GB2598953A - Head mounted display - Google Patents

Abstract

A measurement system consists of a head mounted device (HMD) 1210 with one or more imaging sensors 1212, 1214 that capture an image of at least one eye of a user wearing the HMD. A processing unit 1220 calculates a position, relative to the face of the user, of the centre at least one eye of the user based upon the captured image and a predetermined position of the one or more imaging sensors. Different users may have different vision requirements that may require an HMD to be modified for each user’s vision requirements. For example, an HMD may need to be calibrated for a user’s interpupillary distance (IPD) so that images, or viewpoints displayed in such images, may be appropriately aligned for each eye.

Description

HEAD MOUNTED DISPLAY

Field of Invention

The present invention relates to systems and methods for a head mounted display. Background Head mounted displays (HMDs) have become more affordable and versatile in recent years. An HMD may be used to provide virtual reality (VR) or augmented reality (AR) content to a user.

However, whilst HMDs are becoming more affordable, it is still common for a group of users, such as a family or household, to share the use of one or more HM Ds. Therefore, as each user may require an HMD to be customised to their individual needs, it is desirable to be able to easily customise one or more aspects of the HMD for each user.

It is in this context that the present disclosure arises.

Summary of the Invention

In a first aspect, a measurement system is provided in claim 1.

In another aspect, a measurement method is provided in claim 14.

Further respective aspects and features of the invention are defined in the appended claims.

Embodiments of the present invention will now be described by way of example 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 6a schematically illustrates a plan view of an HMD; - Figure 6b schematically illustrates a near-eye tracking arrangement; - Figure 7 schematically illustrates a remote tracking arrangement; - Figure 8 schematically illustrates a gaze tracking environment; - Figure 9 schematically illustrates a gaze tracking system; - Figure 10 schematically illustrates a human eye; - Figure 11 schematically illustrates a graph of human visual acuity; - Figure 12 schematically illustrates a measurement system; - Figure 13a schematically illustrates a plan view of an HMD; - Figure 13b schematically illustrates a captured image of an eye; - Figure 14 schematically illustrates a plan view of an HMD; - Figure 15 schematically illustrates an example image scaling; - Figure 16 schematically illustrates a measurement method.

Description of the Embodiments

In the following description, a number of specific details are presented in order to provide a thorough understanding of the embodiments of the present invention. It will be apparent, however, to a person skilled in the art that these specific details need not be employed to practice the present invention. Conversely, specific details known to the person skilled in the art are omitted for the purposes of clarity where appropriate.

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. As noted above, many gaze tracking arrangements may be considered particularly suitable for use in HMD systems; however, use with such an HMD system should not be considered essential.

Note that the HMD of Figure 1 may comprise further features, to be described below in connection with other drawings, but which are not shown in Figure 1 for clarity of this initial explanation.

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, as supplied by an external processing device such as a games console in many embodiments. Of course, in some embodiments images may instead (or additionally) be generated by a processor or obtained from memory located at the HMD itself.

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.

The combination of the fact that the user can see only what is displayed by the HMD and, subject to the limitations of the noise blocking or active cancellation properties of the earpieces and associated electronics, can hear only what is provided via the earpieces, mean that this HMD may be considered as a so-called "full immersion" HMD. Note however that in some embodiments the HMD is not a full immersion HMD, and may provide at least some facility for the user to see and/or hear the user's surroundings. This could be by providing some degree of transparency or partial transparency in the display arrangements, and/or by projecting a view of the outside (captured using a camera, for example a camera mounted on the HMD) via the HMD's displays, and/or by allowing the transmission of ambient sound past the earpieces and/or by providing a microphone to generate an input sound signal (for transmission to the earpieces) dependent upon the ambient sound.

A front-facing camera 122 may capture images to the front of the HMD, in use. Such images may be used for head tracking purposes, in some embodiments, while it may also be suitable for capturing images for an augmented reality (AR) style experience. 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. 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 (including one or more batteries and/or being connectable to a mains power outlet) may be linked by a cable to the HMD. Note that the power supply 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 other embodiments, at least part of the functionality associated with generating image and/or audio signals for presentation to the user may be carried out by circuitry and/or processing forming part of the HMD itself. A power supply may be provided as part of the HMD itself.

Some 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.

If one or more cables are used, the physical position at which the cable 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 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 (not full immersion) 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 SO, 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 HMD). 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 needs to track movements with respect to a real or virtual space in which the user is located.

As mentioned above, in some uses of the HMD, such as those associated with virtual reality (VR) or augmented reality (AR) systems, the user's viewpoint needs to track movements with respect to a real or virtual space in which the user is located.

This tracking is carried out by detecting motion of the HMD and varying the apparent viewpoint of the displayed images so that the apparent viewpoint tracks the motion. The detection may be performed using any suitable arrangement (or a combination of such arrangements). Examples include the use of hardware motion detectors (such as accelerometers or gyroscopes), external cameras operable to image the HMD, and outwards-facing cameras mounted onto the HMD.

Turning to gaze tracking in such an arrangement, Figure 6 schematically illustrates two possible arrangements for performing eye tracking on an HMD. The cameras provided within such arrangements may be selected freely so as to be able to perform an effective eye-tracking method. In some existing arrangements, visible light cameras are used to capture images of a user's eyes. Alternatively, infra-red (IR) cameras are used so as to reduce interference either in the captured signals or with the user's vision should a corresponding light source be provided, or to improve performance in low-light conditions.

Figure 6a shows an example of a gaze tracking arrangement in which the cameras are arranged within an HMD so as to capture images of the user's eyes from a short distance. This may be referred to as near-eye tracking, or head-mounted tracking.

In this example, an HMD 600 (with a display element 601) is provided with cameras 610 that are each arranged so as to directly capture one or more images of a respective one of the user's eyes using an optical path that does not include the lens 620. This may be advantageous in that distortion in the captured image due to the optical effect of the lens is able to be avoided. Four cameras 610 are shown here as examples of possible positions that eye-tracking cameras may provided, although it should be considered that any number of cameras may be provided in any suitable location so as to be able to image the corresponding eye effectively. For example, only one camera may be provided per eye or more than two cameras may be provided for each eye.

However it is considered that in a number of embodiments it is advantageous that the cameras are instead arranged so as to include the lens 620 in the optical path used to capture images of the eye. Examples of such positions are shown by the cameras 630. While this may result in processing being required to enable suitably accurate tracking to be performed, due to the deformation in the captured image due to the lens, this may be performed relatively simply due to the fixed relative positions of the corresponding cameras and lenses. An advantage of including the lens within the optical path may be that of simplifying the physical constraints upon the design of an HMD, for example.

Figure 6b shows an example of a gaze tracking arrangement in which the cameras are instead arranged so as to indirectly capture images of the user's eyes. Such an arrangement may be particularly suited to use with IR or otherwise non-visible light sources, as will be apparent from the below description.

Figure 6b includes a mirror 650 arranged between a display 601 and the viewer's eye (of course, this can be extended to or duplicated at the user's other eye as appropriate). For the sake of clarity, any additional optics (such as lenses) are omitted in this Figure -it should be appreciated that they may be present at any suitable position within the depicted arrangement. The mirror 650 in such an arrangement is selected so as to be partially transmissive; that is, the mirror 650 should be selected so as to enable the camera 640 to obtain an image of the user's eye while the user views the display 601. One method of achieving this is to provide a mirror 650 that is reflective to IR wavelengths but transmissive to visible light -this enables IR light used for tracking to be reflected from the user's eye towards the camera 640 while the light emitted by the display 601 passes through the mirror uninterrupted.

Such an arrangement may be advantageous in that the cameras may be more easily arranged out of view of the user, for instance. Further to this, improvements to the accuracy of the eye tracking may be obtained due to the fact that the camera captures images from a position that is effectively (due to the reflection) along the axis between the user's eye and the display.

Of course, eye-tracking arrangements need not be implemented in a head-mounted or otherwise near-eye fashion as has been described above. For example, Figure 7 schematically illustrates a system in which a camera is arranged to capture images of the user from a distance; this distance may vary during tracking, and may take any value in dependence upon the parameters of the tracking system. For example, this distance may be thirty centimetres, a metre, five metres, ten metres, or indeed any value so long as the tracking is not performed using an arrangement that is affixed to the user's head.

In Figure 7, an array of cameras 700 is provided that together provide multiple views of the user 710. These cameras are configured to capture information identifying at least the direction in which a user's 710 eyes are focused, using any suitable method. For example, IR cameras may be utilised to identify reflections from the user's 710 eyes. An array of cameras 700 may be provided so as to provide multiple views of the user's 710 eyes at any given time, or may be provided so as to simply ensure that at any given time at least one camera 700 is able to view the user's 710 eyes. It is apparent that in some use cases it may not be necessary to provide such a high level of coverage and instead only one or two cameras 700 may be used to cover a smaller range of possible viewing directions of the user 710.

Of course, the technical difficulties associated with such a long-distance tracking method may be increased; higher resolution cameras may be required, as may stronger light sources for generating IR light, and further information (such as head orientation of the user) may need to be input to determine a focus of the user's gaze. The specifics of the arrangement may be determined in dependence upon a required level of robustness, accuracy, size, and/or cost, for example, or any other design consideration.

Despite technical challenges including those discussed above, such tracking methods may be considered beneficial in that they allow a greater range of interactions for a user -rather than being limited to HMD viewing, gaze tracking may be performed for a viewer of a television, for instance.

Rather than varying only in the location in which cameras are provided, eye-tracking arrangements may also differ in where the processing of the captured image data to determine tracking data is performed.

Figure 8 schematically illustrates an environment in which an eye-tracking process may be performed. In this example, the user 800 is using an HMD 810 that is associated with the processing unit 830, such as a games console, with the peripheral 820 allowing a user 800 to input commands to control the processing. The HMD 810 may perform eye tracking in line with an arrangement exemplified by Figure 6a or 6b, for example -that is, the HMD 810 may comprise one or more cameras operable to capture images of either or both of the user's 800 eyes. The processing unit 830 may be operable to generate content for display at the HMD 810; although some (or all) of the content generation may be performed by processing units within the HMD 810.

The arrangement in Figure 8 also comprises a camera 840, located outside of the HMD 810, and a display 850. In some cases, the camera 840 may be used for performing tracking of the user 800 while using the HMD 810, for example to identify body motion or a head orientation. The camera 840 and display 850 may be provided as well as or instead of the HMD 810; for example these may be used to capture images of a second user and to display images to that user while the first user 800 uses the HMD 810, or the first user 800 may be tracked and view content with these elements instead of the HMD 810. That is to say, the display 850 may be operable to display generated content provided by the processing unit 830 and the camera 840 may be operable to capture images of one or more users' eyes to enable eye-tracking to be performed.

While the connections shown in Figure 8 are shown by lines, this should of course not be taken to mean that the connections should be wired; any suitable connection method, including wireless connections such as wireless networks or Bluetooth®, may be considered suitable. Similarly, while a dedicated processing unit 830 is shown in Figure 8 it is also considered that the processing may in some embodiments be performed in a distributed manner -such as using a combination of two or more of the HMD 810, one or more processing units, remote servers (cloud processing), or games consoles.

The processing required to generate tracking information from captured images of the user's 800 eye or eyes may be performed locally by the HMD 810, or the captured images or results of one or more detections may be transmitted to an external device (such as a the processing unit 830) for processing. In the former case, the HMD 810 may output the results of the processing to an external device for use in an image generation process if such processing is not performed exclusively at the HMD 810. In embodiments in which the HMD 810 is not present, captured images from the camera 840 are output to the processing unit 830 for processing.

Figure 9 schematically illustrates a system for performing one or more eye tracking processes, for example in an embodiment such as that discussed above with reference to Figure 8. The system 900 comprises a processing device 910, one or more peripherals 920, an HMD 930, a camera 940, and a display 950. Of course, not all elements need be present within the system 900 in a number of embodiments -for instance, if the HMD 930 is present then it is considered that the camera 940 may be omitted as it is unlikely to be able to capture images of the user's eyes.

As shown in Figure 9, the processing device 910 may comprise one or more of a central processing unit (CPU) 911, a graphics processing unit (GPU) 912, storage (such as a hard drive, or any other suitable data storage medium) 913, and an input/output 914. These units may be provided in the form of a personal computer, a games console, or any other suitable processing device.

For example, the CPU 911 may be configured to generate tracking data from one or more input images of the user's eyes from one or more cameras, or from data that is indicative of a user's eye direction. This may be data that is obtained from processing images of the user's eye at a remote device, for example. Of course, should the tracking data be generated elsewhere then such processing would not be necessary at the processing device 910.

The GPU 912 may be configured to generate content for display to the user on which the eye tracking is being performed. In some embodiments, the content itself may be modified in dependence upon the tracking data that is obtained -an example of this is the generation of content in accordance with a foveal rendering technique. Of course, such content generation processes may be performed elsewhere -for example, an HMD 930 may have an on-board GPU that is operable to generate content in dependence upon the eye tracking data.

The storage 913 may be provided so as to store any suitable information. Examples of such information include program data, content generation data, and eye tracking model data. In some cases, such information may be stored remotely such as on a server, and as such a local storage 913 may not be required -the discussion of the storage 913 should therefore be considered to refer to local (and in some cases removable storage media) or remote storage.

The input/output 914 may be configured to perform any suitable communication as appropriate for the processing device 910. Examples of such communication include the transmission of content to the HMD 930 and/or display 950, the reception of eye-tracking data and/or images from the HMD 930 and/or the camera 940, and communication with one or more remote servers (for example, via the internet).

As discussed above, the peripherals 920 may be provided to allow a user to provide inputs to the processing device 910 in order to control processing or otherwise interact with generated content. This may be in the form of button presses or the like, or alternatively via tracked motion to enable gestures to be used as inputs.

The HMD 930 may comprise a number of sub-elements, which have been omitted from Figure 9 for the sake of clarity. Of course, the HMD 930 should comprise a display unit operable to display images to a user. In addition to this, the HMD 930 may comprise any number of suitable cameras for eye tracking (as discussed above), in addition to one or more processing units that are operable to generate content for display and/or generate eye tracking data from the captured images.

The camera 940 and display 950 may be configured in accordance with the discussion of the corresponding elements above with respect to Figure 8.

Turning to the image capture process upon which the eye tracking is based, examples of different cameras are discussed. The first of these is a standard camera, which captures a sequence of images of the eye that may be processed to determine tracking information. The second is that of an event camera, which instead generates outputs in accordance with observed changes in brightness.

It is more common to use standard cameras in such tracking arrangements, given that they are widely available and often relatively cheap to produce. 'Standard cameras' here refer to cameras which capture images of the environment at predetermined intervals which can be combined to generate video content. For example, a typical camera of this type may capture thirty images (frames) each second, and these images may be output to a processing unit for feature detection or the like to be performed so as to enable tracking of the eye.

Such a camera comprises a light-sensitive array that is operable to record light information during an exposure time, with the exposure time being controlled by a shutter speed (the speed of which dictates the frequency of image capture). The shutter may be configured as a rolling shutter (line-by-line reading of the captured information) or a global shutter (reading the captured information of the whole frame simultaneously), for example.

However, in some arrangements it may be considered advantageous to instead use an event camera, which may also be referred to as a dynamic vision sensor. Such cameras do not require a shutter as described above, and instead each element of the light-sensitive array (often referred to as a pixel) is configured to output a signal at any time a threshold brightness change is observed. This means that images are not output in the traditional sense -however an image reconstruction algorithm may be applied that is able to generate an image from the signals output by an event camera.

While there is an increased computational complexity for generating an image from such data, the output of the event camera can be used for tracking without any image generation. One example of how this is performed is that of using an IR-sensitive event camera; when imaged using IR light, the pupil of the human eye displays a much higher level of brightness than the surrounding features. By selecting an appropriate threshold brightness, the motion of the pupil would be expected to trigger events (and corresponding outputs) at the sensor.

Independent of the type of camera that is selected, in many cases it may be advantageous to provide illumination to the eye in order to obtain a suitable image. One example of this is the provision of an IR light source that is configured to emit light in the direction of one or both of the user's eyes; an IR camera may then be provided that is able to detect reflections from the user's eye in order to generate an image. IR light may be preferable as it is invisible to the human eye, and as such does not interfere with normal viewing of content by the user, but it is not considered to be essential. In some cases, the illumination may be provided by a light source that is affixed to the imaging device, while in other embodiments it may instead be that the light source is arranged away from the imaging device.

As suggested in the discussion above, the human eye does not have a uniform structure; that is, the eye is not a perfect sphere, and different parts of the eye have different characteristics (such as varying reflectance or colour). Figure 10 shows a simplified side view of the structure of a typical eye 1000; this Figure has omitted features such as the muscles which control eye motion for the sake of clarity.

The eye 1000 is formed of a near-spherical structure filled with an aqueous solution 1010, with a retina 1020 formed on the rear surface of the eye 1000. The optic nerve 1030 is connected at the rear of the eye 1000. Images are formed on the retina 1020 by light entering the eye 1000, and corresponding signals carrying visual information are transmitted from the retina 1020 to the brain via the optic nerve 1030.

Turning to the front surface of the eye 1000, the sclera 1040 (commonly referred to as the white of the eye) surrounds the iris 1050. The iris 1050 controls the size of the pupil 1060, which is an aperture through which light enters the eye 1000. The iris 1050 and pupil 1060 are covered by the cornea 1070, which is a transparent layer which can refract light entering the eye 1000. The eye 1000 also comprises a lens (not shown) that is present behind the iris 1050 that may be controlled to adjust the focus of the light entering the eye 1000.

The structure of the eye is such that there is an area of high visual acuity (the fovea), with a sharp drop off either side of this. This is illustrated by the curve 1100 of Figure 11, with the peak in the centre representing the foveal region. The area 1110 is the 'blind spot', this is an area in which the eye has no visual acuity as it corresponds to the area where the optic nerve meets the retina. The periphery (that is, the viewing angles furthest from the fovea) is not particularly sensitive colour or detail, and instead is used to detect motion.

As has been discussed above, foveal rendering is a rendering technique that takes advantage of the relatively small size (around 2.5 degrees) of the fovea and the sharp fall-off in acuity outside of that.

The eye undergoes a large amount of motion during viewing, and this motion may be categorised into one of a number of categories.

Saccades, and on a smaller scale micro-saccades, are identified as fast motions in which the eyes rapidly move between different points of focus (often in a jerky fashion). This may be considered as ballistic motion, in that once the movement has been initiated it cannot be altered. Saccades are often not conscious eye motions, and instead are performed reflexively to survey an environment. Saccades may last up to two hundred milliseconds, depending on the distance rotated by the eye, but may be as short as twenty milliseconds. The speed of a saccade is also dependent upon the total rotation angle; typical speeds may be between two hundred and five hundred degrees per second.

'Smooth pursuit' refers to a slower movement type than a saccade. Smooth pursuit is generally associated with a conscious tracking of a point of focus by a viewer, and is performed so as to maintain the position of a target within (or at least substantially within) the foveal region of the viewer's vision. This enables a high-quality view of a target of interest to be maintained in spite of motion. If the target moves too fast, then smooth pursuit may instead require a number of saccades in order to keep up; this is because smooth pursuit has a lower maximum speed, in the region of thirty degrees per second.

The vestibular-ocular reflex is a further example of eye motion. The vestibular-ocular reflex is the motion of the eyes that counteracts head motion; that is, the motion of the eyes relative to the head that enables a person to remain focused on a particular point despite moving their head.

Another type of motion is that of the vergence accommodation reflex. This is the motion that causes the eyes to rotate to converge at a point, and the corresponding adjustment of the lens within the eye to cause that point to come into focus.

Further eye motions that may be observed as a part of a gaze tracking process are those of blinks or winks, in which the eyelid covers the eyes of the user. Such motions may be reflexive or intentional, and can often interfere with eye tracking as they will obscure vision of the eye, and the eye is often not stationary during such a motion.

Different users may have different vision requirements that may require an HMD to be modified for each user's vision requirements. For example, an HMD may need to be calibrated for a user's interpupillary distance (IPD) so that images, or viewpoints displayed in such images, may be appropriately aligned for each eye. However, IPD's are typically measured by a trained professional.

Therefore, it is unlikely, and costly, for a user to have their IPD measured by a professional for calibrating an HMD. Furthermore, if an HMD is used in an educational setting, such as a classroom or museum, users will often not be able to use an HMD that is calibrated for their IPD, as, in these scenarios, the resources to efficiently measure each users IPD are unavailable.

Therefore, it is desirable to provide a system that comprises an HMD comprising an imaging sensor that may capture images of a user's eye in order to calculate the user's IPD.

Accordingly, turning now to figure 12, in an embodiment of the present description, a measurement system 1200 comprises a head mounted device (HMD) 1210 comprising one or more imaging sensors 1212/1214 that are configured to capture an image of at least one eye of a user wearing the HMD, and a processing unit 1216/1220 that is configured to calculate a position, relative to the face of the user, of the centre at least one eye of the user based upon the captured image and a predetermined position of the one or more imaging sensors 1212/1214. In an embodiment of the present description, the HMD 1210 comprises the processing unit 1216. In other embodiments, this processing unit 1220 may be part of a separate device such as a videogame console if this is configured to operate as part of the overall measurement system. Alternatively, the role of the processing unit may be shared between these two processing units.

For example, the position of the one or more imaging sensors relative to the HMD can be known (for example if the imaging sensor is integral to the HMD or otherwise mounted at a predetermined position the HMD) and the position of the HMD relative to a user's face, when the HMD is worn by the user, is also known (for example, assumed to be centrally aligned with the user's nose and/or the user's head).

The position of the centre at least one eye of the user may be calculated from these known positions from the position of the centre at least one eye of the user in an image captured by the one or more imaging sensors, as explained later herein. The position of the centre at least one eye of the user in the image may be determined by analysing the features of the image for example.

Therefore, in an embodiment of the present description, a horizontal distance of the at least one eye to (at least an assumed) centre line of the face of the user may be estimated from the position of the centre of the eye in a respective captured image together with the position on the HMD of the image sensor that captured that respective image.

In an embodiment of the present description, the horizontal distance of the at least one eye to a centre line of the face of the user may be estimated, using a predetermined relationship between the physical position of the user's eye relative to the image sensor and the resulting position of the eye in an image captured by that image sensor to determine from such an image an offset of the position of the centre of the eye of the user from the predetermined distance of the image sensor from a centreline of the H MD.

For example, figure 13a shows an HMD 1300 comprising one or more imaging sensors 1310, the predetermined distance 1312 of the image sensor from a centreline of the HMD, and a predetermined relationship between the physical position 1314 of the user's eye relative to an image sensor.

In figure 13a, the imaging sensors (e.g. cameras) are shown at a non-limiting position. The imaging sensor may be positioned behind one or more optical elements of the HMD, but in this case any shift, magnification or other distortion of the eye image arising from passage through the optics must be accounted for. More typically the imaging sensor(s) are located within the facia of the HMD close to the user's eyes, for example positioned above or below and/or to the left or right of the viewing lens through which the user looks at the image.

Figure 13b shows an image captured by the image sensor and the resulting position 1320 of the eye in an image captured by the image sensor.

It will be appreciated that an imaging sensor in a predetermined positon with a predetermined field of view will therefore capture images within a predetermined region centred on its optical axis. Consequently the position of the user's eye within the image is relative to this predetermined region and optical axis, and hence also to the position of the imaging sensor within the HMD, which in turn is assumed to be positioned centrally relative to the user's nose and/or head. The position of the eye can therefore be related to the notional centre-line of the user's face (based on the user's nose and/or head) by linking these known positions together.

Hence the resulting position of the eye in the image may be converted into a distance that has the same scale as the predetermined distance 1312 of the image sensor from a centreline of the HMD by using the predetermined relationship between the physical position 1314 of the user's eye and the intrinsic properties of the image sensor (e.g. the field of view of the image sensor) for example. An offset 1330 of the position of the centre of the eye of the user from the predetermined distance 1312 of the image sensor from the centreline of the HMD may then be calculated from the converted resulting position and the predetermined distance 1312 of the image sensor from the centreline of the HMD.

In an embodiment of the present description, an interpupillary distance between the eyes of the user may be estimated to be twice the distance between the centre of one eye of the user and a centre line of their face.

For example, in an embodiment of the present description, the HMD may only capture an image of one of the eyes of the user. Therefore, the interpupillary distance may estimated by doubling the value of the distance between the centre of one eye of the user and a centre line of their face, as the positions of a user's eyes may be symmetrical.

In an embodiment of the present description, an interpupillary distance between the eyes of the user may be calculated as the sum of the distances between the centre of each eye of the user and a centre line of their face.

For example, in an embodiment of the present description, the HMD may capture an image of both of the eyes of the user. Therefore, the interpupillary distance may estimated by summing the values of the distance between the centre of each eye of the user and a centre line of their face.

In an embodiment of the present description, the HMD may comprise an optical assembly that may be modified in response to the calculated position of the centre of the pupil of the at least one eye of the user.

For example, as shown in figure 14, an optical assembly 1420 may be modified so that an optical centre 1425 of the optical assembly is positioned in front of the calculated position of each of the user's eye. Positioning the optical centre of the optical assembly in front of the calculated position of each eye may increase the clarity and field of view of the displayed images that are viewed by the user. Similarly optionally, alternatively or in addition respective display units for displaying images may also be repositionable in this manner. Similarly optionally, the optical assembly and display may be fixed within a respective housing, but the housing may be repositionable, in a manner similar to binoculars, to adjust intraocular distance. In any of the above cases, the adjustment may be automatic (e.g. by use of motor(s) or actuator(s)) based on the calculated interpupillary distance. In a case where such repositioning also moves the, or each, image sensor, then the change in predetermined position of the image sensor relative to the centre of the HMD is updated for any future calculations.

In an embodiment of the present description, the HMD may comprise a display portion that may display virtual images, wherein the HMD is configured to modify the displayed virtual images in response to the calculated position of the centre of the pupil of the at least one eye of the user.

For example, in an embodiment of the present description, the displayed virtual images may be modified by modifying the parallax between the virtual images displayed to one eye of the user and the virtual images displayed to the other eye of the user.

In some cases, the position of the centre a user's eye may vary based upon the apparent distance of an object being looked at by a user relative to the user's eye.

Therefore, in an embodiment of the present description, the HMD may comprise a display portion that may display virtual images at a plurality of apparent distances; and a gaze tracker that may detect the apparent distance of an image feature being looked at by the user. In this embodiment, the processing unit may calculate the position of the centre of at least one eye of the user in response to a change of the apparent distance of the image feature being looked at by the user.

To further improve the estimate of the position of an eye of the user, it may be beneficial for the image sensor to be able to measure the distance to the user's eye. For example, the imaging sensor may be a stereoscopic image sensor, and the distance to the eye may be calculated from the image disparity in the stereoscopic images. Alternatively or in addition, any other suitable measurement schemes may be considered such as ultrasound or infrared distance measures, or the capturing within the image of a so-called structured light pattern emitted by an emitter (e.g. a near-infrared emitter), wherein the pitch or spacing of the pattern impinging on the eye is a function of distance from the emitter. In any case, this measurement of the distance between the user's eye and the imaging sensor may then be used, with the position of the user's eye in a captured image and the predetermined position of the imaging sensor, to calculate the position of the centre of the eye of the user relative to the user's face, as described later herein.

Therefore, in an embodiment of the present description, the one or more imaging sensors may be stereoscopic imaging sensors, and the measurement system may calculate a depth of the centre of at the least one eye of the user based upon the one or more captured images. The measurement system may calculate the position of the centre of the eye of the user based upon the depth of the centre of the at least one eye of the user, the captured image and the predetermined position of the one or more imaging sensors.

Estimating the distance to the eye can be additionally beneficial where the image sensor is positioned to the left or right of the user's eye and/or the optical axis of the image sensor is not parallel with the optical axis of the optical assembly providing images to the user.

In these cases the eye is being viewed with an angular offset that means that as the eye varies with distance on the optical axis of the optical assembly providing images to the user (i.e. for different users having different relative eye recess), the eye appears to transit left or right in an image captured by an image sensor that does not share a parallel optical axis.

Consequently the relative angles of the optical axis of the image sensor(s) and the optical assembly providing images to the user, together with the distance of the user's eye from the distance measuring system (whether a stereo camera, structured light emitter or the like), can be used to correct for this offset and hence refine the interpupillary distance offset.

In an embodiment of the present description, the processing unit may estimate a position of the centre of the eye in a respective captured image. For example, in an embodiment of the present description, the processing unit may estimate the position of the centre of the eye in the respective captured image by using information of the properties of the respective imaging sensor. These properties may comprise, for example, the field of view of the imaging sensor.

Figure 15 shows an image 1500 of an eye captured by an image sensor. In this example image the pupil 1510 is facing away from the image sensor. As an eye is approximately spherical, the scale of distances between respective image features in the image may therefore be distorted. A further aspect of the

present description aims to alleviate this issue.

Therefore, in an embodiment of the present description, the processing unit may estimate the position of the centre of the eye in the respective captured image by comparing the relative sizes of predetermined image features in the respective captured image. For example, figure 15 shows the iris of the eye is distorted so that the thickness (e.g. span) of the iris to the left of the pupil 1522 is larger than the thickness of the iris to the right of the pupil 1524. However, for most users the actual thickness of the iris to the left of the pupil is roughly the same as the actual thickness of the iris to the right of the pupil.

Therefore, by comparing these two thicknesses of the iris in the captured image, an estimation of the centre of the user's eye may be improved by calculating a scale function that results in the two thicknesses of the iris 1530 in the scaled image being the same. It will be appreciated that other image features may be used alternatively, or in addition to, the thickness of the iris.

It will be appreciated that the direction of the user's eye may be controlled by the measurement system so that the interpupillary distance is measured when the user is looking straight ahead, so that the pupil indicates the centre of the eye and is at a particularly suitable position to measure the interpupillary distance without further optional correction. For example, the user may be asked to look at a feature placed centrally in a displayed image, so that they are looking directly ahead. Alternatively or in addition, the user may be asked to track a moving object in the displayed image, and an image of the user's eye may be captured when the object passes through the centre of the screen (or at any advantageous position if different views of the eye would be helpful for image analysis or tracking).

Notably in this case, when the user's eye tracks a moving object it does so smoothly, and so a smooth tracking motion of the object as it passes through the desired on-screen position provides a high likelihood that the user is indeed looking in the desired direction.

Figure 16 shows an embodiment of the present description. In an embodiment of the presently claimed invention, a measurement method 1600 may comprise capturing 1610, by one or more imaging sensors mounted on a head mounted display (HMD), an image of at least one eye of a user wearing the HMD; and calculating 1620 a position, relative to the face of the user, of the centre at least one eye of the user based upon the captured image and a predetermined position of the one or more imaging sensors.

In an embodiment of the present description, a computer program may comprise computer executable instructions adapted to cause a computer system to perform a method of the present description.

Summary Embodiment

In a summary embodiment of the present description, a measurement system comprises a head mounted device (HMD) comprising one or more imaging sensors that are configured to capture an image of at least one eye of a user wearing the HMD, and a processing unit that is configured to calculate a position, relative to the face of the user, of the centre at least one eye of the user based upon the captured image and a predetermined position of the one or more imaging sensors, as described elsewhere herein.

In an instance of the summary embodiment, a horizontal distance of the at least one eye to a centre line of the face of the user is estimated from the position of the centre of the eye in a respective captured image together with the position on the HMD of the image sensor that captured that respective image, as described elsewhere herein.

In this instance, optionally the horizontal distance of the at least one eye to a centre line of the face of the user is estimated using a predetermined relationship between the physical position of the user's eye relative to the image sensor and the resulting position of the eye in an image captured by that image sensor to determine from such an image an offset of the position of the centre of the eye of the user from the predetermined distance of the image sensor from a centreline of the HMD, as described elsewhere herein.

Optionally in this embodiment, an interpupillary distance between the eyes of the user is estimated to be twice the distance between the centre of one eye of the user and a centre line of their face, as described elsewhere herein.

Optionally in this embodiment, an interpupillary distance between the eyes of the user is calculated as the sum of the distances between the centre of each eye of the user and a centre line of their face, as described elsewhere herein.

In an instance of the summary embodiment, an optical assembly is configured to be modified in response to the calculated position of the centre of the pupil of the at least one eye of the user, as described elsewhere herein.

In an instance of the summary embodiment, the HMD comprises a display portion that is configured to display virtual images, wherein the HMD is configured to modify the displayed virtual images in response to the calculated position of the centre of the pupil of the at least one eye of the user, as described elsewhere herein.

Optionally in this instance of the summary embodiment, the displayed virtual images are modified by modifying the parallax between the virtual images displayed to one eye of the user and the virtual images displayed the other eye of the user, as described elsewhere herein.

In an instance of the summary embodiment, the HMD comprises a display portion that is configured to display virtual images at a plurality of apparent distances; and a gaze tracker that is configured to detect the apparent distance of an image feature being looked at by the user, in which the processing unit is configured to calculate the position of the centre of the at least one eye of the user in response to a change of the apparent distance of the image feature being looked at by the user, as described elsewhere herein.

In an instance of the summary embodiment, the one or more imaging sensors are stereoscopic imaging sensors, and the processing unit is configured to calculate a depth of the centre of at the least one eye of the user based upon captured image, in which the measurement system calculates the position of the centre of the eye of the user based upon the depth of the centre of the at least one eye of the user, the captured image and the predetermined position of the one or more imaging sensors, as described elsewhere herein.

Optionally in this embodiment, the processing unit is configured to estimate a position of the centre of the eye in a respective captured image, as described elsewhere herein.

Optionally in this instance, the processing unit is configured to estimate the position of the centre of the eye in the respective captured image by using information of the properties of the respective imaging sensor, as described elsewhere herein.

Further optionally in this instance, which the processing unit is configured to estimate the position of the centre of the eye in the respective captured image by comparing the relative sizes of predetermined image features in the respective captured image, as described elsewhere herein.

In a summary embodiment of the present description, a method comprises capturing, by one or more imaging sensors mounted on a head mounted display (HMD), an image of at least one eye of a user wearing the HMD; and calculating a position, relative to the face of the user, of the centre at least one eye of the user based upon the captured image and a predetermined position of the one or more imaging sensors, as described elsewhere herein.

In a summary embodiment of the present description, a computer program comprises computer executable instructions adapted to cause a computer system to perform a method of the present

description, as described elsewhere herein.

The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the invention, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.

Claims (15)

1. CLAIMS1. A measurement system comprising: a head mounted device (HMD), comprising: one or more imaging sensors that are configured to capture an image of at least one eye of a user wearing the HMD; and a processing unit that is configured to calculate a position, relative to the face of the user, of the centre at least one eye of the user based upon the captured image and a predetermined position of the one or more imaging sensors.
2. 2. The measurement system of claim 1, in which: a horizontal distance of the at least one eye to a centre line of the face of the user is estimated from the position of the centre of the eye in a respective captured image together with the position on the HMD of the image sensor that captured that respective image.
3. 3. The measurement system of claim 2, in which: the horizontal distance of the at least one eye to a centre line of the face of the user is estimated using a predetermined relationship between the physical position of the user's eye relative to the image sensor and the resulting position of the eye in an image captured by that image sensor to determine from such an image an offset of the position of the centre of the eye of the user from the predetermined distance of the image sensor from a centreline of the HMD.
4. 4. The measurement system of any preceding claim, in which an interpupillary distance between the eyes of the user is estimated to be twice the distance between the centre of one eye of the user and a centre line of their face.
5. 5. The measurement system of any one of claims 1-3, in which an interpupillary distance between the eyes of the user is calculated as the sum of the distances between the centre of each eye of the user and a centre line of their face.
6. 6. The measurement system according to claim 1, in which the HMD comprises: an optical assembly that is configured to be modified in response to the calculated position of the centre of the pupil of the at least one eye of the user.
7. 7. The measurement system according to claim 1, in which the HMD comprises: a display portion that is configured to display virtual images, wherein the HMD is configured to modify the displayed virtual images in response to the calculated position of the centre of the pupil of the at least one eye of the user.
8. 8. The measurement system according to claim 7, in which the displayed virtual images are modified by modifying the parallax between the virtual images displayed to one eye of the user and the virtual images displayed the other eye of the user.
9. 9. The measurement system according to claim 1, in which the HMD comprises: a display portion that is configured to display virtual images at a plurality of apparent distances; and a gaze tracker that is configured to detect the apparent distance of an image feature being looked at by the user, in which the processing unit is configured to calculate the position of the centre of the at least one eye of the user in response to a change of the apparent distance of the image feature being looked at by the user.
10. 10. The measurement system according to claim 1, in which the one or more imaging sensors are stereoscopic imaging sensors, and the processing unit is configured to calculate a depth of the centre of at the least one eye of the user based upon captured image, in which the measurement system calculates the position of the centre of the eye of the user based upon the depth of the centre of the at least one eye of the user, the captured image and the predetermined position of the one or more imaging sensors.
11. 11. The measurement system according to any preceding claim, in which the processing unit is configured to estimate a position of the centre of the eye in a respective captured image.
12. 12. The measurement system according to claim 11, in which the processing unit is configured to estimate the position of the centre of the eye in the respective captured image by using information of the properties of the respective imaging sensor.
13. 13. The measurement system according to claim 11 or 12, in which the processing unit is configured to estimate the position of the centre of the eye in the respective captured image by comparing the relative sizes of predetermined image features in the respective captured image.
14. 14. A method for measurement comprising: capturing, by one or more imaging sensors mounted on a head mounted display (HMD), an image of at least one eye of a user wearing the HMD; and calculating a position, relative to the face of the user, of the centre at least one eye of the user based upon the captured image and a predetermined position of the one or more imaging sensors.
15. 15. A computer program comprising computer executable instructions adapted to cause a computer system to perform the method of claim 14.