Abstract:
The system according to the present invention includes a light-diffusing screen having a projected image that includes the intrinsic random optical patterns superimposed on a scene image. An electronic photosensor is used to capturing the scene image and an image processor employs algorithms that reduce the random optical patterns that are inherent to the light-diffusing screen.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to the field of digital signal processing, and in particular to artifact reduction in captured images. More specifically, the invention relates to using digital signal processing techniques to reduce the appearance of undesirable random optical patterns inherent to light-diffusing screens.  
       BACKGROUND OF THE INVENTION  
       [0002]     In a camera system, a light-diffusing screen (also referred to as focusing screen) is often used to form a temporary projected image of the scene for pre-visualization through a viewfinder before the scene is captured by film or other types of photosensors. In the motion-picture camera industry, the use of this projected scene image has expanded beyond viewfinders. For example, in U.S. Pat. No. 4,928,171, issued to Kline on May 22, 1990, the inventor describes a video assist system where a small amount of light from the temporary image projected on a light-diffusing screen is captured by an electronic photosensor and converted to a television signal for viewing on a video monitor. In another example, in WO 03/058951, issued to Weigel, et al, on Jul. 17, 2003, the inventors describe an image conversion system using the temporary image projected on a light-diffusing screen as a part of the light path to enable 35 mm camera lens usage on non-35 mm based cameras.  
         [0003]     Common light-diffusing screens exhibit irregular structures that are the result of their construction process. For example, because matte disc-based focusing screens are constructed via a grinding process, they exhibit grain-like irregular structures. As a light-diffusing screen collects light from the desired scene, these irregular structures scatter and modulate incoming light in an undesirable manner, creating random optical patterns in the temporary image that is projected on the screen. Said in another way, the temporary image projected on the light-diffusing screen is the intended scene modulated by the random optical patterns caused by the irregular structures in the screen material.  
         [0004]     When used in devices where suboptimal image quality is acceptable (such as viewfinders for scene framing), random optical patterns created by the light-diffusing screen are a non-issue. However, there are applications where the image quality of the observed image is very important. In such applications, it is important to be able to obtain images that are perceptually free, or nearly free, of the random optical pattern caused by the light-diffusing screen material. That is, ideally, the images obtained should be a close representative of the desired scene as possible.  
         [0005]     One example where image quality matters is the image conversion system described in WO 03/058951. In this conversion system, which enables use of 35 mm lenses on non-35 mm-based cameras, the scene light collected by a 35 mm lens projects a temporary image on a light-diffusing screen. The camera itself, with its own non-35 mm lens, then focuses on the light-diffusing screen, effectively using the projected image as the scene. In this application, it is important to be able to observe images free of random optical patterns caused by the light-diffusing screen material.  
         [0006]     In another example where image quality is crucial is the video assist system described in U.S. application Ser. No. 09/712,639, filed by Eastman Kodak Company by Albadawi et al on Nov. 14, 2000. This invention enables preview of post-production color management while on a movie production set. As this is based on a video assist system, the images used for previewing color management decisions are obtained through the light-diffusing screen, which, again, is characterized by the random optical pattern of the screen. Artifact-laden images are not optimal for judging and making critical decisions on the optical attributes that constitute the projected scene image.  
         [0007]     In both of the above examples, methods for reducing or eliminating the appearance of the random optical pattern are needed to produce images more representative of the intended scene.  
         [0008]     The direct method to reduce or eliminate the appearance of the random optical pattern is to control the irregular structures (striations) in the material used to make the light-diffusing screen. For example, by using super-fine grinding particles in the grinding process to produce matte discs, the irregular physical structure in the discs can be significantly reduced. However, use of fine grinding particles leads to light-diffusing screens that are too transparent (low light scattering) to produce a satisfactory intermediate image. Physical structures in other types of materials can be controlled as well. However, control in physical structure often translates to increase in cost and/or less of light transmission efficiency (for example, those that exhibit Lambertian diffuser properties).  
         [0009]     Another method to reduce the perceived presence of the artifacts is to rapidly move the light-diffusing screen itself, as described in German patent number 2 016 183, issued to Firth et al on Oct. 29, 1970. In U.S. Pat. No. 6,749,304, issued to Jacumet, Jun. 15, 2004, the inventor improves on the concept by using a sandwich structure as one of the embodiments of his invention, with the light-diffusing screen as the middle section, moved by an attached motor. Some drawbacks with this type of solution are results of the fact that these solutions are largely electro-mechanical. The motor required to move the screen requires extra housing and a source of power. An electro-mechanical solution means moving parts, and additional power supply requirements, leading to increased possibility in malfunctions. In addition, the motor generates noise.  
         [0010]     What is needed is a solution that does not increase cost, is compact, and does not use mechanical parts or significantly more power.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is directed to overcoming one or more of the problems set forth above by employing a system for reducing random optical patterns inherent in a light-diffusing screen. The system according to the present invention includes: 
        a) a light-diffusing screen having a projected image that includes a scene optical image with the random optical patterns introduced by the screen;     b) an electronic photosensor for capturing an image; and     c) an image processor for reducing the random optical patterns inherent to the light-diffusing screen. 
 
 Another embodiment of the present invention is directed to a method for reducing random optical patterns inherent in a light-diffusing screen that includes: 
    a) providing a light-diffusing screen having a projected image that includes a scene optical image with the random optical patterns introduced by the screen;     b) capturing an image with an electronic photosensor; and     c) applying image processing algorithms to reduce the random optical patterns that are inherent to the light-diffusing screen.        
 
       Advantageous Effect of the Invention  
       [0018]     The present invention has the following advantages: 
        a) no mechanical parts;     b) no additional mechanical noise; and     c) may be implemented as standalone electronic component or as part of existing electronic component.        
 
         [0022]     These and other features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. Identical reference numerals have been used, where possible, to designate elements that are common to the figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  depicts one version of a generic camera system with a light splitter that splits light between a viewfinder and a digital imaging system containing invention;  
         [0024]      FIG. 2  depicts another version of a generic image conversion system;  
         [0025]      FIG. 3  shows one embodiment of the Image Processor that performs spatial filtering on the luminance channel of the image;  
         [0026]      FIG. 4  shows one embodiment of the Image Processor that performs spatial filtering on three color-channels of the image;  
         [0027]      FIG. 5  shows one embodiment of the Image Processor that performs signal mapping operation on the luminance channel of the image;  
         [0028]      FIG. 6  shows one embodiment of the Image Processor that performs signal mapping operation on three color-channels of the image;  
         [0029]      FIG. 7  shows one embodiment of the Image Processor that performs spatial filtering directly on the photosensor data;  
         [0030]      FIG. 8  shows one embodiment of the Image Processor that performs signal mapping operation directly on the photosensor data;  
         [0031]      FIG. 9  is a block diagram of a common sigma-based filter;  
         [0032]      FIG. 10  is a block diagram of common table-lookup procedure;  
         [0033]      FIG. 11  shows a frontal view of a projected image with a random optical pattern; and  
         [0034]      FIG. 12  shows the steps required to train, or populate, the lookup table. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     In the following description, the present invention will be described in the preferred embodiment as a software program. Those skilled in the art will readily recognize that the equivalent of such software may also be constructed in hardware.  
         [0036]      FIG. 1  and  FIG. 2  show two places in a camera where a light-diffusing screen might be located. In both setups, the scene is projected onto the screen and is subsequently captured by an electronic photosensor. The Image Processor then processes the photosensor data to reduce effects of the inherent optical pattern from the light-diffusing screen.  
         [0037]     Specifically regarding  FIG. 1 , a camera lens  10  receives light (represented by dashed line) and directs the light onto a rotating mirror  12 , whereby the rotating mirror  12  deflects the light onward towards a light-diffusing screen  14 . If the rotating mirror  12  was absent, the light would pass on to a photosensing element (not shown). The light diffusing screen  14  sends the light to a light beam splitter  16 , which diverts the light in at least two directions. One diversion of the light is received by a viewfinder  18 . Viewfinder  18  will allow a person to view a projected image  56  (shown in  FIG. 11 ) that will likely include random optical patterns, because of the light coming from the light diffusing screen  14 , which has inherent optical patterns associated with it. The next diversion of the light is received by a relay lens  20 . The relay lens  20  focuses the light onto electronic photosensor  22 , whose electronic signal output will be an input for an image processor  24 . The image processor  24  reduces the appearance of the inherent random optical patterns by means of digital signal processing shown in greater detail in  FIGS. 3-8 .  
         [0038]     Regarding  FIG. 2 , a camera lens  10  receives light (represented by a solid line) and directs the light onto a light diffusing screen  14 . The light diffusing screen  14  sends the light to a lens of an attached camera  28  that focuses the light onto an electronic photosensor  22 , which provides an electronic signal as an input for image processor  24 . Subsequent to processing the image, image processor  24  directs the light to a second attached camera  26 .  
         [0039]      FIG. 3  shows one embodiment of the Image Processor  24  shown in  FIGS. 1 and 2  that uses spatial filtering to remove the random optical pattern in the light-diffusing screen  14 . First, a converter  30  converts the photosensor data to a common multichannel color format, such as sRGB and YCC, in full frame, interlaced, or subsampled forms. Each color channel is then independently processed by a corresponding spatial filter  32 , reducing the effects of the inherent optical pattern of the light-diffusing screen  14 , which is captured as part of the projected image  56  (shown in  FIG. 11 ).  FIG. 11  shows a frontal view of a projected image  56  with a random optical pattern. Frontal view of light-diffusing screen  54  is shown with the projected image  56  having a random optical pattern  57 . Spatial filters  32  are processing their corresponding red, green, or blue light in accordance with their design. Spatial filters  32  reduce the noisy signal associated with the random optical pattern  57 . In one implementation, spatial filter  32  is a sigma-based filter. This sigma-based filter may be the sigma filter as described by J. Lee in his oft-quoted paper “Digital Image Smoothing and the Sigma Filter” or one that is derived from the sigma filter such as radial sigma filter described in U.S. Pat. No. 6,907,144, issued to Gindele, Jun. 14, 2005, or the more complex multiresolution method described in U.S. Pat. No. 6,937,772, issued to Gindele, Aug. 30, 2005. Lastly, a second converter  34  converts the processed color channel data into the desired output format, such as sRGB for computer monitors, YCC for NTSC video, RGB printing densities for film post-production work, or some other required image color metric or signals.  
         [0040]      FIG. 9  shows essential parts of a sigma-based filter. The main block  46  is the sigma filter itself, which performs the calculations related to the filtering process. Sigma filter  46  gathers additional information from two attached blocks. The scaling factor expands the inclusiveness of the filter. Often, this value is based on the statistical distribution of the data, but can very well be chosen arbitrarily. Signal-dependent sigma information block  48  provides the Sigma based filter  46  with sigma values based on level of received signal. Sigma information, along with scaling factor, determines the neighboring pixels to be included in the averaging process.  
         [0041]      FIG. 4  shows a second embodiment of the Image Processor  24  shown in  FIGS. 1 and 2 . Like the embodiment shown in  FIG. 3 , the Image Processor  24  uses a spatial filter  32 . However, this embodiment shows a special case where only one channel needs to be processed by the spatial filter  32 , thereby reducing the number of calculations required for filtering. First, a converter  36  converts the photosensor data to an image comprising of one luminance data channel and two chrominance data channels. Next, only the luminance channel data is processed by a spatial filter  32 . Since the random optical patterns may be characterized as changes in light intensity, processing only the luminance channel data is a valid practice. Experiments have shown that such is the case. Lastly, a second converter  38  converts the processed luminance data channel and the two unprocessed chrominance data channels to the desired output format, such as sRGB for computer monitors, YCC for NTSC video, RGB printing densities for film post-production work, or some other required image color metric or signal.  
         [0042]      FIG. 5  shows a third embodiment of the Image Processor  24  that uses a signal mapping method to remove the random optical pattern in the light-diffusing screen. First, a converter  30  converts the photosensor data to a common multichannel color format, such as sRGB and YCC, in full frame, interlaced, or subsampled forms. Each channel is then independently processed by a signal mapping algorithm  40 , thus reducing the effects of the inherent optical pattern of the light-diffusing screen, which is captured as part of the projected image  56 . The signal mapping algorithm  40  reduces the appearance of random optical patterns by mapping an input signal to an expected output value. Lastly, a converter  34  transforms the processed channel data into the desired output format, such as sRGB for computer monitors, YCC for NTSC video, RGB printing densities for film post-production work, or some other required image color metric or signal.  
         [0043]     The purpose of the signal mapping algorithm  40  is to transform the input signal value to a desired output signal value, based on prior knowledge about how one input value should be mapped to an output value. This knowledge typically comes from training the system prior to the actual start of scene capture. The signal mapping algorithm  40  itself may be one of several known techniques such as lookup-table mapping, linear interpolation, or cubic interpolation. A person with ordinary skill in the art would recognize these techniques.  
         [0044]     In one implementation of the system, the signal mapping procedure is implemented in the form of a lookup-table mapping procedure. Lookup-table mapping has the distinct advantage in that the procedure is fast, although the technique requires a tremendous amount of hardware memory.  
         [0045]      FIG. 10  shows essential parts of the lookup-table mapping method, as commonly practiced in the art. The lookup-table mapper  50  block uses the input signal as the index into the attached lookup table  52  and returns the signal value stored at the hardware memory location specified by the index.  
         [0046]      FIG. 12  shows the steps required to train, or populate, lookup table  52  shown in shown in  FIG. 10  and used in one implementation of the system. A series of gray patches with known signal values are used. Using a patch as the scene, operation  100  captures the projected image  56  with the electronic photosensor  22 . For each pixel of the captured image operation  110  records the captured signal level and the known signal level. Likewise, for each channel of that pixel operation  120  records the captured signal level and the known signal level. These known signal levels are the output values to which this captured, input, value will be mapped when performing lookup-table signal mapping in operation  130 . Operations  140 ,  150 , and  160  repeat for all gray patches in the chosen set. Typically, a chosen set of patches may not fully populate the lookup table, that is, not all input signal values would have an output map signal value after all patches have been used. To fill in the unknown mapping values, various interpolating methods may be used in operation  170 , such as linear or quadratic interpolation.  
         [0047]     A person familiar with electronic photosensor arrays may recognize that the mapping procedure described above is similar to the common technique used to correct signal variations amongst pixels in a sensor due to differences in dark current (offset) and sensitivity (gain) of each pixel. However, the mapping procedure for the random optical patterns is unique, because the mapping procedure needs to account for the effects caused by interactions between lens light collection aperture and light-diffusing screen, which need not be accounted for in pixel-to-pixel correction.  
         [0048]     One such effect is the non-uniformity in illumination of the light-diffusing screen, which may be a result of optical vignetting, or cosine 4  effects (note: superscript, not a footnote), or both. Vignetting is the optical phenomenon where light intensity tends to fall off towards the edges of the formed image (in the case of this invention, at the diffusing screen) due to the size of the lens aperture. Lens aperture controls the shape of the cone of light collected by the diffusing screen, and as the lens aperture closes down (decrease in aperture size), the cone of light decreases in radius. As this cone becomes small relative to the entire area of the diffusing screen, light starts to fall off towards the edges of the projected image. Even if vignetting is not present, illumination of the light-diffusing screen may still be non-uniform due to cosine 4  effects. Due to geometric factors described in cosine 4  effects, points in the projected image that are off the optical-axis have lower illumination than points that are on the optical axis. An effective implementation of the lookup procedure in the present invention would also be capable of correcting these illumination falloffs due to vignetting and/or non-uniformities due to cosine 4  effects.  
         [0049]     The variations in the random pattern observed in the projected image as a result of changing lens apertures—that is, different lens aperture stop sizes (i.e. f/number) cause different random optical patterns in the projected image—are not inherent of electronic photosensor arrays, and therefore, the non-uniformity pattern correction that is applied in this case is limited to that resulting only from pixel gain-offset differences. An effective implementation of table lookup procedure for reducing the appearance of the random optical patterns would be capable of taking into account these variations in the patterns as lens aperture changes. Electronic photosensors do not exhibit such pattern changes in pixel-to-pixel variations, and thus need not account for such a change.  
         [0050]     A key difference between the random optical patterns and pixel-to-pixel variations is that the light-diffusing screen affects light in a non-linear manner while the sensitivity for each pixel in an electronic sensor may be effectively modeled as linear gain. The non-linear behavior of light in the light-diffusing screen make a lookup table necessary for signal mapping, while signals captured by pixels in the electronic sensor can be modified by multiplying by a gain factor. Memory requirement for a fully (or nearly fully) populated lookup table used for random pattern correction is significantly higher than for gain-offset tables for pixel-to-pixel correction.  
         [0051]      FIG. 6  shows a fourth embodiment of the Image Processor  24 . Like the embodiment shown in  FIG. 5 , this one also uses signal mapping as the method to remove the random optical patterns in the light-diffusing screen. However, this embodiment shows a special case where only one single channel needs to be processed by the signal mapping procedure, thereby reducing the number of mappings required for processing. First, a converter  36  converts the photosensor data to an image comprising of one luminance data channel and two chrominance data channels. Next, signal mapper  40  processes the luminance channel data to remove the random optical pattern captured as part of the image. Lastly, a converter  38  transforms the processed luminance data channel and the two unprocessed chrominance data channels to the desired output format, such as sRGB for computer monitors, YCC for NTSC video, RGB printing densities for film post-production work, or some other required image color metric or signals.  
         [0052]      FIG. 7  shows a fifth embodiment of the Image Processor  24  that performs spatial filtering directly on the photosensor data to reduce the random pattern effects. First, a processing  42  unit separates the photosensor data into independent channels according to the number of channels incorporated into the photosensor. Common photosensors employ three color-channels, and thus is shown accordingly in the diagram. However, this need not be the case. Next, spatial filters  32  process each channel independently, reducing effects of the random optical patterns. It is appreciated that a variety of spatial filters may be used, though, as previously suggested, we favor sigma-based filters for removing random optical patterns captured as part of the projected image. Lastly, an output converter  44  converts the processed channels into the desired output format, such as sRGB for computer monitors, YCC for NTSC video, RGB printing densities for film post-production work, or some other required image color metric or signals. In a preferred implementation, the spatial filter is a sigma-based filter. Refer to  FIG. 9  to see a block diagram of the sigma-based filter.  
         [0053]      FIG. 8  shows a sixth embodiment of the Image Processor that performs signal mapping directly on the photosensor data to reduce the random pattern effects. For each pixel of the photosensor, the signal mapping block  40  maps the input signal to a desired output signal, then a converter  44  converts the processed data to the desired output format, such as sRGB for computer monitors, YCC for NTSC video, RGB printing densities for film post-production work, or some other required image color metric or signals. The signal mapping procedure itself may be one of various known techniques such as lookup-table mapping, linear interpolation, or cubic interpolation. In one implementation, lookup-table mapping is used as the mapping procedure.  FIG. 10  shows a block diagram of the lookup-table mapping procedure.  
         [0054]     The invention has been described with reference to preferred embodiments. However, it will be appreciated that a person of ordinary skill in the art can effect variations and modifications without departing from the scope of the invention.  
       PARTS LIST  
       [0000]    
       
           10  Camera lens  
           12  Rotating mirror  
           14  Light-diffusing screen  
           16  Light beam splitter  
           18  Viewfinder  
           20  Relay lens  
           22  Electronic photosensor  
           24  Image Processor  
           26  Light and processing path of attached camera  
           28  Lens of attached camera  
           30  Photosensor data to multichannel image converter  
           32  Spatial filter  
           34  Multichannel image to output format processor  
           36  Photosensor data to YCC image converter  
           38  YCC image to output format processor  
           40  Signal mapper  
           42  Photosensor data separator  
           44  Photosensor data to output format processor  
           46  Sigma-based filter  
           48  Signal-dependent sigma information  
           50  Table lookup processor  
           52  Lookup table  
           54  Light-diffusing screen  
           56  Projected image including random optical patterns  
           57  Random optical pattern  
           100  operation  
           110  operation  
           120  operation  
           130  operation  
           140  operation  
           150  operation  
           160  operation  
           170  operation