR platform when possible to observe when session visibility has been affected external to the user agent and update the visibility state accordingly.

Note: The XRSession's visibility state does not necessarily imply the visibility of the HTML document. Depending on the system configuration the page may continue to be visible while an immersive session is active. (For example, a headset connected to a PC may continue to display the page on the monitor while the headset is viewing content from an immersive session.) Developers should continue to rely on the [Page Visibility API](https://w3c.github.io/page-visibility/) to determine page visibility.

Each XRSession has a viewer reference space , which is an XRReferenceSpace of type "viewer" with an identity transform origin offset. The viewer reference space has a list of views , which is a list of views corresponding to the views provided by the XR device. If the XRSession's renderState's composition disabled boolean is set to true the list of views MUST contain a single view.

The onend attribute is an Event handler IDL attribute for the end event type.

The oninputsourceschange attribute is an Event handler IDL attribute for the inputsourceschange event type.

The onselectstart attribute is an Event handler IDL attribute for the selectstart event type.

The onselectend attribute is an Event handler IDL attribute for the selectend event type.

The onselect attribute is an Event handler IDL attribute for the select event type.

4.2. XRRenderState

An XRRenderState represents a set of configurable values which affect how an XRSession's output is composited. The active render state for a given XRSession can only change between frame boundaries, and updates can be queued up via updateRenderState().

 dictionary    XRRenderStateInit   {
   double    depthNear  ;
   double    depthFar  ;
   double    inlineVerticalFieldOfView  ;
   XRWebGLLayer ?   baseLayer  ;
};
[ SecureContext ,  Exposed = Window ]  interface    XRRenderState   {
   readonly   attribute   double   depthNear ;
   readonly   attribute   double   depthFar ;
   readonly   attribute   double ?  inlineVerticalFieldOfView ;
   readonly   attribute   XRWebGLLayer ?  baseLayer ;
};

Each XRRenderState has a output canvas , which is an HTMLCanvasElement initially set to null. The output canvas is the DOM element that will display any content rendered for an "inline" XRSession.

Each XRRenderState also has composition disabled boolean, which is initially false. The XRRenderState is considered to be have composition disabled if rendering commands performed for an "inline" XRSession are executed in such a way that they are directly displayed into output canvas, rather than first being processed by the XR Compositor.

Note: At this point the XRRenderState will only have an output canvas if it has composition disabled, but future versions of the specification are likely to introduce methods for setting output canvas' that support more advanced uses like mirroring and layer compositing that will require composition.

The depthNear attribute defines the distance, in meters, of the near clip plane from the viewer. The depthFar attribute defines the distance, in meters, of the far clip plane from the viewer.

depthNear and depthFar is used in the computation of the projectionMatrix of XRViews and determines how the values of an XRWebGLLayer depth buffer are interpreted. depthNear MAY be greater than depthFar.

The inlineVerticalFieldOfView attribute defines the default vertical field of view in radians used when computing projection matrices for "inline" XRSessions. The projection matrix calculation also takes into account the aspect ratio of the output canvas. This value MUST be null for immersive sessions.

The baseLayer attribute defines an XRWebGLLayer which the XR compositor will obtain images from.

4.3. Animation Frames

The primary way an XRSession provides information about the tracking state of the XR device is via callbacks scheduled by calling requestAnimationFrame() on the XRSession instance.

 callback    XRFrameRequestCallback   =  void  ( DOMHighResTimeStamp    time  ,  XRFrame    frame  );

Each XRFrameRequestCallback object has a cancelled boolean initially set to false.

Each XRSession has a list of animation frame callbacks , which is initially empty, and an animation frame callback identifier , which is a number initially be zero.

 let  xrSession  =   null  ;
 function  onWindowAnimationFrame ( time )   {
  window . requestAnimationFrame ( onWindowAnimationFrame );
   // This may be called while an immersive session is running on some devices,
   // such as a desktop with a tethered headset. To prevent two loops from
   // rendering in parallel, skip drawing in this one until the session ends.
   if   (  ! xrSession )   {
    renderFrame ( time ,   null  );
   }
 }
 // The window animation loop can be started immediately upon the page loading.
window . requestAnimationFrame ( onWindowAnimationFrame );
 function  onXRAnimationFrame ( time ,  xrFrame )   {
  xrSession . requestAnimationFrame ( onXRAnimationFrame );
  renderFrame ( time ,  xrFrame );
 }
 function  renderFrame ( time ,  xrFrame )   {
   // Shared rendering logic.
 }
 // Assumed to be called by a user gesture event elsewhere in code.
 function  startXRSession ()   {
  navigator . xr . requestSession (  'immersive-vr'  ). then (( session )   =>   {
    xrSession  =  session ;
    xrSession . addEventListener (  'end'  ,  onXRSessionEnded );
     // Do necessary session setup here.
     // Begin the session’s animation loop.
    xrSession . requestAnimationFrame ( onXRAnimationFrame );
   });
 }
 function  onXRSessionEnded ()   {
  xrSession  =   null  ;
 }

4.4. The XR Compositor

The user agent MUST maintain an XR Compositor which handles presentation to the XR device and frame timing. The compositor MUST use an independent rendering context whose state is isolated from that of any graphics contexts created by the document. The compositor MUST prevent the page from corrupting the compositor state or reading back content from other pages or applications. The compositor MUST also run in separate thread or processes to decouple performance of the page from the ability to present new imagery to the user at the appropriate framerate. The compositor MAY composite additional device or user agent UI over rendered content, like device menus.

Note: Future extensions to this spec may utilize the compositor to composite multiple layers coming from the same page as well.

5. Frame Loop

5.1. XRFrame

An XRFrame represents a snapshot of the state of all of the tracked objects for an XRSession. Applications can acquire an XRFrame by calling requestAnimationFrame() on an XRSession with an XRFrameRequestCallback. When the callback is called it will be passed an XRFrame. Events which need to communicate tracking state, such as the select event, will also provide an XRFrame.

[ SecureContext ,  Exposed = Window ]  interface    XRFrame   {
  [ SameObject ]  readonly   attribute   XRSession   session ;
   XRViewerPose ?  getViewerPose ( XRReferenceSpace    referenceSpace  );
   XRPose ?  getPose ( XRSpace    space  ,  XRSpace    baseSpace  );
};

Each XRFrame has an active boolean which is initially set to false, and an animationFrame boolean which is initially set to false.

The session attribute returns the XRSession that produced the XRFrame.

6. Spaces

A core feature of the WebXR Device API is the ability to provide spatial tracking. Spaces are the interface that enable applications to reason about how tracked entities are spatially related to the user’s physical environment and each other.

6.1. XRSpace

An XRSpace represents a virtual coordinate system with an origin that corresponds to a physical location. Spatial data that is requested from the API or given to the API is always expressed in relation to a specific XRSpace at the time of a specific XRFrame. Numeric values such as pose positions are coordinates in that space relative to its origin. The interface is intentionally opaque.

[ SecureContext ,  Exposed = Window ]  interface    XRSpace   :  EventTarget  {
};

Each XRSpace has a session which is set to the XRSession that created the XRSpace.

Each XRSpace has a native origin that is tracked by the XR device's underlying tracking system, and an effective origin , which is the basis of the XRSpace's coordinate system . The transform from the effective space to the native origin's space is defined by an origin offset , which is an XRRigidTransform initially set to an identity transform.

The effective origin of an XRSpace can only be observed in the coordinate system of another XRSpace as an XRPose, returned by an XRFrame's getPose() method. The spatial relationship between XRSpaces MAY change between XRFrames.

6.2. XRReferenceSpace

An XRReferenceSpace is one of several common XRSpaces that applications can use to establish a spatial relationship with the user’s physical environment.

XRReferenceSpaces are generally expected to remain static for the duration of the XRSession, with the most common exception being mid-session reconfiguration by the user. The native origin for every XRReferenceSpace describes a coordinate system where +X is considered "Right", +Y is considered "Up", and -Z is considered "Forward".

 enum    XRReferenceSpaceType   {
   "viewer" ,
   "local" ,
   "local-floor" ,
   "bounded-floor" ,
   "unbounded" 
};
[ SecureContext ,  Exposed = Window ]
 interface    XRReferenceSpace   :  XRSpace  {
  [ NewObject ]  XRReferenceSpace   getOffsetReferenceSpace ( XRRigidTransform    originOffset  );
   attribute   EventHandler   onreset ;
};

Each XRReferenceSpace has a type , which is an XRReferenceSpaceType.

An XRReferenceSpace is most frequently obtained by calling requestReferenceSpace(), which creates an instance of an XRReferenceSpace or an interface extending it, determined by the XRReferenceSpaceType enum value passed into the call. The type indicates the tracking behavior that the reference space will exhibit:

• Passing a type of viewer creates an XRReferenceSpace instance. It represents a tracking space with a native origin which tracks the position and orientation of the viewer. Every XRSession MUST support "viewer" XRReferenceSpaces.

• Passing a type of local creates an XRReferenceSpace instance. It represents a tracking space with a native origin near the viewer at the time of creation. The exact position and orientation will be initialized based on the conventions of the underlying platform. When using this reference space the user is not expected to move beyond their initial position much, if at all, and tracking is optimized for that purpose. For devices with 6DoF tracking, local reference spaces should emphasize keeping the origin stable relative to the user’s environment.

• Passing a type of local-floor creates an XRReferenceSpace instance. It represents a tracking space with a native origin at the floor in a safe position for the user to stand. The y axis equals 0 at floor level, with the x and z position and orientation initialized based on the conventions of the underlying platform. If the floor level isn’t known it MUST be estimated. If the estimated floor level is determined with a non-default value, it MUST be rounded sufficiently to prevent fingerprinting. When using this reference space the user is not expected to move beyond their initial position much, if at all, and tracking is optimized for that purpose. For devices with 6DoF tracking, local-floor reference spaces should emphasize keeping the origin stable relative to the user’s environment.

  Note: If the floor level of a "local-floor" reference space is adjusted to prevent fingerprinting, rounded to the nearest 1cm is suggested.

• Passing a type of bounded-floor creates an XRBoundedReferenceSpace instance if supported by the XR device and the XRSession. It represents a tracking space with it’s native origin at the floor, where the user is expected to move within a pre-established boundary, given as the boundsGeometry. Tracking in a bounded-floor reference space is optimized for keeping the native origin and boundsGeometry stable relative to the user’s environment.

• Passing a type of unbounded creates an XRReferenceSpace instance if supported by the XR device and the XRSession. It represents a tracking space where the user is expected to move freely around their environment, potentially even long distances from their starting point. Tracking in an unbounded reference space is optimized for stability around the user’s current position, and as such the native origin may drift over time.

Devices that support "local" reference spaces MUST support "local-floor" reference spaces, through emulation if necessary, and vice versa.

The onreset attribute is an Event handler IDL attribute for the reset event type.

Note: It’s expected that some applications will use getOffsetReferenceSpace() to implement scene navigation controls based on mouse, keyboard, touch, or gamepad input. This will result in getOffsetReferenceSpace() being called frequently, at least once per-frame during periods of active input. As a result UAs are strongly encouraged to make the creation of new XRReferenceSpaces with getOffsetReferenceSpace() a lightweight operation.

6.3. XRBoundedReferenceSpace

XRBoundedReferenceSpace extends XRReferenceSpace to include boundsGeometry, indicating the pre-configured boundaries of the users space.

[ SecureContext ,  Exposed = Window ]
 interface    XRBoundedReferenceSpace   :  XRReferenceSpace  {
   readonly   attribute   FrozenArray < DOMPointReadOnly >  boundsGeometry ;
};

The origin of an XRBoundedReferenceSpace MUST be positioned at the floor, such that the y axis equals 0 at floor level. The x and z position and orientation are initialized based on the conventions of the underlying platform, typically expected to be near the center of the room facing in a logical forward direction.

Note: Other XR platforms sometimes refer to the type of tracking offered by a bounded-floor reference space as "room scale" tracking. An XRBoundedReferenceSpace is not intended to describe multi-room spaces, areas with uneven floor levels, or very large open areas. Content that needs to handle those scenarios should use an unbounded reference space.

Each XRBoundedReferenceSpace has a native bounds geometry describing the border around the XRBoundedReferenceSpace, which the user can expect to safely move within. The polygonal boundary is given as an array of DOMPointReadOnlys, which represents a loop of points at the edges of the safe space. The points describe offsets from the native origin in meters. Points MUST be given in a clockwise order as viewed from above, looking towards the negative end of the Y axis. The y value of each point MUST be 0 and the w value of each point MUST be 1. The bounds can be considered to originate at the floor and extend infinitely high. The shape it describes MAY be convex or concave.

Each point in the native bounds geometry MUST be limited to a reasonable distance from the reference space’s native origin.

Note: It is suggested that points of the native bounds geometry be limited to 15 meters from the native origin in all directions.

Each point in the native bounds geometry MUST also be quantized sufficiently to prevent fingerprinting. For user’s safety, quantized points values MUST NOT fall outside the bounds reported by the platform.

Note: It is suggested that points of the native bounds geometry be quantized to the nearest 5cm.

The boundsGeometry attribute is an array of DOMPointReadOnlys such that each entry is equal to the entry in the XRBoundedReferenceSpace's native bounds geometry premultiplied by the inverse of the origin offset. In other words, it provides the same border in XRBoundedReferenceSpace coordinates relative to the effective origin.

Note: Content should not require the user to move beyond the boundsGeometry. It is possible for the user to move beyond the bounds if their physical surroundings allow for it, resulting in position values outside of the polygon they describe. This is not an error condition and should be handled gracefully by page content.

Note: Content generally should not provide a visualization of the boundsGeometry, as it’s the user agent’s responsibility to ensure that safety critical information is provided to the user.

7. Views

7.1. XRView

An XRView describes a single view into an XR scene for a given frame.

Each view corresponds to a display or portion of a display used by an XR device to present imagery to the user. They are used to retrieve all the information necessary to render content that is well aligned to the view's physical output properties, including the field of view, eye offset, and other optical properties. Views may cover overlapping regions of the user’s vision. No guarantee is made about the number of views any XR device uses or their order, nor is the number of views required to be constant for the duration of an XRSession.

A view has an associated internal view offset , which is an XRRigidTransform describing the position and orientation of the view in the viewer reference space's coordinate system.

A view has an associated projection matrix which is a matrix describing the projection to be used when rendering the view, provided by the underlying XR device. The projection matrix MAY include transformations such as shearing that prevent the projection from being accurately described by a simple frustum.

A view has an associated eye which is an XREye describing which eye this view is expected to be shown to. If the view does not have an intrinsically associated eye (the display is monoscopic, for example) this value MUST be set to "none".

Note: Many HMDs will request that content render two views, one for the left eye and one for the right, while most magic window devices will only request one view, but applications should never assume a specific view configuration. For example: A magic window device may request two views if it is capable of stereo output, but may revert to requesting a single view for performance reasons if the stereo output mode is turned off. Similarly, HMDs may request more than two views to facilitate a wide field of view or displays of different pixel density.

 enum    XREye   {
    "none"  ,
    "left"  ,
    "right" 
};
[ SecureContext ,  Exposed = Window ]  interface    XRView   {
   readonly   attribute   XREye   eye ;
   readonly   attribute   Float32Array   projectionMatrix ;
  [ SameObject ]  readonly   attribute   XRRigidTransform   transform ;
};

The eye attribute describes is the eye of the underlying view. This attribute’s primary purpose is to ensure that pre-rendered stereo content can present the correct portion of the content to the correct eye.

The projectionMatrix attribute is the projection matrix of the underlying view. It is strongly recommended that applications use this matrix without modification or decomposition. Failure to use the provided projection matrices when rendering may cause the presented frame to be distorted or badly aligned, resulting in varying degrees of user discomfort. This attribute MUST be computed by obtaining the projection matrix for the XRView.

The transform attribute is the XRRigidTransform of the viewpoint. It represents the position and orientation of the viewpoint in the XRReferenceSpace provided in getViewerPose().

Each XRView has an associated frame which is the XRFrame that produced it.

Each XRView has an associated underlying view which is the underlying view that it represents.

Each XRView has an associated internal projection matrix which stores the projection matrix of its underlying view. It is initially null.

Note: The transform can be used to position camera objects in many rendering libraries. If a more traditional view matrix is needed by the application one can be retrieved by calling view.transform.inverse.matrix.

7.2. XRViewport

An XRViewport object describes a viewport, or rectangular region, of a graphics surface.

[ SecureContext ,  Exposed = Window ]  interface    XRViewport   {
   readonly   attribute   long   x ;
   readonly   attribute   long   y ;
   readonly   attribute   long   width ;
   readonly   attribute   long   height ;
};

The x and y attributes define an offset from the surface origin and the width and height attributes define the rectangular dimensions of the viewport.

The exact interpretation of the viewport values depends on the conventions of the graphics API the viewport is associated with:

• When used with an XRWebGLLayer the x and y attributes specify the lower left corner of the viewport rectangle, in pixels, with the viewport rectangle extending width pixels to the right of x and height pixels above y. The values can be passed to the WebGL viewport function directly.

xrSession . requestAnimationFrame (( time ,  xrFrame )   =>   {
   let  viewer  =  xrFrame . getViewerPose ( xrReferenceSpace );
  gl . bindFramebuffer ( xrWebGLLayer . framebuffer );
   for   ( xrView  of  viewer . views )   {
     let  xrViewport  =  xrWebGLLayer . getViewport ( xrView );
    gl . viewport ( xrViewport . x ,  xrViewport . y ,  xrViewport . width ,  xrViewport . height );
     // WebGL draw calls will now be rendered into the appropriate viewport.
   }
 });

8. Geometric Primitives

8.1. Matrices

WebXR provides various transforms in the form of matrices . WebXR uses the WebGL conventions when communicating matrices, in which 4x4 matrices are given as 16 element Float32Arrays with column major storage, and are applied to column vectors by premultiplying the matrix from the left. They may be passed directly to WebGL’s uniformMatrix4fv function, used to create an equivalent DOMMatrix, or used with a variety of third party math libraries.

[a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15]

{x:X, y:Y, z:Z, w:1}

a0 a4 a8  a12  *  X  =  a0 * X + a4 * Y +  a8 * Z + a12
a1 a5 a9  a13     Y     a1 * X + a5 * Y +  a9 * Z + a13
a2 a6 a10 a14     Z     a2 * X + a6 * Y + a10 * Z + a14
a3 a7 a11 a15     1     a3 * X + a7 * Y + a11 * Z + a15

8.2. Normalization

There are several algorithms which call for a vector or quaternion to be normalized, which means to scale the components to have a collective magnitude of 1.0.

8.3. XRRigidTransform

An XRRigidTransform is a transform described by a position and orientation. When interpreting an XRRigidTransform the orientation is always applied prior to the position.

An XRRigidTransform contains an internal matrix which is a matrix.

[ SecureContext ,  Exposed = Window ]
 interface    XRRigidTransform   {
   constructor ( optional   DOMPointInit    position   = {},  optional   DOMPointInit    orientation   = {});
  [ SameObject ]  readonly   attribute   DOMPointReadOnly   position ;
  [ SameObject ]  readonly   attribute   DOMPointReadOnly   orientation ;
   readonly   attribute   Float32Array   matrix ;
  [ SameObject ]  readonly   attribute   XRRigidTransform   inverse ;
};

The position attribute is a 3-dimensional point, given in meters, describing the translation component of the transform. The position's w attribute MUST be 1.0.

The orientation attribute is a quaternion describing the rotational component of the transform. The orientation MUST be normalized to have a length of 1.0.

The matrix attribute returns the transform described by the position and orientation attributes as a matrix. This attribute MUST be computed by obtaining the matrix for the XRRigidTransform.

Note: This matrix when premultiplied onto a column vector will rotate the vector by the 3D rotation described by orientation, and then translate it by position. Mathematically in column-vector notation, this is M = T * R, where T is a translation matrix corresponding to position and R is a rotation matrix corresponding to orientation.

The inverse attribute returns an XRRigidTransform which, if applied to an object that had previously been transformed by the original XRRigidTransform, would undo the transform and return the object to its initial pose. This attribute SHOULD be lazily evaluated. The XRRigidTransform returned by inverse MUST return the originating XRRigidTransform as its inverse.

An XRRigidTransform with a position of { x: 0, y: 0, z: 0 w: 1 } and an orientation of { x: 0, y: 0, z: 0, w: 1 } is known as an identity transform .

9. Pose

9.1. XRPose

An XRPose describes a position and orientation in space relative to an XRSpace.

[ SecureContext ,  Exposed = Window ]  interface    XRPose   {
  [ SameObject ]  readonly   attribute   XRRigidTransform   transform ;
   readonly   attribute   boolean   emulatedPosition ;
};

The transform attribute describes the position and orientation relative to the base XRSpace.

The emulatedPosition attribute is false when the transform represents an actively tracked 6DoF pose based on sensor readings, or true if its position value includes a computed offset, such as that provided by a neck or arm model.

9.2. XRViewerPose

An XRViewerPose is an XRPose describing the state of a viewer of the XR scene as tracked by the XR device. A viewer may represent a tracked piece of hardware, the observed position of a users head relative to the hardware, or some other means of computing a series of viewpoints into the XR scene. XRViewerPoses can only be queried relative to an XRReferenceSpace. It provides, in addition to the XRPose values, an array of views which include rigid transforms to indicate the viewpoint and projection matrices. These values should be used by the application when render a frame of an XR scene.

[ SecureContext ,  Exposed = Window ]  interface    XRViewerPose   :  XRPose  {
  [ SameObject ]  readonly   attribute   FrozenArray < XRView >  views ;
};

The views array is a sequence of XRViews describing the viewpoints of the XR scene, relative to the XRReferenceSpace the XRViewerPose was queried with. Every view of the XR scene in the array must be rendered in order to display correctly on the XR device. Each XRView includes rigid transforms to indicate the viewpoint and projection matrices, and can be used to query XRViewports from layers when needed.

Note: The XRViewerPose's transform can be used to position graphical representations of the viewer for spectator views of the scene or multi-user interaction.

10. Input

10.1. XRInputSource

An XRInputSource represents an XR input source , which is any input mechanism which allows the user to perform targeted actions in the same virtual space as the viewer. Example XR input sources include, but are not limited to, handheld controllers, optically tracked hands, and gaze-based input methods that operate on the viewer's pose. Input mechanisms which are not explicitly associated with the XR Device, such as traditional gamepads, mice, or keyboards SHOULD NOT be considered XR input sources.

 enum    XRHandedness   {
    "none"  ,
    "left"  ,
    "right" 
};
 enum    XRTargetRayMode   {
   "gaze" ,
   "tracked-pointer" ,
   "screen" 
};
[ SecureContext ,  Exposed = Window ]
 interface    XRInputSource   {
   readonly   attribute   XRHandedness   handedness ;
   readonly   attribute   XRTargetRayMode   targetRayMode ;
  [ SameObject ]  readonly   attribute   XRSpace   targetRaySpace ;
  [ SameObject ]  readonly   attribute   XRSpace ?  gripSpace ;
  [ SameObject ]  readonly   attribute   FrozenArray < DOMString >  profiles ;
};

The handedness attribute describes which hand the XR input source is associated with, if any. Input sources with no natural handedness (such as headset-mounted controls) or for which the handedness is not currently known MUST set this attribute "none".

The targetRayMode attribute describes the method used to produce the target ray, and indicates how the application should present the target ray to the user if desired.

• gaze indicates the target ray will originate at the viewer and follow the direction it is facing. (This is commonly referred to as a "gaze input" device in the context of head-mounted displays.)

• tracked-pointer indicates that the target ray originates from either a handheld device or other hand-tracking mechanism and represents that the user is using their hands or the held device for pointing. The orientation of the target ray relative to the tracked object MUST follow platform-specific ergonomics guidelines when available. In the absence of platform-specific guidance, the target ray SHOULD point in the same direction as the user’s index finger if it was outstretched.

• screen indicates that the input source was an interaction with the canvas element associated with an inline session’s output context, such as a mouse click or touch event.

The targetRaySpace attribute is an XRSpace that has a native origin tracking the position and orientation of the preferred pointing ray of the XRInputSource, as defined by the targetRayMode.

The gripSpace attribute is an XRSpace that has a native origin tracking to the pose that should be used to render virtual objects such that they appear to be held in the user’s hand. If the user were to hold a straight rod, this XRSpace places the native origin at the centroid of their curled fingers and where the -Z axis points along the length of the rod towards their thumb. The X axis is perpendicular to the back of the hand being described, with back of the users right hand pointing towards +X and the back of the user’s left hand pointing towards -X. The Y axis is implied by the relationship between the X and Z axis, with +Y roughly pointing in the direction of the user’s arm.

The gripSpace MUST be null if the input source isn’t inherently trackable such as for input sources with a targetRayMode of "gaze" or "screen".

The profiles attribute is a list of input profile names indicating both the prefered visual representation and behavior of the input source.

An input profile name is a lowercase DOMString containing no spaces, with separate words concatenated with a hyphen (-) character. A descriptive name should be chosen, using the prefered verbiage of the device vendor when possible. If the platform provides an appropriate identifier, such as a USB vendor and product ID, it MAY be used. Values that uniquely identify a single device, such as serial numbers, MUST NOT be used. The input profile name MUST NOT contain an indication of device handedness. If multiple user agents expose the same device, they SHOULD make an effort to report the same input profile name. The WebXR Input Profiles Registry is the recommended location for managing input profile names.

Profiles are given in descending order of specificity. Any input profile names given after the first entry in the list should provide fallback values that represent alternative representations of the device. This may include a more generic or prior version of the device, a more widely recognized device that is sufficiently similar, or a broad description of the device type (such as "generic-trigger-touchpad"). If multiple profiles are given, the layouts they describe must all represent a superset or subset of every other profile in the list.

If the XRSession's mode is "inline", profiles MUST be an empty list.

If the input device cannot be reliably identified, or the user agent wishes to mask the input device being used, it MAY choose to only report generic input profile names or an empty list.

Note: XRInputSources in an XRSession's inputSources array are "live". As such values within them are updated in-place. This means that it doesn’t work to save a reference to an XRInputSource's attribute on one frame and compare it to the same attribute in a subsequent frame to test for state changes, because they will be the same object. Therefore developers that wish to compare input state from frame to frame should copy the content of the state in question.

Each XR input source SHOULD define a primary action . The primary action is a platform-specific action that, when engaged, produces selectstart, selectend, and select events. Examples of possible primary actions are pressing a trigger, touchpad, or button, speaking a command, or making a hand gesture. If the platform guidelines define a recommended primary input then it should be used as the primary action, otherwise the user agent is free to select one.

Sometimes platform-specific behavior can result in a primary action being interrupted or cancelled. For example, a XR input source may be removed from the XR device after the primary action is started but before it ends.

10.2. Transient input

Some XR Devices may support transient input sources , where the XR input source is only meaningful while performing it’s primary action. An example would be mouse, touch, or stylus input against an "inline" XRSession, which MUST produce a transient XRInputSource with a targetRayMode set to screen. Transient input sources are only present in the session’s list of active XR input sources for the duration of the the selectstart, select, and selectend event sequence.

Transient input sources follow a slightly different sequence when firing primary action events:

10.3. XRInputSourceArray

An XRInputSourceArray represents a list of XRInputSources. It is used in favor of a frozen array type when the contents of the list are expected to change over time, such as with the XRSession inputSources attribute.

[ SecureContext ,  Exposed = Window ]
 interface    XRInputSourceArray   {
   iterable < XRInputSource >;
   readonly   attribute   unsigned   long   length ;
   getter   XRInputSource ( unsigned   long    index  );
};

The length attribute of XRInputSourceArray indicates how many XRInputSources are contained within the XRInputSourceArray.

The indexed property getter of XRInputSourceArray retrieves the XRInputSource at the provided index.

11. Layers

Note: While this specification only defines the XRWebGLLayer layer, future extensions to the spec are expected to add additional layer types and the image sources that they draw from.

11.1. XRWebGLLayer

An XRWebGLLayer is a layer which provides a WebGL framebuffer to render into, enabling hardware accelerated rendering of 3D graphics to be presented on the XR device.

 typedef  ( WebGLRenderingContext   or
          WebGL2RenderingContext )   XRWebGLRenderingContext  ;
 dictionary    XRWebGLLayerInit   {
   boolean    antialias   =  true ;
   boolean    depth   =  true ;
   boolean    stencil   =  false ;
   boolean    alpha   =  true ;
   boolean    ignoreDepthValues   =  false ;
   double    framebufferScaleFactor   = 1.0;
};
[ SecureContext ,  Exposed = Window ]
 interface    XRWebGLLayer   {
   constructor ( XRSession    session  ,
              XRWebGLRenderingContext    context  ,
              optional   XRWebGLLayerInit    layerInit   = {});
  // Attributes
   readonly   attribute   boolean   antialias ;
   readonly   attribute   boolean   ignoreDepthValues ;
  [ SameObject ]  readonly   attribute   WebGLFramebuffer   framebuffer ;
   readonly   attribute   unsigned   long   framebufferWidth ;
   readonly   attribute   unsigned   long   framebufferHeight ;
  // Methods
   XRViewport ?  getViewport ( XRView    view  );
  // Static Methods
   static   double   getNativeFramebufferScaleFactor ( XRSession    session  );
};

Each XRWebGLLayer has a context object, initially null, which is an instance of either a WebGLRenderingContext or a WebGL2RenderingContext.

Each XRWebGLLayer has an associated session , which is the XRSession it was created with.

Note: If an XRWebGLLayer's composition disabled boolean is set to true all values on the XRWebGLLayerInit object are ignored, since the WebGLRenderingContext's default framebuffer was already allocated using the context’s actual context parameters and cannot be overridden.

The context attribute is the WebGLRenderingContext the XRWebGLLayer was created with.

Each XRWebGLLayer has a composition disabled boolean which is initially set to false. If set to true it indicates that the XRWebGLLayer MUST NOT allocate its own WebGLFramebuffer, and all properties of the XRWebGLLayer that reflect framebuffer properties MUST instead reflect the properties of the context's default framebuffer.

The framebuffer attribute of an XRWebGLLayer is an instance of a WebGLFramebuffer which has been marked as opaque if composition disabled is false, and null otherwise. The framebuffer size cannot be adjusted by the developer after the XRWebGLLayer has been created.

An opaque framebuffer functions identically to a standard WebGLFramebuffer with the following changes that make it behave more like the default framebuffer:

• An opaque framebuffer MAY support antialiasing, even in WebGL 1.0.

• An opaque framebuffer's attachments cannot be inspected or changed. Calling framebufferTexture2D, framebufferRenderbuffer, deleteFramebuffer, or getFramebufferAttachmentParameter with an opaque framebuffer MUST generate an INVALID_OPERATION error.

• An opaque framebuffer is considered incomplete outside of a requestAnimationFrame() callback. When not in a requestAnimationFrame() callback calls to checkFramebufferStatus outside of a requestAnimationFrame() callback MUST generate a FRAMEBUFFER_UNSUPPORTED error and attempts to clear, draw to, or read from the opaque framebuffer MUST generate an INVALID_FRAMEBUFFER_OPERATION error.

• An opaque framebuffer initialized with depth false will not have an attached depth buffer.

• An opaque framebuffer initialized with stencil false will not have an attached stencil buffer.

• An opaque framebuffer's color buffer will have an alpha channel if and only if alpha is true.

• The XR Compositor will assume the opaque framebuffer contains colors with premultiplied alpha. This is true regardless of the premultipliedAlpha value set in the context's actual context parameters.

Note: User agents are not required to respect true values of depth and stencil, which is similar to WebGL’s behavior when creating a drawing buffer

The buffers attached to an opaque framebuffer MUST be cleared to the values in the table below when first created, or prior to the processing of each XR animation frame. This is identical to the behavior of the WebGL context’s default framebuffer.

Note: Implementations may optimize away the required implicit clear operation of the opaque framebuffer as long as a guarantee can be made that the developer cannot gain access to buffer contents from another process. For instance, if the developer performs an explicit clear then the implicit clear is not needed.

Each XRWebGLLayer has a target framebuffer , which is the framebuffer if composition disabled is false, and the context's default framebuffer otherwise.

The framebufferWidth and framebufferHeight attributes return the width and height of the target framebuffer's attachments, respectively.

The antialias attribute is true if the target framebuffer supports antialiasing using a technique of the UAs choosing, and false if no antialiasing will be performed.

The ignoreDepthValues attribute, if true, indicates the XR Compositor MUST NOT make use of values in the depth buffer attachment when rendering. When the attribute is false it indicates that the content of the depth buffer attachment will be used by the XR Compositor and is expected to be representative of the scene rendered into the layer.

Depth values stored in the buffer are expected to be between 0.0 and 1.0, with 0.0 representing the distance of depthNear and 1.0 representing the distance of depthFar, with intermediate values interpolated linearly. This is the default behavior of WebGL. (See documentation for the depthRange function for additional details.))

Note: Making the scene’s depth buffer available to the compositor allows some platforms to provide quality and comfort improvements such as improved reprojection.

Each XRWebGLLayer MUST have a list of viewports which is a list containing one WebGL viewport for each XRView the XRSession currently exposes. The viewports MUST NOT be overlapping. If composition disabled is true, the list of viewports MUST contain a single WebGL viewport that covers the context's entire default framebuffer.

getViewport() queries the XRViewport the given XRView should use when rendering to the layer.

Each XRSession MUST identify a native WebGL framebuffer resolution , which is the pixel resolution of a WebGL framebuffer required to match the physical pixel resolution of the XR device.

Additionally, the XRSession MUST identify a recommended WebGL framebuffer resolution , which represents a best estimate of the WebGL framebuffer resolution large enough to contain all of the session’s XRViews that provides an average application a good balance between performance and quality. It MAY be smaller than, larger than, or equal to the native WebGL framebuffer resolution.

Note: The user agent is free to use and method of it’s choosing to estimate the recommended WebGL framebuffer resolution. If there are platform-specific methods for querying a recommended size it is recommended that they be used, but not required.

11.2. WebGL Context Compatibility

In order for a WebGL context to be used as a source for immersive XR imagery it must be created on a compatible graphics adapter for the immersive XR device. What is considered a compatible graphics adapter is platform dependent, but is understood to mean that the graphics adapter can supply imagery to the immersive XR device without undue latency. If a WebGL context was not already created on the compatible graphics adapter, it typically must be re-created on the adapter in question before it can be used with an XRWebGLLayer.

Note: On an XR platform with a single GPU, it can safely be assumed that the GPU is compatible with the immersive XR devices advertised by the platform, and thus any hardware accelerated WebGL contexts are compatible as well. On PCs with both an integrated and discrete GPU the discrete GPU is often considered the compatible graphics adapter since it generally a higher performance chip. On desktop PCs with multiple graphics adapters installed, the one with the immersive XR device physically connected to it is likely to be considered the compatible graphics adapter.

Note: "inline" sessions render using the same graphics adapter as canvases, and thus do not need xrCompatible contexts.

 partial   dictionary   WebGLContextAttributes  {
     boolean    xrCompatible   =  null ;
};
 partial   interface   mixin   WebGLRenderingContextBase  {
     Promise < void >  makeXRCompatible ();
};

When a user agent implements this specification it MUST set a XR compatible boolean, initially set to false, on every WebGLRenderingContextBase. Once the XR compatible boolean is set to true, the context can be used with layers for any XRSession requested from the current immersive XR device.

The XR compatible boolean can be set either at context creation time or after context creation, potentially incurring a context loss. To set the XR compatible boolean at context creation time, the xrCompatible context creation attribute must be set to true when requesting a WebGL context.

 function  onXRSessionStarted ( xrSession )   {
   let  glCanvas  =  document . createElement (  "canvas"  );
   let  gl  =  glCanvas . getContext (  "webgl"  ,   {  xrCompatible :   true   });
  loadWebGLResources ();
  xrSession . updateRenderState ({  baseLayer :   new  XRWebGLLayer ( xrSession ,  gl )   });
 }

To set the XR compatible boolean after the context has been created, the makeXRCompatible() method is used.