Reducing Latency in Eye Trackers for Autostereoscopic Display Devices

Abstract

The present invention relates to a method for tracking facial features of a viewer of an autostereoscopic display device, the method comprising the steps of acquiring first sequential data relating to the position of the facial features relative to the autostereoscopic display device, and acquiring second sequential data relating to the velocity and / or acceleration of the facial features relative to the autostereoscopic display device. The first sequential data is acquired with a first latency, and the second sequential data is acquired with a second latency lower than the first latency. As a final step, sensor fusion is performed to combine the first sequential data and the second sequential data to generate final sequential data relating to the position of the facial features relative to the autostereoscopic display device. In this way, the final sequential data is available with a final latency lower than the first latency. A viewer of the autostereoscopic display device may be presented with an improved stereoscopic image and / or have an improved virtual reality experience when moving and rotating his / her head relative to the autostereoscopic display device.

Description

[Technical field]

[0001] <Field of the Invention> The present invention relates to a method for tracking facial characteristics of a viewer of an autostereoscopic display device, a tracking system for tracking facial characteristics of a viewer of an autostereoscopic display device, and an autostereoscopic display device comprising such a tracking system. [Background technology]

[0002] <Background> An eye tracker is a device for measuring a person's real-time eye position and real-time eye movements. It typically captures images of the eyes using photodiodes or cameras. Eye identification is then performed, typically by facial landmark detection, and then eye movements are estimated using signal or image processing methods.

[0003] An important application of eye trackers relates to their use in input devices for human-computer interaction. For example, in autostereoscopic display systems, they provide the real-time position of the viewer's eyes so that the viewer can perceive a realistic three-dimensional image even as he / she moves relative to the display. This occurs by providing only left-eye images to his / her left eye and only right-eye images to his / her right eye. In addition, the eye positions can be used to adapt a perspective view of the three-dimensional scene to the viewer's head / eye position relative to the display (the so-called "look around effect").

[0004] Like virtually any electronic measurement device, autostereoscopic display systems suffer from latency, commonly understood as the time delay between user input and system response. In an autostereoscopic display system setup, this essentially means that there is a delay between the viewer's head / eye movement (user input) and the corresponding adaptation to the displayed content (system response). When latency exceeds a critical threshold, the user's performance and experience degrades. This is usually associated with crosstalk and disturbed look-around effects.

[0005] The eye tracker may be responsible for a significant portion of the latency observed in an autostereoscopic display. For example, a typical total system latency of 85 ms may include an eye tracker latency of 43 ms. Thus, combating latency by simply reducing the latency that finds its origin in the eye tracker would be a step forward. DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0006] <Summary of the Invention> It is therefore an object of the invention to reduce the latency incurred in an eye tracker and, more generally, to improve the viewing experience of a viewer of an autostereoscopic display. [Means for solving the problem]

[0007] It has now been found that one or more of these objectives can be reached by using additional measurement measures.

[0008] Accordingly, the present invention relates to a method for tracking facial features of a viewer of an autostereoscopic display device, the method comprising the step of obtaining first sequential data relating to positions of the facial features relative to the autostereoscopic display device, the first sequential data being obtained with a first latency, the method further comprising: acquiring second sequential data relating to a velocity and / or acceleration of the facial feature relative to the autostereoscopic display device, the second sequential data being acquired at a second latency, the second latency being lower than the first latency; performing sensor fusion to combine the first sequential data and the second sequential data to generate final sequential data related to the positions of the facial features relative to the autostereoscopic display device; The final sequential data is available at a final latency that is lower than the first latency.

[0009] The invention further relates to a tracking system for tracking facial features of a viewer of an autostereoscopic display device, the tracking system comprising: - a tracking camera system configured to acquire first sequential data relating to positions of facial features relative to an autostereoscopic display device, the first sequential data being acquired with a first latency; and a secondary tracking system configured to acquire second sequential data relating to a velocity and / or acceleration of a facial feature relative to the autostereoscopic display device, the second sequential data being acquired with a second latency, the second latency being lower than the first latency; and a sensor fusion module configured to combine the first sequential data and the second sequential data to generate final sequential data related to the positions of the facial features relative to the autostereoscopic display device.

[0010] The invention further relates to an autostereoscopic display device comprising a tracking system as described above. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 shows an exemplary flow diagram of a method according to the invention. [Diagram 2] FIG. 2 shows diagrammatically a first assembly (8) according to the invention. [Diagram 3] FIG. 3 shows diagrammatically a second assembly (8) according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] <Mode for carrying out the invention> Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some elements in the figures may be exaggerated relative to other elements to aid in an understanding of various exemplary embodiments of the present invention. In particular, the relative dimensions of a viewer and an autostereoscopic display device cannot be derived from the drawings.

[0013] Moreover, the terms "first," "second," and the like in the specification and claims, if any, are used generally to distinguish between similar elements and not necessarily to describe a sequential or chronological order.

[0014] In the context of the present invention, the term "viewer" means a person capable of consuming, and in particular viewing, content presented by an autostereoscopic display device. Throughout the text, references to the viewer will be made by masculine terms such as "he," "him," or "his." This is merely for purposes of clarity and brevity, it being understood that feminine terms such as "she" and "her," apply equally.

[0015] In the context of the present invention, velocity is meant to include linear and rotational velocity, and acceleration is meant to include linear acceleration. It is further understood that the position and orientation of an item (e.g., a device, a body part, or a facial feature) can be derived from the item's velocity and acceleration, as well as the item's initially known position and orientation.

[0016] In the context of the present invention, tracking of facial features means tracking the position of a facial feature in time. The obtained position is related to the position of the tracking system performing the tracking and, ultimately, to the position of the autostereoscopic display device.

[0017] In the context of the present invention, latency (or delay time, wait time / latency) means the time delay between user input and system response, which is a result of the limited speed at which any physical interaction can propagate and transfer information.

[0018] In the context of the present invention, sequential data refers to data that contains elements ordered in a sequence by time. It concerns the measurement of a quantity over time, at regular time intervals (a time series).

[0019] Conventional methods for tracking facial features such as eyes typically use one or more cameras to acquire position data about the facial features and track their positions, with the viewer's head within the camera's observation field. In the method of the present invention, this conventional tracking method is supplemented by a second tracking method that is based on the velocity and / or acceleration rather than the position of the facial features.

[0020] Thus, in the method of the invention, at least two types of sequential data are captured regarding the facial features: a first sequential data is obtained regarding the position of the facial features relative to the autostereoscopic display device, while a second sequential data is obtained regarding the velocity and / or acceleration of the facial features relative to the autostereoscopic display device. Thus, the method of the invention requires that the relative velocity and / or acceleration is used as input for determining the second sequential data. Finally, the first and second sequential data are combined using sensor fusion to generate final sequential data regarding the position of the facial features relative to the autostereoscopic display device. These final sequential data form the actual output data of the tracking device (e.g. eye tracker) on which the method of the invention is performed, which form the input for the weaving of the stereoscopic images and, if applicable, to generate a stereoscopic image with the correct viewpoint from the viewer's standpoint (the "look-around effect").

[0021] In the method of the present invention, the facial feature is typically an eye (or two eyes). However, the facial feature may also be a facial feature selected from the group of eyes, nose, ears, head and mouth.

[0022] The first sequential data is preferably obtained according to the prior art by using one or more cameras, which are typically configured to record facial features of a viewer within the observation field of the one or more cameras, which typically further includes recognizing the facial feature within an image recorded by the one or more cameras and determining its position within the image.

[0023] The second sequential data is typically obtained by using a dedicated measurement device capable of determining the velocity and / or acceleration of the facial features relative to the autostereoscopic display device. Such a dedicated measurement device may include a wearable device that is attached to the viewer's head. For example, the wearable device is selected from the group of earphones, headphones, glasses, headbands, tiaras, elastic bands, and headgear. Such a device is typically capable of determining its specific forces (i.e. forces acting on it; accelerations). It may also be capable of determining its angular velocity and / or its orientation. For this purpose, the dedicated measurement device comprises one or more accelerometers and typically also one or more measurement instruments selected from the group of gyroscopes and magnetometers. In the art, a measurement device comprising these instruments is known as an inertial measurement unit (IMU). The velocity of the dedicated measurement device may be derived from the measured accelerations following an initial calibration in which the velocity of the autostereoscopic display device relative to the dedicated measurement device (such as a head-mountable one) is set to a specific value (typically zero).

[0024] A head-mounted IMU preferably includes an accelerometer and a gyroscope, which allows the IMU to distinguish between lateral and rotational movements of the head (specifically rotations characterized as pitch and yaw), which are important for determining head pose. A magnetometer is also preferably present to allow the IMU to distinguish between gravity and certain forces.

[0025] It is understood that the data obtained by the wearable device represents the velocity and / or acceleration of the wearable device and not necessarily the facial features even considering that their relative position is fixed (note that when the wearable device is positioned exactly in or on the facial features, this can be very inconvenient, for example when the eyes are involved). In particular, when the head rotates, the velocity and acceleration of the wearable device may deviate from the velocity and acceleration of the facial features. Therefore, it is preferable that the relative positioning of the wearable device and the facial features is known, so that the velocity and / or acceleration of the facial features themselves can be obtained for the autostereoscopic display device.

[0026] Fig. 1 shows an exemplary flow diagram of the method according to the invention, starting from the upper part of a setup (1) where a viewer (2) is in front of an autostereoscopic display device (3). The autostereoscopic display device (3) is associated with a tracking camera system (4), a secondary tracking system (5) and a sensor fusion module (6). The viewer (2) wears the tracking system (5), which is an earphone. The lower part of the flow diagram shows that first and second sequential data are obtained separately from the setup (1), where the tracking camera system (4) provides the first sequential data and the secondary tracking system (5) provides the second sequential data. Then, both sequential data are combined by sensor fusion performed by the sensor fusion module (6) to generate the final sequential data as output of the method.

[0027] In the method of the present invention, sequential data is acquired at a certain frequency, which is hereafter indicated by the term "sampling rate". The first sequential data is typically acquired at a rate in the range of 10-100 Hz, in particular in the range of 25-75 Hz, for example 30 Hz, 40 Hz, 50 Hz or 60 Hz. The second sequential data is typically acquired at a higher sampling rate than the first sequential data, but may be lower. Of course, it is preferred that the sampling rate is as high as possible. The sampling rate of the second sequential data is, for example, in the range of 20-1.000 Hz, in particular in the range of 50-500 Hz. In particular, the IMU can measure at a high sampling rate, such as a rate in the range of 100-1.000 Hz.

[0028] In some cases, the speed and / or acceleration of the autostereoscopic display device itself must be taken into account when acquiring the second sequential data. In such cases, the autostereoscopic display device is typically configured to generate the third sequential data at its own speed and / or acceleration. This is typically the case when the autostereoscopic display device does not have a fixed position relative to the Earth during use of the device, for example when the device is a handheld (mobile) device such as a mobile phone, tablet, laptop, or game console. Eventually, the absolute acceleration and velocity of the autostereoscopic display device as well as the absolute acceleration and velocity of the facial features need to be available in order to be able to obtain their relative speeds and accelerations. Only when the autostereoscopic display device is truly stationary does it not need to take into account its own absolute acceleration. However, when the viewer is in a car or on a moving boat, it is typically necessary to take into account the acceleration of the autostereoscopic display device, since boats and cars typically undergo acceleration during use. In such a situation, the measurement of the absolute acceleration of the autostereoscopic display device is typically preceded by an initial calibration in which the velocity of the autostereoscopic display device relative to the facial feature is set to a specific value (typically zero). Typically, in such an initial calibration, the orientation of the autostereoscopic display device relative to the facial feature is also set to a specific value. This may involve an initial guess based, for example, on initial camera images.

[0029] The dedicated measurement device can also be a remote device that is not worn on the viewer's head, but is placed at a distance from the head (and consequently moves independently of the head movements). For example, the measurement principle of such a remote device relies on the Doppler effect (in sound or light), time of flight or triangulation. For this purpose, the remote device can be equipped with a source of electromagnetic radiation that can illuminate the head.

[0030] If the autostereoscopic display device and the remote device are not accelerating (i.e., stationary relative to the Earth or moving at a constant speed relative to the Earth), it is sufficient for the remote device to measure the velocity and / or acceleration of the facial features relative to the remote device. This is, for example, the case when the autostereoscopic display device and the remote device are placed or mounted at a particular position in a room where a viewer views the autostereoscopic display device. The remote device may also form an integral part of the autostereoscopic display device.

[0031] When the autostereoscopic display device and / or the remote device accelerate, it is necessary to compensate for such acceleration. This is typically done by measuring the acceleration of the autostereoscopic display device and / or the remote device. Such situations apply, for example, when the autostereoscopic display device is a handheld mobile device or when the viewer is in a car or on a boat.

[0032] For this purpose, the remote device and / or the autostereoscopic display device comprises one or more accelerometers and typically also one or more measuring devices selected from the group of gyroscopes and magnetometers. Preferably, it comprises an IMU. The velocity of the remote device relative to the autostereoscopic display device can be derived from the measured accelerations following an initial calibration in which the velocity of the autostereoscopic display device relative to the remote device is set to a certain value (typically zero). However, preferably, the remote device is integrated with such a handheld device, so that the acceleration of only the handheld device needs to be determined.

[0033] When using remote devices (i.e. devices not worn on the viewer's head), it is preferred that the autostereoscopic display device and the remote device are not accelerating (i.e. stationary with respect to the Earth), since it requires significant processor capacity to take their acceleration into account, and in addition should not run with too much latency. A setting in which this is achieved is, for example, a room dedicated to providing the viewer with a virtual reality experience with the highest possible comfort. This includes the absence of wearables on the head, and the positioning of multiple remote devices that are stationary and accurately acquire second sequential data. Also, the autostereoscopic display device is in a stationary position in the room.

[0034] Traditionally, tracking of facial features (usually eyes) is based on capturing images using a camera that forms part of the tracking system. The capture itself introduces a latency (typically half the shutter speed), which is augmented by the latency resulting from the transfer and processing of the captured frames. However, the present invention provides a way to reduce the latency of the tracking system as a whole to a value lower than the latency caused by camera-based tracking alone.

[0035] In the method of the present invention, first sequential data is obtained, typically via such a camera-based tracking system, representing the positions of facial features relative to an autostereoscopic display device. This is done at a first latency. Second sequential data is obtained via a different means. This is done at a second latency.

[0036] The second sequential data are not position data but velocity and / or acceleration data. These data can be used to predict the position of the facial features in the near future. Thus, in the method of the present invention, the first and second sequential data are combined to obtain final sequential data regarding the position of the facial features relative to the autostereoscopic display device. These data form the actual output of the tracking system (e.g. eye tracking system) in which the method of the present invention is performed. This is done with a final latency.

[0037] It can be seen that the method of the invention advantageously has a final latency that is shorter than the first latency, i.e. by combining the first and second sequential data, the latency of the first sequential data is effectively reduced, as well as advantageously the latency of the tracking device. Thus, the method is advantageously performed in a tracking device of an autostereoscopic display device and can provide an improved stereoscopic image, in particular an improved virtual reality experience, to a viewer when moving and rotating his / her head relative to the autostereoscopic display device, in particular when the autostereoscopic display device is handheld and thus undergoes intense movements, in particular rotations.

[0038] In the method of the present invention, the autostereoscopic display device may be selected from the group of: television, (desktop) monitor, cinema display system, mobile phone, tablet, laptop and game console.

[0039] The combination of the first sequential data with the second sequential data is done via so-called sensor fusion, a commonly known concept as the process of merging data from multiple sensors, thereby overcoming the limitations of the individual sensors. This results in more reliable information and lower uncertainties.

[0040] In the present invention, sensor fusion overcomes the problem of high latency of a first sensor by combining the first sensor with a second sensor that is less accurate but has lower latency. Such sensor fusion thus enables the system to combine the position accuracy of a slow sensor with the velocity and / or acceleration data of a high speed system to provide accurate data with low latency.

[0041] Several sensor fusion techniques are commonly available. Those skilled in the art will know which techniques are suitable for application in the methods of the present invention, and will be able to find how to apply them by routine experimentation and without exerting inventive effort. For example, sensor fusion utilizes a class of algorithms known as Kalman filters.

[0042] The method of the present invention can be advantageously used in a range of applications, generally resulting in reduced latency, as explained above. This is particularly advantageous in the case of handheld autostereoscopic display devices, since such devices undergo rapid motion, especially rotation. The latter results in much more extreme viewer positions, viewer velocities, and viewer accelerations relative to the device than in the case of large stationary devices. Such motion of handheld autostereoscopic display devices easily causes disturbances in the three-dimensional view, since the latency traditionally associated with such devices cannot keep up with the rapid motion of the device.

[0043] A particular advantage of the use of a head-mounted IMU in the method of the invention is that the orientation of the head, and therefore the orientation of the facial features, can be measured faster and more accurately. With a camera alone, it is difficult to distinguish between lateral and rotational movements of the head (particularly rotations characterized as pitch and yaw). Changes in head pose are therefore difficult to detect. However, a head-mounted IMU equipped with an accelerometer and gyroscope (and preferably also a magnetometer) can accurately measure head rotation. Moreover, such measurements are made with lower latency than camera measurements. Thus, viewers who change their head pose can be tracked more accurately and with lower latency by applying the method of the invention.

[0044] The invention further relates to a tracking system (7) for tracking facial features of a viewer of an autostereoscopic display device (3), the tracking system (7) comprising: a tracking camera system (4) configured to acquire first sequential data relating to the positions of the facial features relative to the autostereoscopic display device (3), the first sequential data being acquired with a first latency; a secondary tracking system (5) configured to acquire second sequential data relating to a velocity and / or acceleration of a facial feature relative to the autostereoscopic display device (3), the second sequential data being acquired with a second latency, the second latency being lower than the first latency; - a sensor fusion module (6) configured to combine the first sequential data and the second sequential data to generate final sequential data relating to the positions of the facial features relative to the autostereoscopic display device (3).

[0045] In such an apparatus, the method of the invention as described above can be implemented, where the tracking camera system (4) is configured to acquire first sequential data and the secondary tracking system (5) is configured to acquire second sequential data, the final sequential data being available with a final latency that is shorter than the first latency.

[0046] In a preferred embodiment, a secondary tracking system (5) is designed to be worn on the observer's head and comprises one or more accelerometers and one or more measuring devices, usually selected from the group of gyroscopes and magnetometers, which makes it possible to measure the velocity and / or acceleration of the person's head, including its rotation, and ultimately the velocity and / or acceleration of certain facial features.

[0047] In another preferred embodiment, the secondary tracking system (5) is not worn on the person's head and is designed to measure the speed and / or acceleration of the facial features from an external location, i.e. a location that is not directly linked or associated with the observer's head. For this purpose, the secondary tracking device (5) typically comprises a remote device, configured to measure the speed and / or acceleration of the facial features relative to the remote device, for example by illuminating the facial features with electromagnetic radiation or sound, using the Doppler effect (light or sound), time of flight or triangulation. Such a remote device may be (rigidly) connected to the autostereoscopic display device (3) or may be present at a different location that is not subject to the movements of the autostereoscopic display device (3) and therefore also to the movements of the observer.

[0048] The invention further relates to an autostereoscopic display device (3) comprising a tracking system (7) as described above.

[0049] The invention further relates to an assembly (8) comprising an autostereoscopic display device (3) and a tracking system (7) as described above.

[0050] Figure 2 shows a schematic representation of a first assembly (8) according to the invention. It consists of an autostereoscopic display device (3) and a tracking system (7). The latter consists of a tracking camera system (4), a secondary tracking system (5) and a sensor fusion module (6). The secondary tracking system (5) comprises a set of earphones worn by a viewer of the autostereoscopic display device (3). The earphones capture data relating to the velocity and / or acceleration of the viewer's head and communicate wirelessly with the secondary tracking system (5).

[0051] Figure 3 shows a schematic representation of a second assembly (8) according to the invention. It is composed of an autostereoscopic display device (3) and a tracking system (7). The latter is composed of a tracking camera system (4), a secondary tracking system (5) and a sensor fusion module (6). The secondary tracking system (5) comprises a remote device (identified as remote from the viewer, since the device is not worn by the viewer) that is not subjected to the viewer's movements of the autostereoscopic display device (3). In Figure 3, the secondary tracking system (5) is integrated with the autostereoscopic display device (3) and is rigidly connected thereto, thus lacking the possibility of moving relative to the autostereoscopic display device (3), i.e. being subjected to the movements of the autostereoscopic display device (3).

[0052] In an alternative embodiment (not shown), the secondary tracking system (5) may be physically decoupled from the autostereoscopic display device (3) and made insensitive to movements of the autostereoscopic display device (3).

[0053] The autostereoscopic display device (3) can be selected from the group of: televisions, (desktop) monitors, cinema display systems, mobile phones, tablets, laptops and game consoles.

[0054] Preferably, the secondary tracking system (5), especially the remote device, is integrated into the autostereoscopic display device (3), so that the viewer only engages one device when presented with content from the autostereoscopic display device (3). This makes viewing more convenient and allows the invention to be incorporated into a single handheld autostereoscopic display device (3), such as a mobile phone, tablet, laptop, or game console.

[0055] If the autostereoscopic display device (3) is handheld, its movement and rotation during use needs to be taken into account when carrying out the method of the present invention on the autostereoscopic display device (3). Thus, as mentioned above in the description of the method of the present invention, the autostereoscopic display device (3) is preferably configured to generate third sequential data relating to the speed and / or acceleration of the autostereoscopic display device (3) itself. This is also necessary when the autostereoscopic display device (3) is present in a moving vehicle such as a car or boat, even when the device in such a setup is not handheld.

Claims

1. 1. A method of tracking facial features of a viewer of an autostereoscopic display device, comprising obtaining first sequential data relating to positions of the facial features relative to the autostereoscopic display device, the first sequential data being obtained with a first latency; The method further comprising: - acquiring second sequential data relating to a velocity and / or acceleration of the facial feature relative to the autostereoscopic display device, the second sequential data being acquired with a second latency, the second latency being lower than the first latency; performing sensor fusion to combine the first sequential data and the second sequential data to generate final sequential data regarding the positions of the facial features relative to the autostereoscopic display device, The method, wherein the final sequential data is available at a final latency that is lower than the first latency.

2. 2. The method of claim 1, wherein the first sequential data is acquired by using one or more cameras configured to record facial features of the viewer in a field of view of the one or more cameras.

3. The method of claim 1 or 2, wherein the second sequential data is obtained by using a wearable device worn on the viewer's head, the wearable device configured to generate data regarding a velocity and / or acceleration of the wearable device.

4. the second sequential data is obtained by taking into account the velocity and / or acceleration of the autostereoscopic display device, A method according to any one of claims 1 to 3, wherein the autostereoscopic display device is arranged to generate third sequential data relating to a velocity and / or an acceleration of the autostereoscopic display device.

5. The method of claim 3 or 4, wherein the wearable device is selected from the group of earphones, headphones, glasses, headbands, tiaras, elastic bands, and headgear.

6. The method of claim 1 or 2, wherein the second sequential data is obtained by using a remote device not worn on the viewer's head.

7. The method of claim 6 , wherein the remote device relies on the use of the Doppler effect, triangulation, or time of flight.

8. The method of any one of claims 1 to 7, wherein the facial features are selected from the group of eyes, nose, ears, head and mouth.

9. The method according to any one of claims 1 to 8, wherein the autostereoscopic display device is selected from the group of: televisions, (desktop) monitors, cinema display systems, mobile phones, tablets, laptops and games consoles.

10. The method of any one of claims 1 to 9, wherein the step of performing sensor fusion comprises the use of a Kalman filter.

11. A tracking system (7) for tracking facial features of a viewer of an autostereoscopic display device (3), said tracking system (7) comprising: a tracking camera system (4) configured to acquire first sequential data relating to the positions of the facial features relative to the autostereoscopic display device (3), the first sequential data being acquired with a first latency; a secondary tracking system (5) configured to acquire second sequential data relating to the velocity and / or acceleration of the facial features relative to the autostereoscopic display device (3), the second sequential data being acquired with a second latency, the second latency being lower than the first latency; a sensor fusion module (6) configured to combine the first sequential data and the second sequential data to generate final sequential data relating to the position of the facial features relative to the autostereoscopic display device (3).

12. 12. The tracking system (7) of claim 11, wherein the secondary tracking system (5) comprises a wearable device that can be worn on the viewer's head and comprises one or more measuring instruments selected from the group of one or more accelerometers and optionally gyroscopes and magnetometers.

13. 12. The tracking system (7) of claim 11, wherein the secondary tracking system (5) comprises a remote device configured to measure the speed and / or acceleration of the facial feature relative to the remote device by illuminating the facial feature with electromagnetic radiation or sound, for example by utilizing the Doppler effect, time of flight or triangulation.

14. An assembly (8) comprising an autostereoscopic display device and a tracking system according to any one of claims 11 to 13.

15. Assembly (8) according to claim 14, wherein the secondary tracking system (5) comprises a remote device according to claim 13, the remote device being integrated into the autostereoscopic display device (3).

16. Assembly (8) according to claim 14 or 15, wherein the autostereoscopic display device (3) is configured to generate third sequential data relating to a velocity and / or an acceleration of the autostereoscopic display device (3).

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