Abstract:
A digital image processor is provided. The digital image processor includes a deinterlacing processor that is implemented upon a digital processing unit. The deinterlacing processor is coupled to an input operable to receive an interlaced video stream, a digital memory for storing portions of the interlaced video signal, and an output operable to transmit a deinterlaced video stream. The deinterlacing processor is operable to perform frequency analysis upon the received interlaced video stream in order to generate the deinterlaced video stream having reduced motion artifacts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefits of U.S. Patent Provisional Application No. 60/096,144 filed on Aug. 11, 1998, and is a continuation of and also claims the benefit of U.S. patent application Ser. No. 09/372,713 filed on Aug. 11, 1999 and issued as U.S. Pat. No. 6,489,998 on Dec. 3, 2002, and is related to U.S. patent application Ser. No. 09/167,527 filed on Oct. 6, 1998 and issued as U.S. Pat. No. 6,380,978 on Apr. 30, 2002, all three of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the processing of video images and, more particularly, to techniques for deinterlacing video images. 
     2. Description of the Related Art 
     All major television standards use a raster scanning technique known as “interlacing” or “interlace scanning.” Interlace scanning draws horizontal scan lines from the top of the screen to the bottom of the screen in two passes. Each pass is known as a field. In the National Television System Committee (NTSC) standard used in North America, each field takes approximately 1/60 th  of a second to draw. 
     Interlace scanning depends of the ability of the cathode ray tube (CRT) phosphors to retain an image for a few milliseconds, in effect acting like a “memory” to retain the previous field while the newer interleaved field is being scanned. Interlace scanning provides a benefit in television systems by doubling the vertical resolution of the system without increasing broadcast bandwidth. 
       FIG. 1  shows a number of parallel horizontal scan lines  10  on a conventional television display. A first set of horizontal lines  12  is scanned in a first field period and then a second set of horizontal lines  14  is scanned in a second field period. Thus, the first field is temporarily shifted by 1/60 th  of a second from the second field. When rapidly changing images are being displayed, an object in motion may appear to be fuzzy due to the temporal displacement between the two fields. 
     This temporal displacement typically does not create a problem on conventional television displays, primarily because the image of the “older” field quickly fades in intensity as the light output of the phosphors decays. A secondary reason is that the spatial displacement in the images caused by motion results in a fine detail that television displays resolve well. For these reasons, interlace scanning of motion pictures works acceptably well on conventional television displays. 
       FIG. 2  shows a set of progressively scanned horizontal lines  16 . In progressive scanning, all horizontal lines  16 , are scanned out in one vertical pass  18 , so there is no time displacement of adjacent lines as in interlace scan. Progressive scanning requires a much higher bandwidth signal. Consequently, progressive scanning is typically used for applications where improved image quality and higher resolution are required, relative to conventional television systems. Progressive scanning is widely used in computer CRTs and liquid crystal displays (LCD). 
     If a motion picture formatted for an interlaced monitor device as in  FIG. 1  is to be displayed on a progressively scanned device as in  FIG. 2 , then it must be converted from the interlaced format to the progressive format. This format conversion is known as deinterlacing.  FIG. 3  is a flow diagram of a deinterlace process  19  of the prior art. A first series of interlaced video fields  20  is generated by a video source (not illustrated) at 1/60 th  second intervals. 
     In this example, each of the video fields  20  has a spatial resolution of 720 horizontal by 240 vertical pixels. Each field contains half the vertical resolution of a complete video image. The first series of video fields  20  are input to a deinterlace processor  22 , which converts the 720 by 240 interlaced format to a second series of video fields  24 . In this example, each of the second series of video fields  24  may have 720 by 480 pixels where the fields are displayed at 60 frames per second. 
       FIG. 4  shows a prior art method  25  of deinterlace processing. A video field  26  containing scan lines  30 , and a previous video field  28  containing scan lines  32  is fed into a field combination deinterlace processor  34 . The result is a combined frame  36  with scan lines  38  sourced from video field  26  and scan lines  40  sourced from video field  28 . When this simple deinterlacing of the prior art is performed, and a motion picture formatted for an interlace display is converted to a progressive format, a noticeable “artifact” or error arises because the image content of vertically adjacent lines is time shifted by 1/60 th  second as noted previously. The error is most visible around the edges of objects that are in motion. 
       FIG. 5  shows a deinterlaced image  42  with a stationary object  43  that is rendered without distortion.  FIG. 6  shows an image  44  with the object  43 ′ in motion. The edges of object  43 ′ create artifacts  45  on the edges of the image  44  because of the aforementioned temporal shift. These artifacts  45  are introduced into the image by the conventional field combination deinterlacing method  25  of  FIG. 4 . 
       FIG. 7  is an illustration of an alternative prior art method  46  to deinterlace an image using a single reference field rather than two fields. The method  46  interpolates or doubles the number of lines of one field to produce a progressive frame. A video field  48  is scanned from an image to contain a half set of lines  50 . The half set of lines  50  is deinterlaced by line interpolation in a deinterlacing interpolator  52 . 
     The resulting frame  54  will have all the lines  50  of the original video field  48 . The remaining lines  56  are created by interpolation of lines  50 . The resultant image will not have motion artifacts because all the lines in the image will be created from lines  50  that are time correlated. This alternative method  46  of deinterlacing does not produce motion artifacts, but the vertical resolution of the image is reduced by half. 
     In summary, deinterlacing by combining two fields into a single frame preserves the vertical resolution in an image, but may result in motion artifacts. Deinterlacing by interpolation of a single field to produce a frame eliminates the motion artifacts, but discards half the vertical resolution of the original image. In view of the forgoing, it is desirable to have a method of deinterlacing that provides for preservation of the full resolution of an image, while at the same time eliminating motion artifacts. 
     SUMMARY OF THE INVENTION 
     The present invention fills these needs by providing a method and apparatus for deinterlacing a video input stream while reducing motion artifacts and maintaining vertical resolution in the deinterlaced video stream. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below. 
     In one embodiment of the present invention, a digital image processor is provided. The digital image processor includes a deinterlacing processor that is implemented upon a digital processing unit. The deinterlacing processor is coupled to an input operable to receive an interlaced video stream, a digital memory for storing portions of the interlaced video signal, and an output operable to transmit a deinterlaced video stream. The deinterlacing processor is operable to perform frequency analysis upon the received interlaced video stream in order to generate the deinterlaced video stream having reduced motion artifacts. 
     In another embodiment of the present invention, a method for deinterlacing an interlaced video stream is provided. The method includes receiving a video frame including a number of pixels from an input of the interlaced video stream. The video frame is analyzed for frequency information inherent to the video frame in order to detect motion artifacts. A number of motion artifact detection values is determined for each of the pixels in the video frame. An ultimate detection value is then determined for each motion artifact detection values. The ultimate detection value corresponding to each pixel is mixed with a set of spatially corresponding pixels to generate an output pixel. 
     In yet another embodiment of the present invention, a method for deinterlacing an interlaced video stream is provided. The method includes receiving a first video frame including a number of pixels from an input of the interlaced video stream. The first video frame is analyzed for frequency information inherent to the first video frame in order to detect motion artifacts. A number of motion artifact detection values is determined for each of the pixels in the first video frame from which. An ultimate detection value is then determined for each motion artifact detection value. A second video frame, which includes pixels that spatially correspond to pixels of the first video frame, is determined from the input of the interlaced video stream. The ultimate detection value corresponding to each pixel is then mixed with a set of spatially corresponding pixels in the second video frame to generate an output pixel. 
     An advantage of the present invention is that it allows for detection and reduction of motion artifacts in video images. By reducing the effect of the motion artifact, the video image becomes much clearer and appears to be free of defects. Further, the deinterlacing is accomplished without loss of vertical resolution. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements. 
         FIG. 1  shows a number of parallel horizontal scan lines on a conventional television display. 
         FIG. 2  shows a set of progressively scanned horizontal lines in a prior art progressive scan display. 
         FIG. 3  is an illustration of a deinterlace process of the prior art. 
         FIG. 4  is a further illustration of deinterlace processing of the prior art. 
         FIG. 5  shows a deinterlaced image of the prior art with a stationary object. 
         FIG. 6  shows a deinterlaced image of the prior art with an object in motion, creating undesirable “artifacts.” 
         FIG. 7  is a flow diagram of an alternative prior art method to deinterlace an image using a single reference field. 
         FIG. 8  shows a two-dimensional array of pixel values used to describe the present invention. 
         FIG. 9  is a diagram illustrating a method for using obtaining an output pixel from the two-dimensional array of  FIG. 8  in accordance with the present invention. 
         FIG. 10A  is an illustration used to describe the method of the present invention. 
         FIG. 10B  is a graph of a set of samples from the sampling line of  FIG. 10A . 
         FIG. 10C  is a graph of a sampled cosine wave. 
         FIG. 11  is an illustration used to describe the method of thresholding a detection value of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method and apparatus for a video deinterlace processing is disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. 
       FIGS. 1–7  were discussed with reference to the prior art.  FIG. 4  illustrated the combination of two temporally shifted fields that are adjacent in time, which are combined to create a frame that has double the vertical resolution of each field. For example, if the fields have a resolution of 720 horizontal pixels by 240 vertical pixels, then the combined frame has a resolution of 720 horizontal pixels by 480 vertical pixels. This combined frame will have the full vertical resolution available from the source, but is also prone to having motion artifacts. 
       FIG. 8  shows a two-dimensional array of pixel values  58  that is a subset of the combined frame  36  of  FIG. 4 . The combined frame  36  may be stored in a digital memory unit  59 . Digital memory unit  59  is used to store portions of the interlaced video stream, and is particularly useful for storing temporally adjacent video fields in the present invention. The array  58  is shown having a width of 5 pixels and a height of 7 pixels. The array  58  is labeled across the top C 0  to C 4  indicating columns and is labeled vertically along the left side from the top to bottom R 0  to R 6  indicating rows. The array  58  can be viewed as a moving window that scans across the combined frame  36  from left to right and top to bottom. 
     The array  58  is positioned so that a set of even numbered rows  60  contain pixels from the most recent or “current” field of the original source, and a set of odd numbered rows  62  contain pixels from the previous field. The array  58  is then stepped across the combined frame  36  from left to right horizontally. Each step causes the pixels in each of columns C 1 , C 2 , and C 3  and C 4  to shift to the column to its immediate left. The pixels in column C 0  shift out of the array  58 , and a new column of pixels shifts into column C 4 . 
     After the array  58  has been stepped across all the horizontal positions, it is stepped down vertically by two pixels and returned to the left side of the field. Therefore, even numbered rows  60  contain pixels from the most recent field and odd numbered lines  62  contain pixels from the previous field. The process then repeats itself as array  58  is then stepped across the combined frame  36  again from left to right horizontally. 
       FIG. 9  illustrates a method  64  for using obtaining an output pixel  76  from the two-dimensional array  58 . In an act  66 , a frequency detection value is obtained using the seven pixels of each column of the two-dimensional array  58 . The magnitude of a frequency detection value corresponds to the energy or intensity of the detected motion artifact in a specific pixel. Because there are five columns, there are five frequency detections performed, producing a set of detection values fd 0 , fd 1 , fd 2 , fd 3 , and fd 4 . Next, an act  68  thresholds the set of detection values fd 0 –fd 4 . Then, in act  70 , the set of detection values fd 0 –fd 4  is combined to compute a weighted average. 
     The weighted average is then used in an act  72  to compute an ultimate detection value (UDV). The weighting factors may include variables. One weighting example is in the following Equation 1:
 
 UDV =( fd 0+(2 *fd 1)+(8 *fd 2)+(2 *fd 3)+ fd 4)/14
 
     The weighting causes frequency detection values closest to the center of array  58  to have the greatest influence on UDV. In this way, using five horizontally adjacent frequency detection values results in a low pass filtering act providing smoother transitions between areas within the image  36  where motion artifacts do and do not exist. 
     UDV computed in act  72  is used to control an act  74 , which mixes a pixel with spatially corresponding pixels from the center of array  58  to generate an output pixel. Act  74  preferably implements the following Equation 2:
 
pixelout=( UDV *( pR 2 C 2 +pR 4 C 2)/2)+((1 −UDV )* pR 3 C 2)
 
     where pixelout is the new the output pixel of the deinterlacing act at position pR 2 C 2  is a pixel in the array  58  at location Row  2 , Column  2 , pR 4 C 2  is a pixel in the array  58  at location Row  4 , Column  2 , and pR 3 C 2  is a pixel in the array  58  at location Row  3 , Column  2 . 
     The result of mixing act  74  is that the new value of pixel pR 3 C 2  of the array  58  depends on UDV. If no motion is detected by the calculation of UDV, then the pixel at pR 3 C 2  will be the unmodified value of the pixel at that position in the previous field. If a large UDV, i.e., a value of 1 results, then a strong motion artifact has been detected, and the value of pR 3 C 2  is computed by averaging the values of pR 2 C 3  and pR 4 C 3  of the array  58 . The averaged result will not show motion artifacts because is created from values of the most recent field that are time correlated with the most recent field. Detection values that are between 0 and 1 will cause the pixel at pR 3 C 2  to be a mix of pR 3 C 2  and the average of pR 2 C 3  and pR 4 C 3 . 
       FIG. 10A  illustrates an image  78  showing act  66  in greater detail. Image  78  shows the computation of a single frequency detection value for one column of array  58 . Image  78  includes a distorted object  80  which is effected by an interlace motion artifact. Image  78  is sampled along a line  82 , which is shown for exemplary purposes. This sampling corresponds to one of the columns in two-dimensional array  58 . In this example, line  82  passes through an area where artifacts exist, but in general, a sampling of vertical adjacent pixels may or may not contain artifacts. 
       FIG. 10B  is a graph  84  a set of samples  86  obtained by sampling along line  82  of  FIG. 10A . The set of samples  86  are plotted with the row numbers along the horizontal axis and the brightness or intensity of the pixel along the vertical axis. From graph  84 , it is apparent that in the areas where motion artifacts exist, such as the set of samples  86 , will show a characteristic frequency. This is frequency in space rather than in time and is most conveniently expressed as cycles per line rather than cycles per second or Hertz. The characteristic frequency is 1 cycle/2 lines or 0.5 cycles/line. 
       FIG. 10C  is a graph of a sampled cosine wave  88 . The characteristic frequency created by the motion artifact is detected by multiplying the set of samples  86  by the sampled cosine wave  88 . The sampled cosine wave  88  has a frequency equal to the characteristic frequency of the motion artifact. Then, the result is integrated using the following equation: 
     
       
         
           
             fd 
             = 
             
               
                 ∑ 
                 
                   R 
                   = 
                   0 
                 
                 
                   R 
                   = 
                   6 
                 
               
               ⁢ 
               
                 
                   Y 
                   ⁡ 
                   
                     ( 
                     R 
                     ) 
                   
                 
                 ⁢ 
                 
                   cos 
                   ⁡ 
                   
                     ( 
                     
                       2 
                       ⁢ 
                       R 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                       * 
                       0.5 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       cycles 
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       line 
                     
                     ) 
                   
                 
               
             
           
         
       
     
     where fd is the frequency detection value for one column of array  58 , R is a line index corresponding to the R 0  . . . R 6  of array  58  and has the units “line,” and Y(R) is the set of vertically adjacent samples  86 . 
     The expression cos (2πR*0.5 cycles/line) simplifies to 1 for R=0, 2, 4, and 6 and −1 for R=1, 3, and 5. If 1 and −1 are substituted for R 0  . . . R 6 , the frequency detection equation becomes: fd=(Y6/2+Y4+Y2+Y0/2)−(Y5+Y3+Y1). Note that Y6 and Y0 are divided by 2 because the integration is over the limits 0 to 6. The final fd is the absolute value: fd=Abs(fd). The method  64  of  FIG. 9  is repeated for each column in array  58 , producing the set of frequency detection values  68 . 
       FIG. 11  is a graph  90  of thresholding act  68  in greater detail. Each fd is a number in the range 0 to 1. Graph  90  includes a non-thresholded scale  92  from which values are thresholded to the thresholded scale  94 . Thresholding sets all values above the upper threshold point  96  to the value of 1. All values below the lower threshold point  98  are set to a value of 0. Values between the upper and lower thresholds are expanded to the range 0 to 1. Thresholding can be described with the following equation:
   tdf =( ptfd−LTH )/ UTH   
     where tdf is the thresholded frequency detection value, pthfd is the pre-thresholded frequency detection value (the output of act  66 ), LTH is the lower threshold value and UTH is the upper threshold value. If tfd&gt;1.0, then tfd=1.0. Otherwise, if tfd&lt;0 then tfd=0. 
     While this invention has been described in terms of several preferred embodiments, it will be appreciated that those skilled in the art upon reading the preceding specifications and studying the drawings will realize various alterations, additions, permutations and equivalents thereof. It is therefore intended that the present invention include all such alterations, additions, permutations, and equivalents as fall within the true spirit and scope of the invention. 
     It will therefore be appreciated that the present invention provides a method and apparatus for deinterlacing an interlaced video stream while maintaining the original resolution of the video stream while reducing edge artifacts in moving objects in an output video image. This is accomplished by employing two-field interlacing where the image is relatively static, and employing one-field line doubling where the image is rapidly changing. The combination of these techniques provides a low-artifact, high-resolution deinterlaced image. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Furthermore, certain terminology has been used for the purposes of descriptive clarity, and not to limit the present invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims.