WO2020229793A1

WO2020229793A1 - Virtual reality head mounted display system calibration methods and apparatus

Info

Publication number: WO2020229793A1
Application number: PCT/GB2020/051036
Authority: WO
Inventors: Benjamin CORDES; Paul Grey; Kristian GILLIGAN
Original assignee: Bentley Motors Limited
Priority date: 2019-05-14
Filing date: 2020-04-28
Publication date: 2020-11-19
Also published as: GB2583936A8; GB2583936B; GB201906781D0; GB2583936A

Abstract

In a method of calibrating a VR HMD system (10), a randomised study is carried out in a test environment (22) in which a series of VR representations of a physical reference piece (16) to which a set range of different scaling factors are applied is displayed via the HMD (20) to a number of test subjects (14). The test subjects (14) are asked to identify which VR representation best matches the physical reference piece (16) as seen in the real world and the data analysed to determine which scaling factor the majority of test subjects preferred. The preferred scaling factor is subsequently applied when the using the HMD system to display a VR representation of a design as part of a design development and/or approval process. Apparatus (12) for use in conducting the study includes a calibration table (24) and a plurality of physical reference pieces (16), each mountable to the table at a predetermined position and orientation.

Description

Virtual Reality Head Mounted Display System Calibration Methods and

Apparatus

Technical Field of the Invention

The present invention relates to methods and apparatus for calibrating and/or evaluating a virtual reality head mounted display system. The present invention relates in particular, but not exclusively, to methods and apparatus for calibrating and/or evaluating a virtual reality head mounted display system for use in a product design development and/or approval process. The present invention also relates to methods of using a virtual reality head mounted display system in a product design development and/or approval process.

Background to the Invention

Virtual reality (VR) systems are known which incorporate a head mounted display (HMD) used to present a 3D image to a user. The position and orientation of the user’s head may be tracked and the displayed image varied to take account of changes in the user’s perspective relative to a VR object in the displayed image as they move about in the real world. The system is thereby able to simulate the user moving about a virtual environment and viewing a VR object within it from different angles. In order to track the position and orientation of the user’s head, it is known to use an optical tracking system to track the position and orientation of the HMD worn by the user. In one known system which uses infrared (IR) light, a number of cameras are spaced apart at fixed locations and directed toward a measurement zone in the physical environment. Each camera is equipped with an IR pass filter in front of the lens, and a ring of IR LEDs around the lens to periodically illuminate the measurement zone with IR light. The HMD is equipped with passive retro-reflective markers, which reflect the incoming IR light back to the cameras. The IR reflections are detected by the cameras and the data processed by the optical tracking system. ETsing multiple cameras, the 3D position of each marker can be derived with accuracy and by using multiple markers on the HMD, the position and orientation of the HMD within the measurement zone can be determined. In alternative arrangements, the markers on the HMD are active infra- red sources rather than passive reflectors. In this case, the cameras need not be provided with IR LEDs. The position and orientation of other physical objects within the environment can also be tracked in a similar manner if they are provided with suitable markers.

VR HMD systems are often used for gaming and general entertainment purposes. In such applications, whilst the displayed VR image needs to be realistic, it is not critical that a VR object within the image as it is perceived by the user is an accurate representation of a real object.

It is common practice to produce physical models of a proposed design during a product design and development process. Such physical models can be inspected to ensure a proposed design conforms to requirements, to allow for comparison between different design options, and/or for design approval. However, the production of physical models is expensive and time consuming. The introduction of fast prototyping and 3D printers has reduced the cost and time for producing certain types of physical model but such methods are not always suitable. In any event, even when using fast prototyping and 3D printing techniques, it can be expensive and time consuming to produce large physical models, such as may be required when developing a design for a motor vehicle body for example.

To address these issues, the use of VR 3D displays for product development, approval and inspection is becoming more widespread, as it is possible to produce a computer generated 3D image of a proposed design relatively quickly and inexpensively, often using design information from CAD/CAM systems.

For certain applications, it would be advantageous to use a VR HMD system to display a 3D VR representation of a proposed design (a VR design model) for product development and/or approval purposes. The use of an HMD system provides the user with an immersive display of the VR design model with freedom to move around it and view it from almost any angle. However, in order for VR HMD systems to become more widely adopted in this field, it is necessary to verify that a VR design model as perceived by someone through the HMD is an accurate representation of what would be seen if they viewed a physical model of the design in the real world. In other words, there needs to be an acceptable level of confidence that a VR design model presented to a user using a VR HMD system is an accurate representation of what they would see if they inspected an actual physical model of the design in the real world.

There is a need then for a method of calibrating a VR HMD system which can be used to evaluate the accuracy with which a 3D VR representation of an article as perceived by a user of the system reflects how the user would perceive the physical article in the real world.

There is a further a need for a method of calibrating a VR HMD system which can be used to improve the accuracy with which a 3D representation of an article perceived by a user of the system reflects how the user would perceive the physical article in the real world.

There is a still further need for a method of calibrating a VR HMD system which provides confidence that a 3D VR representation of an article as perceived by a user of the system reflects how the user would perceive the physical article in the real world.

There is also a need for apparatus which can be used to calibrate a VR HMD system in order to evaluate the accuracy with which a 3D representation of an article as perceived by a user of the system reflects how the user would perceive the article in the real world.

There is a further a need for a need for apparatus which can be used to calibrate a VR HMD system in order to improve the accuracy with which a 3D representation of an article as perceived by a user of the system reflects how the user would perceive the physical article in the real world.

There is a still further need for apparatus which can be used to calibrate a VR HMD system in order to provide confidence that a 3D representation of an article as perceived by a user of the system reflects how the user would perceive the physical article in the real world.

Summary of the Invention Aspects of the invention relate to methods and apparatus for calibrating a VR HMD system.

According to a first aspect of the invention, there is provided a method of calibrating a VR HMD system comprising: a. positioning a physical reference piece in a physical test environment at a reference location and orientation within the physical test environment; b. with a test subject positioned within the physical test environment, displaying to the test subject via the VR HMD system under test a VR representation of the test environment which includes a VR representation of the reference piece at said reference position and orientation and in which one of a number of predetermined scaling factors is applied to the size of the VR representation of the reference piece; c. having the test subject compare the VR representation of the reference piece as viewed by them in the HMD with the physical reference piece as seen by them without the HMD; d. repeating steps b and c a number of times applying a different one of the predetermined scaling factors to the size of the VR representation of the reference piece each time and recording which of the VR representations the test subject considers to best match the physical reference piece; e. repeating steps b to d for a number of further test subjects; and f. analysing the data obtained from the test subjects to determine which one of the predetermined scaling factors produces a VR representation of the reference piece which most closely matches the physical reference piece. The scaling factor which produces a VR representation of the reference piece which most closely matches the physical reference piece (referred to as the“preferred” or“optimum” scaling factor”) may be the one which is most frequently applied in the VR representations identified by the test subjects as best matching the physical reference piece.

The method may further comprise subsequently applying the preferred (optimum) scaling factor to a VR object displayed using the HMD system. The method may be part of a design development and/or approval process for an article and the VR object may be a 3D representation of the article. Thus the preferred scaling factor is applied to a 3D VR design model when subsequently using the HMD system to display the 3D VR design model as part of a design development/approval process. This improves the accuracy with which the 3D VR design model as perceived by a user through the HMD reflects what the user would see if they were to view a physical model of the design in the real world.

The method may comprise analysing the data obtained from the test subjects to make an assessment of how accurately the VR HMD system represents the physical reference piece. In particular, the method may comprise analysing the data obtained from the test subjects to make an assessment of how accurately the VR HMD system represents the physical reference piece when the preferred scaling factor is applied.

The order in which the differently sized VR representations of the reference piece are shown to each test subject may be randomised.

The method may comprise using a plurality of differently shaped reference pieces and carrying out steps a to d for each reference piece for each test subject and analysing the data to determine which one one of the predetermined scaling factors produces VR representations of the reference pieces which most closely match the physical reference pieces. Where a plurality of differently shaped reference pieces are used, the order in which the reference pieces are used may be randomised for each test subject. The method is carried out using a group of test subjects and the order in which the test subjects in the group participate may be randomised.

The method may comprise recording for each VR representation of a reference piece shown to a test subject whether the test subject considers it to be larger, smaller or the same size as the physical reference piece.

In an embodiment the, or each, physical reference piece is removably mounted to a support located within the physical test environment, the support and the, or each, physical reference piece having cooperating formations which engage to locate the, or each, physical reference piece to the support at a predetermined position and orientation relative to the support. The VR representation of the test environment may comprise a VR representation of the support. In which case, the method may comprise using a tracking system to align the physical support in the physical test environment with a VR representation of the support within the VR representation of the test environment.

In an embodiment, the method comprises holding an object in contact with a calibration feature on the physical support and tracking the position and orientation of the object in the physical environment, displaying in the HMD the VR representation of the test environment including a VR representation of the object at a position within the VR representation of the test environment which corresponds to the tracked position of the actual object in the physical test environment, and, if the VR representation of the object is not aligned with the respective calibration feature on the VR representation of the support, either moving the actual support in the physical test environment whilst holding the object in contact with the calibration feature or applying a translation to the VR representation data until the VR representation of the object aligns with the corresponding virtual calibration feature of the VR representation of the support. The method may comprise moving the physical support to align it with its VR representation in X and Y axes and applying a translation to the VR representation data to align it in a Z axis. The method may comprise using an optical tracking system and in particular an IR tracking system to track the object. The object may be a HMD controller forming part of the HMD system. The method may comprise tracking the position and orientation of the HMD within the physical test environment and modulating the VR representation of the test environment in response to the tracking data. The method may comprise using an optical tracking system and in particular an IR tracking system to track the HMD.

In accordance with a second aspect of the invention, there is provided a method of displaying a 3D VR representation of a design for an article (a VR design model) using an HMD system, the method comprising: calibrating the HMD system using the method according to the first aspect of the invention as set out above and subsequently applying the preferred scaling factor to the size of the 3D VR representation of the design for the article (the VR design model)displayed using the HMD system.

According to a third aspect of the invention, there is provided apparatus for use in a method of calibrating a VR HMD system, the apparatus comprising a physical support and a physical reference piece mounted to the support at a predetermined position and orientation relative to the support.

The physical reference piece may be removably mountable to the support, the physical reference piece and the support having corresponding reference piece mounting formations which cooperate when the reference piece is mounted to the support so as to locate the reference piece in said predetermined position and orientation relative to the support. In an embodiment, the apparatus comprises a plurality physical reference pieces removably mountable to the support. Each reference piece has a corresponding reference piece mounting formation for cooperation with said cooperating reference piece mounting formation on the support such that each reference piece is locatable in a predetermined position and orientation relative to the support. The plurality of physical reference pieces may be differently shaped from one another.

The support may have a plurality of calibration features for engagement with a HMD controller, each calibration feature being configured such that the HMD controller is located at a predetermined position and orientation relative to the reference piece mounting formation of the support when held in engagement with a respective one of the calibration features. The support may have calibration features for aligning the support in X, Y and Z axes. The support may be a calibration table for location in a test environment. In which case, calibration features for aligning the support in X and Y axes may be provided about a periphery of a top of the table. One or more calibration features for aligning the support in the Z axis (vertically) may be provided in an upper surface of a top of the table. The apparatus may include a tracking system for tracking the position and orientation of the object when held in engagement with a respective calibration feature. The tracking system may be an optical, and in particular an IR, tracking system.

The apparatus according to the third aspect of the invention may be configured for use in the method according to the first aspect of the invention as set out above. The apparatus may include a computing device programmed and arranged to provide a VR representation of a physical test environment including a VR representation of the support within the VR test environment.

According to a fourth aspect of the invention there is provided a VR HMD system comprising a HMD and a computing device programmed and arranged to provide a VR representation of an object to a user through the HMD, wherein the system is configured to apply a preferred scaling factor to the VR representation of an object, wherein the preferred scaling factor is determined using the method according to the first aspect of the invention as set out above.

Detailed Description of the Invention

In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic representation in plan of a physical test environment within which at least part of a method of calibrating a VR HMD system in accordance with an aspect of the invention is carried out. Figure 2 is a perspective view of a calibration table forming part of apparatus for calibrating a VR HMD system according to another aspect of the invention and which is used within the physical test environment of Figure 1 to carry out a part of the method of calibrating a VR HMD system in accordance with the invention.

Figure 3 is a plan view from above of the calibration table of Figure 2.

Figures 4 to 8 are a series of views similar to Figure 2, each showing a differently shaped physical reference piece mounted to the calibration table of Figures 1 and 2.

Figures 9A and 9B are views from above of the physical calibration table of Figure

2 and a corresponding virtual representation of the calibration table respectively, illustrating how the physical table is aligned to its virtual equivalent using an HMD controller held in engagement with calibration formations on the physical table.

Figure 10 is a series of perspective views of a VR representation of the physical reference piece of Figure 4, illustrating how the virtual representation is presented in a range of different sizes as part of a method of calibrating a VR HMD system in accordance with an aspect of the invention.

A method of calibrating a VR HMD system 10 in accordance with an aspect of the invention and apparatus 12 for use in the method will now be described with reference to the accompanying drawings.

The method comprises a number of people, referred to as test subjects 14, comparing a number of differently shaped physical reference pieces 16 as seen by them in the real world with VR representations 16’ of the reference pieces presented using the HMD system being calibrated and viewed by the test subjects through the HMD 20. Each test subject 14 is presented with a series of VR representations 16’ of each reference piece 16. A different scaling factor is applied to the size of the VR representation of the reference piece in each of the series of representations and the test subject is asked to identify which of the VR representations 16’ in the series best matches (visually) the physical reference piece 16. By collating and analysing data from a number of test subjects 14 in a randomised study, it is possible to identify tends in terms of which scaling factor the majority of test subjects preferred and to determine how accurately the VR HMD system 10 displays the physical world. The preferred scaling factor can then be applied when the using the HMD system to display a VR representation of a design (a VR design model) with an acceptable level of confidence that what is seen accurately reflects what would be seen in the real world if a physical model of the design were to be produced.

The method is carried out in a controlled physical test environment 22 illustrated schematically in Figure 1. The physical test environment 22 may be a room, part of a room, or other defined space. Located within the physical test environment 22 is a calibration table 24 adapted to support each of the physical reference pieces 16 used in the method in a predetermined, reference position and orientation relative to the table and the environment. The calibration table 24 includes a base 26 and a table top 28 having an upper reference surface 30 on which each of the reference pieces 16 can be mounted in turn. Centrally located on the upper reference surface 30 is a reference piece mounting feature 32. Each of the physical reference pieces 16 has a correspondingly shaped mounting feature which co-operates with the table top reference piece mounting feature 32 so that each physical reference piece 16 is mounted in a predetermined position and orientation relative to the table top 28. In the present embodiment, the reference piece mounting feature 32 is in the form of a cruciform projection which locates in a correspondingly shaped recess in the base of each of the physical reference pieces 16. The cruciform projection is non- symmetrical in at least one plane, having a cross member 34 which is off-set towards one end of a longitudinal member 36, so that each reference piece can only be mounted to the table top in one orientation. It will be appreciated that other suitable cooperating features could be provided on the table top 28 and the reference pieces to enable each reference piece to be mounted to the table top in a predetermined position and orientation. For example, the projection and recess can have any suitable shape and projections could be provided on the reference pieces with a corresponding recess in the table top. Alternatively, a plurality of corresponding recesses and projections could be provided on the table top and the base of each reference piece.

Figures 4 to 8 illustrate a range of differently shaped physical reference pieces 16 for use in an embodiment of the method, including: a spherical reference piece 16a, a cylindrical reference piece 16b, a cubic reference piece 16c, an L shaped reference piece 16d, and a prism shaped reference piece 16e. Each reference piece 16 is accurately manufactured within very close tolerances. In one embodiment, each reference piece 16 is manufactured to an accuracy of +/- 0.2mm. The reference pieces 16 can be manufactured from any suitable material, which may be a metallic material.

It will be appreciated that more or fewer than five differently shaped reference pieces 16 can be used in the method. The number of differently shaped reference pieces used is selected to provide statistically reliable data without requiring an excessive amount of effort to conduct the study. It will also be appreciated that the shapes of the reference pieces 16 need not be limited to the examples illustrated.

The HMD system 10 includes the HMD 20, a computing device 40 and an IR optical tracking system 42.

The IR tracking system 42 is configured to track the position and orientation of the HMD 20 when worn by a test subject within the environment 22 as a means of tracking the position and orientation of their head, and the overall HMD system 10 is configured to alter the VR image displayed in the HMD as the user moves within the physical test environment to take account of changes in their perspective. This enables the test subject to look at a VR object in the displayed image from different angles. The IR tracking system 42 includes a number of static IR cameras 44 (e.g. a camera with an IR pass filter) spaced apart and directed toward a measurement zone within the physical environment which includes the calibration table 24 and an area adjacent to the calibration table in which a test subject 14 stands. Two cameras 44 are illustrated in Figure 1 for illustrative purposes but any suitable number of cameras 44 can be located in various positions so as to provide adequate coverage of the measurement zone. For example, cameras 44 may be located in front of and/or to the side of and/or behind the measurement zone. A number of tracking markers (not shown) are mounted to the HMD 20. The tracking markers may be passive markers which are not luminescent but which reflect IR light and may take the form of retro-reflective spheres. In this case, the tracking system also comprises a source of IR light directed into the measurement zone volume to periodically illuminate the measurement zone with IR light. The source of IR light may be in the form of a ring of IR LEDs around the lens of each of the cameras 44, though other sources can be used. Alternatively, the tracking markers can be active markers which are luminescent themselves, such as IR LEDs. In this case, no additional IR lighting of the measurement zone is necessary. The cameras 44 detect the IR light (reflected or emitted) from the tracking markers and the image data is processed suing suitable software running on the computing device 40 to periodically determine the position of each of the markers within the measurement zone. Since there are a number of tracking markers on the HMD 20, information relating to the relative positions of the markers can be used to determine both the position and orientation of the HMD 20 within the test environment 22. Whilst IR tracking is a convenient method of tracking the HMD 20 and the user’s head, other tracking technologies can be adopted, including other forms of optical tracking.

The computing device 40 can be any suitable computing device comprising memory and one or more processors and is configured to run software for processing digital VR images displayed in the HMD 20 and for performing IR tracking. The computing device 40 is operatively connected with the IR cameras 44 and with the HMD 20 by any suitable arrangement, which may comprise a wired or wireless connection. The computing device 40 may be located within the physical test environment 22 but could be located externally of the environment.

A digital 3D VR representation of the physical test environment 22 is produced which incorporates a full scale 3D VR representation 24’ of the calibration table 24 and into which can be incorporated 3D VR representations 16’ of each of the physical reference 16 pieces mounted to the calibration table. The VR representations of the physical test environment and reference pieces are generated so as to correspond as closely as possible to the physical test environment 22 and the physical reference pieces 16. In one embodiment, the VR representation of the test environment 22 is produced by mapping a high dynamic range (HDR) 360 degree photograph of the physical test environment to a semi sphere. This provides an accurate representation of the light, reflections and shadows of the physical test environment 22. The VR representations 16’ of the references pieces 16 will typically include material finishes which correspond as closely as possible to those of the physical reference pieces 16 and which may be provided by means of scanned materials. In an alternative embodiment, the VR representation of the physical test environment could be produced by modelling it in 3D.

The position of the VR representation 16’ each reference piece 16 within the VR representation of the test environment and the position of the physical reference piece 16 in the physical test environment 22 are aligned to one another. Where the calibration table 24 is movable, it is necessary to position and align the physical calibration table 24 within the physical test environment to the VR representation 24’ of the calibration table within the VR representation of the test environment 22’ prior to conducting the study using the test subjects. In the present embodiment, this is achieved by providing a number of calibration features on the calibration table which are engaged by a HMD controller 46 forming part of the HMD system and using the IR tracking system 28 to track the position of the controller 46. For aligning the calibration table in X and Y co-ordinates, two pairs of calibration features are provided about the periphery of the table top 28. A first pair of X-axis calibration features 48 are located diametrically opposite one another and a second pair of Y-axis calibration features 50 are located diametrically opposite one another and offset by 90 degrees to the X-axis calibration features. Each of the X-axis and Y-axis calibration features is in the form of a recess shaped for engagement with an end of the HMD controller 46, to accurately position the controller relative to the table top 28.

The calibration table 24 is initially positioned within the physical test environment 22, which may be provided with markings on the floor to indicate the correct location and orientation of the table. To fully align the physical calibration table 24 with its VR representation 24’, the HMD controller 46 is placed in contact with one of the X-axis calibration features 48 as illustrated in Figure 9A. The position and orientation of the physical HMD controller 46 is tracked using the IR tacking system and a VR representation of the controller 46’ is displayed in the VR representation of the test environment in the HMD 20 at the tracked position. Initially this will usually not be aligned correctly with the VR representation of the calibration table 24’ and the physical calibration table 24 is moved with the physical hand controller held in contact with the calibration feature 48 until the VR representation of the controller 46’ is brought into proper alignment with the respective calibration feature of the VR calibration table 24’ as illustrated in Figure 9B. This procedure is repeated with one of the Y-axis calibration features 50 to bring the calibration table into alignment along both X and Y axes. To provide rotational alignment, the position of the calibration table 24 is checked against all four X-axis and Y-axis calibration features 48, 50. It will be appreciated that the order in which the calibration features 48, 50 are used can be varied.

In addition to aligning the calibration table in X and Y axes, the upper surface 30 is aligned in the Z-axis. For this, a number of Z-axis calibration features 52 are provided in the upper surface 30 of the calibration table top 28 for engagement with the HMD controller. The Z-axis calibration features 52 are in the form of recesses in the table top surface 30 with which the controller 46 is engaged. Once the physical calibration table 24 is aligned in X and Y axes and rotationally, alignment in the Z axis is checked by engaging the HMD controller 46 with one of the Z-axis calibration features and reviewing the VR representation in the HMD to see if there is any misalignment between the VR controller and the VR calibration table. If there is misalignment, a translation is applied to the VR representation data until the VR controller 46’ lines up with the VR calibration table 24. The other three Z-axis calibration features 52 are then used to check the table for flatness and the digital data translated as required. Once the physical calibration table 24 has been correctly aligned with the VR calibration table 24’ , the test environment is ready to conduct the statistical study using a number of test subjects 14.

It will be appreciated that the physical test environment 22 may include a physical calibration table 24 or some other support device for supporting the physical reference pieces 16 which is permanently positioned for alignment with the VR calibration table or support device. In this case, the alignment procedure described above may not be required.

During the study, each test subject 14 stands at predetermined position relative to the calibration table 24 and the reference pieces 16 when mounted on the calibration table, and which standing position is marked on the floor. The standing position may be a measured distance from the centre of the calibration table. Typically, the distance between the standing position and the centre of the calibration table is in the range of one to two meters and more preferably is about 1.5 meters. A first one of the physical reference pieces 16 is mounted to the calibration table 24 and the test subject is shown a series of VR representations of the test environment which include a VR representation of the respective reference piece mounted to the calibration table through the HMD. In each of the series of VR representations for a particular reference piece, a different scaling factor is applied to the size of VR representation 16’ of the reference piece 16. Figure 10 illustrates a series of five VR representations 16a’of a spherical reference piece 16a to which different scaling factors have been applied. In this case the scaling factors are -10%, nominal, +5%, +10% and + 15%. In tests it was found advantageous to apply the scaling factor to both the VR calibration table 24’ and the VR reference piece 16’ , as it was found that test subjects tended to judge the relationship between table and reference piece. However, the scaling factor could be applied only to the reference pieces. Where the test environment 22 is fully modelled in the VR representation, the scaling factors could be applied to the whole of the VR representation of the test environment.

Each test subject 14 views each of the series of VR representations 16’ with the HMD 20 on and compares it with the physical reference piece 16 by temporarily removing the HMD whilst remaining on the marked standing position. The test subject 14 may remove and replace the HMD a number of times to obtain a good comparison. Once the test subject as reviewed all of the differently sized VR representations 16’ of the test piece, an evaluation is made of which they consider to be the best match (visually) to the physical reference piece 16. The test subject may be asked which they consider to be the best match after viewing them all. In addition, or alternatively, the test subject 14 may be asked to indicate for each representation in the series whether the VR reference piece appears larger, smaller, or the same size as the physical reference piece in order to arrive at a conclusion regarding which of the differently sized VR representations is the best match. This process is then repeated with the same test subject for each of the differently shaped reference pieces 16a- 16e.

The order in which the test subjects 14 participate in the study is randomised as is the order in which the differently shaped reference pieces 16a-16e are used for each test subject, and the order in which the differently sized VR representations are presented to a given subject for each reference piece.

Once the data from all the test subjects has been collected, it is statistically analysed to identify tends in terms of which scaling factor the majority of test subjects preferred (that is to say which scaling factor the majority of the test subjects considered produced VR representations of the reference pieces that when viewed through the HMD most closely match the physical reference pieces when viewed without the HMD) and to determine how accurately the VR HMD system 10 displays the physical world. Where a particular scaling factor is identified as producing the most accurate VR representations of the reference pieces, this“preferred” or“optimum” scaling factor can subsequently be applied when using the HMD system to display a VR representation of a design as part of a design development and/or approval process with an acceptable level of confidence that what is visualised through the HMD is an accurate representation of how a model of the design would be seen in the real world. This will enable the HMD system 10 to be used to review a VR model of the design instead of producing a physical model for at least part of the design and/or approval process, thus saving time and cost.

The method in accordance with the invention enables the accuracy with which a given HMD system 10 reproduces the real world to be improved by identifying an appropriate scaling factor to compensate for any inherent tendency that the system may have to produce VR images of objects which are generally perceived as being either smaller or bigger than the real world equivalent objects. Furthermore, the method can be used to provide confidence that the system can be used instead of producing physical models. The method is carried out for each HMD system 10 of interest, e.g. for a particular HMD and software combination, and repeated if there are significant changes to the system such as a software update and/or a change of HMD.

The number of test subjects 14 used, the number and shapes of the physical reference pieces 16, and the number and resolution of the of scaling factors applied are all selected to provide statistically meaningful data. The number of scaling factors applied and the differentiation between the scaling factors may be varied for use in calibrating different HMD systems. For example, the range of scaling factors described above may be suitable for use with an HMD system in which VR objects are generally perceived as being smaller than equivalent real word objects. However, for use with an HMD system in which VR objects are generally perceived as being bigger than real world objects it may be appropriate to include a larger number of negative scaling factors in the study to compensate. A small pilot study might be carried out to assess the appropriateness of an initial set of scaling factors which can then be varied accordingly for use in a main study.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims. For example, the calibration features in the calibration table 24 can be modified for use with a given HMD controller or other trackable object. Furthermore, the support for the physical reference pieces need not be in the form of a table.

Claims

1. A method of calibrating a VR HMD system comprising: a. positioning a physical reference piece in a physical test environment at a reference location and orientation within the physical test environment; b. with a test subject positioned within the physical test environment, displaying to the test subject via the VR HMD system under test a VR representation of the test environment which includes a VR representation of the reference piece at said reference position and orientation and in which one of a number of predetermined scaling factors is applied to the size of the VR representation of the reference piece; c. having the test subject compare the VR representation of the reference piece as viewed by them in the HMD with the physical reference piece as seen by them without the HMD; d. repeating steps b and c a number of times applying a different one of the predetermined scaling factors to the size of the VR representation of the reference piece each time and recording which of the VR representations the test subject considers to best match the physical reference piece; e. repeating steps b to d for a number of test subjects; and f. analysing the data obtained from the test subjects to determine which one of the predetermined scaling factors produces a VR representation of the reference piece which most closely matches the physical reference piece.

2. A method as claimed in claim 1, wherein the method comprises subsequently applying the scaling factor identified in step f as producing a VR representation of the reference piece which most closely matches the physical reference piece to a VR object when displaying the VR object using the HMD system.

3. A method as claimed in claim 2, wherein the method is part of a design development and/or approval process for an article and the VR object is a 3D representation of the article.

4. A method as claimed in any one of claims 1 to 3, wherein the method comprises analysing the data obtained from the test subjects to make an assessment of how accurately the VR HMD system represents the physical reference piece.

5. A method as claimed in anyone of claims 1 to 4, wherein the order in which the differently sized VR representations of the reference piece are shown to each test subject is randomised.

6. A method as claimed in any one of claims 1 to 5, wherein the method comprises using a plurality of differently shaped reference pieces and carrying out steps a to d for each reference piece for each test subject.

7. A method as claimed in claim 6, wherein the order in which the differently shaped reference pieces are used is randomised for each test subject.

8. A method as claimed in any one of the preceding claims, in which the method is carried out using a group of test subjects and the order in which the test subjects in the group participate is randomised.

9. A method as claimed in any one of claims 1 to 8, wherein the method comprises recording for each VR representation of a reference piece shown to a test subject whether the test subject considers it to be larger, smaller or the same size as the physical reference piece.

10. A method as claimed in any one of claims 1 to 9, wherein the, or each, physical reference piece is removably mounted to a support located within the physical test environment, the support and the, or each, physical reference piece having cooperating formations which engage to locate the, or each, physical reference piece to the support at a predetermined position and orientation relative to the support.

11. A method as claimed in claim 10, the method comprising using an optical tracking system to position the support within the physical test environment so as to align the physical support in the physical test environment with a VR representation of the support within the VR representation of the test environment.

12. A method as claimed in claim 11, the method comprising holding an object in contact with a calibration feature on the physical support and tracking the position and orientation of the object in the physical environment, displaying in the HMD the VR representation of the test environment a VR representation of the object at a position and orientation within the VR representation of the test environment which corresponds to the tracked position of the actual object in the physical test environment, and, if the VR representation of the object is not aligned with the respective calibration feature on the VR representation of the support, moving the actual support in the physical test environment whilst holding the object in contact with the calibration feature until the VR representation of the object aligns with the corresponding virtual calibration feature of the VR representation of the support.

13. A method as claimed in claim 11 or claim 12, the method comprising holding an object in contact with a calibration feature on the physical support and tracking the position and orientation of the object in the physical environment, displaying in the HMD the VR representation of the test environment a VR representation of the object at a position and orientation within the VR representation of the test environment which corresponds to the tracked position of the actual object in the physical test environment, and, if the VR representation of the object is not aligned with the respective calibration feature on the VR representation of the support, applying a translation to the VR representation data whilst holding the physical object in contact with the calibration feature on the physical support until the VR representation of the object aligns with the corresponding virtual calibration feature of the VR representation of the support.

14. A method as claimed in any one of claims 1 to 13, wherein the method comprises tracking the position and orientation of the HMD within the physical test environment and modulating the VR representation of the test environment in response to the tracking data.

15. Apparatus for use in a method of calibrating a VR HMD system, the apparatus comprising a support and a physical reference piece mounted to the support at a predetermined position and orientation relative to the support.

16. Apparatus as claimed in claim 15, wherein the physical reference piece is removably mountable to the support, the physical reference piece and the support having corresponding reference piece mounting formations which cooperate when the reference piece is mounted to the support so as to locate the reference piece in said predetermined position and orientation relative to the support.

17. Apparatus as claimed in claim 16, wherein the apparatus comprises a plurality physical reference pieces, each reference piece being removably mountable to the support and having a corresponding mounting formation for cooperation with said cooperating reference piece mounting formation on the support to locate the reference piece in said predetermined position and orientation relative to the support.

18. Apparatus as claimed in any one of claims 15 to 17, wherein the support comprises calibration features for engagement with a HMD controller, each calibration feature being configured such that the HMD controller is located at a predetermined position and orientation relative to the reference piece mounting formation of the support when held in engagement with a respective one of the calibration features.

19. Apparatus as claimed in claim 18, wherein the support comprises calibration features for aligning the support in X, Y and Z axes.

20. Apparatus as claimed in any one of claims 15 to 19, wherein the support is a calibration table.