WO2012175939A1

WO2012175939A1 - Apparatus and method for displaying images

Info

Publication number: WO2012175939A1
Application number: PCT/GB2012/051376
Authority: WO
Inventors: Gordon Derek LOVE; Martin Scott BANKS
Original assignee: University Of Durham; The Regents Of The University Of California
Priority date: 2011-06-23
Filing date: 2012-06-15
Publication date: 2012-12-27
Also published as: GB201110661D0

Abstract

An apparatus and method for displaying images are described, the apparatus comprising: (i) a display device for displaying an image representing a three-dimensional scene, wherein a depth in said scene is specified as a depth of interest; and (ii) control means for providing a signal for controlling a focal length of lens means for viewing said image, said signal depending on said depth of interest. The apparatus may be adapted to display a left-eye image and a right-eye image to provide a stereoscopic image of the three-dimensional scene. Thereby, the focal length of the lens means may be controlled to reduce a difference between vergence distance and focal distance when a viewer fixates and focuses on a part of the scene at the depth of interest.

Description

APPARATUS AND METHOD FOR DISPLAYING IMAGES

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for displaying images, and a storage medium comprising data for displaying images. In particular, but not exclusively, the present invention relates to an apparatus and method for displaying stereoscopic images, and a storage medium comprising data for displaying stereoscopic images.

BACKGROUND OF THE INVENTION

Stereoscopic displays present different two- dimensional (2D) images to the two eyes and thereby create a compelling three-dimensional (3D) sensation. They are being developed for numerous applications including cinema, television, virtual prototyping, and medical imaging.

Despite the clear advantages of stereoscopic displays, there are some well-known problems. Stereoscopic displays cause perceptual distortions, performance decrements, and visual fatigue. These problems occur because some of the presented depth cues (i.e. perspective and binocular disparity) specify the intended 3D scene, while focus cues (blur and accommodation) are determined only by the fixed distance of the display itself.

Figure 1 illustrates the differences between viewing the real world and viewing a conventional stereoscopic display. Figure la shows a plan view of a viewer's left and right eyes (indicated as circles towards the bottom of the figure) and two objects in the natural environment. In natural viewing, images arrive at the eyes with varying binocular disparity, so that a viewer must adjust the eyes' vergence angle (the angle between the lines of sight) as he looks from one point to another. In Figure la the viewer is fixating the far object and not the near object. The lines of sight to the far object intersect at the far object. The distance from the viewer to the point at which the lines of sight intersect is the vergence distance. The viewer also adjusts the focal power of the lens in each eye (i.e. accommodates) appropriately for the fixated part of the scene (i.e. where the eyes are looking) . The distance to which the eye must be focused to create a sharp retinal image is the focal distance; the distance to which the eyes are focussed is the accommodation distance. Variations in focal distance create differences in image sharpness. Figure lc shows a photograph of two objects like the ones depicted in Figure la, with the camera focussed on the far object. The near object and the nearer parts of the ground plane appear blurred.

Vergence and accommodation responses are neurally coupled: that is, changes in vergence drive changes in accommodation (vergence accommodation) and changes in accommodation drive changes in vergence (accommodative vergence) . This is advantageous in natural viewing because vergence distance and focal distance are nearly always identical.

In conventional stereoscopic displays, binocular disparity in the images simulates changes in vergence as happens in natural viewing, but the focal distance remains fixed at the display distance. Thus, the natural correlation between vergence and focal distance is disrupted. Figure lb shows a simulation of a conventional stereoscopic display of the same pair of objects as in Figure la. The display screen is at the same distance as the simulated far object so the vergence and focal distance of the image of the far object are the same as in Figure la. However, the near object is presented on the display screen so its focal distance is no longer equal to the vergence distance, resulting in vergence-accommodation conflict and incorrect blur. Figure Id shows a photograph of the two objects in which the focal distance is effectively the same as in a conventional stereoscopic display. The image of the near object and the ground plane are sharp.

Disruption of the natural correlation between vergence and focal distance causes several problems. Firstly, perceptual distortions occur due to the conflict between the binocular disparity and the focus information. Secondly, difficulties in simultaneously fusing and focusing a stimulus occur because the viewer must now adjust vergence and accommodation to different distances. If accommodation is accurate, he/she will see the object clearly, but may see double images; if vergence is accurate, the viewer will see one fused object, but it may be blurred. Thirdly, visual discomfort and fatigue occur as the viewer attempts to adjust vergence and accommodation appropriately. Fig. le is a plot showing the range of vergence-accommodation conflicts that can be handled without discomfort. The abscissa represents the simulated distance and the ordinate represents the focal distance. Stimuli which fall within the shaded zone will be comfortable to fuse and focus. Conflicts large enough to cause discomfort are commonplace with near viewing.

Because of these problems, there has been increasing interest in creating stereoscopic displays that minimize the conflict between simulated distance cues and focus cues. Several approaches have been taken to constructing such displays, but they fall into two categories: 1 ) wave-front reconstructing displays and 2) volumetric displays. To date, none of these approaches are widely used due to some significant limitations.

Wave-front reconstructing displays, such as holograms, present correct focus information but require extraordinary resolution, computation, and optics that make them currently impractical.

Volumetric displays present scene illumination as a volume of light sources and have been implemented as a swept-volume display by projecting images on to rotating display screen, and with a stack of liquid-crystal panels. Each illumination point naturally provides correct disparity and focus cues, so the displays do not require knowledge of the viewer's gaze direction or accommodative state. However, they prevent correct handling of view-dependent lighting effects such as specular highlights and occlusions for more than a single point, and these displays require a huge number of addressable voxels, which limits their spatial and temporal resolution and restricts their workspace.

By restricting the viewing position, these displays become fixed-viewpoint volumetric displays. By fixing the viewpoint, the graphics engineer can separate the simulated 3D scene into a 2 D projection and a depth associated with each pixel. The 2 D resolution of the human visual system is approximately 50 cpd (cycles per degree visual angle) ; by industry standards the 2 D resolution of an adequate display system is about half that value. Under optimal conditions, viewers can discriminate changes in focal distance of -1/ 3D , so the focal-depth resolution of an adequate display can be relatively coarse. Thus the number of voxels that must be computed for a fixed-viewpoint display is a small fraction of that needed for a multiple-viewpoint display, which requires high resolution in all three dimensions. Presenting the light sources at different focal distances in fixed-viewpoint, volumetric displays has been done in various ways, using: a deformable mirror to change the focal distance of parts of the image, a set of three displays combined at the viewer' s eyes via beam splitters, a translating micro-display, a translating lens between the viewer and display, and a non- translating lens that changes focal power. Solutions based on transmissive optics are more desirable if the device is to be miniaturized to be made wearable. The translating micro-display, deformable mirror, and translating lens require mechanical movements that greatly limit the size of the workspace and the speed of changes in focal distance. In all of these designs, it would be very challenging, if not impossible, to miniaturize them sufficiently to produce a practical, wearable device.

A fixed-viewpoint, volumetric, stereoscopic display system is described in Gordon D. Love et al., "High-speed switchable lens enables the development of a volumetric stereoscopic display", Optics Express Vol. 17, No.18, pp. 15716-15725, 31 August 2009. In that system, the scene to be displayed is divided into discrete depths, or depth planes corresponding to different ranges of distances in the simulated scene (i.e. distances from the viewer). Instead of displaying the whole image simultaneously, as is done on conventional displays, each depth plane is displayed at a different time. A stationary, switchable lens assembly placed in front of the eye is synchronized to the graphic display to be consistent with the presentation of the appropriate depth plane. In this way, a temporally-multiplexed image is constructed. The system can be constructed either using two lens assemblies and one CRT display, presenting separate images to the two eyes in a time sequential fashion, or using two CRTs and lens assemblies, a pair for each eye, presenting separate images simultaneously to the two eyes .

When the most distant parts of the scene are displayed, the lens assembly is switched to its shortest focal length, so that the eyes have to accommodate far to create sharp retinal images. When near parts of the scene are displayed, the lens system is switched to longer focal lengths so that the eye must accommodate to closer distances to create sharp images. It is not necessary to know where the viewer' s eye is focussed in order for the correct focus cues to be generated. If the viewer accommodates far, the distant parts of the displayed scene are sharply focused on the retinas and the near parts are blurred. If the viewer accommodates near, distant parts are blurred and near parts are sharp. In this way, focus cues - blur in the retinal image and accommodation - are nearly correct. The mismatch between vergence and accommodation is eliminated because the focal state matches the image content, thereby significantly reducing visual discomfort.

Since a finite number of focal planes are used, depth-weighted blending is used to assign image intensities to the depth planes: the image intensity at each focal plane is weighted according to the dioptric distance of the point from the plane. For example, image points representing an object at the dioptric midpoint between two focal planes are illuminated at half intensity on the two planes. The corresponding pixels on the two planes lie along a line of sight so they sum in the retinal image to form an approximation of the image that would occur when viewing a real object at that distance. In this system, depth-weighted blending is crucial to simulating continuous 3D scenes without visible discontinuities between focal planes.

A problem with this system is that the display depends on light from different focal distances being imaged in the appropriate position on the viewer' s retina. The viewpoint must therefore be fixed. If the viewer moves the head, thereby changing the position of the eye relative to the incoming light, the light is not imaged in the correct position and this leads to undesirable effects. The problem is most evident at object boundaries: so-called occlusions. With head movements, pixels from the occluded background that should not be visible become visible, so the region no longer behaves like an occlusion boundary. In other words, breaks in the images become evident.

Another limitation of this system is the switching speed of the lens assembly and refresh rate of the display. Using two CRTs (one for each eye) and a CRT frame rate of 180Hz, the four focal states are presented at 45Hz per eye. With a single CRT the focal states are presented at 22.5 Hz per eye, producing fairly noticeable flicker.

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an apparatus for displaying images, the apparatus comprising:

(i) a display device for displaying an image representing a three-dimensional scene, wherein a depth in said scene is specified as a depth of interest; and

(ii) control means for providing a signal for controlling a focal length of lens means for viewing said image, said signal depending on said depth of interest.

Advantageously, by providing a signal for controlling a focal length of the above-mentioned lens means in dependence on a specified depth of interest, the display provides focus cues which can be used by a viewer to perceive a depth.

The display device may be arranged to display a left-eye image and a right-eye image to provide a stereoscopic image representing said three-dimensional scene . Advantageously, by applying the present invention to stereoscopic displays, discomfort and fatigue can be reduced for viewing objects at the depth of interest in the displayed scene, thereby providing a significant improvement over conventional stereoscopic displays. By specifying a depth of interest, rather than attempting to correct the vergence-accommodation conflict for all depths within the scene, the apparatus of the present invention is less complex and much more practical than the prior art system described above because it is not necessary to divide the image into separate sub-frames corresponding to different depth ranges. As a result, the display can be viewed from multiple angles without breaks at occlusion boundaries. In addition, the focal length of the lens means need only be adjusted at the rate of change of the depth of interest, so will be limited by the rate of change of the image content and/or the rate at which a viewer can change his accommodation distance. Since it is not necessary to adjust the focal length of the lens means with each sub-frame as in the prior art system described above, the apparatus can be used to control lens means having a relatively low switching speed without introducing limitations such as flicker .

The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images corresponds to a vergence distance for fixating on an object at the depth of interest.

Advantageously, this restores a correspondence between vergence and accommodation distance for viewing objects at the depth of interest, reducing problems associated with vergence-accommodation conflict.

The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images is substantially equal to a vergence distance for fixating on an object at the depth of interest.

Advantageously, this restores the natural relationship between vergence and accommodation distance for viewing objects at the depth of interest, substantially reducing vergence-accommodation conflict.

The signal may be for controlling the focal length of said lens means such that a difference between a vergence distance and a focal distance for respectively fixating and focussing on a part of the image corresponding to the depth of interest is reduced.

Advantageously, this reduces problems associated with vergence-accommodation conflict.

The specified depth of interest may be a depth of a predefined object of interest within the three- dimensional scene.

Advantageously, providing a signal for controlling the focal length of said lens means in dependence on the depth of an object of interest within the scene has the effect that a viewer fixating and focussing on an object of interest (for example a user-controlled cursor or tool in a CAD application, or a main character in a film) must accommodate to different distances as the object of interest changes depth. Similarly, if a different object at a different depth within the scene is specified as the object of interest, a viewer must accommodate to a different distance to focus on that object.

The depth of interest may have a fixed value for a sequence of images. Advantageously, this makes the apparatus relatively simple to implement, because the focal length of the lens means is adjusted less frequently. For example, it may be set once for the duration of an image sequence, for example for the duration of an entire film. The lens means focal state could be chosen based on the overall properties of the image contents and the viewing distance. For example, the depth of interest may be set to correspond to a depth where most of the action takes place in film clip.

The specified depth of interest may be an average depth in the three-dimensional scene.

Advantageously, this feature can be implemented to ensure that, on average, accommodation-vergence conflict is reduced.

The depth of interest may be predefined. The depth of interest may be determined in real time .

The scene may comprise a plurality of objects at different depths. Blurring may be applied to areas of the image corresponding to parts of the scene at depths other than the depth of interest.

Advantageously, this may help to guide a viewer towards objects at the depth of interest, for example to an object of interest.

The apparatus may further comprise said lens means for viewing said image.

The lens means may comprise at least one respective lens assembly for viewing each of said left-eye image and said right-eye image.

At least one lens assembly may comprise at least one lens .

According to a second aspect of the present invention, there is provided a method for displaying an image, the method comprising:

(i) displaying an image representing a three- dimensional scene, wherein a depth in said scene is specified as a depth of interest; and

(ii) providing a signal for controlling a focal length of lens means for viewing said image, said signal depending on said depth of interest.

The step of displaying an image representing a three-dimensional scene may comprise displaying a left- eye image and a right-eye image to provide a stereoscopic image representing said three-dimensional scene. The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images corresponds to a vergence distance for fixating on an object at the depth of interest.

The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images is substantially equal to a vergence distance for fixating on an object at the depth of interest. The signal may be for controlling the focal length of said lens means such that a difference between a vergence distance and a focal distance for respectively fixating and focussing on a part of the image corresponding to the depth of interest is reduced.

The specified depth of interest may be a depth of a predefined object of interest within the three- dimensional scene. The specified depth of interest may be an average depth in the three-dimensional scene.

The depth of interest may have a fixed value for a sequence of images.

The depth of interest may be predefined. The method may further comprise a step of determining said depth of interest based on the content of said image. The method may further comprise the steps of:

identifying an object of interest in said scene, determining a depth of said object of interest, and specifying said depth as said depth of interest. The method may further comprise a step of applying blurring to at least one area of the image corresponding to a part of the scene at depths other than the specified depth of interest. The scene may comprise a plurality of objects at different depths.

The method may further comprise a step of controlling the focal length of said lens means for viewing said image, based on said signal.

The step of controlling the focal length of said lens means may comprise controlling the focal length of at least one respective lens assembly for viewing each of said right-eye image and said left-eye image.

According to a third aspect of the present invention, there is provided a computer program for controlling an apparatus for displaying images, the program comprising:

(i) first computer code for causing said apparatus to display an image representing a three-dimensional scene, wherein a depth in said scene is specified as a depth of interest; and (ii) second computer code for causing said apparatus to provide a signal for controlling a focal length of lens means for viewing said image, said signal depending on said depth of interest.

The first computer code may comprise computer code for causing said apparatus to display a left-eye image and a right-eye image to provide a stereoscopic image representing said three-dimensional scene.

The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images substantially matches a vergence distance for fixating on an object at the depth of interest. The signal may be for controlling the focal length of said lens means such that a difference between a vergence distance and a focal distance for respectively fixating and focussing on a part of the image corresponding to the depth of interest is reduced.

The depth of interest may have a fixed value for a sequence of images.

The depth of interest may be predefined.

The program may further comprise third computer code for specifying said depth of interest.

The program may further comprise fourth computer code for determining said depth of interest based on the content of said image.

The program may further comprise:

fifth computer code for identifying an object of interest in said scene,

sixth computer code for determining a depth of said object of interest, and

seventh computer code for specifying said depth as said depth of interest.

The program may further comprise eighth computer code for applying blurring to areas of the image corresponding to at least one part of the scene at a depth other than the specified depth of interest.

The scene may comprise a plurality of objects at different depths.

The program may further comprise ninth computer code for causing said apparatus to control the focal length of said lens means for viewing said image, based on said signal .

The program may further comprise tenth computer code for causing said apparatus to control the respective focal length of at least one respective lens assembly for viewing each of said right-eye image and said left-eye image .

According to a fourth aspect of the present invention, there is provided a data storage medium, comprising

(i) image data corresponding to an image representing a three-dimensional scene; and

(ii) depth data specifying a depth of interest in said three-dimensional scene.

The image data may correspond to a right-eye image and a left-eye image for providing a stereoscopic image representing said three-dimensional scene.

According to a fifth aspect of the present invention, there is provided a method for generating image data for an apparatus as defined above, comprising the steps of:

providing image data corresponding to an image representing a three-dimensional scene;

specifying a depth of interest in said scene; and writing depth data, corresponding to said depth of interest, and said image data to a data file.

The image data may correspond to a left-eye image and a right-eye image for providing a stereoscopic image representing said three-dimensional scene. The depth of interest may be determined based on the content of said image data. The method may further comprise the steps of:

identifying an object of interest in said scene, determining a depth of said object of interest, and specifying said depth as said depth of interest. The method may further comprise a step of applying blurring to at least one area of the image corresponding to a part of the scene at a depth other than the specified depth of interest. According to a sixth aspect of the present invention, there is provided a computer program for generating image data for an apparatus as defined above, the program comprising:

first computer code for providing image data corresponding to an image representing a three- dimensional scene;

second computer code for specifying a depth of interest in said scene; and

third computer code for writing depth data, corresponding to said depth of interest, and said image data to a data file.

The image data may correspond to a left-eye image and a right-eye image for providing a stereoscopic image representing said three-dimensional scene.

The depth of interest may be determined based on the content of said image data. The program may further comprise:

fourth computer code for identifying an object of interest in said scene; and

fifth computer code for determining a depth of said object of interest, and

sixth computer code for specifying said depth as said depth of interest.

The program may further comprise seventh computer code for applying blurring to at least one area of the image corresponding to a part of the scene at depths other than the specified depth of interest.

According to a seventh aspect of the present invention, there is provided an apparatus for displaying images, the apparatus comprising:

(i) a display device for displaying a plurality of images for providing a composite image representing a three-dimensional scene, wherein each image is associated with a different depth range of the scene; and

(ii) control means for providing a signal for controlling a focal length of lens means for viewing said images, in dependence on said depth range associated with the image currently displayed, wherein each image represents the same scene, differing in that, in each image, blurring is applied to at least one area of the image corresponding to a part of the scene at a depth outside of the associated depth range. Advantageously, by providing plural images representing the same scene, differing only in the blurring applied to the images, the display can be viewed from multiple viewing angles without breaks appearing at occlusion boundaries. In one embodiment of said apparatus, said display device is arranged for displaying a plurality of image pairs, wherein a plurality of said image pairs each comprises a left-eye image and a right-eye image for providing a stereoscopic image representing the three- dimensional scene, wherein each pair of images is associated with a different depth range of said scene; and said control means is arranged for providing a signal for controlling a focal length of said lens means for viewing said left-eye images and said right-eye images, in dependence on said depth range associated with the image pair currently displayed; wherein each image pair provides a stereoscopic image representing the same scene, differing in that, in each image pair, blurring is applied to at least one area of the image corresponding to a part of the scene at a depth outside of the associated depth range. ^■ The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said image pairs corresponds to a vergence distance for fixating a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said image pairs is substantially equal to a vergence distance for fixating on a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

The images may be displayed sequentially so that a viewer perceives a the plurality of images as a single composite image.

The left-eye images and the right-eye images of the plurality of image pairs may each be displayed sequentially so that a viewer perceives a single right- eye image and a single left-eye image, which combine to provide a stereoscopic composite image.

The apparatus may further comprise said lens means for viewing said images.

Said lens means may comprise a respective lens assembly for viewing each of said left-eye image and said right-eye image.

At least one lens assembly may comprise at least one lens .

According to an eighth aspect of the present invention, there is provided a method for displaying images, the method comprising the following steps:

(i) displaying a plurality of images for providing a composite image representing a three-dimensional scene, wherein each image is associated with a different depth range of the scene; and

(ii) providing a signal for controlling a focal length of lens means for viewing said images, in dependence on said depth range associated with the image currently displayed, wherein each image represents the same scene, differing in that, in each image, blurring is applied to at least one area of the image corresponding to at least one part of the scene at a depth outside of the associated depth range.

In one embodiment of said method, the step of displaying a plurality of images comprises displaying a plurality of image pairs, wherein a plurality of said image pairs each comprises a left-eye image and a right- eye image for providing a stereoscopic image representing the three-dimensional scene, wherein each pair of images is associated with a different depth range of the scene; wherein each image pair provides a stereoscopic image representing the same scene, differing in that, in each image pair, blurring is applied to at least one area of the image corresponding to at least one part of the scene at a depth outside of the associated depth range.

The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said plurality of image pairs corresponds to a vergence distance for fixating a part of the image corresponding to a part of the scene at the depth range associated with that image pair .

The images may be each displayed sequentially so that a viewer perceives a single composite image.

The left-eye images and the right-eye images of the plurality of image pairs may be each displayed sequentially so that a viewer perceives a single composite right-eye image and a single composite left-eye image, which combine to provide a composite stereoscopic image .

According to a ninth aspect of the present invention, there is provided a computer program for controlling an apparatus for displaying images, the program comprising:

(i) first computer code for causing said apparatus to display a plurality of images for providing a composite image representing a three-dimensional scene, wherein each image is associated with a different depth range of the scene; and

(ii) second computer code for causing said apparatus to provide a signal for controlling a focal length of lens means for viewing said images, in dependence on said depth range associated with the image currently displayed, wherein each image represents the same scene, differing in that, in each image, blurring is applied to at least one area of the image corresponding to at least one part of the scene at a depth outside of the associated depth range.

According to one embodiment of the program, said first computer code comprises computer code for causing said apparatus to display a plurality of image pairs, wherein each a plurality of said image pairs each comprises a left-eye image and a right-eye image for providing a stereoscopic image representing a three- dimensional scene, wherein each image pair is associated with a different depth range of the scene; wherein each image pair provides a stereoscopic image representing the same scene, differing in that, in each image pair, blurring is applied to at least one area of the image corresponding to at least one part of the scene at a depth outside of the associated depth range.

The signal may be for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said plurality of image pairs corresponds to a vergence distance for fixating a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

The images may be displayed sequentially so that a viewer perceives a single composite image. The left-eye images and the right-eye images of the plurality of image pairs may be each displayed sequentially so that a viewer perceives a single right- eye image and a single left-eye image, which combine to provide a composite stereoscopic image.

According to a tenth aspect of the present invention, there is provided a method for generating image data for an apparatus as defined above, comprising the steps of:

providing image data corresponding to an image representing a three-dimensional scene; and

creating a plurality of versions of said image, each of said versions being associated with a different range of depths in the three-dimensional scene, wherein, in each image, blurring is applied to at least one area of the image corresponding to a part of the scene at a depth outside of the respective depth range associated with the respective image.

In one embodiment of the method, the step of providing image data comprises providing image data corresponding to a pair of images comprising a left-eye image and a right-eye image for providing a stereoscopic image representing the three-dimensional scene; and the step of creating a plurality of versions of said image comprises creating a plurality of versions of said pair of images, each of said versions being associated with a different range of depths in the three-dimensional scene, wherein, in each pair of images, blurring is applied to at least one area of the images corresponding to a part of the scene at a depth outside of the respective depth range associated with the respective pair of images. According to an eleventh aspect of the present invention, there is provided a computer program for generating image data for an apparatus as defined above, the program comprising:

first computer code for providing image data corresponding to an image representing a three- dimensional scene; and

second computer code for creating a plurality of versions of said image, each of said versions being associated with a different range of depths in the three- dimensional scene, wherein, in each image, blurring is applied to at least one area of the image corresponding to a part of the scene at a depth outside of the respective depth range associated with the respective image.

In one embodiment of the program, said first computer code comprises computer code for providing image data corresponding to a pair of images comprising a left- eye image and a right-eye image for providing a stereoscopic image representing the three-dimensional scene; and said second computer code comprises computer code for creating a plurality of versions of said pair of images, each of said versions being associated with a different range of depths in the three-dimensional scene, wherein, in each pair of images, blurring is applied to at least one area of the images corresponding to a part of the scene at a depth outside of the respective depth range associated with the respective pair of images.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

Figure 1 illustrates the difference between viewing the real world and viewing a conventional stereoscopic display;

Figure 2 shows a schematic representation of an apparatus according to a first embodiment of the present invention;

Figure 3 shows a schematic representation of a lens assemb1 ;

Figure 4 shows a schematic representation of an apparatus according to a second embodiment of the present invention; and

Figure 5 illustrates a third embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 2 shows an apparatus 10 for displaying stereoscopic images, according to a first embodiment of the invention. The apparatus 10 includes a display device, in the form of a computer 14 coupled to a CRT monitor 16, which displays stereoscopic images of a three-dimensional scene on the monitor 16.

Separate images are displayed for the left eye 18a and right eye 18b. In this embodiment, the left- and right- eye images are presented time-sequentially to the two eyes 18a, 18b by using liquid-crystal shutter glasses 20a, 20b that alternately block and pass light to the respective eye 18a, 18b. A photosensor 22 detects light from the monitor 16 to enable a shutter controller 24 to synchronize the shutter glasses 20 with the monitor 16. With the monitor 16 running at 180 Hz, the refresh rate is therefore 90 Hz per eye.

As in conventional stereoscopic displays, binocular disparity between the left-and right-eye images simulates depth in the image, because a viewer's eyes 18a, 18b must converge to different depths to fixate objects at different depths in the scene. Thus the vergence distance for fixating an object indicates the relative depth of that object in the 3D scene. Other depth cues such as disparity and occlusion may be used to enhance this effect.

The entire 3D scene is presented to each eye 18a, 18b in a single respective two-dimensional (2D) image. This simplifies the apparatus compared to the prior art described above in which the scene is divided into four separate images or sub-frames containing parts of the original image corresponding to parts of the scene at different depth ranges.

The image displayed on the monitor 16 is viewed through two switchable lens assemblies 26a, 26b, one for each eye 18, 18b. A lens controller 28 controls the focal lengths of the lens assemblies 26a, 26b in response to a signal received from the computer 14. The focal lengths of the two lens assemblies 26a, 26b are controlled in dependence on a specified depth of interest of the displayed scene. This enables a reduction in the vergence-accommodation conflict experienced by a viewer.

The depth of interest is specified with reference to the content of the scene, and corresponds to a depth of parts of the scene at which a viewer is most likely to fixate their view. The depth of interest may correspond to the depth of an object of interest in the displayed scene, and will depend on the application. Here are some examples :

(a) In the case of a 3D movie, the object of interest would usually be defined by the cinematographer, and may correspond to a main character in a scene consisting of the character and other foreground and background objects. The object of interest could be identified during movie production and information corresponding to the depth of the object of interest (i.e. the depth of interest) attached to the film as met- data. Alternatively, salience-extraction algorithms could be used to perform an estimation of the object of interest automatically; the depth of the object of interest could then be computed and used to set the focal power of the lens assemblies 26a, 26b.

(b) In a CAD/CAM application, a user is generally working on a particular part of the scene with a tool. The depth of the tool could therefore be used as the depth of interest used for setting the focal power of the lens assemblies 26a, 26b.

(c) In remote surgery, the surgeon is generally working with a tool in a particular part of the viewable area, so the depth of the tool could be used to set the focal power of the lens assemblies 26a, 26b.

(d) In a video game, object of interest is usually the player's target at a given moment. The depth of the target could therefore be extracted and used to set the focal power of the lens assemblies 26a, 26b.

(e) In 3D imaging, for example medical imaging, the object of interest may be the object of interest may be a user-controlled cursor, such that the depth of interest is the depth of parts of the scene next to the cursor. The lens controller 28 enables the focal length of the lens assemblies 26a, 26b to be adjusted in response to a change of the depth of interest. For example, the lens assemblies 26a, 26b may be controlled so that a viewer's focal distance for focussing on an object of interest must change as it moves to different depth, for example as a main character in a 3D movie moves from the foreground towards the background of a scene. In addition, the lens assemblies 26a, 26b may be controlled so that a viewer's focal distance must change to focus on a new object of interest when the object of interest switches to a new object of interest at a different depth, such as might occur when a new character enters the scene, or when there is a change of scene.

Parts of the image' which are not perceived at the depth of interest, or parts of the image other than an object of interest, may be rendered with blur so that the viewer is guided to fixate on the object of interest.

Alternatively, the depth of interest may have a fixed value for the duration of a sequence of images. Instead of switching focal state from moment to moment as the content of displayed image frames varies, the lens assemblies 26a, 26b would remain at a fixed focal length for the duration of an image sequence. The depth of interest may be specified based on the overall contents of the image sequence, for example an average depth of the images or of objects of interest in the images. If the focal power of the lens assemblies 26a, 26b does not change from moment to moment, the apparatus will still provide an improvement of the accommodation-vergence conflict, on average, compared to a conventional stereoscopic display. However, individual frames may generate a large vergence-accommodation conflict as occurs with conventional displays. This approach has the advantage of simplicity of implementation and use. The focal power of the lens assemblies 26a, 26b may be set manually or automatically prior to viewing the image sequence .

The focal length of the lens assemblies 26a, 26b is controlled so that the lens assemblies 26a, 26b reduce a viewer's focal distance for focussing on the image on the monitor 16 if the vergence distance for fixating an object at the depth of interest is less than the distance from the eyes 18a, 18b to the monitor 16 (i.e. if the object of interest appears to be in front of the 16 screen on which the image is displayed) , and increases the focal distance for focussing on the image if the vergence distance for fixating an object at the depth of interest is greater than the distance from the viewer to the 16 (i.e. if the object of interest appears to be behind the monitor screen 16 on which the image is displayed) . In this way, the difference between vergence distance and focal distance for respectively fixating and focussing on a part of the images corresponding to the depth of interest is reduced.

Ideally, the lens assemblies 26a, 26b are controlled so that the focal distance for focussing on the object of interest (or on parts of the scene located at the depth of interest) is substantially equal to the vergence distance for fixating the object of interest, so that the vergence-accommodation conflict becomes zero and visual discomfort is minimized when a viewer accommodates to the object of interest. In a multiple viewpoint situation, this may require information regarding a viewer' s head position, in particular a viewing distance, to be input to the apparatus in order for the focal distance to be correctly adjusted. Alternatively, in a multiple viewer context such as a cinema, the lens assemblies 26a, 26b may be controlled so that the focal distance is substantially equal to the vergence distance for the object of interest as viewed from a typical viewing distance. The vergence-accomrnodation conflict would then become zero for viewers at the typical viewing distance, but there may remain some vergence-accomrnodation conflict for other viewers. Nonetheless, the establishment of a correspondence between focal distance and vergence distance will improve viewing comfort.

With reference to Figures 2 and 3, each lens assembly 26a, 26b comprises a polarizer 30, two fixed birefringent lenses 32, 34 and two polarization modulators 36, 38 in the form of ferroelectric liquid- crystal modulators (FLCs) . Birefringent materials have two refractive indices (ordinary and extraordinary) depending on the polarization of the incident light, so the lens has two focal lengths that are selected with a polarization modulator. If a birefringent lens is arranged such that the extraordinary axis is vertical and the ordinary axis is horizontal, incoming vertically polarized light is focused at a distance corresponding to the extraordinary refractive index. If the light's polarization axis is rotated to horizontal before the lens, it is focused at the distance corresponding to the ordinary index. The FLCs 34, 36 are used to switch the polarization orientation of the light prior to each lens 30, 32. They act like half-wave plates whose optical axis can be rotated through -45°. They are arranged so that the incident polarization is either aligned with or at 45° to the optical axis so that the output polarization is rotated by either 0° or 90°. The first polarizer 30 produces vertically polarized light, which is then either rotated or not through 90° by the first FLC 36. The first lens 32 focuses the two polarization states differently. The second FLC 38 and second lens 34 produce two more possible focal lengths for each of the polarisation states, creating four focal states in total, as illustrated in Figure 3. The switching between focal lengths can occur very quickly (<lms) . More lenses and polarizations modulators can be stacked to provide finer control of focal length: by stacking N lenses and polarization modulators, a lens assembly having 2N focal lengths can be achieved.

The lens material is calcite, which has the advantages of transparency, high birefringence (0.172) in the visible light range, and machinability . The lenses are plano-convex with a diameter of 20mm. The convex surfaces have radii of curvature of 143.3 and 286.7mm, so the four focal powers are 5.09, 5.69, 6.29, and 6.89 diopters (D), and the separations are 0.6D. A fixed glass lens (not shown) allows adjustment of the whole focal range. The number and dioptric separation of the focal states are important design features. With N focal states and average separations of Δ, the workspace is (2^N - 1) N Δ, which in the illustrated apparatus is 1.8D. The separations Δ can be unequal, which might be advantageous for some applications. The separations Δ can be reduced by increasing the number of focal lengths of the lens assemblies 26a, 26b, to reduce any apparent discontinuities between the focal states of the lens assemblies. However, it may not be necessary to use a lens assembly 26a, 26b capable of switching at such high-speed in all applications of the present embodiment, because the required change in focal length of the lens assembly will vary relatively slowly in most envisaged applications, since it depends on the rate of change of the content of the displayed image sequence (e.g. the forward/rearward speed of a character in a movie scene, or of a user-controlled tool in a graphics software application) . This contrasts with the prior art apparatus described above, in which the maximum lens speed and CRT refresh rate imposed a limitation on the number of sub-frames which could be time-multiplexed without producing noticeable flicker. As a result, other types of switchable lens assemblies can be used.

The skilled person will appreciate that the lens assembly described above is given as an example only, and that various other types of lenses may be used. Other suitable types of lenses include liquid lenses (based on electrowetting) , switchable liquid crystal diffractive lenses, and other types of liquid crystal lenses. Switchable lenses are commercially available from companies such as Varioptic SA (www . varioptic . com) and PixelOptics, Inc. (www .pixeloptics. com) . The lens assembly may comprise one or more switchable lenses, rather than the stack of lenses and polarization modulators described above.

The lens controller 28 receives a control signal for controlling the focal power of the lens assemblies 26a, 26b from the computer 14, the control signal being synchronised with the content of the displayed image. The signal is transmitted wirelessly to the lens controller 28, for example using RF or optical/IR transmission . In order to provide an appropriate signal for controlling the focal length of the lens assemblies 26a, 26b, it is necessary to specify the depth of interest in the three-dimensional scene. The depth of interest may be determined manually or automatically. Automatic determination of the depth of interest in real-time is appropriate for interactive applications where the depth of interest depends on interaction by a user, such as CAD applications, remote surgery and some video games. For a film, the object of interest in each frame or group of frames may be predefined by the cinematographer manually and/or automatically during the post-production stage. Automatic specification of the depth of interest may be performed by identifying an object of interest using a salience-extraction algorithm, then calculating the depth of that object based on the disparity between the left and right images. In both cases, information corresponding to the depth of the object of interest (i.e. the depth of interest) could be distributed with the image/video data as metadata. When displaying the film for example in a cinema, this metadata can be used to send the appropriate control signal to the lens controller 28 so that the focal power of the lens assemblies 26a, 26b are set and adjusted in synchronisation with the displayed video content. However, the depth of interest information could also be determined retro-actively for conventional 3D films, and the apparatus could be equipped with software for processing the film data to automatically extract the depth data. Once the depth of interest has been identified, the required focal power of the 26a, 26b assemblies can be determined, and an appropriate signal output to the lens controller 28 for adjusting the focal length of the lens assemblies 26a, 26b.

One of the advantages of the present invention over the prior art system described above is that it is multi- viewpoint, so can be viewed by multiple viewers, for example in a cinema. Although Figure 2 shows only a single pair of lens assemblies 26a, 26b, the skilled person will appreciate that the apparatus can include many pairs of lens assemblies, e.g. one per viewer, each controlled by the same signal.

The embodiment of the present invention described above retains the advantages of the prior art system described above, compared to conventional stereoscopic displays. In particular, it creates the appropriate relationship between accommodation distance (focal distance) and depth so that, as the viewer looks to different parts of the scene, the correct relationship between accommodation and blur is established. It reduces or eliminates the vergence-accommodation conflict that causes visual discomfort when viewing stereoscopic displays .

However, the embodiment described above also has several advantages compared to the prior art system described above. By specifying a single depth of interest in each 3D scene, it does not require the left- and right- eye images of the 3D scene to be each divided into sub-frames corresponding to different depth ranges to be displayed sequentially. As a result, the image generation becomes much easier than in the prior art system described above because the conventional techniques can be used. Furthermore, flicker and motion artifacts become less visible because the effective frame rate is higher. Because the entire scene is displayed in a single image, no image break-up occurs as the viewer's head moves. As a result, head position does not have to be constrained or tracked. Finally, there is no longer a requirement for fast lens-switching. In the prior art system described above, the lens had to switch very quickly so that sub-frames corresponding to different depth ranges could be presented at a high enough rate to avoid noticeable flicker and motion artifacts. With the present invention, the lens assemblies 26a, 26b only need to be adjusted as fast as the eye can accommodate, and this allows a much slower switching rate.

An apparatus 10' according to a second embodiment of the invention is shown in Figure 4. The apparatus 10' is similar to the apparatus 10 of the first embodiment described above, but the left-eye and right-eye images are shown simultaneously on separate CRTs 16a, 16b, one for each eye 18a, 18b, rather than being time-multiplexed on a single CRT 16. Light from the each of the two CRTs 16a, 16b is reflected towards the respective eye 18a, 18b by a respective prism 40a, 40b. This embodiment therefore does not require any shutter glasses or shutter controller. In this embodiment, the refresh rate for each eye is equal to the refresh rate of the CRT, that is, twice as high as in the first embodiment.

With reference to Figure 5, an apparatus and method according to a third embodiment of the invention will now be described. The set up for the third embodiment may be configured as shown in Figures 2 and 4 , and differs from the first and second embodiments only in its method of operation. Instead of displaying a single respective image for each of the left-eye 18a and right-eye 18b, four pairs of images or sub-frames are displayed in a time-multiplexed manner, pair of images comprising a right-eye image and a left-eye image which combine to provide a stereoscopic image of a three-dimensional scene. Each pair of images is associated with a different depth range of the scene. Each retinal image is a composite image based on the sum of the four images or sub-frames over time. Each of the four pairs of images provides a stereoscopic image of the same simulated scene, but is rendered with different blurring. In each image pair, regions of the images corresponding to parts of the scene outside the depth range associated with the respective image pair are blurred. As a result, each image pair provides a stereoscopic image of the entire simulated scene, but only parts of the scene at the depth range associated with that image pair are rendered clearly. Other parts of the scene shown in that image pair are rendered blurred.

The images are viewed through the lens assemblies 26a, 26b described above. The lens assemblies 26a, 26b are switched between the four focal states (illustrated in Figure 3) in synchronization with the display so that the focal length of the lens assembly depends on the depth range associated with the image pair currently displayed . Because the lens assemblies 26a, 26b are set to a different focal length when each of the image pairs is displayed, the viewer must accommodate to a different distance in order to focus on each image pair. This is illustrated in Figure 5. Each image pair is associated with a respective depth range of the scene: "far", "far- mid", "mid-near" and "near". Although each image pair is displayed on the screen of the monitor 16, the viewer's eyes 18 must accommodate to a corresponding distance, labelled "far", "far-mid", "mid-near" and "near", to focus on the corresponding image pair.

When the image pair associated with the "far" depth range is displayed, the lens assemblies 26a, 26b are switched to their shortest focal length, so that the eyes 18a, 18b have to accommodate far to create sharp retinal images of the displayed image pair. When an image pair associated with the "near" depth range is displayed, parts of the scene is displayed, the lens assemblies 26a, 26b are switched to longer focal lengths so that the eyes 18a, 18b must accommodate closer to focus on the displayed image pair.

Thus, if the viewer accommodates near, a sharp retinal image of the image pair associated with the "near" depth range is created, while the image pairs associated with more distant depth ranges will be blurred at the retina. In addition, as a result of the blurring applied to the "near" image pair, parts of the "near" image pair which correspond to more distant depth ranges of the scene will appear blurred. Therefore, the viewer will only perceive sharp images of parts of the scene located at the "near" depth range. Likewise, if the viewer accommodates to the "far" position, only parts of the scene corresponding to the "far" depth range appear sharp: in the "far" image pair, on which the viewer is focussed, the other depth ranges of the scene are rendered blurred, while other image pairs are out-of-focus because of the distance to which the eye is accommodated and therefore blurred in the retinal image. The visual system suppresses the blurred parts of the image, so the viewer will perceive approximately correct focus cues - i.e. accommodation and blur in the retinal image. In other words, when a viewer looks at a given object in the scene, he will be presented with four copies of the scene, only one of which will be sharp, the others having been treated by the blurring effect. The vergence distance for viewing the object is the same for each image pair, but the focal distance is different for focussing on each image pair. The vergence-accommodation conflict will be minimised when focussing on the sharp image. The viewer will therefore suppress the blurred versions and will focus on the image pair for which the fixated object appears sharp. This third embodiment of the invention therefore simulates approximately correct blur and accommodation, in a similar manner to that of the prior art system described above. However, it provides a significant advantage relative to that system, in that it permits viewing from multiple viewpoints. In the prior art system, the image was divided into four sub-frames, one for each depth range. As a result, breaks became apparent in the overall image if the viewpoint was not controlled precisely. In contrast, the present invention displays the entire scene in each image pair, so that breaks between the image pairs cannot arise.

Since a finite number of focal planes are used, depth-weighted blending can be used to assign image intensities to the depth planes, and/or to vary the amount of blurring. The image intensity and/or amount of blurring at each focal plane may be varied according to the dioptric distance of the point from the plane.

Although this embodiment has been described in terms of four image pairs, and four focal states of the lens assemblies, the skilled person will appreciate that this can be increased by including more lenses and polarizations modulators in the lens assemblies 26a, 26b to increase the number of focal states.

The skilled person will appreciate that, although the above embodiments include one or two CRTs for displaying the stereoscopic images, other types of display device may be used, for example LCD or OLED displays, DLP and LCOS projectors, or head-mounted displays. The left- and right-eye images may be presented simultaneously on a single screen, for example in different colours and viewed through filters.

Although the embodiments described above include a respective lens assembly for viewing each of the left-eye and right-eye images, the invention could be implemented using a single lens assembly for some applications.

The first to third embodiments described above relate to stereoscopic displays, but the skilled person will appreciate that the present invention is applicable to displays in general, including those which may be viewed using one eye only (monocular displays) . The eye/brain receives information indicating depth from many cues. The two major cues are binocular disparity and focus. Conventional 3D displays show the correct disparity, but ignore focus. The embodiments described above aim to provide focus cues, in addition to showing the correct disparity. However, alternative embodiments of the present invention based on monocular systems may ignore disparity, but present focus cues. That is, by viewing the above-described embodiments with only one eye (i.e. viewing only the left-eye images or only the right- eye images), a viewer would perceive depth as indicated by the focal distance for focusing on the image (or, in the case of the third embodiment, the focal distances for focusing on the plurality of sub-frames).

Any of the first to third embodiments described above may be modified to display only one set of images, instead of displaying pairs of images comprising a left- eye image and a right-eye image. The single set of images may be viewed using one eye only. Alternatively, the same image could be displayed to the left and right eyes. The lens assembly or assemblies would be controlled in the same way as described above. That is, the focal length of the lens assembly could be controlled in dependence on a depth of interest, as in the first and second embodiments. Alternatively, the focal length may be controlled in dependence on the depth range associated with a currently displayed image or sub-frame, as in the third embodiment.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.

Claims

1. An apparatus for displaying images, the apparatus comprising :

2. An apparatus according to claim 1, wherein said display device is arranged to display a left-eye image and a right-eye image to provide a stereoscopic image representing said three-dimensional scene.

3. An apparatus according to claim 2, wherein said signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images corresponds to a vergence distance for fixating on an object at the depth of interest.

4. An apparatus according to claim 2 or claim 3, wherein said signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images substantially matches a vergence distance for fixating on an object at the depth of interest.

5. An apparatus according to any of claims 2 to 4, wherein said signal is for controlling the focal length of said lens means such that a difference between a vergence distance and a focal distance for respectively fixating and focussing on a part of the image corresponding to the depth of interest is reduced.

6. An apparatus according to any of the preceding claims, wherein the specified depth of interest is a depth of a predefined object of interest within the three-dimensional scene.

7. An apparatus according to any of claims 1 to 5, wherein the depth of interest has a fixed value for a sequence of images.

8. An apparatus according to any of claims 1 to 5 or 7, wherein the specified depth of interest is an average depth in the three-dimensional scene.

9. An apparatus according to any of the preceding claims, wherein said depth of interest is predefined.

10. An apparatus according to any of the preceding claims, wherein said depth of interest is determined in real time.

11. An apparatus according to any of the preceding claims, wherein the scene comprises a plurality of objects at different depths.

12. An apparatus according to any of the preceding claims, wherein blurring is applied to areas of the image corresponding to parts of the scene at depths other than the specified depth of interest.

13. An apparatus according to any one of the preceding claims, further comprising said lens means for viewing said image.

14. An apparatus according to claim 13 and claim 2, wherein said lens means comprises at least one respective lens assembly for viewing each of said left-eye image and said right-eye image.

15. An apparatus according to claim 14, wherein at least one said lens assembly comprises at least one lens.

16. A method for displaying an image, the method comprising :

17. A method according to claim 16, wherein the step of displaying an image representing a three-dimensional scene comprises displaying a left-eye image and a right- eye image to provide a stereoscopic image representing said three-dimensional scene.

18. A method according to claim 17, wherein said signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images corresponds to a vergence distance for fixating on an object at the depth of interest.

19. A method according to claim 17 or claim 18, wherein said signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images is substantially equal to a vergence distance for fixating on an object at the depth of interest .

20. A method according to any of claims 17 to 19, wherein said signal is for controlling the focal length of said lens means such that a difference between the vergence distance and a focal distance for respectively fixating and focussing on a part of the image corresponding to the depth of interest is reduced.

21. A method according to any of claims 16 to 20, wherein the specified depth of interest is a depth of a predefined object of interest within the three- dimensional scene.

22. A method according to any of claims 16 to 20, wherein the specified depth of interest is an average depth in the three-dimensional scene.

23. A method according to any of claims 16 to 20 or 22, wherein the depth of interest has a fixed value for a sequence of images.

24. A method according to any of claims 16 to 23, wherein said depth of interest is predefined.

25. A method according to any of claims 16 to 24, further comprising a step of determining said depth of interest based on the content of said image.

26. A method according to any of claims 16 to 25, further comprising the steps of:

identifying an object of interest in said scene, determining a depth of said object of interest, and specifying said depth as said depth of interest.

27. A method according to any of claims 16 to 26, further comprising a step of applying blurring to at least one area of the image corresponding to a part of the scene at a depth other than the specified depth of interest .

28. A method according to any of claims 16 to 27, wherein the scene comprises a plurality of objects at different depths.

29. A method according to any of claims 16 to 28, further comprising a step of controlling the focal length of said lens means for viewing said image, based on said signal .

30. A method according to claim 29 and claim 17, wherein the step of controlling the focal length of said lens means includes controlling a respective focal length of at least one respective lens assembly for viewing each of said right-eye image and said left-eye image.

31. A computer program, for controlling an apparatus for displaying images, the program comprising:

32. A program according to claim 31, wherein said first computer code comprises computer code for causing said apparatus to display a left-eye image and a right-eye image to provide a stereoscopic image representing said three-dimensional scene.

33. A program according to claim 32, wherein said signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images corresponds to a vergence distance for fixating on an object at the depth of interest.

34. A program according to claim 32 or claim 33, wherein said signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic image through said lens means, a focal distance for focussing on said images substantially matches a vergence distance for fixating on an object at the depth of interest.

35. A program according to any of claims 32 to 34, wherein said signal is for controlling the focal length of said lens means such that a difference between a vergence distance and a focal distance for respectively fixating and focussing on a part of the image corresponding to the depth of interest is reduced.

36. A program according to any of claims 31 to 35, wherein the specified depth of interest is a depth of a predefined object of interest within the three- dimensional scene.

37. A program according to any of claims 31 to 35, wherein the specified depth of interest is an average depth in the three-dimensional scene.

38. A program according to any of claims 31 to 35 or 37, wherein the depth of interest has a fixed value for a sequence of images.

39. A program according to any of claims 31 to 38, wherein said depth of interest is predefined.

40. A program according to any of claims 31 to 39, further comprising third computer code for specifying said depth of interest.

41. A program according to any of claims 31 to 40, further comprising fourth computer code for determining said depth of interest based on the content of said image .

42. A program according to any of claims 31 to 41, further comprising:

fifth computer code for identifying an object of interest in said scene,

sixth computer code for determining a depth of said object of interest, and

seventh computer code for specifying said depth as said depth of interest.

43. A program according to any of claims 31 to 42, further comprising eighth computer code for applying blurring to at least one area of the image corresponding to a part of the scene at a depth other than the specified depth of interest.

44. A program according to any of claims 31 to 43, wherein the scene comprises a plurality of objects at different depths.

45. A program according to any of claims 31 to 44, further comprising ninth computer code for causing said apparatus to control the focal length of said lens means for viewing image, based on said signal.

46. A program according to claim 32, or any of claims 33 to 45 and claim 32, further comprising tenth computer code for causing said apparatus to control the respective focal length of at least one respective lens assembly for viewing each of said right-eye image and said left-eye image .

47. A data storage medium, comprising

(ii) depth data specifying a depth of interest in said three-dimensional scene.

48. A data storage medium according to claim 47, wherein said image data corresponds to a right-eye image and a left-eye image for providing a stereoscopic image representing said three-dimensional scene.

49. A method for generating image data for an apparatus according to any one of claims 1 to 15, the method comprising the steps of: providing image data corresponding to an image representing a three-dimensional scene;

50. A method according to claim 49, wherein said image data corresponds to a left-eye image and a right-eye image for providing a stereoscopic image representing said three-dimensional scene.

51. A method according to claim 49 or claim 50, wherein said depth of interest is determined based on the content of said image data.

52. A method according to any of claims 49 to 51, further comprising the steps of:

identifying an object of interest in said scene, ■determining a depth of said object of interest, and specifying said depth as said depth of interest.

53. A method according to any of claims 49 to 52, further comprising a step of applying blurring to areas of the image corresponding to parts of the scene at depths other than the specified depth of interest.

5 . A computer program for generating image data for an apparatus according to any one of claims 1 to 15, the program comprising:

second computer code for specifying a depth of interest in said scene; and third computer code for writing depth data, corresponding to said depth of interest, and said image data to a data file.

55. A program according to claim 54, wherein said image data corresponds to a left-eye image and a right-eye image for providing a stereoscopic image representing said three-dimensional scene.

56. A program according to claim 54 or claim 55, wherein said depth of interest is determined based on the content of said image data.

57. A program according to any of claims 54 to 56, further comprising:

fourth computer code for identifying an object of interest in said scene; and

fifth computer code for determining a depth of said object of interest, and

sixth computer code for specifying said depth as said depth of interest.

58. A program according to any of claims 54 to 57, further comprising seventh computer code for applying blurring to at least one area of the image corresponding to a part of the scene at depths other than the specified depth of interest.

59. An apparatus for displaying images, the apparatus comprising:

(i) a display device for displaying a plurality of images for providing a composite image representing a three-dimensional scene, wherein each image is associated with a different depth range of the scene; and (ii) control means for providing a signal for controlling a focal length of lens means for viewing said images, in dependence on said depth range associated with the image currently displayed;

wherein each image represents the same scene, differing in that, in each image, blurring is applied to at least one area of the image corresponding to a part of the scene at a depth outside of the associated depth range .

60. An apparatus according to claim 59,

wherein said display device is arranged for displaying a plurality of image pairs, wherein a plurality of said image pairs each comprises a left-eye image and a right-eye image for providing a stereoscopic image representing the three-dimensional scene, wherein each pair of images is associated with a different depth range of the scene; and

said control means is arranged for providing a signal for controlling a focal length of lens means for viewing said left-eye images and said right-eye images, in dependence on said depth range associated with the image pair currently displayed;

wherein each image pair provides a stereoscopic image representing the same scene, differing in that, in each image pair, blurring is applied to at least one area of the image corresponding to a part of the scene at a depth outside of the associated depth range.

61. An apparatus according to claim 60, wherein the signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said plurality of image pairs corresponds to a vergence distance for fixating a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

62. An apparatus according to claim 60 or claim 61, wherein the signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said image pairs is substantially equal to a vergence distance for fixating on a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

63. An apparatus according to any one of claims 59 to 62, wherein the images are displayed sequentially so that a viewer perceives said plurality of images as a single composite image.

64. An apparatus according to any one of claims 60 to 62, wherein the left-eye images and the right-eye images of the plurality of image pairs are each displayed sequentially so that a viewer perceives a single composite right-eye image and a single composite left-eye image, which combine to provide a composite stereoscopic image.

65. An apparatus according to any of claims 59 to 64, further comprising said lens means for viewing said images .

66. An apparatus according to claim 65 and claim 60, wherein said lens means comprises at least one respective lens assembly for viewing each of said left-eye image and said right-eye image.

67. An apparatus according to claim 66, wherein at least one said lens assembly comprises at least one lens.

68. A method for displaying images, the method comprising the following steps:

69. A method according to claim 68,

wherein the step of displaying a plurality of images comprises displaying a plurality of image pairs, wherein a plurality of said image pairs each comprises a left-eye image and a right-eye image for providing a stereoscopic image representing the three-dimensional scene, wherein each pair of images is associated with a different depth range of the scene; and

wherein each image pair provides a stereoscopic image represents the same scene, differing in that, in each image pair, blurring is applied to at least one area of the image corresponding to at least one part of the scene at a depth outside of the associated depth range.

70. Ά method according to claim 69, wherein the signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said plurality of image pairs corresponds to a vergence distance for fixating a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

71. A method according to any of claim 69 or claim 70, wherein the signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said image pairs is substantially equal to a vergence distance for fixating on a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

72. A method according to any one of claims 68 to 71, wherein the plural images are displayed sequentially so that a viewer perceives said plurality of images as a single composite image.

73. A method according to any one of claims 69 to 71, wherein the left-eye images and the right-eye images of the plurality of image pairs are each displayed sequentially so that a viewer perceives a single composite right-eye image and a single composite left-eye image, which combine to provide a composite stereoscopic image.

74. A computer program for controlling an apparatus for displaying images, the program comprising: (i) first computer code for causing said apparatus to display a plurality of images for providing a composite image representing a three-dimensional scene, wherein each image is associated with a different depth range of the scene; and

(ii) second computer code for causing said apparatus to provide a signal for controlling a focal length of lens means for viewing said images, in dependence on said depth range associated with the image currently displayed, wherein each image represents the same scene, differing in that, in each image pair, blurring is applied to at least one area of the image corresponding to at least one part of the scene at a depth outside of the associated depth range.

75. A computer program according to claim 74,

wherein said first computer code comprises computer code for causing said apparatus to display a plurality of image pairs, wherein each a plurality of said image pairs each comprises a left-eye image and a right-eye image for providing a stereoscopic image representing a three- dimensional scene, wherein each image pair is associated with a different depth range of the scene; and

wherein each image pair provides a stereoscopic image representing the same scene, differing in that, in each image pair, blurring is applied to at least one area of the image corresponding to at least one part of the scene at a depth outside of the associated depth range.

76. A program according to claim 75, wherein the signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said plurality of image pairs corresponds to a vergence distance for fixating a part of the image corresponding to a part of the scene at the depth range associated with that image pair.

77. A program according to any of claim 75 or claim 76, wherein the signal is for controlling the focal length of said lens means such that, when viewing the stereoscopic composite image through said lens means, a focal distance for focussing on one of said image pairs is substantially equal to a vergence distance for fixating on a part of the image corresponding to a part o the scene at the depth range associated with that image pair.

78. A program according to any one of claims 74 to 77, wherein the plural images are displayed sequentially so that a viewer perceives said plurality of images as a single composite image.

79. A program according to any one of claims 75 to 77, wherein the left-eye images and the right-eye images of the plurality of image pairs are each displayed sequentially so that a viewer perceives a single composite right-eye image and a single composite left-eye image, which combine to provide a composite stereoscopic image.

80. A method for generating image data for an apparatus according to any one of claims 59 to 67, the method comprising the steps of:

81. A method according to claim 80,

wherein the step of providing image data comprises providing image data corresponding to a pair of images comprising a left-eye image and a right-eye image for providing a stereoscopic image representing the three- dimensional scene; and

wherein the step of creating a plurality of versions of said image comprises creating a plurality of versions of said pair of images, each of said versions being associated with a different range of depths in the three- dimensional scene, wherein, in each pair of images, blurring is applied to at least one area of the images corresponding to a part of the scene at a depth outside of the respective depth range associated with the respective pair of images.

82. A computer program for generating image data for an apparatus according to any one of claims 59 to 67, the program comprising:

second computer code for creating a plurality of versions of the image, each of said versions being associated with a different range of depths in the three- dimensional scene, wherein, in each image, blurring is applied to at least one area of the image corresponding to a part of the scene at a depth outside of the respective depth range associated with the respective image .

83. A program according to claim 82,

wherein said first computer code comprises computer code for providing image data corresponding to a pair of images comprising a left-eye image and a right-eye image for providing a stereoscopic image representing the three-dimensional scene; and

said second computer code comprises computer code for creating a plurality of versions of said pair of images, each of said versions being associated with a different range of depths in the three-dimensional scene, wherein, in each pair of images, blurring is applied to at least one area of the images corresponding to a part of the scene at a depth outside of the respective depth range associated with the respective pair of images.

84. An apparatus for displaying images, substantially as described herein with reference to any of Figures 2 to 5.

85. A method for displaying images, substantially as described herein with reference to any of Figures 2 to 5.