Abstract:
A plurality of bandpass filters extract high frequency components of a digital image signal sampled every predetermined unit, thus generating a plurality of high frequency signals. The high frequency signals are added to produce a resultant high frequency signal. On the basis of the value of the resultant high frequency signal, the maximum value or minimum value detected from the high frequency signals is selected. The resultant high frequency signal and the selected maximum or minimum value are added according to determined weights, so that an enhancement signal is generated. Edge enhancement is performed using the enhancement signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2003-377118 filed Nov. 6, 2003, which is hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an edge enhancement system and method for performing edge enhancement of image signals. 
   2. Description of the Related Art 
     FIG. 5  is a block diagram of a conventional edge enhancement circuit  513 . In  FIG. 5 , a luminance-signal generation circuit (MainY)  501  generates a main luminance signal derived from image data on which edge enhancement is to be applied. A horizontal bandpass filter (H-BPF)  502  detects a horizontal edge component of the main luminance signal generated from the luminance-signal generation circuit  501  to generate a horizontal edge signal. A gain circuit  505  applies an arbitrary gain to the horizontal edge signal generated from the horizontal bandpass filter  502  to control the amplitude of the signal. 
   Similarly, a vertical bandpass filter (V-BPF)  503  detects a vertical edge component of the main luminance signal to generate a vertical edge signal, and a diagonal bandpass filter (D-BPF)  504  detects a diagonal edge component of the main luminance signal to generate a diagonal edge signal. Gain circuits  506  and  507  control the amplitudes of the corresponding edge signals, respectively. 
   Adders  508  and  509  add the horizontal, vertical, and diagonal edge signals to generate an edge enhancement signal. A gain circuit  510  controls the amplitude of the edge enhancement signal. Finally, an adder  511  adds the main luminance signal generated from the luminance-signal generation circuit  501  to the edge enhancement signal generated from the gain circuit  510 , thus generating an edge-enhanced luminance signal. 
   In the conventional edge enhancement circuit  513 , the horizontal, vertical, and diagonal edge signals are added, thus generating a resultant edge signal. When the amplitudes of the respective edge signals are large, the amplitudes overlap each other. Thus, the amplitude of the resultant edge signal is too large. This can result in the degradation of quality of an image subjected to edge enhancement. In this instance, a spatial frequency domain with the problem of amplitude overlap resulting in abnormally large amplitude will now be described.  FIG. 6  shows regions occupied by the horizontal, vertical, and diagonal edge signals in the spatial frequency domain. Referring to  FIG. 6 , the abscissa denotes a horizontal spatial frequency axis and the ordinate denotes a vertical spatial frequency axis. As a region comes closer to the origin, its frequency becomes lower. Referring to  FIG. 6 , regions H are detected by the horizontal bandpass filter  502 , regions V are detected by the vertical bandpass filter  503 , and regions D are detected by the diagonal bandpass filter  504 . A horizontal edge signal overlaps a diagonal edge signal in the region shown by the arrow a of  FIG. 6 . Thus, the above-mentioned problem occurs. 
     FIG. 7  shows an example of the generation of a resultant edge signal having an abnormally large amplitude in the region shown by arrow a of  FIG. 6 . Referring to  FIG. 7 , the horizontal bandpass filter  502  generates a horizontal edge signal H having a waveform  701  in the region shown by the arrow a of  FIG. 6 . The diagonal bandpass filter  504  generates a diagonal edge signal D having a waveform  702  in the region shown by the arrow a of  FIG. 6 . In this case (the amplitude of a vertical edge signal is zero), as shown in  FIG. 7 , the maximum amplitude of a resultant edge signal Mix having a waveform  703  is too large compared to a predetermined value obtained by superimposing the amplitude of the diagonal edge signal on that of the horizontal edge signal. The disadvantage is that unnatural edge enhancement is caused by the above resultant edge signal (enhancement signal) having the abnormally large amplitude. 
   SUMMARY OF THE INVENTION 
   The present invention is made in consideration of the above-mentioned problems by providing an edge enhancement system and method capable of preventing unnatural edge enhancement in images such as complex video images. 
   According to an aspect of the present invention, an edge enhancement system for images is disclosed. The edge enhancement system includes: a plurality of bandpass filters for extracting high frequency signals of a digital image signal sampled every predetermined unit; an adder for adding the high frequency signals generated from the respective bandpass filters to produce a first high frequency signal; a maximum value detector for detecting the maximum value of the high frequency signals for each sampling to produce a second high frequency signal; a minimum value detector for detecting the minimum value of the high frequency signals for each sampling to produce a third high frequency signal; and a weight adder for determining weights for the first, second, and third high frequency signals on the basis of the value of the first high frequency signal and adding the first high frequency signal to the second or third high frequency signal according to the weights to produce a fourth high frequency signal. 
   The present invention further provides an edge enhancement method including the steps of receiving a sampled digital image signal and then extracting high frequency signals from the sampled digital image signal. Next, a first high frequency signal is produced by combining the extracted high frequency signals, and then a maximum value of the high frequency signals for each sampling is determined to produce a second high frequency signal. A minimum value of the high frequency signals for each sampling to produce a third high frequency signal is determined and weights for the first, second, and third high frequency signals based on the first high frequency signal are also determined. Based on the determined weights, the method adds the first high frequency signal to either the second or third high frequency signal to produce a fourth high frequency signal. 
   According to another aspect of the present invention, when the first high frequency signal has an abnormally large amplitude, the weight for the first high frequency signal is set to a low value and the weight for the second or third high frequency signal is set to a high value. Accordingly, the fourth high frequency signal obtained by adding the above signals according to the weights can be set to the optimum value that is equal to or lower than a predetermined value, namely, a value that is not so extraordinarily large. Thus, in edge enhancement for complex video images, advantageously, unnatural edge enhancement caused by an enhancement signal having an abnormally large amplitude can be prevented and high quality images can be obtained. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a schematic block diagram of an edge enhancement circuit  413  according to an embodiment of the present invention. 
       FIG. 2A  shows output signal waveforms of components of the edge enhancement circuit  413  shown in  FIG. 1 . 
       FIG. 2B  shows output signal waveforms of components of the edge enhancement circuit  413  in  FIG. 1 . 
       FIG. 3  is a diagram showing the input-output characteristic of a weighted additional coefficient calculation circuit included in a weighted average circuit  116 . 
       FIG. 4  is a schematic block diagram of an imaging system having an edge enhancement circuit in accordance with an embodiment of the present invention. 
       FIG. 5  is a block diagram of a conventional edge enhancement circuit  513 . 
       FIG. 6  shows a spatial frequency domain including regions occupied by horizontal, vertical, and diagonal edge signals. 
       FIG. 7  shows an example of the generation of a resultant edge signal having an abnormally large amplitude within the region of arrow a of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Embodiments of the present invention will be described in detail in accordance with the accompanying drawings. An imaging system in which an edge enhancement circuit is employed will now be described. 
     FIG. 4  is a schematic block diagram of an imaging system having an edge enhancement circuit in accordance with an embodiment of the present invention. In  FIG. 4 , a lens  401  and an optical filter  402  form an object image on an imaging device  403 . The optical filter  402  includes an optical low-pass filter and an infrared cut filter. The imaging device  403  includes a CCD (charge coupled device) sensor or a CMOS (complementary metal oxide semiconductor) sensor. In this case, the imaging device  403  has RGB (red, green, and blue) color filters. 
   An analog-to-digital (A/D) converter  404  converts the analog signal output from the imaging device  403  into a digital signal (image data). The image data is subjected to dark noise correction by an optical black circuit (OB)  405  after which a white balance circuit (WB)  406  performs white balance processing on the image data generated from the optical black circuit  405 . A color interpolation circuit  407  performs interpolation calculation for each color pixel of the image data output from the white balance circuit  406 . A color conversion matrix (MXT) circuit (ColorMTX)  408  converts the image data (R, G, and B signals), subjected to interpolation calculation and generated from the color interpolation circuit  407 , into YUV data. The YUV data consists of a Y signal representing the luminance signal, a U signal, which is the difference between the luminance signal and the R component, and a V signal, which represents the difference between the luminance signal and the B component. 
   A chroma suppress circuit (CSup)  409  processes the YUV data to suppress the colors of chroma components within a saturation region. A UV gain circuit (UVGain)  410  applies arbitrary gains to the U and V signals included in the YUV data generated from the chroma suppress circuit  409 , respectively. 
   In addition, the image data generated from the white balance circuit  406  is also supplied to a luminance-signal (Y-signal) generation circuit (MakeY)  412 . The Y-signal generation circuit  412  performs interpolation calculation on the image data to produce a luminance signal. An edge enhancement circuit (Enhance)  413  uses the produced luminance signal to perform edge enhancement processing (e.g., edge emphasis). 
   A luminance-signal generation circuit (MainY)  101 , shown in  FIG. 1 , generates a main luminance signal based on a luminance signal generated from the Y-signal generation circuit  412  in  FIG. 4 . A horizontal bandpass filter (H-BPF)  102  detects a horizontal edge component of the main luminance signal generated from the luminance-signal generation circuit  101  to generate a horizontal edge signal. A vertical bandpass filter (V-BPF)  103  detects a vertical edge component of the main luminance signal of the luminance-signal generation circuit  101  to generate a vertical edge signal. A diagonal bandpass filter (D-BPF)  104  detects a diagonal edge component of the main luminance signal of the luminance-signal generation circuit  101  to generate a diagonal edge signal. 
   A gain circuit  105  applies an arbitrary gain to the horizontal edge signal generated from the horizontal bandpass filter  102  to control the amplitude of the signal and then generates an amplitude-controlled horizontal edge signal Yh. Similarly, a gain circuit  106  applies an arbitrary gain to the vertical edge signal generated from the vertical bandpass filter  103  to control the amplitude of the signal and generates an amplitude-controlled vertical edge signal Yv. A gain circuit  107  applies an arbitrary gain to the diagonal edge signal generated from the vertical bandpass filter  104  to control the amplitude of the signal and generates an amplitude-controlled diagonal edge signal Yd. 
   An RGB conversion circuit (ConvRGB)  411  converts the U and V signals generated from the UV gain circuit  410  and the luminance signal generated from the edge enhancement circuit  413  into RGB data. A gamma correction circuit (GammaRGB)  414  performs gamma correction on the RGB data generated from the RGB conversion circuit  411 . A JPG conversion circuit (JPG)  415  converts the RGB data generated from the gamma correction circuit  414  into JPEG (Joint Photographic Experts Group) image data. The JPEG image data is then recorded onto a recording medium (not shown) that is detachable from the imaging system. 
   First Embodiment 
   First, the schematic structure of an edge enhancement system included in an imaging system (for example, like a digital camera) according to an embodiment of the present invention.  FIG. 1  shows an edge enhancement circuit according to an embodiment of the present invention. This edge enhancement circuit  413  in  FIG. 1  is employed in the imaging system of  FIG. 4 . 
   A luminance-signal generation circuit (MainY)  101  generates a main luminance signal based on a luminance signal generated from the Y-signal generation circuit  412  in  FIG. 4 . A horizontal bandpass filter (H-BPF)  102  detects a horizontal edge component of the main luminance signal generated from the luminance-signal generation circuit  101  to generate a horizontal edge signal. A vertical bandpass filter (V-BPF)  103  detects a vertical edge component of the main luminance signal of the luminance-signal generation circuit  101  to generate a vertical edge signal. A diagonal bandpass filter (D-BPF)  104  detects a diagonal edge component of the main luminance signal of the luminance-signal generation circuit  101  to generate a diagonal edge signal. 
   A gain circuit  105  applies an arbitrary gain to the horizontal edge signal generated from the horizontal bandpass filter  102  to control the amplitude of the signal and generates an amplitude-controlled horizontal edge signal Yh. Similarly, a gain circuit  106  applies an arbitrary gain to the vertical edge signal generated from the vertical bandpass filter  103  to control the amplitude of the signal and generates an amplitude-controlled vertical edge signal Yv. A gain circuit  106  applies an arbitrary gain to the diagonal edge signal generated from the vertical bandpass filter  104  to control the amplitude of the signal and generates an amplitude-controlled diagonal edge signal Yd. 
   An adder  108  adds the horizontal edge signal Yh to the vertical edge signal Yv. An adder  109  adds the output of the adder  108  to the diagonal edge signal Yd to generate a resultant edge signal Ymix. In other words, the adders  108  and  109  add the horizontal edge signal Yh, the vertical edge signal Yv, and the diagonal edge signal Yd shown in the following Expression 1 to produce the resultant edge signal Ymix.
 
 Ymix=Yh+Yv+Yd   (Expression 1)
 
   For every pixel, a maximum value detection circuit  110  detects the maximum value from values of the horizontal edge signal Yh, the vertical edge signal Yv, and the diagonal edge signal Yd to generate a maximum signal Ymax. A minimum value detection circuit  111  detects the minimum value from values of the horizontal edge signal Yh, the vertical edge signal Yv, and the diagonal edge signal Yd every pixel to generate a minimum signal Ymin. A gain circuit  112  controls the amplitude of the maximum signal Ymax. A gain circuit  113  controls the amplitude of the minimum signal Ymin. When the edge signal Ymix is positive, a selection circuit  114  selects the amplitude-controlled maximum signal Ymax generated from the gain circuit  112  and outputs the selected signal as a selection signal Ysel. When the edge signal Ymix is negative, the selection circuit  114  selects the amplitude-controlled minimum signal Ymin generated from the gain circuit  113  and outputs the selected signal as the selection signal Ysel. 
   A subtractor  115  subtracts the selection signal Ysel from the edge signal Ymix to produce a difference signal Ydiff. According to the present embodiment, the difference signal Ydiff indicates the absolute value of the difference between the edge signal Ymix and the selection signal Ysel. 
   A weighted average circuit  116  applies weighting and averaging to the edge signal Ymix and the selection signal Ysel on the basis of the difference signal Ydiff to produce an enhancement signal Yenhance. Specifically, to calculate a weighted additional coefficient α (0≦α≦1) on the basis of the difference signal Ydiff, the weighted average circuit  116  includes a weighted additional coefficient calculation circuit (not shown) having the input-output characteristic shown in  FIG. 3 . The weighted additional coefficient calculation circuit calculates the weighted additional coefficient α to determine the ratio of the edge signal Ymix to the selection signal Ysel to be added on the basis of the difference signal Ydiff. As shown in  FIG. 3 , as the difference signal Ydiff becomes larger, the weighted additional coefficient calculation circuit obtains the weighted additional coefficient α so that the coefficient α approaches 0. As the difference signal Ydiff becomes smaller, the weighted additional coefficient calculation circuit calculates the weighted additional coefficient α so that the coefficient α approaches 1. The weighted average circuit  116  calculates the following Expression 2 using the edge signal Ymix, the selection signal Ysel, and the weighted additional coefficient α which is obtained based on the difference signal Ydiff, thus producing the enhancement signal Yenhance.
 
 Yenhance=Ysel×α+Ymix ×(1−α)  (Expression 2)
 
   In other words, when the difference signal Ydiff is large, the weighted average circuit  116  determines that the edge signal Ymix has an abnormally large amplitude in a region of the spatial frequency domain where the outputs of the bandpass filters overlap each other as shown by the arrow a of  FIG. 6  and uses Expression 2 and the weighted additional coefficient α=1.0 to obtain the enhancement signal Yenhance. Since the addition ratio of the selection signal Ysel is large and the addition ratio of the edge signal Ymix is small, the enhancement signal Yenhance can be produced without being influenced by the edge signal Ymix having the abnormally large amplitude. When the difference signal Ydiff is small, the weighted average circuit  116  determines that the amplitude of the edge signal Ymix is small in the region of the spatial frequency domain where the outputs of the bandpass filters do not overlap each other or even where the outputs of the bandpass filters overlap each other, and calculates Expression 2 using the weighted additional coefficient α=0.0 to obtain the enhancement signal Yenhance. In this case, the addition ratio of the selection signal Ysel is small and the addition ratio of the edge signal Ymix is large in the enhancement signal Yenhance, resulting in effective edge enhancement using the value of the edge signal Ymix. 
   The input-output characteristic of the weighted additional coefficient calculation circuit is not limited to that shown in  FIG. 3 . The following input-output characteristic may be used: As the difference signal Ydiff becomes larger, the weighted additional coefficient α approaches 1. As the difference signal Ydiff is smaller, the weighted additional coefficient α approaches 0. 
   The operation of the edge enhancement circuit  413  in  FIG. 1  will now be described with reference to  FIGS. 2A and 2B .  FIGS. 2A and 2B  show examples of output signal waveforms of the edge enhancement circuit  413  in  FIG. 1 . The output signal waveforms are generated when a luminance signal in, for example, the frequency region shown by the arrow a of  FIG. 6  is processed by the edge enhancement circuit  413 . Note that the luminance signal is obtained by capturing a vertically striped object, and the output Yv of the vertical bandpass filter  103  indicates zero. In FIGS.  2 A and  2 B, the luminance signal and various signals obtained by processing the luminance signal are digital. Each digital signal consists of discrete values sampled every pixel. 
   In  FIG. 2A , the horizontal edge signal Yh has a signal waveform  201 . The horizontal edge signal Yh is generated from the gain circuit  105 , which controls the amplitude of the output signal of the horizontal bandpass filter  102 . The diagonal edge signal Yd has a signal waveform  202 . The diagonal edge signal Yd is generated from the gain circuit  107  that controls the amplitude of the output signal of the diagonal bandpass filter  104 . Referring to  FIG. 2B , the resultant edge signal, Ymix is a combination of Yh, Yv and Yd and also has a signal waveform  203 . Specifically, the edge signal Ymix is generated by adding the horizontal edge signal Yh (signal waveform  201 ) and the diagonal edge signal Yd (signal waveform  202 ) by using the adders  108  and  109 . 
   Referring to  FIG. 2A , the maximum signal Ymax has a signal waveform  204 . The maximum signal Ymax is output by the maximum value detection circuit  110  based on inputs from Yh and Yd (signal waveforms  201  and  202 , respectively). The minimum signal Ymin has a signal waveform  205 . The minimum signal Ymin is output by the minimum value detection circuit  111  based on inputs from Yh and Yd ( 201  and  202 ). The maximum signal Ymax, subjected to amplitude control through the gain circuit  112 , has a signal waveform  206 . The minimum signal Ymin, subjected to amplitude control through the gain circuit  113 , has a signal waveform  207 . 
   When the edge signal Ymix is positive, the selection circuit  114  selects the signal waveform  206 . On the other hand, when the edge signal Ymix is negative, the selection circuit  114  selects the signal waveform  207 . In  FIG. 2B , the selection signal Ysel has a signal waveform  208 . The selection signal Ysel is obtained by combining the maximum and minimum values of the signals generated from the respective bandpass filters. 
   The subtractor  115  generates the difference signal Ydiff indicating the absolute value of the difference between the signal waveform  203  of the edge signal Ymix and the signal waveform  208  of the selection signal Ysel. Subsequently, the weighted average circuit  116  performs weighting and averaging on the edge signal Ymix and the selection signal Ysel on the basis of the difference signal Ydiff to produce the enhancement signal Yenhance having a signal waveform  209  shown in  FIG. 2B . When the edge signal Ymix with the signal waveform  203  has an abnormally large amplitude, the enhancement signal Yenhance can be equal to or approximate to the value of the selection signal Ysel with the signal waveform  208 , which has a normal amplitude. 
   OTHER EMBODIMENTS 
   The imaging system is not limited to that shown in  FIG. 4 . So long as edge enhancement can be performed on an image signal such as a luminance signal and on RGB signals, any imaging system within the spirit and scope of the present invention may be used. An object to be subjected to edge enhancement is not limited to a luminance signal. RGB signals and a complementary color signal can be subjected to edge enhancement. Although shown with three filters, the edge enhancement circuit  413  of  FIG. 1  can employ two or more filters. The gain circuits are arranged in various locations in the edge enhancement circuit  413  in  FIG. 1 . If amplitude control is not needed, any gain circuit can be omitted. 
   Furthermore, after the program read from the storage medium is written to a function expansion board inserted into the computer or to a memory provided in a function expansion unit connected to the computer, a CPU or the like mounted on the function expansion board or function expansion unit performs all or a part of the actual processing so that the functions of the foregoing embodiments can be implemented by this processing. 
   As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.