Abstract:
Methods and apparatus for displaying an image with enhanced image depth are disclosed. In one method, a range of depth values of picture elements (pixels) of an image is divided into multiple depth layers, and pixels in the different depth layers are displayed in a phased manner relative to the frame start time. For each image frame, objects with increasing depth in a scene are displayed with increasing delays relative to the image frame start time. The resulting illusion of depth is believed to be attributable to the edge-detection response of the human visual system, which reacts strongly to the alternating illumination on each side of an object&#39;s edge. In some implementations, the display device includes multiple pixel units that are individually activated dependent upon corresponding pixel depth data.

Description:
BACKGROUND 
       [0001]    Depth perception is the visual ability to perceive the world in three dimensions, and provides an observer the ability to accurately gauge distances to objects and displacements between objects. In many higher animals, depth perception relies heavily on binocular vision, but also uses many monocular cues to form the final integrated perception. Human beings have two eyes separated by about 2.5 inches. Light rays entering each eye are brought to focus on the retina. Photoreceptor nerve cells in the retina respond to the presence and intensity of the light rays by producing electrical impulses which are transmitted to the brain. Each eye has a slightly different viewpoint, and sends impulses conveying a slightly different two-dimensional image to the brain. The brain fuses the two different two-dimensional images together, resulting in a single image with apparent depth. The brain uses differences in the two-dimensional images from the eyes to interpret depth, thereby producing three-dimensional or stereoscopic vision. 
         [0002]    Conventional three-dimensional (3D) display techniques provide each of an observer&#39;s eyes with a slightly different image. The observer&#39;s brain then uses the differences in the images to produce a single image with apparent depth. Known 3D display techniques rely on polarized light, different colors (anaglyph), alternating columns (lenticular lens), alternating images (shuttering), separate displays, or volumetric constructions. 
         [0003]    All of the known 3D display techniques require special apparatus for providing each of an observer&#39;s eyes with a slightly different image. For example, in known polarized light techniques, the observer wears glasses with polarized lenses that allow only a left eye image to enter the left eye, and only a right eye image to enter the right eye. Similarly, known different-color (anaglyph) techniques require that the observer wear glasses with a different colored lens for each eye (e.g., one red lens and one green lens). The different colored lenses allow only a left eye image to enter the left eye, and only a right eye image to enter the right eye. Known alternating-column (lenticular lens) techniques include special optics that allow only a left eye image to be visible to an observer&#39;s left eye, and only a right eye image to be visible to the observer&#39;s right eye. 
       SUMMARY 
       [0004]    The problems identified above are at least partly addressed by herein described display methods and apparatus for enhancing a viewer&#39;s perception of depth. In contrast to known 3D techniques, the disclosed methods and apparatus do not require that each of an observer&#39;s eyes be provided with a slightly different image. Rather, a display screen presents different portions of an image in a phased manner that enhances the viewer&#39;s perception of depth. For each image frame, objects with increasing depth in a scene are displayed with increasing delays relative to the image frame start time. The resulting illusion of depth is believed to be attributable to the edge-detection response of the human visual system, which reacts strongly to the alternating illumination on each side of an object&#39;s edge. 
         [0005]    Some disclosed method embodiments for displaying an image containing multiple objects include: displaying multiple portions of the image alternately and in timed sequence such that periods of time between consecutive displays of the portions fall within a selected range of time. Each of the multiple portions of the image contains a different one of the objects, and the range of time is selected such that a human observer of the image perceives depth in the image as the portions of the image are displayed. The image may be made up of multiple picture elements (pixels) having associated depth values. The display method may include dividing the pixels into multiple depth layers, including at least a first depth layer and a second depth layer. The pixels having depth values within the first depth layer are displayed at a start time, and after a selected period of time from the start time, the pixels having depth values within the second layer are displayed. The selected period of time is selected such that a human observer of the image perceives depth in the image as the pixels of the image are displayed. 
         [0006]    Some system implementations include a display device having a display screen, and a memory system storing color/intensity data and depth data for each of multiple pixels of an image to be displayed on the display screen. The image is divided into multiple depth layers. A display processor of the display system is coupled between the memory system and the display device, and is configured to access the color/intensity data and the depth data stored in the memory system, to use the color/intensity data and the depth data to generate a display signal, and to provide the display signal to the display device. The display signal causes the display device to display the depth layers of the image on the display screen alternately and in timed sequence such that a human observer of the image preceives depth in the image. 
         [0007]    Some display device embodiments include multiple pixel units, wherein each of the pixel units includes: a pixel cell configured to display a pixel dependent upon color/intensity data of the pixel, a color/intensity data buffer for storing the color/intensity data, a depth data buffer for storing depth data of the pixel, a pixel switch element coupled to the color/intensity data buffer, and a timing circuit coupled to the depth data buffer and to the pixel switch element. The pixel switch element is coupled to receive a signal from the timing circuit, and configured to provide the color/intensity data from the color/intensity data buffer to the pixel cell in response to the signal from the timing circuit. The timing circuit is configured to provide the signal to the pixel switch element dependent upon the depth data stored in the depth data buffer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    A better understanding of the various disclosed embodiments can be obtained when the detailed description is considered in conjunction with the following drawings, in which: 
           [0009]      FIG. 1  is a diagram of an image to be displayed on a display screen, wherein the image includes several different objects; 
           [0010]      FIG. 2  is a diagram of a first portion of the image of  FIG. 1  being displayed on the display screen; 
           [0011]      FIG. 3  is a diagram of a second portion of the image of  FIG. 1  being displayed on the display screen; 
           [0012]      FIG. 4  is a diagram of a third portion of the image of  FIG. 1  being displayed on the display screen; 
           [0013]      FIG. 5  is a timing diagram for a method for displaying the image of  FIG. 1  on the display screen so as to provide a human observer with a perception of depth in the image; 
           [0014]      FIG. 6  is a diagram of a three-dimensional space defined for picture elements (pixels) making up the image of  FIG. 1 ; 
           [0015]      FIG. 7  is a flow chart of a method for displaying an image such that an observer of the image perceives depth in the image; 
           [0016]      FIG. 8  is a diagram of one embodiment of a three-dimensional space defined for pixels making up the image displayed by the method of  FIG. 7 ; 
           [0017]      FIG. 9  is a timing diagram for the method of  FIG. 7 ; 
           [0018]      FIG. 10  is a diagram of one embodiment of a display system for displaying an image such that an observer of the image perceives depth in the image; 
           [0019]      FIG. 11  is a diagram of one embodiment of a display device of the display system of  FIG. 10 , wherein the display device includes multiple pixel units that are individually activated dependent upon corresponding pixel depth data; and 
           [0020]      FIG. 12  is a diagram of a representative one of the pixel units of the display device of  FIG. 11 . 
       
    
    
       [0021]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0022]      FIGS. 1-4  will be used to illustrate one embodiment of a method for displaying an image such that a human observer of the image perceives depth in the image.  FIG. 1  is a diagram of an image  10  to be displayed on a display screen, wherein the image  10  includes several different objects: a chair  14 , a potted plant  16 , a floor  18 , a picture  20 , and a wall  22 . The objects in the image  10  are positioned about each other in three-dimensional space. The chair  14  and the potted plant  16  are resting on the floor  18 , and the picture  20  is hanging on the wall  22 . The chair  14  is closest to an observer of the image  10 , and farthest from the wall  22 . The plant  16  is farther from the observer than the chair  14 , and closer to the wall  22  than the chair  14 . 
         [0023]    In the image  10  of  FIG. 1 , a portion of the plant  16  is located behind the chair  14 , and that portion of the plant  16  is not visible in the image  10 . Similarly, portions of the floor  18  and the wall  22  are located under and behind the chair  14  and the plant  16  and are not visible in image  10 . 
         [0024]      FIGS. 2-4  illustrate how portions of the image  10  of  FIG. 1  may be displayed on the display screen alternately and in timed sequence such that the observer perceives depth in the image  10 .  FIG. 2  is a diagram of a first portion of the image  10  of  FIG. 1  being displayed on a display screen  24 . The first portion of the image  10  includes the chair  14  by itself. A portion  26  of the image  10  surrounding the chair  14  is preferably a selected fill color. The selected till color is preferably black as a black fill color induces the least amount of activation in photoreceptors of the observer&#39;s eyes. The fill color may also serve to create artificial discontinuities or “edges” about the chair  14 , thereby enhancing an edge detection response in the visual processing center of the observer&#39;s brain. The display screen  24  may be or may include a liquid crystal display (LCD) screen, or a portion of a cathode ray tube (CRT). 
         [0025]    A selected period of time after the first portion of the image  10  is displayed, a second portion of the image  10  is displayed.  FIG. 3  is a diagram of a second portion of the image  10  of  FIG. 1  being displayed on the display screen  24 . The second portion of the image  10  includes a visible portion of the plant  16  by itself. A portion  28  of the image  10  surrounding the visible portion of the plant  16  is preferably the selected fill color for the reasons described above. 
         [0026]    A selected period of time after the second portion of the image  10  is displayed, a third portion of the image  10  is displayed.  FIG. 4  is a diagram of a third portion of the image  10  of  FIG. 1  being displayed on the display screen  24 . The third portion of the image  10  includes the floor  18 , the picture  20 , and the wall  22  by themselves. A portion  30  of the image  10  includes the portions of the image  10  occupied by the chair  14  and the plant  16 . The portion  30  is preferably the selected till color for the reasons described above. A selected period of time after the third portion of the image  10  is displayed, the cycle of displaying the different portions of the image  10  alternately and in timed sequence is repeated, and the first portion of the image  10  shown in  FIG. 2  is again displayed. 
         [0027]    As described in more detail below, the selected periods of time between the displays of the portions of the image  10  are generally selected such that the observer has time to “see” one portion of the image  10  before another portion of the image  10  is displayed. As a result of displaying the portions alternately and in timed sequence, the observer perceives depth in the image  10 . It is believed that this perception of depth is due to an interaction between the activation of visual receptors in the eyes and the visual processing center of the human brain, wherein the human brain processes the temporal discrepancies in the displayed portions of the image  10  as depth. 
         [0028]      FIG. 5  is a timing diagram for the above described method for displaying the image  10  of  FIG. 1  on the display screen so as to provide the observer with a perception of depth in the image.  10 . As indicated in  FIG. 5 , the chair  14  is first displayed. (See  FIG. 2  and the above description of  FIG. 2 .) A time period ‘t 1 ’ after the chair  14  is displayed, the visible portion of the plant  16  is displayed. (See  FIG. 3  and the above description of  FIG. 3 .) A time period ‘t 1 ’ after the visible portion of the plant  16  is displayed, the floor  18 , the picture  20 , and the wall  22  are displayed. (See  FIG. 4  and the above description of  FIG. 4 .) A time period ‘t 1 ’ after the floor  18 , the picture  20 , and the wall  22  are displayed, the cycle of displaying the portions of the image  10  alternately and in timed sequence is repeated as described above. 
         [0029]    As described above, the time period ‘t 1 ’ between the displays of the portions of the image  10  is generally selected such that the observer has time to “see” one portion of the image  10  before another portion of the image  10  is displayed. In general, the time period ‘t 1 ’ is about 1/60th of a second (0.0167 sec.) as it is believed that the human eye has a natural frequency of 60 hertz (Hz). The time period ‘t 1 ’ between displays preferably ranges from about 11 milliseconds (0.011 seconds) to approximately 17 milliseconds (0.017 seconds). 
         [0030]    In the embodiment of  FIG. 5 , each portion of the image  10  of  FIG. 1  is displayed for a time period ‘t 2 ’ followed by a time period ‘t 3 ’ during which the portion of the image  10  is not displayed. As indicated in  FIG. 5 , the time periods ‘t 2 ’ and ‘t 3 ’ may be varied to achieve desired qualities of the displayed image  10 , such as image brightness. As the time period ‘t 2 ’ is increased, the time period ‘t 3 ’ decreases. In addition, the time periods ‘t 2 ’ and ‘t 3 ’ may vary as a function of image intensity or spatial position. Assuming a 60 Hz (cycles per second) refresh rate with display for 9 cycles and non-display for 1 cycle, exemplary values for the time periods ‘t 2 ’ and ‘t 3 ’ are 0.1500 seconds and 0.0167 seconds, respectively. It is noted that the refresh rate, the number of cycles that a portion of an image is displayed, and the number of cycles that the portion of the image is not displayed are all variable. 
         [0031]    Also evident in  FIG. 5  is that fact that the displays of the portions of the image  10  of  FIG. 1  may overlap. In  FIG. 5 , the display of the second portion of the image  10 , including the visible portion of the plant  16  (see  FIG. 3 ), begins before the display of the first portion of the image  10 , including the chair  14  (sec  FIG. 2 ), ends. Thus for a period of time the chair  14  and the visible portion of the plant  16  may be displayed on the display screen  24  simultaneously. Similarly, during the display of the second portion of the image  10 , including the visible portion of the plant  16 , the display of the third portion of the image  10 , including the floor  18 , the picture  20 , and the wall  22 , is initiated. Thus for a period of time the visible portion of the plant  16 , the floor  18 , the picture  20 , and the wall  22  are displayed simultaneously. It is also possible that the time period ‘t 2 ’ is less than the time period ‘t 1 ’ such that the displays of the portions of the image  10  of  FIG. 1  do not overlap. 
         [0032]      FIG. 6  is a diagram of a three-dimensional space defined for pixels making up the image  10  of  FIG. 1 . Each pixel has an ‘X’ value representing a distance along an indicated ‘X’ axis and a ‘Y’ value representing a distance along an indicated ‘Y’ axis. In general, the ‘X’ and ‘Y’ values correspond to a specific location of the pixel on the display screen  24  (see  FIGS. 2-4 ). Each pixel also has a ‘Z’ value representing a distance along an indicated ‘Z’ axis that is orthogonal to a plane defined by the ‘X’ and ‘Y’ axes. The ‘Z’ value represents a distance of the pixel from the plane defined by the ‘X’ and ‘Y’ axes; that is, a depth of the pixel within the three-dimensional space relative to the display screen  24 . 
         [0033]    In general, the ‘X,’ ‘Y,’ and ‘Z’ values of the pixels making up the image  10  of  FIG. 1  vary over predetermined ranges. The ‘Z’ values of the pixels vary within a depth value range as indicated in  FIG. 6 . In  FIG. 6 , the depth value range is divided into three sections or layers: a layer  1 , a layer  2 , and a layer  3 . The portion of the image  10  including the chair  14  (see  FIG. 2 ) may include pixels having depth values within the layer  1 . The portion of the image  10  including the visible part of the plant  16  (see  FIG. 3 ) may include pixels having depth values within the layer  2 , and the portion of the image  10  including the floor  18 , the picture  20 , and the wall  22  may include pixels having depth values within the layer  3 . 
         [0034]    For example, the image  10  may be a computer-generated image, generated in such a way that the pixels forming the chair  14  (see  FIG. 2 ) have depth values within the layer  1 , the pixels forming the visible part of the plant  16  (see  FIG. 3 ) have depth values within the layer  2 , and the pixels forming the floor  18 , the picture  20 , and the wall  22  have depth values within the layer  3 . 
         [0035]    Referring back to  FIGS. 1-5 , the portions of the image  10  may be displayed on the display screen  24  such that pixels having depth values within the layer  1  are first activated. As a result, the first portion of the image  10  including the chair  14  is first displayed. (See  FIG. 2 .) The time period ‘t 1 ’ after the pixels having depth values within the layer  1  are activated, pixels having depth values within the layer  2  may be activated. As a result, the second portion of the image  10  including the visible portion of the plant  16  is displayed. (See  FIG. 3 .) The time period ‘t 1 ’ after the pixels having depth values within the layer  2  are activated. pixels having depth values within the layer  3  may be activated. Accordingly, the third portion of the image  10  including the floor  18 , the picture  20 , and the wall  22  is displayed. (See  FIG. 4 .) The time period ‘t 1 ’ after the pixels having depth values within the layer  3  are activated, the cycle of activating the pixels having depth values within the three depth layers is repeated. As the pixels of the image  10  are triggered in this manner, the observer of the image  10  expectedly has a perception of depth in the image  10 . 
         [0036]      FIG. 7  is a flow chart of a method  40  for displaying an image such that an observer of the image perceives depth in the image. During a first step  42  of the method  40 , a range of depth values of the pixels is divided into a plurality of depth layers.  FIG. 8  is a diagram of one embodiment of a three-dimensional space defined for pixels making up the image displayed by the method  40  of  FIG. 7 . As indicated in  FIG. 8 , the ‘Z’ values of the pixels vary within a predefined depth value range, wherein the depth value range is divided into ‘n’ sections or layers. Three of the n layers, a layer  1 , a layer  2 , and a layer n, are shown in  FIG. 8 . 
         [0037]    Referring back to  FIG. 7 , a counter index ‘k’ is set to ‘1’ during a step  44 . During a step  46 , pixels having depth values within the depth layer k are displayed beginning at a start time. A step  48  involves waiting a selected period of time after the start time. During a decision step  50 , a decision is made as to whether all of the n depth layers have been displayed. If all of the n depth layers have been displayed, the step  44  is repeated. If all of the n depth layers have not been displayed during the decision step  50 , the counter index k is incremented during a step  52 , and the step  46  is repeated. 
         [0038]    During the method  40 , the depth layers may be displayed on a display screen. The display screen may be or may include a liquid crystal display (LCD) screen, or a portion of a cathode ray tube (CRT). In general, pixels that are activated produce light (e.g., according to corresponding color/intensity data), and pixels that are not activated do not produce light. 
         [0039]      FIG. 9  is a timing diagram for the method  40  of  FIG. 7 . In  FIG. 9 , the selected period of time is the time period ‘t 1 ’ described above. As the method  40  is carried out, the image is displayed such that pixels having depth values within the layer  1  are first displayed (see  FIG. 8 ). As a result, a first portion of the image is displayed, wherein the first portion of the image preferably includes a first object. The time period ‘t 1 ’ after the pixels having depth values within the layer  1  are displayed, pixels having depth values within the layer  2  are displayed (see  FIG. 8 ). As a result, a second portion of the image is displayed, wherein the second portion of the image preferably includes a second object. This process continues until the pixels having depth values within the layer n are displayed (see  FIG. 8 ). Following the display of the pixels having depth values within the layer n, the pixels having depth values within the layer  1  are displayed again as the cycle repeats. As the pixels of the image are displayed in this manner, the observer of the image expectedly perceives depth in the image. 
         [0040]      FIG. 10  is a diagram of one embodiment of a display system  60  for displaying an image such that an observer of the image perceives depth in the image. The image may, for example, include multiple objects (see  FIG. 1 ). In the embodiment of  FIG. 10 , the system  60  includes a computer system  62 , a display processor  72 , and a display device  76  including a display screen  78 . The computer system  62  includes a processor  64  coupled to a memory system  66 . In general, the processor  64  generates color/intensity data  68  and depth data  70  for each of multiple pixels of an image to be displayed on the display screen  78  of the display device  76 , and stores the color/intensity data  68  and the depth data  70  in the memory system  66 . 
         [0041]    In general, the display processor  72  is coupled to the memory system  66  of the computer system  62 , and accesses the color/intensity data  68  and the depth data  70  stored in the memory system  66 . The display processor  72  uses the color/intensity data  68  and the depth data  70  retrieved from the memory system  66  to generate a display signal  74 , and provides the display signal  74  to the display device  76  as indicated in  FIG. 10 . The display signal  74  may be, for example, a video signal. As indicated in  FIG. 10 , the display processor  72  may be part of the computer system  62 . The display screen  78  may be or may include a liquid crystal display (LCD) screen, or a portion of a cathode ray tube (CRT). 
         [0042]    In general, the display signal  74  produced by the display processor  72  causes the display device  76  to display multiple portions of the image alternately and in timed sequence on the display screen  78  such that periods of time between consecutive displays of the portions fall within a selected range of time. Each of the portions of the image preferably contains a different one of multiple objects of the image. As described above, the range of time is selected such that a human observer of the image displayed on the display screen  78  perceives depth in the image as the portions of the image are displayed. The display processor  72  may, for example, carry out the method  40  shown in  FIG. 7  and described above. 
         [0043]      FIG. 11  is a diagram of one embodiment of the display device  76  of  FIG. 10  wherein the display device  76  is a liquid crystal display (LCD) with multiple pixel units that are individually activated dependent upon corresponding pixel depth data. In the embodiment of  FIG. 11 , the display device  76  includes a control unit  80 , and the display screen  78  of the display device  76  includes multiple pixel units  82  coupled to the control unit  80 . The display signal  74  generated by the display processor  62  (see  FIG. 10 ) and received by the display device  76  includes color/intensity data (from the color/intensity data  68  of  FIG. 10 ), depth data (from the depth data  70  of  FIG. 10 ), and one or more timing signals. 
         [0044]    A typical video signal conveys an image made up of a stream of frames, wherein each frame is made up of a series of horizontal lines, and each line is made up of a series of pixels. In a video graphics array (VGA) signal, the lines in each frame are transmitted in order from top to bottom (VGA is not interlaced), and the pixels in each line are transmitted from left to right. Separate horizontal and vertical synchronization signals are used to define the ends of each line and frame, respectively. A “line time” for displaying a line exists between two consecutive horizontal synchronization signals, and a “frame time” for displaying a frame exists between two consecutive vertical synchronization signals. 
         [0045]    In general, the control unit  80  uses the one or more timing signals to generate a clock signal, and provides corresponding color/intensity data, corresponding depth data, and the clock signal to each of the pixel units  82 . In the method  40  of  FIG. 7  described above, a range of depth values of pixels of an image are divided into n layers, and the n layers are displayed alternately in timed sequence. In the embodiment if  FIG. 11 , the control unit  80  produces the clock signal such that the clock signal has a period that is 1/n times the frame time so that all n layers are displayed during the frame time. 
         [0046]      FIG. 12  is a diagram of a representative one of the pixel units  82  of the display device  76  of  FIG. 11 . In the embodiment of  FIG. 12 , each pixel unit  82  includes a pixel cell  84 , a pixel switch element  86 , a color/intensity data buffer  88 , a depth data buffer  90 , and a timing circuit  92 . The pixel cell  84  is a typical thin film transistor (TFT) light control element; essentially a small capacitor with a liquid crystal material disposed between two optically transparent and electrically conductive layers. The pixel cell  84  is controlled by the pixel switch clement  86  and the timing circuit  92 . 
         [0047]    As described above and indicated in  FIG. 12 , the control unit  80  ( FIG. 11 ) provides corresponding color/intensity data, corresponding depth data, and the clock signal to the pixel unit  82 . When the pixel  82  receives the corresponding color/intensity data and the corresponding depth data, the pixel  82  stores the color/intensity data in the color/intensity data buffer  88 , and stores the depth data in the depth data buffer  90 . 
         [0048]    In one embodiment, the depth data stored in the depth buffer  90  specifies one of n depth layers in which the pixel resides (see  FIG. 8 ). The timing circuit  92  may include, for example, a modulo-n counter that counts from 1 to n during each frame time. When the value of the counter matches the depth data stored in the depth buffer  90 , the timing circuit  92  sends a signal to the pixel switch element  86 , thereby activating the pixel cell  84 . In general, the pixel switch element  86  activates the pixel cell  84  in accordance with the color/intensity data from the color/intensity data buffer  88  in response to the signal from the timing circuit  92 . For example, in response to the signal from the timing circuit  92 , the pixel switch element  86  may provide the color/intensity data from the color/intensity data buffer  88  to the pixel cell  84 . In receiving the color/intensity data from the color/intensity data buffer  88 , the pixel cell  84  is activated according to the color/intensity data. 
         [0049]    In general, the pixel cell  84  alternates between an active state and an inactive state. Once the pixel cell  84  is activated, the timing circuit  92  determines an amount of time that the pixel cell  84  remains active. At the end of a selected active time period, the timing circuit  92  disables the pixel switch element  86 , thereby deactivating the pixel cell  84 . The amount of time that the pixel cell  84  remains active is generally selected to achieve a desired level of pixel saturation and hue intensity. The timing circuit  92  may control the amount of time the pixel cell  84  remains active to achieve, for example, a desired active-to-inactive time ratio. 
         [0050]    Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, it is well known that the human visual system also employs size and intensity cues when evaluating object distances. When such additional visual cues are available, the depth layer sequence may be re-ordered or even reversed without significantly impacting a viewer&#39;s perception of depth. It is intended that the following claims be interpreted to embrace all such variations and modifications.