Abstract:
A method and apparatus for enhancing an edge transient contained in a video signal include identifying the edge transient and the edge center of the edge transient in the video signal; selecting a side of the edge transient, relative to an edge center thereof, to enhance; and adding an edge change signal to the video signal to asymmetrically enhance the selected edge transient.

Description:
FIELD OF THE INVENTION 
     This invention relates to video signal processing, and more particularly, to a method and apparatus for enhancing edge transients contained in a video signal to improve displayed video image sharpness by asymmetrically steepening the edge transients. 
     BACKGROUND OF THE INVENTION 
     Improvements in the perceived sharpness of a displayed video image produced from a video signal, are typically accomplished with edge transient sharpness enhancement methods that employ an enhancement signal which is added to the video signal to steepen edge transients contained therein. Utilizing such an enhancement signal steepens the rise in a transition from a dark image region to a light image region, or alternatively, steepens the fall in a transition from a light image region to a dark image region. 
     Some prior art edge transient sharpness enhancement methods employ linear techniques which enhance frequencies that are already within the video image by using a linear high frequency boosting filter. Other prior art edge transient sharpness enhancement methods employ highly non-linear techniques that attempt to steepen the edge transients without introducing so-called under and overshoots which are introduced by the linear techniques. As such, truly new high frequencies are introduced into the displayed video image. 
     The linear techniques are used in some prior art edge transient sharpness enhancement methods to symmetrically modify video signal edge transients on both sides of the edge transient center by steepening both sides of the edge transient substantially equally. Other prior art edge transient sharpness enhancement methods use the non-linear technique to steepen the sides of the edge transient, but all in a symmetrical manner. 
       FIG. 1  is an enhanced edge transient generated by an exemplary prior art edge transient enhancement method that localizes the edge center of the edge transient of a video signal and steepens the edge transient on both sides of the edge center symmetrically. Solid line  10  represents an edge transient of a video signal having an edge center  12 , a first edge side portion  14  and a second edge side portion  16 . Solid line  18  represents an edge enhancement signal or edge change signal. The edge change signal  18  is generally symmetrical around its zero crossing, the position of which is located at the edge center  12  of the edge transient  10 . The edge change signal  18  is added to the original video signal  10  to arrive at a video signal with an enhanced edge transient represented by dashed line  20 . The enhanced edge transient  20  includes an edge center  22  which is substantially identical to edge center  12  of the original edge transient video signal  10 , a first enhanced or steepened edge side portion  24  and a second enhanced or steepened edge side portion  26 . 
     There are problems associated with these prior art edge transient enhancement methods. Thin (white) lines in the displayed video image appear wider than the same lines in the original video image, and if the enhancement is performed robustly, the resulting video image may appear very unnatural. 
     Accordingly, improvements in edge transient enhancement is needed to overcome the problems associated with prior art edge transient enhancement methods. 
     SUMMARY OF THE INVENTION 
     A method and apparatus is disclosed herein for enhancing an edge transient contained in a video signal. The method comprises identifying the edge transient in the video signal, and asymmetrically enhancing the edge transient of the video signal. The apparatus comprises means for identifying the edge transient in the video signal, and means for asymmetrically enhancing the edge transient of the video signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with accompanying drawings where like numerals are used to identify like elements and wherein: 
         FIG. 1  graphically depicts an enhanced edge transient generated by a prior art edge transient enhancement method; 
         FIG. 2  graphically depicts an enhanced edge transient generated by the method of the present invention; and 
         FIG. 3  is a block diagram of an edge enhancement apparatus according to an exemplary embodiment of the invention. 
     
    
    
     It should be understood that the drawings are for purposes of illustrating the concepts of the invention and are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention achieves improved displayed video image sharpness by asymmetrically steepening edge transients contained in a video signal.  FIG. 2  graphically depicts an enhanced edge transient generated by the method of the present invention. Solid line  30  represents an edge transient of a video signal having an edge center  32 , a first edge side portion  34 , which is the darker portion of the transient  30 , and a second edge side portion  36 , which is the brighter portion of the transient  30 . Solid line  38  represents an edge enhancement signal or edge change signal. In accordance with the present invention, the edge change signal  38  includes a zero crossing  39  which is located at the edge center  32  of the edge transient  30 . The edge change signal  38  is generally asymmetrical around the zero crossing  39 . 
     The edge change signal  38  is added to the video signal to asymmetrically enhance the edge transient  30  of that signal, i.e., to arrive at a video signal with an asymmetrically enhanced edge transient represented by dashed line  40 . The enhanced edge transient  40  includes an edge center  42  which is located at the edge center  32  of the edge transient  30 . The enhanced edge transient  40  further includes an enhanced or steepened first edge side portion  44 , i.e., the darker portion of the edge transient, and a second edge side portion  46 , i.e., the brighter portion of the edge transient, which is not changed, or is steepened significantly less than the first edge side portion  44 . As a result, thin (white) lines are preserved in the displayed video image thereby resulting in a much more natural image. Additionally, small errors in the enhancement are generally less visible in the darker portion  44  of the edge transient  40  than in the brighter portion  46 . Accordingly, robustness is improved. 
     Although, in most applications it is more advantageous to limit the amount of steepening applied to the brighter side of the edge transient, the invention is not limited to this embodiment. For example, there may be applications where it is advantageous to steepen the brighter side of the edge transient and limit the amount of steepening applied to the darker side of the edge transient. 
     The result of the introduced asymmetry is a more natural looking edge enhancement when the video image is displayed on a CRT. Any CRT inherently follows a gamma curve that relates the input luminance level to the perceived intensity level. As the gamma is larger than 1 (one), a change in the brighter side portion of the edge transient will cause a larger perceived change in intensity levels compared to the same luminance level change for the darker side portion of the edge transient. The asymmetrical edge enhancement method of the present invention at least partially compensates for this in the luminance domain. 
       FIG. 3  is a block diagram of an edge enhancement apparatus  50 , according to an exemplary embodiment of the invention, that asymmetrically steepens edge transients in video signals. In general, the apparatus  50  can be implemented in software, hardware, or a combination of both. The apparatus  50  comprises an edge center detector  52 , an edge side detector  54 , a first change signal generator  56 , a second change signal generator  58 , a switch  60 , and an adder  62 . The first change signal generator  56  generates an asymmetrical edge change signal that steepens the darker portion of an edge transient. The second change signal generator  58  generates an asymmetrical edge change signal that steepens the brighter portion of an edge transient. The change signal generators  56 ,  58  may be implemented using conventional linear techniques. By way of example and not limitation, the change signal generators  56 ,  58  may be implemented using linear highpass or bandpass filters. In another example, the change signal generators  56 ,  58  may be implemented using a non-linear luminance transient improvement (LTI) algorithm. 
     In operation, the edge center detector  52  detects the spatial location of an edge transient contained in an inputted video signal and the edge transient&#39;s center position, and outputs this data to the edge side detector  54 . The edge side detector  54  determines on which side of the edge transient center the current pixel is, i.e., the darker (lower) or brighter (higher) portion of the edge transient, and outputs this information to the switch  60 . The output of the edge side detector  54  is a binary signal that is used to select between the output of the first and second change signal generators  56 ,  58 . The output of one of the first and second change signal generators  56 ,  58  is selected by the switch  60  in accordance with signal outputted by the edge side detector  54  (which includes an identification of what side of the edge center the current pixel was located), and added to the original video signal via the adder  62  to asymmetrically enhance the edge transient of the original video signal thereby arriving at a video signal with an asymmetrically enhanced edge transient. 
     While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. For example, other methods can be used for asymmetrically steepening edge transients contained in a video signal. One method may involve shifting a linear change signal (such as the edge change signal  18  shown in  FIG. 1 ) somewhat towards the edge peak of the video signal, i.e., out of the center. As such, the edge center changes also, and the edge has been asymmetrically steepened with respect to the input edge transient signal. This and all other such modifications and changes are considered to be within the scope of the appended claims.