WO2014108675A1

WO2014108675A1 - Head-mounteable apparatus detecting cable tangle

Info

Publication number: WO2014108675A1
Application number: PCT/GB2014/050031
Authority: WO
Inventors: Simon Benson; Ian Bickerstaff; Sharwin Raghoebardayal
Original assignee: Sony Computer Entertainment Europe Limited
Priority date: 2013-01-08
Filing date: 2014-01-07
Publication date: 2014-07-17
Also published as: GB201300289D0; GB2509551A

Abstract

A head-mountable apparatus (20) comprises a frame which is configured to be mounted on a user's head, the frame supporting one or more electrical and/or optical devices (122); a cable (82), (84) connecting the one or more electrical and/or optical devices to an external device (300) separate to the head-mountable apparatus (20); and a detector for detecting head movements of a magnitude which, assuming a substantially stationary external device, may cause the cable (82), (84) to become at least partially wrapped around the user's body.

Description

HEAD-MOUNTEABLE APPARATUS DETECTING CABLE

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the earlier filing date of GB1300289.4 filed in the United Kingdom Intellectual Property Office on 8 January 2013, the entire contents of which application are incorporated herein by reference.

BACKGROUND

Field

This invention relates to head-mountable apparatus.

Description of Related Art

The "background" description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as prior art against the present disclosure.

A head-mountable display (HMD) is one example of a head-mountable apparatus. Audio headphones comprising a frame supporting one or more audio transducers are another example of a head-mountable apparatus. A head-mounted torch or light is a further example of a head-mountable apparatus. The following background discussions will relate mainly to HMDs, but the principles are also applicable to other types of head-mountable apparatus.

In an HMD, an image or video display device is provided which may be worn on the head or as part of a helmet. Either one eye or both eyes are provided with small electronic display devices.

Some HMDs allow a displayed image to be superimposed on a real-world view. This type of HMD can be referred to as an optical see-through HMD and generally requires the display devices to be positioned somewhere other than directly in front of the user's eyes. Some way of deflecting the displayed image so that the user may see it is then required. This might be through the use of a partially reflective mirror placed in front of the user's eyes so as to allow the user to see through the mirror but also to see a reflection of the output of the display devices. In another arrangement, disclosed in EP-A-1 731 943 and US-A-2010/0157433, a waveguide arrangement employing total internal reflection is used to convey a displayed image from a display device disposed to the side of the user's head so that the user may see the displayed image but still see a view of the real world through the waveguide. Once again, in either of these types of arrangement, a virtual image of the display is created (using known techniques) so that the user sees the virtual image at an appropriate size and distance to allow relaxed viewing. For example, even though the physical display device may be tiny (for example, 10 mm x 10 mm) and may be just a few millimetres from the user's eye, the virtual image may be arranged so as to be perceived by the user at a distance of (for example) 20 m from the user, having a perceived size of 5 m x 5m.

Other HMDs, however, allow the user only to see the displayed images, which is to say that they obscure or at least substantially obscure the user's view of the real world environment surrounding the user. This type of HMD can position the actual display devices in front of the user's eyes, in association with appropriate lenses or other optical components which place a virtual displayed image at a suitable distance for the user to focus in a relaxed manner - for example, at a similar virtual distance and perceived size as the optical see-through HMD described above. This type of device might be used for viewing movies or similar recorded content, or for viewing so-called virtual reality content representing a virtual space surrounding the user. It is of course however possible to display a real-world view on this type of HMD, for example by using a forward-facing camera (such as a video camera) to generate images for display on the display devices.

Although the original development of HMDs was perhaps driven by the military and professional applications of these devices, HMDs are becoming more popular for use by casual users in, for example, computer game or domestic computing applications.

SUMMARY

This invention provides a head-mountable apparatus comprising:

a frame which is configured to be mounted on a user's head, the frame supporting one or more electrical and/or optical devices;

a cable connecting the one or more electrical and/or optical devices to an external device separate to the head-mountable apparatus; and

a detector for detecting head movements of a magnitude which, assuming a substantially stationary external device, may cause the cable to become at least partially wrapped around the user's body.

The invention recognises that with a cabled head-mountable apparatus, there is a risk that the user will move or rotate so as to cause potential cable problems. Embodiments of the invention provide a system to detect, and optionally to warn a user of a wired or cabled head- mountable apparatus such as a head-mountable display (HMD), about the possibility of the HMD cable becoming tangled or otherwise wrapped around the user's neck or other part of the body. In embodiments of the invention the HMD may have a motion detection and/or tracking arrangement such as one or more gyros or optical sensors so that the HMD (or a device that is driving the HMD) has awareness of the orientation or the HMD. If the HMD is rotated excessively in one direction (such as 360 degrees clockwise) then the HMD or driving device can judge this to be an increased risk that the cable connected to the HMD could be at risk of wrapping or tangling around the user. The system could then alert the user to this risk. This invention is therefore intended to increase the safety and comfort of HMD usage. Note that similar issues may occur in other types of head-mountable apparatus (apart from HMDs), connected by a cable to an external device such as a signal source (or processor) and/or a power supply.

Various further aspects and features of the present invention are defined in the appended claims and include a method of operating a head-mountable apparatus as well as a computer program.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates an HMD worn by a user;

Figure 2 is a schematic plan view of an HMD;

Figure 3 schematically illustrates the formation of a virtual image by an HMD;

Figure 4 schematically illustrates another type of display for use in an HMD;

Figure 5 schematically illustrates a pair of stereoscopic images;

Figure 6 schematically illustrates a user wearing an HMD connected to a Sony® PlayStation 3® games console;

Figure 7 schematically illustrates a tangle detector using an accelerometer;

Figure 8 schematically illustrates a tangle detector using a camera;

Figure 9 schematically illustrates a cable having sensor portions;

Figure 10 schematically illustrates a cable strain detector;

Figure 11 schematically illustrates a tangle detector using a directional Bluetooth® antenna;

Figure 12 is a schematic flowchart illustrating a tangle detection and warning process;

Figure 13 schematically illustrates a warning display;

Figure 14 is a schematic flowchart illustrating a tangle detection and warning process; and

Figure 15 schematically illustrates a displayed camera view.

Referring now to Figure 1 , a user 10 is wearing an HMD 20 (as an example of a generic head-mountable apparatus - other examples including audio headphones or a head-mountable light source) on the user's head 30. The HMD comprises a frame 40, in this example formed of a rear strap and a top strap, and a display portion 50.

The HMD of Figure 1 completely (or at least substantially completely) obscures the user's view of the surrounding environment. All that the user can see is the pair of images displayed within the HMD. The HMD has associated headphone audio transducers or earpieces 60 which fit into the user's left and right ears 70. The earpieces 60 replay an audio signal provided from an external source, which may be the same as the video signal source which provides the video signal for display to the user's eyes. One or both ears may be provided with such transducers. The external source may therefore provide audio and/or video signals to the HMD and may optionally act as a power supply to the HMD.

A front-facing camera 122 such as a video camera may capture images to the front of the HMD (that is to say, of the surroundings of the HMD), in use. One or more such cameras may be provided. A Bluetooth® antenna 124 may provide communication facilities or may simply be arranged as a directional antenna to allow a detection of the direction of a nearby Bluetooth transmitter.

In operation, a video signal is provided for display by the HMD. This could be provided by an external video signal source 80 such as a video games machine or data processing apparatus (such as a personal computer), in which case the signals could be transmitted to the HMD by a wired or a wireless connection 82. Examples of suitable wireless connections include Bluetooth® connections. Audio signals for the earpieces 60 can be carried by the same connection. Similarly, any control signals passed from the HMD to the video (audio) signal source may be carried by the same connection. Furthermore, a power supply 83 (including one or more batteries and/or being connectable to a mains power outlet) may be linked by a cable 84 to the HMD. Note that the power supply 83 and the video signal source 80 may be separate units or may be embodied as the same physical unit. There may be separate cables for power and video (and indeed for audio) signal supply, or these may be combined for carriage on a single cable (for example, using separate conductors, as in a USB cable, or in a similar way to a "power over Ethernet" arrangement in which data is carried as a balanced signal and power as direct current, over the same collection of physical wires). The video and/or audio signal may be carried by, for example, an optical fibre cable. In general terms, embodiments of the invention are applicable to an HMD having at least one electrical and/or optical cable linking the HMD to another device, such as a power supply and/or a video (and/or audio) signal source. So, embodiments of the invention can include, for example:

(a) an HMD having its own power supply (as part of the HMD arrangement) but a cabled connection to a video and/or audio signal source;

(b) an HMD having a cabled connection to a power supply and to a video and/or audio signal source, embodied as a single physical cable or more than one physical cable;

(c) an HMD having its own video and/or audio signal source (as part of the HMD arrangement) and a cabled connection to a power supply; or

(d) an HMD having a wireless connection to a video and/or audio signal source and a cabled connection to a power supply. Instead of an HMD, another type of head-mountable apparatus may be used, but the same considerations apply.

The physical position at which the cable 82 and/or 84 enters or joins the HMD is not particularly important from a technical point of view. Aesthetically, and to avoid the cable(s) brushing the user's face in operation, it would normally be the case that the cable(s) would enter or join the HMD at the side or back of the HMD (relative to the orientation of the user's head when worn in normal operation). Accordingly, the position of the cables 82, 84 relative to the HMD in Figure 1 should be treated merely as a schematic representation.

Accordingly, the arrangement of Figure 1 provides an example of a head-mountable display system comprising a frame to be mounted onto an observer's head, the frame defining one or two eye display positions which, in use, are positioned in front of a respective eye of the observer and a display element mounted with respect to each of the eye display positions, the display element providing a virtual image of a video display of a video signal from a video signal source to that eye of the observer.

Figure 1 shows just one example of an HMD. Other formats are possible: for example an

HMD could use a frame more similar to that associated with conventional eyeglasses, namely a substantially horizontal leg extending back from the display portion to the top rear of the user's ear, possibly curling down behind the ear. In other examples, the user's view of the external environment may not in fact be entirely obscured; the displayed images could be arranged so as to be superposed (from the user's point of view) over the external environment. An example of such an arrangement will be described below with reference to Figure 4.

In the example of Figure 1 , a separate respective display is provided for each of the user's eyes. A schematic plan view of how this is achieved is provided as Figure 2, which illustrates the positions 100 of the user's eyes and the relative position 110 of the user's nose. The display portion 50, in schematic form, comprises an exterior shield 120 to mask ambient light from the user's eyes and an internal shield 130 which prevents one eye from seeing the display intended for the other eye. The combination of the user's face, the exterior shield 120 and the interior shield 130 form two compartments 140, one for each eye. In each of the compartments there is provided a display element 150 and one or more optical elements 160. The way in which the display element and the optical element(s) cooperate to provide a display to the user will be described with reference to Figure 3.

Referring to Figure 3, the display element 150 generates a displayed image which is (in this example) refracted by the optical elements 160 (shown schematically as a convex lens but which could include compound lenses or other elements) so as to generate a virtual image 170 which appears to the user to be larger than and significantly further away than the real image generated by the display element 150. As an example, the virtual image may have an apparent image size (image diagonal) of more than 1 m and may be disposed at a distance of more than 1 m from the user's eye (or from the frame of the HM D). In general terms, depending on the purpose of the HMD, it is desirable to have the virtual image disposed a significant distance from the user. For example, if the HMD is for viewing movies or the like, it is desirable that the user's eyes are relaxed during such viewing, which requires a distance (to the virtual image) of at least several metres. In Figure 3, solid lines (such as the line 180) are used to denote real optical rays, whereas broken lines (such as the line 190) are used to denote virtual rays.

An alternative arrangement is shown in Figure 4. This arrangement may be used where it is desired that the user's view of the external environment is not entirely obscured. However, it is also applicable to HMDs in which the user's external view is wholly obscured. In the arrangement of Figure 4, the display element 150 and optical elements 200 cooperate to provide an image which is projected onto a mirror 210, which deflects the image towards the user's eye position 220. The user perceives a virtual image to be located at a position 230 which is in front of the user and at a suitable distance from the user.

In the case of an HMD in which the user's view of the external surroundings is entirely obscured, the mirror 210 can be a substantially 100% reflective mirror. The arrangement of Figure 4 then has the advantage that the display element and optical elements can be located closer to the centre of gravity of the user's head and to the side of the user's eyes, which can produce a less bulky HMD for the user to wear. Alternatively, if the HMD is designed not to completely obscure the user's view of the external environment, the mirror 210 can be made partially reflective so that the user sees the external environment, through the mirror 210, with the virtual image superposed over the real external environment.

In the case where separate respective displays are provided for each of the user's eyes, it is possible to display stereoscopic images. An example of a pair of stereoscopic images for display to the left and right eyes is shown in Figure 5. The images exhibit a lateral displacement relative to one another, with the displacement of image features depending upon the (real or simulated) lateral separation of the cameras by which the images were captured, the angular convergence of the cameras and the (real or simulated) distance of each image feature from the camera position.

Note that the lateral displacements in Figure 5 could in fact be the other way round, which is to say that the left eye image as drawn could in fact be the right eye image, and the right eye image as drawn could in fact be the left eye image. This is because some stereoscopic displays tend to shift objects to the right in the right eye image and to the left in the left eye image, so as to simulate the idea that the user is looking through a stereoscopic window onto the scene beyond. However, some HMDs use the arrangement shown in Figure 5 because this gives the impression to the user that the user is viewing the scene through a pair of binoculars. The choice between these two arrangements is at the discretion of the system designer. In some situations, an HMD may be used simply to view movies and the like. In this case, there is no change required to the apparent viewpoint of the displayed images as the user turns the user's head, for example from side to side. In other uses, however, such as those associated with virtual reality (VR) or augmented reality (AR) systems, the user's viewpoint need to track movements with respect to a real or virtual space in which the user is located.

Figure 6 schematically illustrates a user wearing an H M D connected to a Sony® PlayStation 3® games console 300. The games console 300 is connected to a mains power supply 310 and (optionally) to a main display screen (not shown). A cable, acting as the cables 82, 84 discussed above (and so acting as both power supply and signal cables), links the HMD 20 to the games console 300 and is, for example, plugged into a USB socket 320 on the console 300. Note that in the present embodiments, a single physical cable is provided which fulfils the functions of the cables 82, 84. In Figure 6, the user is also shown holding a hand-held controller 330 which may be, for example, a Sony® Move® controller which communicates wirelessly with the games console 300 to control (or to contribute to the control of) game operations relating to a currently executed game program.

The video displays in the HMD 20 are arranged to display images generated by the games console 300, and the earpieces 60 in the HMD 20 are arranged to reproduce audio signals generated by the games console 300. Note that if a USB type cable is used, these signals will be in digital form when they reach the HMD 20, such that the HMD 20 comprises a digital to analogue converter (DAC) to convert at least the audio signals back into an analogue form for reproduction.

Images from the camera 122 mounted on the HMD 20 are passed back to the games console 300 via the cable 82, 84. Similarly, if motion or other sensors are provided at the HMD 20, signals from those sensors may be at least partially processed at the HMD 20 and/or may be at least partially processed at the games console 300. The use and processing of such signals will be described further below.

The USB connection from the games console 300 also provides power to the HMD 20, according to the USB standard.

A risk can occur when a user is wearing an HMD of this type, particularly a so-called "full immersion" HM D which reduces or removes the opportunity of the user to sense his or her surroundings, by virtue of substantially obscuring the user's view of the surroundings and inhibiting the user from hearing the surroundings by reproducing an audio signal directly into the user's ears. This risk is that the user may move, in the course of enjoying the game or other content which is being reproduced by the HMD, such that the cable 82, 84 may become entangled and/or at least partially wrapped around the user's body. The latter situation can be particularly hazardous if the cable were to wrap around the user's neck. To address this potential problem, in the context of a head-mountable apparatus such as the HMD 20 discussed above, having a frame configured to be mounted on a user's head, the frame supporting one or more electrical and/or optical devices, and an electrical and/or optical cable connecting the one or more electrical and/or optical devices to an external device (such as the games console 300) separate to the head-mountable apparatus, a detector is provided for detecting head movements of a magnitude which, assuming a substantially stationary external device, may cause the cable to become at least partially wrapped around the user's body.

Various techniques for providing this detection will be discussed below. In the discussions which follow, the expression "cable tangle" will be used as a generic expression to represent the cable tangling with itself and/or the cable at least partially wrapping around the user's body. In other words, a "cable tangle" is indicative of a generic event relating to the cable which is undesirable for the user of a head-mountable apparatus connected to an external device by a cable.

Figure 7 schematically illustrates a cable tangle detector using an accelerometer and/or a gyroscope 400 to act as a motion detector to detect movement such as rotational motion of the HMD in use. Accordingly, the accelerometer and/or gyroscope 400 may be mounted on the frame of the HMD 20 or may be mounted on the cable but at a position sufficiently close to the user's head (in operation) that the rotational motion of the user's head or a good approximation of that motion is detected by the accelerometer and/or gyroscope 400.

In common with other arrangements to be described below, the processing of signals acquired by a sensor such as the accelerometer and/or gyroscope 400 may be carried out at least partially at the HMD and/or at least partially at the external device (in this case, the games console 300). Carrying out the processing at the HMD has the advantage that a small amount of data transfer is required between the HM D and the external device, at least in respect of the detection process. Carrying out the processing at the games console 300 has the advantage that the games console has a higher power processor and memory arrangement, as well as fewer restrictions on power consumption. For the purposes of the present description, it is (apart from the aspects discussed above) immaterial whether the processing is carried out at the HMD, at the external device or both.

The output of the accelerometer/gyroscope 400 is passed to an integrator 410. So, changes in motion or position detected by the accelerometer/gyroscope 400 are converted into absolute positions (relative to an arbitrary starting point) by the integrator 410. The output of the integrator 410 is passed to a detector 420 which detects whether the rotational position of the HMD 20 (and therefore of the users head) has changed by at least a predetermined (threshold) angle (such as 240° or 360°, for example as measured with respect to a horizontal plane), and thereby that there has been a head movement of at least the predetermined angle. The measures taken if such a detection is made will be described further below.

It will be appreciated that Figure 7 is a schematic representation of the detection process. It may be that the detector 420 operates directly on the "change of position" data produced by the accelerometer/gyroscope 400, so that the integrator 410 is not in fact required.

Figure 8 schematically illustrates a tangle detector using a camera such as the front- facing camera 122 on the HMD 20. A tracker 430, acting on the images captured by the front- facing camera 122, optically detects rotational movement of the HMD 20 by comparing images captured by the camera 122, thereby detecting the relative movement of the camera against the surroundings around the HMD. A full (360°) turn can be detected by the tracker 430 acting as a comparator to generate a comparison of the image captured at the end of the rotation with the image captured at the beginning of the rotation, on the basis that there has been rotational movement relative to the surroundings in between these two images and thereby to compare captured images so as to detect motion of the head-mountable apparatus relative to the surroundings. If the two images are substantially identical, it may be assumed that the HMD has rotated by a full 360°. If the image has moved by a multiple of the angular field of view of the camera (which may be known, as, for example, 55°) then the angular rotation of the camera relative to the surroundings may be detected and compared as above with a threshold rotation angle. The outcome, which may be indicative of a rotation of more than the threshold angle, is detected by a detector 440.

Note that a similar but inverse arrangement could be provided by using a stationary camera, for example a camera associated with the games console 300 (not shown) to track a light source associated with the user. In the embodiment illustrated schematically in Figure 6, such a light source can be provided by the illuminated end of the motion controller 330 (and indeed, an accelerometer/gyroscope function can also be provided by the motion controller 330 although, as the motion controller 330 may be used in motion to control aspects of the gameplay, the motion of the controller 330 may not correspond well to the actual motion of the user's body). However, using just a single light source can have a disadvantage that if the user turns his or her back on the fixed camera, the light source will not be visible to the camera. So, if a stationary camera and light source arrangement is used, embodiments of the invention provide multiple light sources (which may be infrared light sources and therefore the illumination can be invisible to the human eye) mounted on and spaced around the HMD, such as a light source 450 shown in Figure 1.

In other embodiments, tangling or self-overlap of the cable (the cable overlapping itself) caused, for example, buy head movements, can be detected directly rather than implied from rotation of the user's body or head. Figure 9 schematically illustrates a cable 460 having sensor portions 470 surrounding a core region 480 in which the cable's conductors and/or optical fibres are housed. An example of the sensor portions may be that they detect stress, for example by providing for a change in electrical resistance of the sensor portions (and so a change in current-carrying properties relating to a sensor current provided, for example, by the HMD all the games console 300) when they are compressed, so that an overlap of the cable 82, 84 with itself leads to compression of the outer sensor portions 470 which can in turn be detected as a potential problem situation relating to the cable. Another example of sensor portions may be a proximity detector for sensing the proximity of an overlapping portion of the cable, for example formed by a set of conductors carrying a high-frequency test signal (such as a 100 kHz test signal), with those or other conductors in the outer sensor portions detecting induced currents from the test signal. The induced currents may change if one part of the cable overlies another part of the cable, thereby indicating an overlap, tangle or potential problem situation.

Figure 10 schematically illustrates a cable strain detector which indirectly detects a potential problem situation relating to the cable 82, 84 by detecting instances of the cable being placed under longitudinal strain. The cable strain detector 500 shown in Figure 10 is schematically illustrated near to a USB plug 510 (and so at the games console end of the cable) but could in fact be positioned anywhere along the cable. In the embodiment shown, the strain detector 500 comprises a looped region of the cable 520, with parts of the loop being connected by a strain detector 530 such as a solid state electrical strain detector. The detection of a longitudinal cable strain of at least a predetermined threshold amount by the strain detector can be used to detect a potential cable problem such as a tangle or a problem caused by excessive or inappropriate user movement.

Figure 1 1 schematically illustrates a tangle detector using a directional Bluetooth® or other radio frequency communication antenna such as the antenna 124 shown schematically in Figure 1. The antenna 124 can be made directional in its response by known techniques. It can be used either to receive signals transmitted by a signal source such as the games console 300, or to transmit signals to a receiver which is assumed to be stationary such as a receiver associated with the games console 300, or both. In each case of radio frequency communication with an external device, an indication of direction is given by changes in the relative strength of the communicated radiofrequency signal. The signal strength will be at a high level when the directional properties of the antenna 124 are aligned with the direction between the HMD and the other communicating device such as the games console 300, and will be at a lower level when the directional properties of the antenna 124 are not aligned with the direction between the HMD and the other communicating device. As many directional antennas have a symmetrical response (in other words, two peaks in response separated by 180°) this arrangement may, in itself, give a partially ambiguous result. For example, if the response starts at the peak dips to a trough, returns to a peak, dips to a trough and returns to peak again, this could represent a 360° turn of the HMD or could instead represent a 180° turn in one direction followed by a 180° turn back in the opposite direction to the original starting point. The former could represent a problem situation relating to the cable, whereas the latter situation may well not represent any problem relating to the cable. Accordingly, if there is this potential ambiguity in the detection results, embodiments of the invention may combine a directional radiofrequency detection with other motion or angle detection techniques. Having said this, in general terms, the signal strength from the directional antenna 124 (if it is used as a receiver) or from a fixed antenna at the games console 300 is passed to a motion tracker 540 which detects motion as described above, and from there to a detector 550 which detects whether the motion has exceeded a threshold likely to cause cable problems as discussed above.

Figure 12 is a schematic flowchart illustrating a tangle (or potential tangle) detection and warning process, a warning or alert being provided in the event of a detection of a potential cable tangle problem. In these example arrangements, the video displays of the HMD provide an alert device to alert the user. In other embodiments, the earpieces (and a suitable sound generator - not shown) could provide an audible alert device. I n other embodiments, a vibrational element associated with the HMD could cause a vibration to be felt by the user in the case of a cable problem (and of course combinations of these alert techniques could be used). If two such vibrational elements are provided, on different sides of the HMD, they could also provide an indication to the user (without otherwise interrupting the user's enjoyment of the content being replayed or otherwise provided by the HM D) of which way to turn so as to alleviate the detected cable problem. For example, if the user needs to turn or rotate to the user's left to alleviate a cable problem, a transducer on the left side of the HMD could be caused to vibrate. The amplitude, duration or repetition rate of the vibrational warning(s) could be used to indicate the severity of the potential cable problem, with a higher amplitude, a longer duration and/or a more frequent repetition being used to indicate a more severe potential problem. In the case of, for example, the head-mountable apparatus being embodied as headphones, the vibrational elements could be positioned at the user's ears. A similar directional warning could be provided using sounds generated for reproduction in one or other of the earpieces. In other words, the alert (which may be a visual and/or audible and/or vibrational alert) is arranged so as to indicate, to the user, which way to move the user's head in order to alleviate the instance (the cable tangle situation) which led to the alert. The detection of which way to turn the head can be based upon the most recent rotational head movement (or in some embodiments, the most recently detected head rotation of at least a threshold net rotational angle, even if the movement paused or even partially reversed during its implementation), in that the indication of which way to turn is the opposite rotational direction to the most recently detected such head movement. Accordingly, the user alert may be provided configured to alert the user to the detection of a head rotation of at least the predetermined magnitude, thereby warning the user that the cable may become at least partially wrapped around the user's body, and/or configured to alert the user to the detection of a longitudinal cable strain of at least the predetermined magnitude, thereby warning the user that the cable may become at least partially wrapped around the user's body.

Each of the processes described above leads to a detection of a potential tangle by the respective detector. These processes are illustrated generically by a step 600. At a step 610, the system waits for a predetermined time T (such as 0.7 seconds) in case the user moves again so as to remove the potential problem. If the user does not move so as to remove the potential cable problem within the predetermined time, then at a step 620 a warning is displayed and/or sounded.

Figure 13 schematically illustrates a warning display which may be displayed on the video displays of the HMD so the user sees the warning display. In the example shown, a textual warning 630, which may be in a coloured and/or flashing font, is superimposed over the images which are already being displayed on the video displays of the HMD, for a predetermined time (such as 10 seconds) or until the potential cable problem is at least alleviated by counteracting movements of the user. In embodiments of the invention, an indication 640 may be provided to tell the user which way to move (rotate) in order to alleviate the potential cable problem.

Figure 14 is a schematic flowchart illustrating another tangle detection and warning process. Once again, the various detection techniques discussed above are summarised by a generic detection step 700. In response to the detection of a potential tangle, the video content currently being displayed by the video displays of the HMD may optionally be paused, and in its place a view from the forward-facing camera 122 is displayed at a step 710. This has the effect of returning the user from the fully immersive environment of the HMD operation to a real-world view, allowing the user to make appropriate changes in the user's position and/or orientation so as to alleviate any cable-related problems. Figure 15 schematically illustrates an example of such a displayed camera view. Warning messages such as those schematically illustrated in Figure 13 may be superimposed over the view of the forward-facing camera. After a predetermined period, or when the cable problem has been detected to have been alleviated, the system can return to displaying the previous video content on the displays of the HMD 20, un-pausing the content if necessary.

The techniques described above may be implemented in hardware, software or combinations of the two. In the case that a software-controlled data processing apparatus is employed to implement one or more features of the embodiments, it will be appreciated that such software, and a storage or transmission medium such as a non-transitory machine- readable storage medium (such as a non-volatile memory or a magnetic or optical disk) by which such software is provided, are also considered as embodiments of the invention.

Numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described in this document, without departing from the scope of the claims.

Respective embodiments of the present invention are defined by the following numbered clauses:

1. A head-mountable apparatus comprising:

2. A head-mountable apparatus according to clause 1 , in which the detector is operable to detect head movements which represent a head rotation of at least a predetermined angle.

3. A head-mountable apparatus according to clause 2, in which the detector comprises a camera operable to capture images of the surroundings of the head-mountable apparatus, and a comparator to compare captured images so as to detect motion of the head-mountable apparatus relative to the surroundings.

4. A head-mountable apparatus according to clause 2, in which the detector comprises a motion detector for detecting movement of the head-mountable apparatus.

5. A head-mountable apparatus according to clause 4, in which the motion detector comprises an accelerometer and/or a gyroscope.

6. A head-mountable apparatus according to clause 2, in which the detector comprises a directional radio frequency antenna for communicating, via radio frequencies, with a device external to the head-mountable apparatus.

7. A head-mountable apparatus according to clause 1 , in which the detector is operable to detect a strain, in the cable, of at least a threshold amount.

8. A head-mountable apparatus according to clause 1 , in which the detector is operable to detect head movements which cause the cable to overlap itself.

9. A head-mountable apparatus according to clause 8, in which the cable comprises a sensor to detect the proximity of an overlapping portion of the cable. 10. A head-mountable apparatus according to any one of the preceding clauses, comprising an alert device for providing an alert to a user of the head-mountable apparatus in response to a detection by the detector.

1 1. A head-mountable apparatus according to clause 10, in which the alert is an audible alert and/or a visual alert and/or a vibrational alert.

12. A head-mountable apparatus according to clause 1 1 , in which the alert is arranged so as to indicate, to the user, which way to move the user's head in order to alleviate the instance which led to the alert.

13. A head-mountable apparatus according to any one of the preceding clauses, in which the one or more electrical and/or optical devices comprise one or more video displays.

14. A head-mountable apparatus according to clause 13, in which the one or more video displays are arranged with respect to the user's eyes, in use, so as to substantially obscure the user's view of the user's surroundings.

15. A head-mountable apparatus according to any one of the preceding clauses, in which the one or more electrical and/or optical devices comprise one or more audio transducers.

16. A head-mountable apparatus according to any one of the preceding clauses, in which the one or more electrical and/or optical devices comprise one or more video cameras.

17. A head-mountable apparatus according to any one of the preceding clauses, in which the cable comprises either or both of an electrical cable and an optical cable.

18. A head-mountable apparatus according to any one of the preceding clauses, in which the external device is a video and/or audio signal source.

19. A head-mountable apparatus according to any one of the preceding clauses, in which the external device is a power supply.

20. A system comprising a head-mountable apparatus according to any one of the preceding clauses, connected via the cable to the external device.

21. A method of operation of a head-mountable apparatus having a frame which is configured to be mounted on a user's head, the frame supporting one or more electrical and/or optical devices and a cable connecting the one or more electrical and/or optical devices to an external device separate to the head-mountable apparatus;

the method comprising:

detecting head movements of a magnitude which, assuming a substantially stationary external device, may cause the cable to become at least partially wrapped around the user's body.

22. A method of operation of a head-mountable apparatus having a frame which is configured to be mounted on a user's head, the frame supporting one or more devices selected from the group consisting of electrical devices and optical devices and a cable connecting the one or more devices to an external device separate to the head-mountable apparatus; the method comprising:

detecting a head rotation relative to the external device of at least a predetermined magnitude; and

providing a user alert to alert the user to the detection of a head rotation of at least the predetermined magnitude, thereby warning the user that the cable may become at least partially wrapped around the user's body.

23. A method of operation of a head-mountable apparatus having a frame which is configured to be mounted on a user's head, the frame supporting one or more devices selected from the group consisting of electrical devices and optical devices and a cable connecting the one or more devices to an external device separate to the head-mountable apparatus;

the method comprising:

detecting a longitudinal cable strain of at least a predetermined magnitude; and providing a user alert to alert the user to the detection of a longitudinal cable strain of at least the predetermined magnitude, thereby warning the user that the cable may become at least partially wrapped around the user's body.

24. Computer software which, when executed by a computer, causes the computer to execute the method of any one of clauses 21 to 23.

25. A non-transitory, machine-readable storage medium on which computer software according to clause 24 is stored.

26. A head-mountable apparatus comprising:

a frame which is configured to be mounted on a user's head, the frame supporting one or more devices selected from the group consisting of electrical devices and optical devices; a cable connecting the one or more devices to an external device separate to the head- mountable apparatus; and

a detector configured to detect a head rotation relative to the external device of at least a predetermined magnitude; and

a user alert device configured to alert the user to the detection of a head rotation of at least the predetermined magnitude, thereby warning the user that the cable may become at least partially wrapped around the user's body.

27. A head-mountable apparatus comprising:

a detector configured to detect a longitudinal cable strain of at least a predetermined magnitude; and a user alert device configured to alert the user to the detection of a longitudinal cable strain of at least the predetermined magnitude, thereby warning the user that the cable may become at least partially wrapped around the user's body.

28 A head-mountable apparatus according to clause 26, in which the detector is operable to detect head movements which represent a head rotation of at least the predetermined angle. 29. A head-mountable apparatus according to clause 28, in which the detector comprises a camera operable to capture images of the surroundings of the head-mountable apparatus, and a comparator to compare captured images so as to detect motion of the head-mountable apparatus relative to the surroundings.

30. A head-mountable apparatus according to clause 28, in which the detector comprises a motion detector for detecting movement of the head-mountable apparatus.

31. A head-mountable apparatus according to clause 30, in which the motion detector comprises an accelerometer and/or a gyroscope.

32. A head-mountable apparatus according to clause 28, in which the detector comprises a directional radio frequency antenna for communicating, via radio frequencies, with a device external to the head-mountable apparatus.

33. A head-mountable apparatus according to clause 26, in which the detector is operable to detect head movements which cause the cable to overlap itself.

34. A head-mountable apparatus according to clause 33, in which the cable comprises a sensor to detect the proximity of an overlapping portion of the cable.

35. A head-mountable apparatus according to any one of clauses 26 to 34, in which the alert is an audible alert and/or a visual alert and/or a vibrational alert.

36. A head-mountable apparatus according to clause 35, in which the alert is arranged so as to indicate, to the user, which way to move the user's head in order to alleviate the instance which led to the alert.

37. A head-mountable apparatus according to any one of clauses 26 to 36, in which the one or more electrical and/or optical devices comprise one or more video displays.

38. A head-mountable apparatus according to clause 37, in which the one or more video displays are arranged with respect to the user's eyes, in use, so as to substantially obscure the user's view of the user's surroundings.

39. A head-mountable apparatus according to any one of clauses 26 to 38, in which the one or more electrical and/or optical devices comprise one or more audio transducers.

40. A head-mountable apparatus according to any one of clauses 26 to 39, in which the one or more electrical and/or optical devices comprise one or more video cameras.

41. A head-mountable apparatus according to any one of clauses 26 to 40, in which the cable comprises either or both of an electrical cable and an optical cable. 42. A head-mountable apparatus according to any one of clauses 26 to 41 , in which the external device is a video and/or audio signal source.

43. A head-mountable apparatus according to any one of clauses 26 to 42, in which the external device is a power supply.

44. A system comprising a head-mountable apparatus according to any one of clauses 26 to 43, connected via the cable to the external device.

Claims

1. A head-mountable apparatus comprising:

2. A head-mountable apparatus according to claim 1 , in which the detector is operable to detect head movements which represent a head rotation of at least a predetermined angle.

3. A head-mountable apparatus according to claim 2, in which the detector comprises a camera operable to capture images of the surroundings of the head-mountable apparatus, and a comparator to compare captured images so as to detect motion of the head-mountable apparatus relative to the surroundings.

4. A head-mountable apparatus according to claim 2, in which the detector comprises a motion detector for detecting movement of the head-mountable apparatus.

5. A head-mountable apparatus according to claim 4, in which the motion detector comprises one or more selected from the group consisting of an accelerometer and a gyroscope.

6. A head-mountable apparatus according to claim 2, in which the detector comprises a directional radio frequency antenna for communicating, via radio frequencies, with a device external to the head-mountable apparatus.

7. A head-mountable apparatus according to claim 1 , in which the detector is operable to detect a strain, in the cable, of at least a threshold amount.

8. A head-mountable apparatus according to claim 1 , in which the detector is operable to detect head movements which cause the cable to overlap itself.

9. A head-mountable apparatus according to claim 8, in which the cable comprises a sensor to detect the proximity of an overlapping portion of the cable.

10. A head-mountable apparatus according to any one of the preceding claims, comprising an alert device for providing an alert to a user of the head-mountable apparatus in response to a detection by the detector.

1 1. A head-mountable apparatus according to claim 10, in which the alert comprises one or more selected from the group consisting of: an audible alert; a visual alert; and a vibrational alert.

12. A head-mountable apparatus according to claim 11 , in which the alert is arranged so as to indicate, to the user, which way to move the user's head in order to alleviate the instance which led to the alert.

13. A head-mountable apparatus according to any one of the preceding claims, in which the one or more devices comprise one or more video displays.

14. A head-mountable apparatus according to claim 13, in which the one or more video displays are arranged with respect to the user's eyes, in use, so as to substantially obscure the user's view of the user's surroundings.

15. A head-mountable apparatus according to any one of the preceding claims, in which the one or more devices comprise one or more audio transducers.

16. A head-mountable apparatus according to any one of the preceding claims, in which the one or more devices comprise one or more video cameras.

17. A head-mountable apparatus according to any one of the preceding claims, in which the cable comprises one or more selected from the group consisting of an electrical cable and an optical cable.

18. A head-mountable apparatus according to any one of the preceding claims, in which the external device comprises one or more selected from the group consisting of a video source and an audio signal source.

19. A head-mountable apparatus according to any one of the preceding claims, in which the external device is a power supply.

20. A system com prisi ng a head-mountable apparatus according to any one of the preceding claims, connected via the cable to the external device.

21 . A head-mountable apparatus comprising:

22. A head-mountable apparatus comprising:

a detector configured to detect a longitudinal cable strain of at least a predetermined magnitude; and

a user alert device configured to alert the user to the detection of a longitudinal cable strain of at least the predetermined magnitude, thereby warning the user that the cable may become at least partially wrapped around the user's body.

the method comprising:

24. A method of operation of a head-mountable apparatus having a frame which is configured to be mounted on a user's head, the frame supporting one or more devices selected from the group consisting of electrical devices and optical devices and a cable connecting the one or more devices to an external device separate to the head-mountable apparatus;

the method comprising:

25. A method of operation of a head-mountable apparatus having a frame which is configured to be mounted on a user's head, the frame supporting one or more devices selected from the group consisting of electrical devices and optical devices and a cable connecting the one or more devices to an external device separate to the head-mountable apparatus;

the method comprising:

26. Computer software which, when executed by a computer, causes the computer to execute the method of any one of claims 23 to 25.

27. A non-transitory, machine-readable storage medium on which computer software according to claim 26 is stored.