Abstract:
A method for adapting 3D video content to a display under different viewing conditions is disclosed. The method has the steps of:
       retrieving a stereoscopic image pair;   obtaining a maximum disparity value for the stereoscopic image pair;   determining a largest allowable shift for the stereoscopic image pair using the obtained maximum disparity value;   calculating an actual shift for a left image and a right image of the stereoscopic image pair using the determined largest allowable shift; and   shifting the left image and the right image in accordance with the calculated actual shift.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of displaying 3D video content, and more specifically to the adaptation of 3D video content for display under different viewing conditions. 
       BACKGROUND OF THE INVENTION 
       [0002]    The production of 3D video is to a large extent determined by the targeted viewing conditions, e.g. cinema projection in a theatre or display on a domestic 3D-TV display. The main parameters that are taken into account during production are is the width of the targeted screen and the distance between the viewer and the screen. 
         [0003]    When 3D video content designed for specific viewing conditions shall be displayed under different viewing conditions, the 3D video content should be modified to fit these new viewing conditions. Otherwise the 3D experience quality may be rather low due to shallow 3D effects or discomfort and visual fatigue. Despite this problem, today generally no kind of adaptation is performed. This sometimes leads to very poor 3D effects, e.g. when playing 3D movies excerpts or trailers on a 3D-TV display. 
         [0004]    With the current growth of the 3D Cinema market the adaptation of 3D video content will become an important issue for the replication and distribution of 3D-DVD (Digital Versatile Disc) and 3D-BD (BluRay Disc). The goal is to avoid the need to handle several masters for the same 3D video content. 
         [0005]    Today the most primarily investigated approach for adaptation of 3D video content consists in synthesizing new “virtual” views located at the ideal camera positions for the targeted viewing conditions. This view synthesis enables pleasing 3D effects without altering the structure of the scene shot. However, view synthesis is complex and expensive in terms of computations. It requires the delivery of high quality disparity maps along with color video views, as the use of poor quality disparity maps induces unacceptable artifacts in the synthesized views. Though for computer-generated content the generation of the required disparity maps is rather easy, for natural video contents this is a rather challenging task. Up to now no reliable chain from disparity estimation to view synthesis is available. 
         [0006]    Even if improved solutions for disparity estimation become available, it still remains desirable to provide a reasonable, low-complex adaptation solution, e.g. for 3D set-top boxes. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, it is an object of the present invention to propose a solution for adaptation of 3D video content to different viewing conditions, which can be implemented with low complexity. 
         [0008]    According to the invention, this object is achieved by a method for adapting 3D video content to a display, which has the steps of:
       retrieving a stereoscopic image pair;   obtaining a maximum disparity value for the stereoscopic image pair;   determining a largest allowable shift for the stereoscopic image pair using the obtained maximum disparity value;   calculating an actual shift for a left image and a right image of the stereoscopic image pair using the determined largest allowable shift; and   shifting the left image and the right image in accordance with the calculated actual shift.       
 
         [0014]    Similarly, an apparatus for adapting 3D video content to a display has:
       an input for retrieving a stereoscopic image pair;   a disparity determination unit for obtaining a maximum disparity value for the stereoscopic image pair;   a maximum shift determination unit for determining a largest allowable shift for the stereoscopic image pair from the obtained maximum disparity value;   an actual shift calculation unit for calculating an actual shift for a left image and a right image of the stereoscopic image pair from the determined largest allowable shift; and   an image shifting unit for shifting the left image and the right image in accordance with the calculated actual shift.       
 
         [0020]    The invention proposes an adaptation of the 3D content by performing a view shifting on a frame-by-frame basis. The 3D effect is increased by moving back the scene with regard to the screen, i.e. by moving the views apart. To this end, in order to adapt a 3D movie to a 3D-TV the left view is shifted to the left and the right view is shifted to the right. Though this alters the scene structure with regard to what the director of the movie originally chose, the 3D effect is optimized. A real-time control adapted to the content, or more specifically adapted to the amount of disparity of each stereoscopic image pair, is implemented to ensure that the resulting depth remains in the visual comfort area. For this purpose advantageously the highest disparity value is transmitted for each stereoscopic image pair. Alternatively, the highest disparity value is obtained by a search for the maximum value within a complete disparity map that is transmitted for the stereoscopic image pair. As a further alternative, the highest disparity value is obtained by a disparity estimation feature. In this case a coarse, block-based implementation of the disparity estimation is sufficient. 
         [0021]    The solution according to the present invention allows a reliable and fast adaptation of 3D video content to a display device. The 3D effect is optimized while granting the viewer comfort without implementing a depth-based synthesis, which is expensive in terms of computation and hazardous when poor quality depth maps are used. 
         [0022]    Advantageously, the successive shifting steps are complemented with a temporal filtering, e.g. Kalman filtering, which is a second order filtering, to smoothen the temporal behavior of the display adaptation. Temporal filtering allows to prevent annoying jittering 3D artifacts in the resulting 3D content. Such artifacts are especially likely when depth estimation is required. For natural content, disparity maps may present frame-by-frame estimation errors, which could harm the final depth perception. By temporal filtering a smooth variation of the pixel shift is achieved. For CGI contents (Computer-Generated Imagery) supplied with their own depth maps, temporal filtering is not necessarily performed. 
         [0023]    Preferably, the viewer has the possibility to adjust the shift of the left view and the right view with an interface, e.g. an interface similar to the volume or the contrast bar. 
         [0024]    Advantageously, the shifted left image and the shifted right image are sent directly to the display. Alternatively, the shifted left image and the shifted right image are stored on a storage medium for displaying them later. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    For a better understanding the invention shall now be explained in more detail in the following description with reference to the figures. It is understood that the invention is not limited to this exemplary embodiment and that specified features can also expediently be combined and/or modified without departing from the scope of the present invention as defined in the appended claims. In the figures: 
           [0026]      FIG. 1  shows a stereoscopic image pair; 
           [0027]      FIG. 2  depicts depth maps of the stereoscopic image pair of  FIG. 1 ; 
           [0028]      FIG. 3  gives an explanation of the vergence-accommodation conflict; 
           [0029]      FIG. 4  depicts the depth situation for a cinema movie scene; 
           [0030]      FIG. 5  shows the depth situation when the movie scene is displayed on a domestic 3D-TV panel; 
           [0031]      FIG. 6  shows a flow chart of an adaptation method according to the invention; and 
           [0032]      FIG. 7  schematically illustrates an apparatus for performing the adaptation method according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    In  FIG. 1  a stereoscopic image pair is shown. The image pair consists of a left view  40  and a right view  50 . Each image  40 ,  50  has a width of 1024 pixels and a height of 768 pixels. The two images  40 ,  50  were taken with a camera pair having an inter-camera distance of t c =10 cm and a focal length of f=2240 pixels. The distance of the convergence plane from the camera basis was Z conv =+ ∞ . The near clipping plane was located at Z near =4.48 m, the far clipping plane at Z far =112.06 m. The maximum distance of the objects in the images  40 ,  50  was Z max ≈12 m, the minimum distance Z min ≈5 m. 
         [0034]    The depth maps  41 ,  51  of the stereoscopic image pair of  FIG. 1  are depicted in  FIG. 2 . An object located in the near clipping plane would correspond to pure white values. Accordingly, an object located in the far clipping plane would correspond to pure black values. The disparity d(Z) for a given depth Z is given by 
         [0000]    
       
         
           
             
               d 
                
               
                 ( 
                 Z 
                 ) 
               
             
             = 
             
               
                 t 
                 c 
               
               × 
               f 
               × 
               
                 
                   ( 
                   
                     
                       1 
                       
                         Z 
                         conv 
                       
                     
                     - 
                     
                       1 
                       Z 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
         [0035]    With Z conv =+ ∞ , the above equation simplifies to 
         [0000]    
       
         
           
             
               d 
                
               
                 ( 
                 Z 
                 ) 
               
             
             = 
             
               
                 - 
                 
                   
                     t 
                     c 
                   
                   Z 
                 
               
               × 
               
                 f 
                 . 
               
             
           
         
       
     
         [0036]    Therefore, for Z conv =+ ∞  the maximum disparity is negative, i.e. d max &lt;0. Using the above formula, the minimum depth Z min  results in a minimum disparity of d min ≈−44.8 pixels, whereas the maximum depth Z max  results in a maximum disparity of d max ≈−18.7 pixels. As a rule parallax and disparity are positive for objects located behind the screen (Z&gt;Z conv ), whereas they are negative for objects located in front of the screen (Z&lt;Z conv ). 
         [0037]    To look at a three-dimensional object in real life, the eyes of a viewer need to do two things. Firstly they must verge, i.e. they must rotate slightly inward or outward so that the projection of an image is always in the center of both retinas. Secondly, the eyes must accommodate, i.e. change the shape of each lens to focus the image on the retinas. Artificial 3D, however, causes a vergence-accommodation conflict. The viewer must focus at one distance, where the light is emitting from the screen, but verge at another distance, namely the spatial position of the 3D object. This vergence-accommodation conflict may lead to headaches and other discomforts. 
         [0038]    The vergence-accommodation conflict is schematically illustrated in  FIG. 3 . The viewer, whose eyes are separated by an inter-ocular distance t, focuses on a screen  1  with a width W screen . As long as the viewer verges on an object  6  located in the plane of the screen  1 , there is no vergence-accommodation conflict. In this case the vergence distance D conv  is equal to the accommodation distance of the eyes. If, however, the viewer verges to an object  6 ′ located before the screen or an object  6 ″ located behind the screen, the vergence distance D conv  is different from the accommodation distance of the eyes. 
         [0039]    Due to this vergence-accommodation conflict there are a lower parallax bound and an upper parallax bound, which limit the depth range where objects may be located. The lower parallax bound designates the largest distance to the front of the screen where an object may be displayed, whereas the upper parallax bound designates the corresponding distance to the back of the screen. 
         [0040]      FIG. 4  illustrates the depth situation for a cinema movie scene. Drawn is the depth perceived by a viewer against the depth of the objects in real world. The figure is based on a cinema movie scene without any particular effect, i.e. there is a linear relationship between the depth that has been shot and the depth perceived by the viewers. In the figure, the thick black line  1  corresponds to the position of the cinema screen. Typically, the cinema screen is located at a distance of 10 m from the viewer. The thick dark grey line  2  corresponds to the lower bound for negative parallax values. In cinema there is no upper parallax bound because the screen is far enough away from the viewer. The viewer can look into the infinite without feeling any accommodation pain. As the movie scene under consideration does not present any specific 3D effect, i.e. there is no stereoscopic distortion, the depth perceived by the viewer, which is illustrated by the dashed black line  3 , corresponds to the depth of objects that have been shot. 
         [0041]      FIG. 5  illustrates the corresponding depth situation when the movie scene is displayed on a domestic 3D-TV panel. In this case the distance to the screen changes to typically about 3 m. As a consequence, an upper parallax bound appears. The upper parallax bound is illustrated by the light grey line  4 . Obtaining 3D effects that are comparable to the 3D effects that are achieved in a cinema is impossible because of the limited visual comfort area. Indeed, at home the viewer is located too close to the screen. As a consequence looking into the infinite while still accommodating on the screen yields visual fatigue. If no adaptation is performed, the movie scene only presents poor 3D effects, which is illustrated by the dashed black line  3 . The solution according to the present invention, which moves the scene further to the back behind the screen, allows to increase the depth perception, without exceeding the visual comfort area. This is shown by the dash-dotted black line  5 . 
         [0042]    In the following the basis for the adaptation process that is performed in order to achieve the increased depth perception illustrated by the dash-dotted black line  5  in  FIG. 5  shall be described. 
         [0043]    A stereoscopic image pair of a frame t with a disparity d max (t) is assumed. The value d max (t) denotes the highest disparity value in pixels of the stereoscopic image pair. A priori d max (t)&gt;0. The value d max (t) is either transmitted as metadata for the stereoscopic image pair or obtained by a search for the maximum value within a complete disparity map that is transmitted for the stereoscopic image pair. Alternatively, a disparity estimation feature is implemented in the 3D-TV display or a connected set-top box. In this case a coarse, block-based implementation is sufficient. 
         [0044]    Consider 
         [0000]    
       
         
           
             
               
                 d 
                 ∞ 
               
               = 
               
                 
                   
                     N 
                     row 
                   
                   
                     W 
                     screen 
                   
                 
                 × 
                 
                   t 
                   e 
                 
               
             
             , 
           
         
       
     
         [0000]    where N row  denotes the number of pixels per line, W screen  is the width of the domestic screen in meters, and t e  denotes the viewer&#39;s inter-ocular distance. For an adult t e =0.065 m, whereas for a child t e =0.04 m. 
         [0045]    Let D stand for the distance from viewer to the 3D-TV screen. The highest disparity amount d MAX   display  that is allowable for these viewing conditions is given by: 
         [0000]    
       
         
           
             
               
                 d 
                 MAX 
                 display 
               
               = 
               
                 min 
                  
                 
                   { 
                   
                     
                       
                         d 
                         ∞ 
                       
                       × 
                       
                         D 
                         M 
                       
                     
                     ; 
                     
                       d 
                       ∞ 
                     
                   
                   } 
                 
               
             
             , 
           
         
       
     
         [0000]    where the value 1/M in diopters corresponds to the vergence-accommodation conflict tolerance that is admitted by the manufacturer of the set-top box or the 3D-TV display. Advantageously, a HDMI connection is used for this purpose. Consequently, the largest allowable shift h MAX (t) for a frame t is given by: 
         [0000]        h   MAX ( t )= d   MAX   display   −d   max ( t ). 
         [0046]    The actual shift h(t) may be any value between 0 and h MAX (t), according to the viewer&#39;s preferences, with a shift of h(t)/2 pixels to the left for the left view and a shift of h(t)/2 pixels to the right for the right frame. Advantageously the viewer has the possibility to adjust the shift with an interface similar to the volume or the contrast bar. This adjustment is expressed by a factor α, which may assume values between 0 and 1. 
         [0000]        h ( t )=α× h   MAX ( t ) αε[0;1]
 
         [0047]    In practice shift values h(t) up to ˜60 pixels, i.e. 30 pixels per view, are obtained. This corresponds to about 3% of the horizontal resolution, which is an acceptable value. 
         [0048]    Preferably, a temporal filtering feature is implemented to smoothen temporal variations of d max . It has been found that such temporal filtering, e.g. Kalman filtering, is feasible and remains unnoticeable to the viewer. 
         [0049]    An adaptation method according to the invention is shown in  FIG. 6 . In a first step  10  a stereoscopic image pair is received. Then the maximum disparity value d max (t) is obtained  11 , either from metadata transmitted together with the stereoscopic image pair or by a disparity estimation. In the next step  12  the largest allowable shift h MAX (t) is determined, e.g. as described above. From the value h MAX (t), and advantageously also from the user settable shift adjustment parameter α, the final shift h(t) for the frame, or rather the shift value h(t)/2 for the left image and the right image, are calculated  13 . The left image and the right image are the shifted  14  accordingly and sent  15  to a display or stored  16  on a storage medium. 
         [0050]    An apparatus  20  for performing the adaptation method according to the invention is schematically illustrated in  FIG. 7 . The apparatus  20  comprises an input  21  for receiving  10  a stereoscopic image pair. A disparity determination unit  22  obtains  11  the maximum disparity value d max (t), either from metadata transmitted together with the stereoscopic image pair using a metadata evaluation unit  32  or by a disparity estimation using a disparity estimator  33 . An optional temporal filter  31  downstream of the disparity determination unit  22  performs a temporal filtering on the maximum disparity value d max (t). A maximum shift determination unit  23  determines  12  the largest allowable shift h MAX (t). Preferably the apparatus  20  has a user interface  24 , which enables the viewer to set a shift adjustment parameter α. An actual shift calculation unit  25  calculates the final shift h(t) for the frame, or rather the shift value h(t)/2 for the left image and the right image, taking into account the shift adjustment parameter α. An image shifting unit  26  shifts  14  the left image and the right image accordingly. Finally, outputs  27 ,  28  are provided for sending  15  the shifted images to a display  29  or for storing  16  the shifted images on a storage medium  30 . Apparently, the different units may likewise be incorporated into a single processing unit. This is indicated by the dashed rectangle. Also, the user interface  24  does not necessarily need to be integrated in the apparatus  20 . It is likewise possible to connect the user interface  24  to the apparatus  20  via an input. For example, when the adaptation method according to the invention is performed in a set-top box, the user interface  24  may be provided by a connected display or a personal computer, which then transmits the adjustment parameter α to the set-top box.