Patent Description:
Virtual environments are ubiquitous in computing environments, finding use in video games (in which a virtual environment may represent a game world); maps (in which a virtual environment may represent terrain to be navigated); simulations (in which a virtual environment may simulate a real environment); digital storytelling (in which virtual characters may interact with each other in a virtual environment); and many other applications. Modern computer users are generally comfortable perceiving, and interacting with, virtual environments. However, users' experiences with virtual environments can be limited by the technology for presenting virtual environments. For example, conventional displays (e.g., 2D display screens) and audio systems (e.g., fixed speakers) may be unable to realize a virtual environment in ways that create a compelling, realistic, and immersive experience.

Virtual reality ("VR"), augmented reality ("AR"), mixed reality ("MR"), and related technologies (collectively, "XR") share an ability to present, to a user of an XR system, sensory information corresponding to a virtual environment represented by data in a computer system. This disclosure contemplates a distinction between VR, AR, and MR systems (although some systems may be categorized as VR in one aspect (e.g., a visual aspect), and simultaneously categorized as AR or MR in another aspect (e.g., an audio aspect)). As used herein, VR systems present a virtual environment that replaces a user's real environment in at least one aspect; for example, a VR system could present the user with a view of the virtual environment while simultaneously obscuring his or her view of the real environment, such as with a light-blocking head-mounted display. Similarly, a VR system could present the user with audio corresponding to the virtual environment, while simultaneously blocking (attenuating) audio from the real environment.

VR systems may experience various drawbacks that result from replacing a user's real environment with a virtual environment. One drawback is a feeling of motion sickness that can arise when a user's field of view in a virtual environment no longer corresponds to the state of his or her inner ear, which detects one's balance and orientation in the real environment (not a virtual environment). Similarly, users may experience disorientation in VR environments where their own bodies and limbs (views of which users rely on to feel "grounded" in the real environment) are not directly visible. Another drawback is the computational burden (e.g., storage, processing power) placed on VR systems which must present a full 3D virtual environment, particularly in real-time applications that seek to immerse the user in the virtual environment. Similarly, such environments may need to reach a very high standard of realism to be considered immersive, as users tend to be sensitive to even minor imperfections in virtual environments - any of which can destroy a user's sense of immersion in the virtual environment. Further, another drawback of VR systems is that such applications of systems cannot take advantage of the wide range of sensory data in the real environment, such as the various sights and sounds that one experiences in the real world. A related drawback is that VR systems may struggle to create shared environments in which multiple users can interact, as users that share a physical space in the real environment may not be able to directly see or interact with each other in a virtual environment.

As used herein, AR systems present a virtual environment that overlaps or overlays the real environment in at least one aspect. For example, an AR system could present the user with a view of a virtual environment overlaid on the user's view of the real environment, such as with a transmissive head-mounted display that presents a displayed image while allowing light to pass through the display into the user's eye. Similarly, an AR system could present the user with audio corresponding to the virtual environment, while simultaneously mixing in audio from the real environment. Similarly, as used herein, MR systems present a virtual environment that overlaps or overlays the real environment in at least one aspect, as do AR systems, and may additionally allow that a virtual environment in an MR system may interact with the real environment in at least one aspect. For example, a virtual character in a virtual environment may toggle a light switch in the real environment, causing a corresponding light bulb in the real environment to turn on or off. As another example, the virtual character may react (such as with a facial expression) to audio signals in the real environment. By maintaining presentation of the real environment, AR and MR systems may avoid some of the aforementioned drawbacks of VR systems; for instance, motion sickness in users is reduced because visual cues from the real environment (including users' own bodies) can remain visible, and such systems need not present a user with a fully realized 3D environment in order to be immersive. Further, AR and MR systems can take advantage of real world sensory input (e.g., views and sounds of scenery, objects, and other users) to create new applications that augment that input.

<CIT> discloses a method and a corresponding system, comprising: receiving, via a microphone of a wearable head device, an audio signal; determining an envelope of the audio signal; estimating a reverberation time based on the envelope of the audio signal;; and presenting, via a speaker of a wearable head device, an audio content, wherein the audio content is based on the reverberation time.

It can be desirable for MR systems to interface with as many human senses as possible to create an immersive mixed reality environment for a user. Visual displays of virtual content can be important to a mixed reality experience, but audio signals can also be valuable in creating immersion in the mixed reality environment. Similar to visually displayed virtual content, virtual audio content can also be adapted to simulate sounds from a real environment. For example, virtual audio content presented in a real environment with echoes may also be rendered as echoing, even though the virtual audio content may not actually be echoing in the real environment. This adaptation can help blend virtual content with real content such that a distinction between the two is not obvious or even imperceptible to an end-user. To effectively blend virtual audio content with real audio content, it can be desirable to understand acoustic properties of the real environment so that virtual audio content can simulate the characteristics of real audio content.

The invention is directed to a method according to claim <NUM>, a system according to claim <NUM> and a medium according to claim <NUM>. Further developments of the invention are according to dependent claims <NUM>-<NUM> and <NUM>-<NUM>.

In the following description of examples, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the invention.

Like all people, a user of a mixed reality system exists in a real environment - that is, a three-dimensional portion of the "real world," and all of its contents, that are perceptible by the user. For example, a user perceives a real environment using one's ordinary human senses - sight, sound, touch, taste, smell - and interacts with the real environment by moving one's own body in the real environment. Locations in a real environment can be described as coordinates in a coordinate space; for example, a coordinate can comprise latitude, longitude, and elevation with respect to sea level; distances in three orthogonal dimensions from a reference point; or other suitable values. Likewise, a vector can describe a quantity having a direction and a magnitude in the coordinate space.

A computing device can maintain, for example in a memory associated with the device, a representation of a virtual environment. As used herein, a virtual environment is a computational representation of a three-dimensional space. A virtual environment can include representations of any object, action, signal, parameter, coordinate, vector, or other characteristic associated with that space. In some examples, circuitry (e.g., a processor) of a computing device can maintain and update a state of a virtual environment; that is, a processor can determine at a first time t0, based on data associated with the virtual environment and/or input provided by a user, a state of the virtual environment at a second time t1. For instance, if an object in the virtual environment is located at a first coordinate at time t0, and has certain programmed physical parameters (e.g., mass, coefficient of friction); and an input received from user indicates that a force should be applied to the object in a direction vector; the processor can apply laws of kinematics to determine a location of the object at time t1 using basic mechanics. The processor can use any suitable information known about the virtual environment, and/or any suitable input, to determine a state of the virtual environment at a time t1. In maintaining and updating a state of a virtual environment, the processor can execute any suitable software, including software relating to the creation and deletion of virtual objects in the virtual environment; software (e.g., scripts) for defining behavior of virtual objects or characters in the virtual environment; software for defining the behavior of signals (e.g., audio signals) in the virtual environment; software for creating and updating parameters associated with the virtual environment; software for generating audio signals in the virtual environment; software for handling input and output; software for implementing network operations; software for applying asset data (e.g., animation data to move a virtual object over time); or many other possibilities.

Output devices, such as a display or a speaker, can present any or all aspects of a virtual environment to a user. For example, a virtual environment may include virtual objects (which may include representations of inanimate objects; people; animals; lights; etc.) that may be presented to a user. A processor can determine a view of the virtual environment (for example, corresponding to a "camera" with an origin coordinate, a view axis, and a frustum); and render, to a display, a viewable scene of the virtual environment corresponding to that view. Any suitable rendering technology may be used for this purpose. In some examples, the viewable scene may include only some virtual objects in the virtual environment, and exclude certain other virtual objects. Similarly, a virtual environment may include audio aspects that may be presented to a user as one or more audio signals. For instance, a virtual object in the virtual environment may generate a sound originating from a location coordinate of the object (e.g., a virtual character may speak or cause a sound effect); or the virtual environment may be associated with musical cues or ambient sounds that may or may not be associated with a particular location. A processor can determine an audio signal corresponding to a "listener" coordinate - for instance, an audio signal corresponding to a composite of sounds in the virtual environment, and mixed and processed to simulate an audio signal that would be heard by a listener at the listener coordinate - and present the audio signal to a user via one or more speakers.

Because a virtual environment exists only as a computational structure, a user cannot directly perceive a virtual environment using one's ordinary senses. Instead, a user can perceive a virtual environment only indirectly, as presented to the user, for example by a display, speakers, haptic output devices, etc. Similarly, a user cannot directly touch, manipulate, or otherwise interact with a virtual environment; but can provide input data, via input devices or sensors, to a processor that can use the device or sensor data to update the virtual environment. For example, a camera sensor can provide optical data indicating that a user is trying to move an object in a virtual environment, and a processor can use that data to cause the object to respond accordingly in the virtual environment.

A mixed reality system can present to the user, for example using a transmissive display and/or one or more speakers (which may, for example, be incorporated into a wearable head device), a mixed reality environment ("MRE") that combines aspects of a real environment and a virtual environment. In some embodiments, the one or more speakers may be external to the head-mounted wearable unit. As used herein, a MRE is a simultaneous representation of a real environment and a corresponding virtual environment. In some examples, the corresponding real and virtual environments share a single coordinate space; in some examples, a real coordinate space and a corresponding virtual coordinate space are related to each other by a transformation matrix (or other suitable representation). Accordingly, a single coordinate (along with, in some examples, a transformation matrix) can define a first location in the real environment, and also a second, corresponding, location in the virtual environment; and vice versa.

In a MRE, a virtual object (e.g., in a virtual environment associated with the MRE) can correspond to a real object (e.g., in a real environment associated with the MRE). For instance, if the real environment of a MRE comprises a real lamp post (a real object) at a location coordinate, the virtual environment of the MRE may comprise a virtual lamp post (a virtual object) at a corresponding location coordinate. As used herein, the real object in combination with its corresponding virtual object together constitute a "mixed reality object. " It is not necessary for a virtual object to perfectly match or align with a corresponding real object. In some examples, a virtual object can be a simplified version of a corresponding real object. For instance, if a real environment includes a real lamp post, a corresponding virtual object may comprise a cylinder of roughly the same height and radius as the real lamp post (reflecting that lamp posts may be roughly cylindrical in shape). Simplifying virtual objects in this manner can allow computational efficiencies, and can simplify calculations to be performed on such virtual objects. Further, in some examples of a MRE, not all real objects in a real environment may be associated with a corresponding virtual object. Likewise, in some examples of a MRE, not all virtual objects in a virtual environment may be associated with a corresponding real object. That is, some virtual objects may solely in a virtual environment of a MRE, without any real-world counterpart.

In some examples, virtual objects may have characteristics that differ, sometimes drastically, from those of corresponding real objects. For instance, while a real environment in a MRE may comprise a green, two-armed cactus - a prickly inanimate object - a corresponding virtual object in the MRE may have the characteristics of a green, two-armed virtual character with human facial features and a surly demeanor. In this example, the virtual object resembles its corresponding real object in certain characteristics (color, number of arms); but differs from the real object in other characteristics (facial features, personality). In this way, virtual objects have the potential to represent real objects in a creative, abstract, exaggerated, or fanciful manner; or to impart behaviors (e.g., human personalities) to otherwise inanimate real objects. In some examples, virtual objects may be purely fanciful creations with no real-world counterpart (e.g., a virtual monster in a virtual environment, perhaps at a location corresponding to an empty space in a real environment).

Compared to VR systems, which present the user with a virtual environment while obscuring the real environment, a mixed reality system presenting a MRE affords the advantage that the real environment remains perceptible while the virtual environment is presented. Accordingly, the user of the mixed reality system is able to use visual and audio cues associated with the real environment to experience and interact with the corresponding virtual environment. As an example, while a user of VR systems may struggle to perceive or interact with a virtual object displayed in a virtual environment - because, as noted above, a user cannot directly perceive or interact with a virtual environment - a user of an MR system may find it intuitive and natural to interact with a virtual object by seeing, hearing, and touching a corresponding real object in his or her own real environment. This level of interactivity can heighten a user's feelings of immersion, connection, and engagement with a virtual environment. Similarly, by simultaneously presenting a real environment and a virtual environment, mixed reality systems can reduce negative psychological feelings (e.g., cognitive dissonance) and negative physical feelings (e.g., motion sickness) associated with VR systems. Mixed reality systems further offer many possibilities for applications that may augment or alter our experiences of the real world.

<FIG> illustrates an example real environment <NUM> in which a user <NUM> uses a mixed reality system <NUM>. Mixed reality system <NUM> may comprise a display (e.g., a transmissive display) and one or more speakers, and one or more sensors (e.g., a camera), for example as described below. The real environment <NUM> shown comprises a rectangular room 104A, in which user <NUM> is standing; and real objects 122A (a lamp), 124A (a table), 126A (a sofa), and 128A (a painting). Room 104A further comprises a location coordinate <NUM>, which may be considered an origin of the real environment <NUM>. As shown in <FIG>, an environment/world coordinate system <NUM> (comprising an x-axis 108X, a y-axis 108Y, and a z-axis 108Z) with its origin at point <NUM> (a world coordinate), can define a coordinate space for real environment <NUM>. In some embodiments, the origin point <NUM> of the environment/world coordinate system <NUM> may correspond to where the mixed reality system <NUM> was powered on. In some embodiments, the origin point <NUM> of the environment/world coordinate system <NUM> may be reset during operation. In some examples, user <NUM> may be considered a real object in real environment <NUM>; similarly, user <NUM>'s body parts (e.g., hands, feet) may be considered real objects in real environment <NUM>. In some examples, a user/listener/head coordinate system <NUM> (comprising an x-axis 114X, a y-axis 114Y, and a z-axis 114Z) with its origin at point <NUM> (e.g., user/listener/head coordinate) can define a coordinate space for the user/listener/head on which the mixed reality system <NUM> is located. The origin point <NUM> of the user/listener/head coordinate system <NUM> may be defined relative to one or more components of the mixed reality system <NUM>. For example, the origin point <NUM> of the user/listener/head coordinate system <NUM> may be defined relative to the display of the mixed reality system <NUM> such as during initial calibration of the mixed reality system <NUM>. A matrix (which may include a translation matrix and a Quaternion matrix or other rotation matrix), or other suitable representation can characterize a transformation between the user/listener/head coordinate system <NUM> space and the environment/world coordinate system <NUM> space. In some embodiments, a left ear coordinate <NUM> and a right ear coordinate <NUM> may be defined relative to the origin point <NUM> of the user/listener/head coordinate system <NUM>. A matrix (which may include a translation matrix and a Quaternion matrix or other rotation matrix), or other suitable representation can characterize a transformation between the left ear coordinate <NUM> and the right ear coordinate <NUM>, and user/listener/head coordinate system <NUM> space. The user/listener/head coordinate system <NUM> can simplify the representation of locations relative to the user's head, or to a head-mounted device, for example, relative to the environment/world coordinate system <NUM>. Using Simultaneous Localization and Mapping (SLAM), visual odometry, or other techniques, a transformation between user coordinate system <NUM> and environment coordinate system <NUM> can be determined and updated in real-time.

<FIG> illustrates an example virtual environment <NUM> that corresponds to real environment <NUM>. The virtual environment <NUM> shown comprises a virtual rectangular room 104B corresponding to real rectangular room 104A; a virtual object 122B corresponding to real object 122A; a virtual object 124B corresponding to real object 124A; and a virtual object 126B corresponding to real object 126A. Metadata associated with the virtual objects 122B, 124B, 126B can include information derived from the corresponding real objects 122A, 124A, 126A. Virtual environment <NUM> additionally comprises a virtual monster <NUM>, which does not correspond to any real object in real environment <NUM>. Real object 128A in real environment <NUM> does not correspond to any virtual object in virtual environment <NUM>. A persistent coordinate system <NUM> (comprising an x-axis 133X, a y-axis 133Y, and a z-axis 133Z) with its origin at point <NUM> (persistent coordinate), can define a coordinate space for virtual content. The origin point <NUM> of the persistent coordinate system <NUM> may be defined relative/with respect to one or more real objects, such as the real object 126A. A matrix (which may include a translation matrix and a Quaternion matrix or other rotation matrix), or other suitable representation can characterize a transformation between the persistent coordinate system <NUM> space and the environment/world coordinate system <NUM> space. In some embodiments, each of the virtual objects 122B, 124B, 126B, and <NUM> may have their own persistent coordinate point relative to the origin point <NUM> of the persistent coordinate system <NUM>. In some embodiments, there may be multiple persistent coordinate systems and each of the virtual objects 122B, 124B, 126B, and <NUM> may have their own persistent coordinate point relative to one or more persistent coordinate systems.

With respect to <FIG> and <FIG>, environment/world coordinate system <NUM> defines a shared coordinate space for both real environment <NUM> and virtual environment <NUM>. In the example shown, the coordinate space has its origin at point <NUM>. Further, the coordinate space is defined by the same three orthogonal axes (108X, 108Y, 108Z). Accordingly, a first location in real environment <NUM>, and a second, corresponding location in virtual environment <NUM>, can be described with respect to the same coordinate space. This simplifies identifying and displaying corresponding locations in real and virtual environments, because the same coordinates can be used to identify both locations. However, in some examples, corresponding real and virtual environments need not use a shared coordinate space. For instance, in some examples (not shown), a matrix (which may include a translation matrix and a Quaternion matrix or other rotation matrix), or other suitable representation can characterize a transformation between a real environment coordinate space and a virtual environment coordinate space.

<FIG> illustrates an example MRE <NUM> that simultaneously presents aspects of real environment <NUM> and virtual environment <NUM> to user <NUM> via mixed reality system <NUM>. In the example shown, MRE <NUM> simultaneously presents user <NUM> with real objects 122A, 124A, 126A, and 128A from real environment <NUM> (e.g., via a transmissive portion of a display of mixed reality system <NUM>); and virtual objects 122B, 124B, 126B, and <NUM> from virtual environment <NUM> (e.g., via an active display portion of the display of mixed reality system <NUM>). As above, origin point <NUM> acts as an origin for a coordinate space corresponding to MRE <NUM>, and coordinate system <NUM> defines an x-axis, y-axis, and z-axis for the coordinate space.

In the example shown, mixed reality objects comprise corresponding pairs of real objects and virtual objects (i.e., 122A/122B, 124A/124B, 126A/126B) that occupy corresponding locations in coordinate space <NUM>. In some examples, both the real objects and the virtual objects may be simultaneously visible to user <NUM>. This may be desirable in, for example, instances where the virtual object presents information designed to augment a view of the corresponding real object (such as in a museum application where a virtual object presents the missing pieces of an ancient damaged sculpture). In some examples, the virtual objects (122B, 124B, and/or 126B) may be displayed (e.g., via active pixelated occlusion using a pixelated occlusion shutter) so as to occlude the corresponding real objects (122A, 124A, and/or 126A). This may be desirable in, for example, instances where the virtual object acts as a visual replacement for the corresponding real object (such as in an interactive storytelling application where an inanimate real object becomes a "living" character).

In some examples, real objects (e.g., 122A, 124A, 126A) may be associated with virtual content or helper data that may not necessarily constitute virtual objects. Virtual content or helper data can facilitate processing or handling of virtual objects in the mixed reality environment. For example, such virtual content could include two-dimensional representations of corresponding real objects; custom asset types associated with corresponding real objects; or statistical data associated with corresponding real objects. This information can enable or facilitate calculations involving a real object without incurring unnecessary computational overhead.

In some examples, the presentation described above may also incorporate audio aspects. For instance, in MRE <NUM>, virtual monster <NUM> could be associated with one or more audio signals, such as a footstep sound effect that is generated as the monster walks around MRE <NUM>. As described further below, a processor of mixed reality system <NUM> can compute an audio signal corresponding to a mixed and processed composite of all such sounds in MRE <NUM>, and present the audio signal to user <NUM> via one or more speakers included in mixed reality system <NUM> and/or one or more external speakers.

Example mixed reality system <NUM> can include a wearable head device (e.g., a wearable augmented reality or mixed reality head device) comprising a display (which may comprise left and right transmissive displays, which may be near-eye displays, and associated components for coupling light from the displays to the user's eyes); left and right speakers (e.g., positioned adjacent to the user's left and right ears, respectively); an inertial measurement unit (IMU)(e.g., mounted to a temple arm of the head device); an orthogonal coil electromagnetic receiver (e.g., mounted to the left temple piece); left and right cameras (e.g., depth (time-of-flight) cameras) oriented away from the user; and left and right eye cameras oriented toward the user (e.g., for detecting the user's eye movements). However, a mixed reality system <NUM> can incorporate any suitable display technology, and any suitable sensors (e.g., optical, infrared, acoustic, LIDAR, EOG, GPS, magnetic). In addition, mixed reality system <NUM> may incorporate networking features (e.g., Wi-Fi capability) to communicate with other devices and systems, including other mixed reality systems. Mixed reality system <NUM> may further include a battery (which may be mounted in an auxiliary unit, such as a belt pack designed to be worn around a user's waist), a processor, and a memory. The wearable head device of mixed reality system <NUM> may include tracking components, such as an IMU or other suitable sensors, configured to output a set of coordinates of the wearable head device relative to the user's environment. In some examples, tracking components may provide input to a processor performing a Simultaneous Localization and Mapping (SLAM) and/or visual odometry algorithm. In some examples, mixed reality system <NUM> may also include a handheld controller <NUM>, and/or an auxiliary unit <NUM>, which may be a wearable beltpack, as described further below.

<FIG> illustrate components of an example mixed reality system <NUM> (which may correspond to mixed reality system <NUM>) that may be used to present a MRE (which may correspond to MRE <NUM>), or other virtual environment, to a user. <FIG> illustrates a perspective view of a wearable head device <NUM> included in example mixed reality system <NUM>. <FIG> illustrates a top view of wearable head device <NUM> worn on a user's head <NUM>. <FIG> illustrates a front view of wearable head device <NUM>. <FIG> illustrates an edge view of example eyepiece <NUM> of wearable head device <NUM>. As shown in <FIG>, the example wearable head device <NUM> includes an example left eyepiece (e.g., a left transparent waveguide set eyepiece) <NUM> and an example right eyepiece (e.g., a right transparent waveguide set eyepiece) <NUM>. Each eyepiece <NUM> and <NUM> can include transmissive elements through which a real environment can be visible, as well as display elements for presenting a display (e.g., via imagewise modulated light) overlapping the real environment. In some examples, such display elements can include surface diffractive optical elements for controlling the flow of imagewise modulated light. For instance, the left eyepiece <NUM> can include a left incoupling grating set <NUM>, a left orthogonal pupil expansion (OPE) grating set <NUM>, and a left exit (output) pupil expansion (EPE) grating set <NUM>. Similarly, the right eyepiece <NUM> can include a right incoupling grating set <NUM>, a right OPE grating set <NUM> and a right EPE grating set <NUM>. Imagewise modulated light can be transferred to a user's eye via the incoupling gratings <NUM> and <NUM>, OPEs <NUM> and <NUM>, and EPE <NUM> and <NUM>. Each incoupling grating set <NUM>, <NUM> can be configured to deflect light toward its corresponding OPE grating set <NUM>, <NUM>. Each OPE grating set <NUM>, <NUM> can be designed to incrementally deflect light down toward its associated EPE <NUM>, <NUM>, thereby horizontally extending an exit pupil being formed. Each EPE <NUM>, <NUM> can be configured to incrementally redirect at least a portion of light received from its corresponding OPE grating set <NUM>, <NUM> outward to a user eyebox position (not shown) defined behind the eyepieces <NUM>, <NUM>, vertically extending the exit pupil that is formed at the eyebox. Alternatively, in lieu of the incoupling grating sets <NUM> and <NUM>, OPE grating sets <NUM> and <NUM>, and EPE grating sets <NUM> and <NUM>, the eyepieces <NUM> and <NUM> can include other arrangements of gratings and/or refractive and reflective features for controlling the coupling of imagewise modulated light to the user's eyes.

In some examples, wearable head device <NUM> can include a left temple arm <NUM> and a right temple arm <NUM>, where the left temple arm <NUM> includes a left speaker <NUM> and the right temple arm <NUM> includes a right speaker <NUM>. An orthogonal coil electromagnetic receiver <NUM> can be located in the left temple piece, or in another suitable location in the wearable head unit <NUM>. An Inertial Measurement Unit (IMU) <NUM> can be located in the right temple arm <NUM>, or in another suitable location in the wearable head device <NUM>. The wearable head device <NUM> can also include a left depth (e.g., time-of-flight) camera <NUM> and a right depth camera <NUM>. The depth cameras <NUM>, <NUM> can be suitably oriented in different directions so as to together cover a wider field of view.

In the example shown in <FIG>, a left source of imagewise modulated light <NUM> can be optically coupled into the left eyepiece <NUM> through the left incoupling grating set <NUM>, and a right source of imagewise modulated light <NUM> can be optically coupled into the right eyepiece <NUM> through the right incoupling grating set <NUM>. Sources of imagewise modulated light <NUM>, <NUM> can include, for example, optical fiber scanners; projectors including electronic light modulators such as Digital Light Processing (DLP) chips or Liquid Crystal on Silicon (LCoS) modulators; or emissive displays, such as micro Light Emitting Diode (µLED) or micro Organic Light Emitting Diode (µOLED) panels coupled into the incoupling grating sets <NUM>, <NUM> using one or more lenses per side. The input coupling grating sets <NUM>, <NUM> can deflect light from the sources of imagewise modulated light <NUM>, <NUM> to angles above the critical angle for Total Internal Reflection (TIR) for the eyepieces <NUM>, <NUM>. The OPE grating sets <NUM>, <NUM> incrementally deflect light propagating by TIR down toward the EPE grating sets <NUM>, <NUM>. The EPE grating sets <NUM>, <NUM> incrementally couple light toward the user's face, including the pupils of the user's eyes.

In some examples, as shown in <FIG>, each of the left eyepiece <NUM> and the right eyepiece <NUM> includes a plurality of waveguides <NUM>. For example, each eyepiece <NUM>, <NUM> can include multiple individual waveguides, each dedicated to a respective color channel (e.g., red, blue and green). In some examples, each eyepiece <NUM>, <NUM> can include multiple sets of such waveguides, with each set configured to impart different wavefront curvature to emitted light. The wavefront curvature may be convex with respect to the user's eyes, for example to present a virtual object positioned a distance in front of the user (e.g., by a distance corresponding to the reciprocal of wavefront curvature). In some examples, EPE grating sets <NUM>, <NUM> can include curved grating grooves to effect convex wavefront curvature by altering the Poynting vector of exiting light across each EPE.

In some examples, to create a perception that displayed content is three-dimensional, stereoscopically-adjusted left and right eye imagery can be presented to the user through the imagewise light modulators <NUM>, <NUM> and the eyepieces <NUM>, <NUM>. The perceived realism of a presentation of a three-dimensional virtual object can be enhanced by selecting waveguides (and thus corresponding the wavefront curvatures) such that the virtual object is displayed at a distance approximating a distance indicated by the stereoscopic left and right images. This technique may also reduce motion sickness experienced by some users, which may be caused by differences between the depth perception cues provided by stereoscopic left and right eye imagery, and the autonomic accommodation (e.g., object distance-dependent focus) of the human eye.

<FIG> illustrates an edge-facing view from the top of the right eyepiece <NUM> of example wearable head device <NUM>. As shown in <FIG>, the plurality of waveguides <NUM> can include a first subset of three waveguides <NUM> and a second subset of three waveguides <NUM>. The two subsets of waveguides <NUM>, <NUM> can be differentiated by different EPE gratings featuring different grating line curvatures to impart different wavefront curvatures to exiting light. Within each of the subsets of waveguides <NUM>, <NUM> each waveguide can be used to couple a different spectral channel (e.g., one of red, green and blue spectral channels) to the user's right eye <NUM>. (Although not shown in <FIG>, the structure of the left eyepiece <NUM> is analogous to the structure of the right eyepiece <NUM>.

<FIG> illustrates an example handheld controller component <NUM> of a mixed reality system <NUM>. In some examples, handheld controller <NUM> includes a grip portion <NUM> and one or more buttons <NUM> disposed along a top surface <NUM>. In some examples, buttons <NUM> may be configured for use as an optical tracking target, e.g., for tracking six-degree-of-freedom (6DOF) motion of the handheld controller <NUM>, in conjunction with a camera or other optical sensor (which may be mounted in a head unit (e.g., wearable head device <NUM>) of mixed reality system <NUM>). In some examples, handheld controller <NUM> includes tracking components (e.g., an IMU or other suitable sensors) for detecting position or orientation, such as position or orientation relative to wearable head device <NUM>. In some examples, such tracking components may be positioned in a handle of handheld controller <NUM>, and/or may be mechanically coupled to the handheld controller. Handheld controller <NUM> can be configured to provide one or more output signals corresponding to one or more of a pressed state of the buttons; or a position, orientation, and/or motion of the handheld controller <NUM> (e.g., via an IMU). Such output signals may be used as input to a processor of mixed reality system <NUM>. Such input may correspond to a position, orientation, and/or movement of the handheld controller (and, by extension, to a position, orientation, and/or movement of a hand of a user holding the controller). Such input may also correspond to a user pressing buttons <NUM>.

<FIG> illustrates an example auxiliary unit <NUM> of a mixed reality system <NUM>. The auxiliary unit <NUM> can include a battery to provide energy to operate the system <NUM>, and can include a processor for executing programs to operate the system <NUM>. As shown, the example auxiliary unit <NUM> includes a clip <NUM>, such as for attaching the auxiliary unit <NUM> to a user's belt. Other form factors are suitable for auxiliary unit <NUM> and will be apparent, including form factors that do not involve mounting the unit to a user's belt. In some examples, auxiliary unit <NUM> is coupled to the wearable head device <NUM> through a multiconduit cable that can include, for example, electrical wires and fiber optics. Wireless connections between the auxiliary unit <NUM> and the wearable head device <NUM> can also be used.

In some examples, mixed reality system <NUM> can include one or more microphones to detect sound and provide corresponding signals to the mixed reality system. In some examples, a microphone may be attached to, or integrated with, wearable head device <NUM>, and may be configured to detect a user's voice. In some examples, a microphone may be attached to, or integrated with, handheld controller <NUM> and/or auxiliary unit <NUM>. Such a microphone may be configured to detect environmental sounds, ambient noise, voices of a user or a third party, or other sounds.

<FIG> shows an example functional block diagram that may correspond to an example mixed reality system, such as mixed reality system <NUM> described above (which may correspond to mixed reality system <NUM> with respect to <FIG>). As shown in <FIG>, example handheld controller 400B (which may correspond to handheld controller <NUM> (a "totem")) includes a totem-to-wearable head device six degree of freedom (6DOF) totem subsystem 404A and example wearable head device 400A (which may correspond to wearable head device <NUM>) includes a totem-to-wearable head device 6DOF subsystem 404B. In the example, the 6DOF totem subsystem 404A and the 6DOF subsystem 404B cooperate to determine six coordinates (e.g., offsets in three translation directions and rotation along three axes) of the handheld controller 400B relative to the wearable head device 400A. The six degrees of freedom may be expressed relative to a coordinate system of the wearable head device 400A. The three translation offsets may be expressed as X, Y, and Z offsets in such a coordinate system, as a translation matrix, or as some other representation. The rotation degrees of freedom may be expressed as sequence of yaw, pitch and roll rotations, as a rotation matrix, as a quaternion, or as some other representation. In some examples, the wearable head device 400A; one or more depth cameras <NUM> (and/or one or more non-depth cameras) included in the wearable head device 400A; and/or one or more optical targets (e.g., buttons <NUM> of handheld controller 400B as described above, or dedicated optical targets included in the handheld controller 400B) can be used for 6DOF tracking. In some examples, the handheld controller 400B can include a camera, as described above; and the wearable head device 400A can include an optical target for optical tracking in conjunction with the camera. In some examples, the wearable head device 400A and the handheld controller 400B each include a set of three orthogonally oriented solenoids which are used to wirelessly send and receive three distinguishable signals. By measuring the relative magnitude of the three distinguishable signals received in each of the coils used for receiving, the 6DOF of the wearable head device 400A relative to the handheld controller 400B may be determined. Additionally, 6DOF totem subsystem 404A can include an Inertial Measurement Unit (IMU) that is useful to provide improved accuracy and/or more timely information on rapid movements of the handheld controller 400B.

In some examples, it may become necessary to transform coordinates from a local coordinate space (e.g., a coordinate space fixed relative to the wearable head device 400A) to an inertial coordinate space (e.g., a coordinate space fixed relative to the real environment), for example in order to compensate for the movement of the wearable head device 400A relative to the coordinate system <NUM>. For instance, such transformations may be necessary for a display of the wearable head device 400A to present a virtual object at an expected position and orientation relative to the real environment (e.g., a virtual person sitting in a real chair, facing forward, regardless of the wearable head device's position and orientation), rather than at a fixed position and orientation on the display (e.g., at the same position in the right lower corner of the display), to preserve the illusion that the virtual object exists in the real environment (and does not, for example, appear positioned unnaturally in the real environment as the wearable head device 400A shifts and rotates). In some examples, a compensatory transformation between coordinate spaces can be determined by processing imagery from the depth cameras <NUM> using a SLAM and/or visual odometry procedure in order to determine the transformation of the wearable head device 400A relative to the coordinate system <NUM>. In the example shown in <FIG>, the depth cameras <NUM> are coupled to a SLAM/visual odometry block <NUM> and can provide imagery to block <NUM>. The SLAM/visual odometry block <NUM> implementation can include a processor configured to process this imagery and determine a position and orientation of the user's head, which can then be used to identify a transformation between a head coordinate space and another coordinate space (e.g., an inertial coordinate space). Similarly, in some examples, an additional source of information on the user's head pose and location is obtained from an IMU <NUM>. Information from the IMU <NUM> can be integrated with information from the SLAM/visual odometry block <NUM> to provide improved accuracy and/or more timely information on rapid adjustments of the user's head pose and position.

In some examples, the depth cameras <NUM> can supply 3D imagery to a hand gesture tracker <NUM>, which may be implemented in a processor of the wearable head device 400A. The hand gesture tracker <NUM> can identify a user's hand gestures, for example by matching 3D imagery received from the depth cameras <NUM> to stored patterns representing hand gestures. Other suitable techniques of identifying a user's hand gestures will be apparent.

In some examples, one or more processors <NUM> may be configured to receive data from the wearable head device's 6DOF headgear subsystem 404B, the IMU <NUM>, the SLAM/visual odometry block <NUM>, depth cameras <NUM>, and/or the hand gesture tracker <NUM>. The processor <NUM> can also send and receive control signals from the 6DOF totem system 404A. The processor <NUM> may be coupled to the 6DOF totem system 404A wirelessly, such as in examples where the handheld controller 400B is untethered. Processor <NUM> may further communicate with additional components, such as an audio-visual content memory <NUM>, a Graphical Processing Unit (GPU) <NUM>, and/or a Digital Signal Processor (DSP) audio spatializer <NUM>. The DSP audio spatializer <NUM> may be coupled to a Head Related Transfer Function (HRTF) memory <NUM>. The GPU <NUM> can include a left channel output coupled to the left source of imagewise modulated light <NUM> and a right channel output coupled to the right source of imagewise modulated light <NUM>. GPU <NUM> can output stereoscopic image data to the sources of imagewise modulated light <NUM>, <NUM>, for example as described above with respect to <FIG>. The DSP audio spatializer <NUM> can output audio to a left speaker <NUM> and/or a right speaker <NUM>. The DSP audio spatializer <NUM> can receive input from processor <NUM> indicating a direction vector from a user to a virtual sound source (which may be moved by the user, e.g., via the handheld controller <NUM>). Based on the direction vector, the DSP audio spatializer <NUM> can determine a corresponding HRTF (e.g., by accessing a HRTF, or by interpolating multiple HRTFs). The DSP audio spatializer <NUM> can then apply the determined HRTF to an audio signal, such as an audio signal corresponding to a virtual sound generated by a virtual object. This can enhance the believability and realism of the virtual sound, by incorporating the relative position and orientation of the user relative to the virtual sound in the mixed reality environment - that is, by presenting a virtual sound that matches a user's expectations of what that virtual sound would sound like if it were a real sound in a real environment.

In some examples, such as shown in <FIG>, one or more of processor <NUM>, GPU <NUM>, DSP audio spatializer <NUM>, HRTF memory <NUM>, and audio/visual content memory <NUM> may be included in an auxiliary unit 400C (which may correspond to auxiliary unit <NUM> described above). The auxiliary unit 400C may include a battery <NUM> to power its components and/or to supply power to the wearable head device 400A or handheld controller 400B. Including such components in an auxiliary unit, which can be mounted to a user's waist, can limit the size and weight of the wearable head device 400A, which can in turn reduce fatigue of a user's head and neck.

While <FIG> presents elements corresponding to various components of an example mixed reality system, various other suitable arrangements of these components will become apparent to those skilled in the art. For example, elements presented in <FIG> as being associated with auxiliary unit 400C could instead be associated with the wearable head device 400A or handheld controller 400B. Furthermore, some mixed reality systems may forgo entirely a handheld controller 400B or auxiliary unit 400C. Such changes and modifications are to be understood as being included within the scope of the disclosed examples.

Presenting virtual audio content to a user can be advantageous in creating an immersive augmented/mixed reality experience. An immersive augmented/mixed reality experience can further blend real content with virtual content when convincing audio is presented in addition to convincing video. Displaying convincing virtual video content (e.g., aligned with and/or inseparable from real content) can include mapping a real, and sometimes unknown, environment while simultaneously estimating a MR system's location and orientation within the real environment to accurately display virtual video content within the real environment. Displaying convincing virtual video content can further include rendering two sets of the same virtual video content from two different perspectives so that a stereoscopic image can be presented to a user to simulate three-dimensional virtual video content. Similar to displaying convincing virtual video content, presenting virtual audio content in a convincing matter can also include complex analyses of a real environment. For example, it can be desirable to understand acoustic properties of a real environment in which a MR system is being used so that virtual audio content can be rendered in a way that it simulates real audio content. Acoustic properties of a real environment can be used by a MR system (e.g., MR system <NUM>, <NUM>) to modify a rendering algorithm such that the virtual audio content sounds as if it originated from or otherwise belongs in the real environment. For example, a MR system used in a room with hard flooring and exposed walls may produce virtual audio content that mimics an echo that real audio content may have. Playing virtual audio content in a static manner as a user changes real environments (which may have different acoustic properties) may detract from an experience's immersion. It can be especially beneficial to render virtual audio content to mimic characteristics of real audio content if real audio content and virtual audio content may interact with each other (e.g., a user can speak to a virtual companion, and the virtual companion may speak back to the user). To do so, a MR system may determine acoustic characteristics of the real environment and apply those acoustic characteristics to virtual audio content (e.g., by altering a rendering algorithm for the virtual audio content). Additional details may be found in <CIT>.

One parameter that may characterize a real environment's acoustic properties can be a reverberation time (e.g., a T60 time). A reverberation time can include a length of time required for a sound to decay by a certain amount (e.g., by <NUM> decibels). Sound decay can be a result of sound reflecting off surfaces in a real environment (e.g., walls, floors, furniture, etc.) whilst losing energy due to, for example, geometric spreading. A reverberation time can be influenced by environmental factors. For example, absorbent surfaces (e.g., cushions) may absorb sound in addition to geometric spreading, and a reverberation time may be reduced as a result. In some embodiments, it may not be necessary to have information about an original source to estimate an environment's reverberation time.

Another parameter that may characterize a real environment's acoustic properties can be a reverberation gain. A reverberation gain can include a ratio of a sound's direct/source/original energy to the sound's reverberation energy (e.g., energy of a reverberation resulting from the direct/source/original sound) where a listener and the source are substantially co-located (e.g., a user may clap their hands, producing a source sound that may be considered substantially co-located with one or more microphones mounted on a head-wearable MR system). For example, an impulse (e.g., a clap) may have an energy associated with the impulse, and the reverberation sound from the impulse may have an energy associated with the reverberation of the impulse. The ratio of the original/source energy to the reverberation energy may be a reverberation gain. A real environment's reverberation gain may be influenced by, for example, absorbent surfaces that can absorb sound and thereby reduce a reverberation energy.

The reverberation time and the reverberation gain can collectively be referred to as a reverberation fingerprint. In some embodiments, a reverberation fingerprint can be passed to an audio rendering algorithm as one or more input parameters, which may allow the audio rendering algorithm to present virtual audio content with the same or similar characteristics as real audio content in a real environment.

A reverberation fingerprint can be useful because it may characterize a real environment's acoustic properties independent of a sound source's position and/or orientation in the real environment. For example, a standard interior room with four walls, a floor, and a ceiling may exhibit the same (or substantially the same) reverberation time and/or reverberation gain regardless if a source is located at a corner of the room, in the center of the room, or along any of the room's walls/edges. As another example, a sound source directly facing a corner of the room, the center of the room, or a wall in the room may all behave the same (or substantially the same) according to a real environment's reverberation fingerprint. A reverberation fingerprint can also be useful because it may characterize a real environment's acoustic properties independent of characteristics of a sound source. For example, a sound source (e.g., a person talking) at a low frequency, middle frequency, or high frequency may all behave the same (or substantially the same) according to a real environment's reverberation time and/or reverberation gain. Similarly, an impulse sound source (e.g., a clap) and a non-impulse sound source may behave the same (or substantially the same) according to a real environment's reverberation fingerprint (e.g., reverberation time and/or reverberation gain). As another example, a loud sound source and a quiet sound source (e.g., in terms of amplitude) may behave the same (or substantially the same) according to a real environment's reverberation fingerprint (e.g., reverberation time and/or reverberation gain). The independence of a reverberation fingerprint from characteristics and/or location of a sound source can make the reverberation fingerprint a useful tool to render virtual audio content in a computationally efficient manner (e.g., the rendering algorithm can be the same as long as a user does not change environments, for example, by moving to a different room). In some embodiments, a reverberation fingerprint may apply to "well-behaved" rooms (e.g., a standard interior room with four walls, a floor, and a ceiling), and may not apply to "misbehaved" rooms (e.g., a long corridor) that may have special acoustic properties.

In some embodiments, it can be desirable to perform a "blind" estimation of a real environment's reverberation fingerprint. A blind estimation can be an estimation of a reverberation fingerprint where information about a sound source may not be required. For example, a reverberation fingerprint may be estimated based simply on human conversation, where information on the original speech may not be provided to the estimation algorithm. Pauses during human speech can provide enough time for a reverberation fingerprint to be estimated using blind estimation. It can be beneficial to perform a blind estimation because such an estimation can be done without requiring a lengthy setup process and/or user interaction. In some embodiments, a reverberation time can be blindly estimated and may not require information about an original sound source. In some embodiments, a blind estimation may not be performed on a reverberation gain, which may include information about an original sound source.

<FIG> illustrates an example process <NUM> of estimating a reverberation fingerprint, according to some embodiments. The example process shown can be implemented using one or more components of a mixed reality system, such as one or more of wearable head device <NUM>, handheld controller <NUM>, and auxiliary unit <NUM> of the example mixed reality system <NUM> described above; or by a system (e.g., a system comprising a cloud server) in communication with mixed reality system <NUM>. At step <NUM> of process <NUM>, an input <NUM> can be split into one or more filtered components, which may then be individually processed. For example, at step <NUM>, a bandpass filter can be applied to an input <NUM>, which can be an audio signal from one or more microphones (e.g., one or more microphones mounted on a MR system). A bandpass filter can preferentially allow certain frequency ranges through the filter and/or suppress frequencies outside the frequency range. Bandpass filters can break a signal into smaller component pieces that may be easier to process for computational efficiency. Bandpass filters can also improve a signal-to-noise ratio of a signal by removing unwanted noise at frequencies outside the frequency range. In some embodiments, bandpass filters can be used to separate an audio signal into six frequency ranges. A reverberation fingerprint (e.g., a reverberation time and a reverberation gain) can be estimated for each frequency range. This can be used to create a continuous frequency response curve such that each frequency can have an associated reverberation time and/or reverberation gain (e.g., a reverberation time and/or reverberation gain may be interpolated from calculated values that may be centered at a frequency range separated by a bandpass filter). Although six frequency ranges are discussed, an audio signal may be separated into any number of frequency ranges (e.g., using any number of bandpass filters). In some embodiments, octave filters can be applied to the input signal. In some embodiments, <NUM>/<NUM> octave filters can be applied to the input signal. In some embodiments, signals with frequencies that are too low (e.g., less than <NUM>) may not be analyzed for a reverberation fingerprint (e.g., because low frequencies may not sufficiently reverberate to conduct a reverberation fingerprint analysis).

At step <NUM>, frequency band boosting can optionally be applied. Frequency band boosting may be applied to low frequencies (e.g., less than <NUM>) that may have a low signal-to-noise ratio, but the signal-to-noise ratio may still be sufficiently high to determine a reverberation fingerprint (e.g., the signal-to-noise ratio may be higher than a signal-to-noise ratio for frequencies less than <NUM>). Frequency band boosting may be applied to other frequency bands, or not at all.

At step <NUM>, a running energy estimation can be performed on a signal. A running energy estimation can be performed in the frequency domain, time domain, spectral domain, and/or any other suitable domain. Signal energy may be estimated by determining an area under a squared magnitude of the signal in a time domain or by using other appropriate methods.

At step <NUM>, envelope detection can be run on the signal and may be based on a running energy (estimate) of the signal. A signal envelope can be a characterization of signal peaks and/or troughs and may define upper and/or lower boundaries of a signal (e.g., an oscillating signal). Envelope detection can be performed using a Hilbert transform, a leaky integrator based root mean square detector, and/or other suitable methods.

At step <NUM>, peak picking can be run on a signal envelope. Peak picking can identify local peaks in a signal envelope based on an amplitude of a previously detected peak and/or based on local maxima.

At step <NUM>, a free decay region estimation can be run on a signal envelope. A free decay region can be a region of a signal envelope where the envelope decreases (e.g., after a local peak). This can be the result of a reverberation where new sound may not be detected and only previous sound continues to reverberate in a real environment, resulting in a decrease in the signal envelope. At step <NUM>, a linear fit can be determined for each of one or more free decay regions in a signal. A linear fit may be appropriate where a signal envelope is measured on a decibel scale due to an exponential decay of sound energy, and a decibel scale measuring on a logarithmic scale.

At step <NUM>, a reverberation time can be estimated. A reverberation time may be estimated based on a free decay region or a portion of a free decay region with the fastest decaying slope, which can be determined from a linear fit determined for each free decay region (or portion of a free decay region). In some embodiments, a threshold amount of time (e.g., <NUM>) after a local peak may be ignored in determining a linear fit. This can be beneficial to avoid short-term reverberations (which may behave differently) and/or to help ensure the regression is fit to exclusively reverberant sound and not source sound. A linearly fitted slope can represent an amount by which the signal envelope decreases in decibels per unit of time (e.g., per second).

In some embodiments, multiple linear fits can be applied to a single free decay region. For example, a linear regression may only be applied within a time range that the regression is sufficiently accurate (e.g., a correlation of <NUM>% or greater). If a linear regression no longer fits the remainder of the duration of a free decay region, one or more additional/alternate linear regressions may be applied. Accuracy in a reverberation time estimate can be increased by using only the fastest decaying slope within a free decay region because the associated portion of the free decay region may most accurately represent only reverberant sounds. For example, a portion of a free decay region with a slower decaying slope may capture a small amount of non-reverberant (e.g., original/source) sound, which may artificially slow a measured decay rate. Based on the fastest decaying linearly fitted slope, a reverberation time (which can be a time required for a signal to decay by <NUM> decibels) can be extrapolated.

<FIG> illustrates an example process <NUM> for estimating a reverberation time. Example process <NUM> may correspond to step <NUM> of example process <NUM> described above. Example process <NUM> can be implemented using one or more components of a mixed reality system, such as one or more of wearable head device <NUM>, handheld controller <NUM>, and auxiliary unit <NUM> of the example mixed reality system <NUM> described above; or by a system (e.g., a system comprising a cloud server) in communication with mixed reality system <NUM>. At step <NUM> of example process <NUM>, a local peak may be determined (e.g., a local peak from a signal envelope). At step <NUM>, a linear regression can be fit to part or all of a free decay region. A free decay region can be a region of a signal envelope where the envelope decreases (e.g., after a local peak). In some embodiments, a linear regression may not account for a portion of time after a local peak (e.g., <NUM> after a local peak). At step <NUM>, it can be determined whether the linear fit is sufficiently accurate (e.g., has a sufficiently low root mean square error). If it is determined that the linear fit is not sufficiently accurate, at step <NUM> the next free decay region or portion of a free decay region may be examined. If it is determined that the linear fit is sufficiently accurate, at step <NUM> it can be determined if the decay region occurs over a sufficiently long period of time (e.g., ><NUM>). If it is determined that the decay region does not occur over a sufficiently long period of time, the next free decay region or portion of a free decay region may be examined at step <NUM>. If it is determined that the decay region does occur over a sufficiently long period of time, at step <NUM> it can be determined if the decay slope from the linear regression is the fastest decay slope for the entire free decay region. If it is determined that the decay slope is not the fastest decay slope for the entire free decay region, the next free decay region or portion of a free decay region can be examined at step <NUM>. If it is determined that the decay slope is the fastest decay slope for the entire free decay region, a reverberation time can be extrapolated based on the fastest decay slope at step <NUM>.

In some embodiments, a reverberation time can be estimated using converging (or approximately converging) measurements. For example, a reverberation time can be declared after a threshold number of consecutive free decay regions have decay slopes within a threshold value of each other. An average decay slope may then be determined and declared as a reverberation time. In some embodiments, decay slopes associated with free decay regions can be weighted according to a quality estimate for each measured decay slope. In some embodiments, a decay slope may be determined to be more accurate when an associated portion of a free decay region lasts for a threshold amount of time (e.g., <NUM>), which can increase an accuracy of the decay slope estimation. In some embodiments, a decay slope may be determined to be more accurate if it has a relatively accurate linear fit (e.g., a low root mean square error). Decay slopes that are more accurate can be assigned higher weights in a weighted average to determine a reverberation time. In some embodiments, a single decay slope that is determined to be the most accurate (e.g., based on decay length and/or linear fit accuracy) can be used to determine a reverberation time, which may be a reverberation time for a given frequency range (e.g., a frequency range selected by a bandpass filter at step <NUM>).

Referring back to <FIG> and process <NUM>, at step <NUM>, according to the invention, a time difference between the reverberation time and a previously estimated reverberation time is determined. It is also determined whether the time difference is greater than a time threshold, and in accordance with a determination that the time difference is greater than the time threshold, the reverberation time estimate is declared. At step <NUM>, a confidence value may be determined and associated with a reverberation time. A confidence value may be determined based on various factors. For example, a confidence value can be based on a number of convergent decay slopes, a linear fit accuracy of utilized decay slopes, a decay length of utilized decay slopes, or any combination of these and/or other factors. In some embodiments, a reverberation time estimate with an associated confidence may not be declared if the confidence value is below a threshold value (e.g., because insufficient free decay regions were detected for convergence). If a reverberation time estimate is not declared, other reverberation time estimates for other frequency ranges (e.g., frequency ranges separated at step <NUM> using a bandpass filter) may still be declared (e.g., if those reverberation time estimates have sufficiently high confidence values). The reverberation time estimate for a missing frequency range may be interpolated from declared reverberation times at other frequency ranges.

At step <NUM>, a direct sound energy estimation can be performed. A direct sound energy estimation may utilize information on the direct/source sound. For example, if a direct/source sound is known, a direct sound energy estimation can estimate the energy of the direct/source sound (e.g., by integrating an area under a signal envelope peak including the direct/source sound). This can be achieved by using impulse sounds, which may be easier to separate a direct/source sound from a reverberant sound. In some embodiments, a user may be prompted (e.g., by a MR system) to clap their hands to produce an impulse sound. In some embodiments, a speaker, for example one that is mounted on an MR system, may play an impulse sound. In some embodiments, an impulse sound can be used to estimate both a direct sound energy and a reverberation time estimate. In some embodiments, a direct sound estimation can be blindly estimated (e.g., if a blind estimate can separate a direct/source sound from a reverberant sound without prior knowledge of the direct/source sound).

At step <NUM>, a reverberation sound energy can be estimated. The reverberation sound energy can be estimated by integrating a signal envelope from an end of a direct/source sound until the reverberant sound is no longer detected and/or the reverberant sound falls below a certain gain threshold (e.g., -90dB).

At step <NUM>, a reverberation gain can be estimated based on the direct sound energy estimation and the reverberation energy estimation. In some embodiments, the reverberation gain is calculated by taking a ratio of the reverberation energy to the direct sound energy. In some embodiments, the reverberation gain is calculated by taking a ratio of the direct sound energy to the reverberation energy. A reverberation gain estimate can be declared (e.g., passed to an audio rendering algorithm). In some embodiments, a confidence level may be associated with a reverberation gain estimate. For example, if a peak is detected in a reverberation energy estimation, it may indicate that new direct/source sound was introduced, and the reverberation gain estimate may no longer be accurate. In some embodiments, a reverberation gain estimate may only be declared if a confidence level is at or above a certain threshold.

In addition to using a reverberation fingerprint to more realistically render virtual audio content, a reverberation fingerprint can also be used to identify a real environment and/or identify a change in a real environment. For example, a user may calibrate a MR system in a first room (e.g., a first acoustic environment) and then move to a second room. The second room may have different acoustic properties than the first room (e.g., a different reverberation time and/or a different reverberation gain). A MR system may blindly estimate a reverberation time in the second room, determine that the reverberation time is sufficiently different than a previously declared reverberation time, and conclude that the user has changed rooms. A MR system may then declare a new reverberation time and/or a new reverberation gain (e.g., by asking the user to clap again, by playing an impulse through an external speaker, and/or doing a blind estimate of the reverberation gain). As another example, a user may calibrate a MR system in a room, and the MR system may determine a reverberation fingerprint of the room. The MR system may then identify the room based on the reverberation fingerprint and/or other factors (e.g., location determined through GPS and/or WiFi networks, or via one or more sensors such as described above with respect to the example mixed reality system <NUM>). The MR system may access a remote database of previously mapped rooms and, using the reverberation fingerprint and/or other factors, identify the room as previously mapped. The MR system may download assets related to the room (e.g., a previously generated three-dimensional map of the room).

<FIG> illustrates an example process for identifying a change in acoustic properties of a real environment. The example process shown can be implemented using one or more components of a mixed reality system, such as one or more of wearable head device <NUM>, handheld controller <NUM>, and auxiliary unit <NUM> of the example mixed reality system <NUM> described above; or by a system (e.g., a system comprising a cloud server) in communication with mixed reality system <NUM>. At step <NUM> of the example process, a new reverberation time can be determined (e.g., using process <NUM> and/or process <NUM>). At step <NUM>, the new reverberation time can be compared to a previously declared reverberation time. At step <NUM>, it can be determined if the new reverberation time is sufficiently different than the previously declared reverberation time. The difference can be evaluated in any number of ways. For example, a difference may be sufficient if a new reverberation time for a frequency range has a difference from a declared reverberation time for the frequency range greater than a specified threshold (e.g., <NUM>%, which may be a sufficient difference for human listeners to perceive a difference). As another example, a sufficient difference may be determined if a threshold number of reverberation times for given frequency ranges differs from a threshold number of declared reverberation times for those frequency ranges. As another example, an absolute value of a difference between a new frequency response curve (which can include interpolated points between declared reverberation times for tested frequency ranges) and a declared frequency response curve can be integrated. If the integrated area is above a certain threshold, it may be determined that the new reverberation times are sufficiently different from the declared reverberation times.

If the new reverberation time is determined to be insufficiently different from a declared reverberation time, a MR system may continue to determine new reverberation times at step <NUM>. If the new reverberation time is determined to be sufficiently different from a declared reverberation time, at step <NUM> it can be determined if a sufficient number of sufficiently different reverberation times has been detected. For example, three consecutive reverberation time estimates that are all sufficiently different from a declared reverberation item for a given frequency range may be a sufficient number of sufficiently different reverberation times. Other thresholds may also be used (e.g., three out of five most recent reverberation time estimates). If it is determined that a sufficient number of sufficiently different reverberation times has not been detected, a MR system may continue to determine new reverberation times at step <NUM>. If it is determined that a sufficient number of sufficiently different reverberation times has been detected, a new reverberation time may be declared at step <NUM>. In some embodiments, step <NUM> can also include initiating a new reverberation gain estimate, which can prompt a user to clap or play an impulse sound from an external speaker. In some embodiments, step <NUM> can also include accessing a remote database to identify a new real environment based on the new reverberation fingerprint and/or other information available to a MR system (e.g., location determined from GPS and/or WiFi connections, or via one or more sensors such as described above with respect to the example mixed reality system <NUM>).

Claim 1:
A method (<NUM>) comprising:
receiving (<NUM>), via a microphone of a wearable head device, an audio signal;
determining (<NUM>) an envelope of the audio signal;
estimating (<NUM>) a reverberation time (T60[f]) based on the envelope of the audio signal;
determining a time difference between the reverberation time and a previously estimated reverberation time;
determining whether the time difference is greater than a time threshold; and
in accordance with a determination that the time difference is greater than the time threshold, presenting, via a speaker (<NUM>, <NUM>) of the wearable head device (400A), audio content,
wherein the audio content is based on the reverberation time (T60[f]).