Abstract:
A method of constructing a channel mask for an autostereoscopic display, the display including an electronically controlled screen covered by a parallax filter device that includes a refractive medium and is configured to obscure certain areas on the screen for a left eye of a viewer and to obscure certain other areas on the screen for a right eye of the viewer, the channel mask being a two-dimensional geometric object that permits to assign to each point on the screen one of a number of viewing positions of an eye of the viewer, the method including the step of constructing the channel mask by tracing light rays that propagate from selected points on the screen and are refracted at the parallax filter device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method of constructing a channel mask for an autostereoscopic display, the display comprising an electronically controlled screen covered by a parallax filter device that includes a refractive medium and is configured to obscure certain areas on the screen for a left eye of a viewer and to obscure certain other areas on the screen for a right eye of the viewer, the channel mask being a two-dimensional geometric object that permits to assign one of a number of pre-defined viewing positions of an eye of the viewer to each point on the screen. 
         [0002]    According to the general principles of stereoscopy, an impression of spatial depth is generated by presenting to the two eyes of a viewer two different images that show the same scene from slightly different perspectives which represent the parallax difference between the left right and eye of the viewer. 
         [0003]    Conventional systems for presenting different images to the left and right eyes of the user employ headsets or shutter glasses which, however, are quite disturbing for the user. 
         [0004]    U.S. Pat. No. 8,077,195 B2 describes a system which permits to view autostereoscopic images “with the naked eye”, so that stereoscopic images can for example be produced on a screen of a computer monitor or the like. To that end, the image information of the left and right channels, i.e. the information intended for the left and right eye, respectively, of the user, is displayed on the screen in the form of segments, i.e. vertical or slanted stripes, which alternatingly belong to the left and to the right channel, and a parallax filter device, e.g. in the form of a lens array of cylindrical lenses is arranged in front of the screen and is carefully positioned relative to the pixel raster of the screen, so that the light emitted from the various screen pixels is deflected such that, for a specific position of the viewer, the information of each channel is visible only for one eye. A head tracking or eye tracking system may be employed for adapting the image representation on the screen to changing positions of the viewer. When a specific viewing position has been determined, a channel mask is constructed in order to appropriately assign the correct channel to each pixel. 
         [0005]    The concept of channel masks may be extended to multi-view systems wherein the screen can be watched by one or more viewers from a number of different view positions. Then, the channel mask will define three or more channels, one for each of the envisaged viewing directions, i.e. the envisaged positions of an eye of a viewer. 
         [0006]    The channel masks may be defined in an object plane, i.e. the plane that forms the surface of the screen, and in a principal plane of the parallax filter, which principal plane is somewhat offset from the object plane towards the viewer(s). For example, in case of a parallax filter in the form of an array of cylindrical lenses, the principal plane may be the plane that contains the apex lines of the cylindrical lenses. 
         [0007]    When a viewer watches an area of the screen at right angles, one half of each cylindrical lens will deflect the light from the underlying screen pixels towards the left side of the user&#39;s face, and the other half of the lens will deflect the light of the underlying pixels towards the right side of the user&#39;s face. Consequently, the alternating pattern of apex lines of the cylindrical lenses and border lines between adjacent lenses will naturally define a channel mask in the principal plane, and a corresponding channel mask in the object plane can simply be obtained by an orthogonal projection in the direction normal to the screen. 
         [0008]    However, if the screen is viewed under a certain skew angle, the skew angle being defined as an angle between the line of sight from the viewer to a point on the screen and a normal to the screen at this point, then, for this area of the screen, the channel mask in the object plane will be laterally offset relative to the channel mask in the principal plane. The amount of this offset will also be influenced by the refraction of the light rays at the apex of each lens. 
         [0009]    Since the skew angles under which the screen is seen will generally be relatively small, the effect of the refraction can be compensated with reasonable accuracy by defining an auxiliary object plane between the object plane and the principal plane, the position of this auxiliary object plane being determined by the ratio between the refractive indices of the surrounding medium (air) and of the glass forming the lens, such that the channel mask in the object plane can be obtained by a central projection of the channel mask in the principal plane onto the auxiliary object plane, with the viewing position as projection center. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of the invention to improve the optical quality of the autostereoscopic display. 
         [0011]    In order to achieve this object, according to the invention, the channel mask is constructed by tracing light rays that propagate from selected points on the screen and are refracted at the parallax filter device. 
         [0012]    According to the invention, the steps of constructing a channel mask in the principal plane and then applying a central projection onto the auxiliary object plane are integrated into a single step of tracing light rays from the surface of the screen through the refractive medium of the parallax filter to a given viewing position or, equivalently, tracing back the light rays from the viewing position through the refractive medium onto the screen surface. This permits to construct a channel mask directly in the object plane, whereas the corresponding channel mask in the principal plane is defined only implicitly. 
         [0013]    The invention has the advantage that a refraction-corrected channel mask is obtained which is more accurate in particular for large skew angles. This permits to provide an autostereoscopic display with high image quality even for larger screen dimensions. Moreover, this method permits to adapt the system more easily to different types of parallax filter devices and also to displays with curved screens, for example. 
     
    
     
         [0014]    Preferred embodiments of the invention will now be described in conjunction with the drawings, wherein: 
         BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram of an autostereoscopic system; 
           [0016]      FIG. 2  a schematic perspective view of a portion of a screen and a superposed parallax filter device; 
           [0017]      FIG. 3  is a diagram illustrating a construction of a channel mask for a viewing position on a normal to the screen; 
           [0018]      FIG. 4  is a diagram illustrating a construction of a channel mask for a viewing position with a non-zero skew angle; 
           [0019]      FIG. 5  is a diagram illustrating the construction of the channel mask for an even larger skew angle; 
           [0020]      FIG. 6  is a diagram illustrating a relation between channel masks in an object plane and a principal plane of a parallax filter device; 
           [0021]      FIG. 7  is a schematic perspective view illustrating a ray tracing step for a plurality of points in the principal plane and the object plane; 
           [0022]      FIG. 8  is a perspective view showing a comparison between different construction methods for a channel mask in the principal plane; 
           [0023]      FIG. 9  illustrates a ray tracing process for a display with a parallax filter device in the form of a stripe array; 
           [0024]      FIG. 10  illustrates a ray tracing process for a display with a controllable parallax filter device; and 
           [0025]      FIG. 11  is a schematic top plan view of an autostereoscopic display with a curved screen. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The system shown in  FIG. 1  comprises a computer system  10 , e.g. a personal computer, including a graphics card  12  which drives a display  14  so as to display images on a screen  16  of the display. A parallax filter device, e.g. a lens array  18  is disposed in front of the screen  16 , and a video camera forming part of an eye tracking or head tracking system  20  is attached to the display  14  and communicates with the computer system  10 . 
         [0027]    The graphics card  12  has access to two texture maps L and R that are stored in a memory of the computer system  10 . The texture map L stores image information of a left channel, i.e. image information that is to be displayed to the left eye of a user (not shown) who looks at the screen  16  through the lens array  18 . Similarly, the texture map R stores image information of the right channel for the right eye of the user. The head tracking system  20  keeps track of any movements of the head of the user and signals these movements to the computer system, which will then adapt the information displayed on the screen  16  to the changed position of the user. 
         [0028]      FIG. 2  is a schematic perspective view showing a part of the lens array  18  and a part of the screen  16 . The surface of the screen  16  is divided into an array of individually controllable light emitting pixels  22 . In case of a color screen, the pixels  22  will actually be sub-pixels in the basic colors red, green and blue. 
         [0029]    The lens array  18  is constituted by a layer of glass or another transparent refractve medium and has a large number of cylindrical lenses  24  disposed in parallel and side-by-side. The cylindrical lenses  24  either extend approximately in vertical direction of the screen or may be inclined relative to the vertical and, accordingly, relative to the raster of the pixels  22  on the screen. In  FIG. 2 , the cylindrical lenses  24  are also shown to be slightly curved. This may be due to manufacturing tolerances, or the curvature may have been formed on purpose e.g. in order to further reduce Moiré effects resulting from a superposition of the regular lens raster on an also regular pixel raster. The width of the cylindrical lenses  24  is typically an integer or fractional multiple, of the width of an individual sub-pixel, typically at least three times its width. Each cylindrical lens has an apex line  26  shown as a dashed line in  FIG. 2 . 
         [0030]    Right and left eyes  28 ,  30  of a viewer have been shown schematically in  FIG. 2 . A point U at the center between the two eyes  28 ,  30  marks a reference position of a viewer watching the screen  16 . Two light rays  32 ,  34  have been shown to symbolize light that is emitted from two neighboring pixels  22  on the screen  16 , is collimated by one of the cylindrical lenses  24  and deflected to the right eye and the left eye, respectively, of the user. The distance between the top surface of the lens array  18  and the surface of the screen  16  may be equal to, smaller or even larger than the focal length of the lenses  24 , so that each eye will see an enlarged image of a corresponding pixel or parts thereof. The enlarged images of the pixels  22  that are visible for the eye  28 , for example, through all the cylindrical lenses  24  fill the entire field of view, whereas other pixels remain invisible for that eye. On the other hand, there are pixels that are visible for the other eye  30  but invisible for the eye  28 . Thus, it is possible to control the pixels  22  such that different images are presented to the right eye  28  and the left eye  30  of the user, so that the user perceives a three-dimensional image. 
         [0031]      FIG. 3  is a cross-sectional view of two of the cylindrical lenses  24 . A top surface of the screen  16  is designated as an object plane o of the lens array, and the plane passing through the apex lines  26  of the lenses  24  is designated as a principal plane p. When the screen  16  is viewed at rights angles, i.e. from a position on a normal to the screen, light that is emitted from a part of the screen below the left half of a lens  24  will be deflected towards the left eye of the viewer, as is symbolized by rays  34  in  FIG. 3 , whereas light that is emitted from areas of the screen  16  underneath the right half of a lens  24  is deflected towards the right eye, so that it can be seen only by the right eye of the user, as symbolized by rays  32  in  FIG. 3 . 
         [0032]    Consequently, a channel mask Mo can be defined which divides the surface of the screen  16  into left channel zones  36  that are visible only by the left eye (or are not visible at all) and right channel zones  38  that are visible only by the right eye (or not visible at all). A first-type boundary between the zones  36 ,  38  corresponds to the position of the apex of a lens  24 , and a second-type boundary  42  corresponds to the boundary between two adjacent lenses  24 . Since, thus, the channel mask is defined by the geometry of the lens array  18 , it is convenient to consider a channel mask Mp in the principal plane. When the pertinent part of the screen  16  is watched at right angles, as in  FIG. 3 , the channel masks Mp and Mo are congruent. 
         [0033]      FIG. 4  illustrates a situation where the part of the screen  16  is seen by a viewer under a certain skew angle a. More precisely,  FIG. 4  shows rays  44  which each connect the reference point U ( FIG. 2 ) to an apex of one of the lenses  24 . It is assumed in  FIG. 4  that the reference point U is so far away from the principal plane p that the rays  44  are practically parallel. 
         [0034]    Due to the non-zero skew angle a, the channel mask Mo in the object plane is laterally offset from the channel mask Mp in the principal plane. However, the offset is mitigated due to the fact that the light rays are refracted at the apex of each lens  24  in accordance with Snell&#39;s law, with the skew angle a as incident angle and an emergent angle β. In the example shown, it is assumed that the refractive medium forming the lens array  18  has a refractive index  2 , as compared to a refractive index  1  of the ambient air. In this case, it follows from Snell&#39;s Law that 
         [0000]      sin(α)/sin(β)=2.
 
         [0035]    The effect of this refraction can be approximated by considering an auxiliary object plane o′ half way between the object plane o and the principal plane p, and by using a central projection, with the reference point U as the center, to project Mp onto the auxiliary object plane o′. This results in a channel mask Mo′ which is at least approximately congruent with Mo. 
         [0036]    However, when the skew angle α is larger, as in  FIG. 5 , it is preferable to construct the channel mask Mo directly by tracing the rays  44  on the basis of Snell&#39;s law. Of course, by tracing the rays from the reference position U through the apex of each cylindrical lens, one obtains only the first-type boundaries  40  of the channel mask Mo. The second-type boundaries  42  can however be found just by taking the center position between adjacent first-type boundaries  40 . 
         [0037]    For comparison, the result of the approximative method using a central projection onto the auxiliary object plane o′, as in  FIG. 4 , has been illustrated in dashed lines in the left part of  FIG. 5 . It can be seen, that there is a considerable offset between the channel mask Mo′ obtained in this way and the channel mask Mo obtained by ray tracing. 
         [0038]    It will be observed that the channel mask Mp in the principal plane p is not actually needed for constructing the channel mask Mo. Nevertheless, the channel mask Mo implicitly defines also a corresponding channel mask Mp, as has been shown in  FIG. 6 . This channel mask Mp can be constructed “reversely” by moving the channel mask Mo into the auxiliary object plane o′ and then drawing straight lines  46  from the boundaries of the channel zones in the object plane o′ to the reference point U. The points where these lines  46  pass through the principal plane p define the boundaries of the channel zones of the channel mask Mp. Thus, the construction of the channel mask Mo by ray tracing is equivalent to defining the channel mask Mp in a suitable position relative to the lens array  18  (with channel zone boundaries offset from the apex lines of the cylindrical lenses) and then using the central projection to construct the channel mask Mo′. 
         [0039]    If the cylindrical lenses  24  can be considered to be straight with sufficient accuracy, it is sufficient to calculate the refracted rays  44  only once for each cylindrical lens in order to construct the channel mask Mo. However, when the apex lines  26  of the lenses are curved, as in  FIG. 2 , it is preferred to trace a band of rays  44  passing through several points along the apex line  26 , as has been shown in  FIG. 7 . 
         [0040]      FIG. 8  shows a comparison between a corrected channel mask Mp constructed in accordance with the invention, i.e. by ray tracing, and a non-corrected channel mask as defined directly by the apex lines  26  of the cylindrical lenses. It can be seen that the offset between the corrected and uncorrected channel masks can amount to more than the width of an individual channel zone  38 . Thus, when the uncorrected channel mask, as defined directly by the apex lines  26 , would be used for constructing a channel mask  48  in the object plane o by means of central projection onto the auxiliary object plane o′, the viewer would perceive an inverted three dimensional image when looking at the screen under this large skew angle. The invention permits to avoid this effect even for displays with a large screen and/or for short viewing distances. 
         [0041]    The invention is not limited to systems in which a lens array is used as parallax filter device.  FIG. 9  shows an example, where the parallax filter device is formed by a stripe array  50  constituted by a glass plate  52  with non-transparent stripes  54  printed on the top surface thereof. For a suitable viewing distance, the stripes  54  will obscure the pixels in a left channel zone for the right eye of the user and the pixels in a right channel zone for the left eye of the user. In this example, the glass plate  52  is separated from the screen  16  by an air gap  56  (or possibly a transparent layer with a refractive index different from that of the glass plate). 
         [0042]    In this embodiment, the rays  44  passing through the edges of the stripes  54  are traced on their way through the glass plate  52 , where they are refracted twice, so as to find the boundaries of the channel zones of the channel mask Mo in the object plane o. 
         [0043]    Of course, an air gap similar to the air gap  56  may also be present between the cylindrical lens  18  and the screen  16  in the previous embodiment, and then a refraction of the rays  44  at the boundary of this air gap would also have to be taken into consideration in the ray tracing process. 
         [0044]    In a similar way the invention can deal with arbitrary stacks of optical media consisting of an number of layers, each of them of individual thickness and with an individual refraction index, including the possibility of layers that are positioned on the viewer&#39;s side of principal plane p. 
         [0045]      FIG. 10  illustrates yet another embodiment where a parallax filter device is formed by a controllable gradient lens array  58 . This lens array is formed by a transparent plane-parallel plate  60  made of a material the refractive index of which varies when an electric field is applied. Fine patterns of electrodes  62  and  64  are formed on the opposite surfaces of the plate  60 , and when a voltage is applied between one of the electrodes  62  and an opposing one of the electrodes  64 , the column of the material between these electrodes will change its refractive index. Consequently, by appropriately controlling the voltages applied to the pairs of electrodes  62 ,  64 , it is possible to create a gradient in the refractive index of the plate  60 . This gradient will cause a refraction of the rays  44 , as has been shown in  FIG. 10 . 
         [0046]    In this way, it is possible to create a pattern of stripe-shaped lenses  66  the boundaries of which have been indicated by dashed lines in  FIG. 10 . When the refractive index increases from the boundary  66  towards the center of the lens and then decreases again towards the opposite boundary, the effect is equivalent to the effect of a cylindrical lens. with the only difference that the refraction of the ray  44  does not only occur at the surface of the lens but also continuously within the material of the lens. Nevertheless, it is possible to calculate the refraction and to trace the rays  44  so as to construct the channel mask Mo. 
         [0047]    This statement holds also for the range of other approaches for switchable or non-switchable lens arrays, e.g. switchable or non-switchable anisotropic lens, switchable LCD-lens and others. 
         [0048]    The invention is even applicable to autostereoscopic displays which have a curved screen  16 ′ and a curved lens array  18 ′, as shown in  FIG. 11 . The effect of the curvature may easily be taken into account when tracing the rays  44 . 
         [0049]    In a practical embodiment, a ray tracing software will be implemented in the computer system  10  ( FIG. 1 ) for calculating the rays through the various points of the lens array and the screen. When the eye tracking system  20  detects a shift in the reference position U of the user, the calculations will be updated for the two new viewing positions of the left eye and the right eye. 
         [0050]    In an alternative embodiment, it is possible to perform ray tracing calculations for a plurality of different viewing positions. In that case, a viewing position may be defined as a position of a single eye, regardless of whether this eye is a left eye or a right eye of the user. Then, the necessary ray tracing calculations need to be made only once, and the views for which the image information is presented on the screen are selected in accordance with the information provided by the head tracking system  20 . This embodiment may also be used in a multi-view system permitting two or more users to watch a three-dimensional scene simultaneously. A multi-view system of this type has been described in applicants co-pending European patent application EP 14 200 536.2. 
         [0051]    In yet another embodiment, it is also possible to make actual measurements for ray tracing rather than calculating the rays. For example, two optical sensors may be arranged in the positions of the eyes  28 ,  30  of the viewer in  FIG. 11 , and then the pixels on the screen  16 ′ may be energized one after the other to see whether at least one of the two sensors detects a corresponding optical signal. Then, depending upon which of the two sensors has received a signal, the pixel that has been energized can be assigned to one of the channel zones. 
         [0052]    These measurements may be made for a single viewing position or for several viewing positions. 
         [0053]    In general, ray tracing by measurement has the advantage that any manufacturing tolerances in the production of the lens array and/or the mounting of the lens array on the screen are eliminated automatically. 
         [0054]    A measurement for a single viewing position will normally not be sufficient to derive the exact shape and position of the lens array. However, by making measurements for two or more viewing positions, it will generally be possible to remove the ambiguity in the data, so that the exact shape and position of the lens array can be calculated from the measurement results. Then channel masks for other viewing positions which have not been measured, may be constructed by ray tracing calculations.