Abstract:
The present invention provides method for producing a substantially seamless video image on a display surface. The method comprises the steps of separately projecting at least a first and a second video image onto a display surface such that a seam is defined by overlapping portions of said first and second video images. Inside the seam, the brightness of video image is adjusted by adjusting pixels of said overlapping portions in accordance with a modulus determined by the number of overlapping portions defining the seam.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional application Ser. No. 60/751,336 filed Dec. 16, 2005, incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to projection displays, and more particularly to projection display systems comprising a plurality of projectors arranged to display tiled images on a display screen. 
       BACKGROUND OF THE INVENTION 
       [0003]    The quality of a projected image can be described by reference to a number of image characteristics. Each characteristic represents a potential source of distortion in the displayed image. Brightness and brightness uniformity are important characteristics of displayed images. Brightness distortion, also referred to as luminance distortion degrades the quality of a projected image to a viewer of the image. Most projectors do not project images at a constant luminance level across the entire display screen. Therefore, brightness distortion is a common problem in the design of projector display systems. 
         [0004]    Brightness distortion has many possible sources. A common source of brightness distortion is due to inherent optical characteristics of lenses used in projection displays. This non uniformity is due to the design of the optics within the light engines of the projectors. Another possible cause for non uniformity is the projection lamps themselves. Regardless of the source of the luminance distortion, non-uniformity in luminance detracts from the displayed image in the eyes of a viewer of the image. 
         [0005]    The luminance non-uniformity of a projector and its associated lens can become more pronounced when a plurality of projectors are employed in combination to display a single image on a display screen. Such an arrangement of projectors is as “tiling”. Tiling projectors and projector images on a display screen provides a larger image with higher overall resolution than can be obtained from a single projector. However, the technique of tiling images for display has drawbacks. Non-uniformity in luminance is often much more apparent in a composite image created by multiple projectors whose individual images are tiled together. This is particularly a problem in “seam” areas of the displayed image. Seams are created in those areas where images from a plurality of projectors overlap each other on the display screen. Brightness non uniformity in seam areas of a displayed tiled image is distracting to viewers and degrades the quality of the displayed image. Therefore, systems and methods for maintaining brightness uniformity in seam areas of tiled images are needed. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides systems and methods for maintaining brightness uniformity in seam areas of tiled images. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Preferred embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings, in which: 
           [0008]      FIG. 1  is a block diagram of an example display system including two projectors according to an embodiment of the invention. 
           [0009]      FIG. 2  is a block diagram of an example display system including four projectors according to an embodiment of the invention 
           [0010]      FIG. 3  is a flow chart illustrating steps of a method for smoothing seams in a tiled display according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  illustrates a display system  10  for projecting a video image  20  onto a display screen  40 . System  10  comprises a plurality of video projectors ( 160 ,  260 ) arranged such that each projector ( 160 ,  260 ) separately projects first and second portions ( 158 ,  258  respectively) of image  20  onto display screen  40 . In the embodiment illustrated in  FIG. 1 , projectors  160  and  260  comprise spatial light modulator (SLM) type projectors. According to one example embodiment of the invention projectors  160  and  260  comprise DLP™ projectors. DLP is a trademark of Texas Instruments. The light engine for each projector comprises any suitable technology, such as one or more Liquid Crystal Display (LCD) panels, Digital Light Processing (DLP) or Liquid Crystal on Silicon (LCOS). Nonetheless, those skilled in the art will recognize-that the present invention may be used. with other projectors, including those using other types of image generation technologies. 
         [0012]    Each separately projected image portion  158 ,  258  comprises a corresponding unique image portion ( 159 ,  259 ) respectively of image  20 . Each separately projected image portion  158 ,  258  further comprises a common image portion ( 60 L and  60 R) respectively of image  20 . For example, a first projector  160  projects first portion  158  of image  20  onto screen  40 . First portion  158  of image  20  comprises a unique image portion  159 , i.e., a portion of image  20  that is not projected by any other projector. First portion  158  of image  20  also comprises a common portion  60 L. Portion  60 L is a duplicate of image portion  60 R of image portion  258 . Common portion  60 L and common portion  60 R are projected to overlap each other on display  40 . The overlapping common portions  60 L and  60 R define a seam  88  of projected image  20 . 
         [0013]    System  10  further comprises a video image separator  90 . A video signal representing video image  20  is provided to video image separator  90 . A processor  95  of image separator  90  separates the incoming video image signal into video image signal portions  106  and  206 . Video image signal portions  106 ,  206  represent video image portions  158  and  258  respectively. 
         [0014]    Video image separator further comprises a pixel brightness adjuster  27 . Pixel brightness adjuster  27  examines pixel values within overlapping image portions (e.g.,  60 L and  60 R) of image signal portions  106  and  206 . Brightness adjuster  27  divides pixel values corresponding to overlapping pixels of overlapping image portions  60 L and  60 R (for example pixel  32  and  22 ) by the number of overlapping image portions comprising seam  88 . In the system illustrated in  FIG. 1  seam  88  comprises two image overlapping image portions  60 L and  60 R. Therefore pixel brightness adjuster divides the brightness value for pixel  60 L and  60 R by two. When projected onto display  40  the combined brightness of pixels  60 L and  60 R will approximate the intended brightness corresponding to pixel  60  of input image  20 . 
         [0015]    However, pixel values are represented by a limited number of bits. For example pixel values are commonly represented by 8 bits. Each of the 256 combinations of 8 bits corresponds to a different brightness level. If a brightness level does is not evenly divisible by the number of projectors it is not possible to accurately represent the original brightness value by a combination of equal lower values. In order to more closely approximate the original brightness value, pixel adjuster  27  determines the modulus (n) of the pixel brightness value to be adjusted, where n is the number of projectors comprising system  100 . Pixel brightness adjuster  27  adjusts the pixel brightness value for each projector based on the modulus (n) it determines. 
         [0016]    In a two projector system pixel brightness adjuster  27  determines the modulus (2) of pixel brightness values of overlapping pixels. If the modulus (2) is 0, the original brightness value is evenly divisible by the number of projectors. In that case dividing the value by two and assigning equal values to each overlapping pixel will provide the original brightness value when the overlapping pixels are displayed. 
         [0017]    If the modulus is 1, one of the overlapping pixels is assigned the integer portion of the original brightness value divided by the number of projectors. The other overlapping pixel is assigned a brightness value equal to the brightness value assigned to the other plus 1. 
         [0018]      FIG. 2  illustrates a projector system  100  according to an alternative embodiment of the invention. Projector system  100  comprising four projectors  160 ,  260 ,  360  and  460 . Incoming video signal  12  is provided to video image separator  90 . Video signal  12  represents an image  20  to be displayed on screen  40 . Image separator  90  comprises a processor  95  and a pixel brightness adjuster  27 . Image separator  90  separates incoming video signal into video signal portions  81 ,  82 ,  83  and  84 . Each video signal portion represents a portion of image  20 . Similar to the embodiment of  FIG. 1  each image portion  159 ,  259 ,  359  and  459  of image  20  comprises a unique image portion and an overlapping image portion. Overlapping image portions define seams  55 ,  66 ,  77  and  88  of displayed image  20 . In a configuration comprising four projectors arranged in accordance with  FIG. 2 , an image area  73  comprises four overlapping image portions. Accordingly the brightness of a given pixel, for example pixel  800 , in image area  73  will be a combination of four pixel brightness values, one value supplied by each video signal portion  81 ,  82 ,  83  and  84 . 
         [0019]    Pixel adjuster  27  of video separator  90  compensates for distortions in brightness by dividing the brightness value P of each pixel in area  72 , for example the brightness value of pixel  800 , by four (the number of projectors providing a pixel value for pixel  800 ). A value of P/4 is assigned to each overlapping pixel. To avoid loss of dynamic range pixel adjuster  27  also determines the modulus (n) for pixel brightness values of overlapping pixels, where n=4. If the modulus (4) of the brightness value is 0 each of the four overlapping pixel values is assigned a brightness value equal to the original brightness value divided by 4 (P/4). If modulus (4) of the brightness value is 1 a binary 1 is added to P/4 for one of the four overlapping pixels. If the modulus (4) of the brightness value is 2 a binary 1 is added to P/4 for two of the overlapping pixels. If the modulus 4 of the brightness value is 3 a binary 1 is added to three of the overlapping pixels. By adjusting pixel brightness values in accordance with the modulus (n) of the brightness of overlapping pixels, pixel brightness adjuster improves the dynamic range in the seams of image  20 .