Processor for emphasizing edges of a video signal
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a processing apparatus for emphasizing the edges of a video signal and, more particularly, to a processing apparatus for emphasizing the edges of a video signal to sharpen the contours within a picture produced thereby. 
     2. Description of the Prior Art 
     In the imaging art, it is a common practice to feed a television (TV) signal or like video signal to a TV receiver and other output devices after applying non-linear correction such as gamma correction to the video signal. In this instance, both highlight and shadow portions of a gamma-corrected video signal are processed for edge emphasis and, then, subjected to reverse gamma correction to produce a final picture. A problem with a prior art edge emphasizing system is that it excessively emphasizes edges on the highlight side of a picture while not sufficiently emphasizing them on the shadow side, as shown in FIG. 3 of the drawings. 
     Such uneven edge emphasis stems from the fact that because gamma correction and other kinds of non-linear correction are generally implemented with compansion processing: comparison processing applies the same degree of edge emphasis to both the highlight side and the shadow side of a video signal at the companded level and thus causes an edge component to appear enlarged on the highlight side and reduced on the shadow side in the eventual reverse gamma-corrected picture. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to eliminate the drawback particular to the prior art edge emphasis processing as discussed above and provide a processing apparatus which is capable of emphasizing the edges of a video signal to the same degree on both of the highlight and the shadow sides thereof. 
     In accordance with the present invention, a processing apparatus for emphasizing the edges of a video signal which is applied thereto and which is non-linear relative to density or luminance levels adapted for the reproduction of a vide signal includes non-linear converting means for converting the non-linear video signal into a linear video signal, edge emphasizing means for applying edge emphasis to the linear video signal to produce a liner edge-emphasized video signal, and non-linear converting means for applying non-linear processing to the linear edge-emphasized video signal. 
     Further, in accordance with the present invention, a processing apparatus for emphasizing the edges of a video signal which is applied thereto and which is non-linear relative to density or luminance levels adapted for the reproduction of a video signal includes linear converting means for converting the non-linear video signal into a linear video signal, edge-emphasized signal producing means for producing a linear edge-emphasized signal from the linear video signal, non-linear converting means for converting the edge-emphasized signal into a non-liner edge-emphasized signal, and adding means for adding the non-linear edge-emphasized signal to the non-linear video signal. 
     It is to be noted that the word &#34;linear&#34; which frequently appears in the instant specification is representative of the correspondence of signal levels to output levels at the final picture output stage. For example, it refer to the correspondence of signal levels to, in the case of a hard copy, the density levels of a recording paper, e.g., photosensitive material and, in the case of a cathode ray tube (CRT), the luminance levels of a fluorescent material provided on the CRT screen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic block diagram showing an embodiment of a processing apparatus for emphasizing the edges of a video signal in accordance with the present invention; 
     FIG. 2 is a schematic block diagram showing an alternative embodiment of the present invention which is a simplified version of the embodiment as shown in FIG. 1; and 
     FIG. 3 is a graph demonstrating the fact that when edge emphasis processing is applied to a gamma-corrected video signal by a prior art device, the result is different between the highlight side and the shadow side of a picture. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, a processing apparatus for emphasizing the edges of a video signal embodying the present invention is shown. In this particular embodiment, the highlight and shadow portions of a gamma-corrected digital video signal are subjected to substantially the same degree of edge emphasis processing. 
     As shown in FIG. 1, the processing apparatus is made up of a linear converting circuit 3, a sharpness processing circuit 4, adding circuits 5 and 9, non-linear converting circuits 6 and 7, and a substracting circuit 8. While signals appearing in various sections of the processing apparatus as designated by circled small letters a to h are represented by analog waveforms, such is merely to facilitate an understanding of the present invention and, in practice, they are digital signals. 
     The processing apparatus has an input terminal A which is connected to the linear converting circuit 3. The linear converting circuit 3 functions to transform a gamma-corrected non-linear digital video signal a, or input original signal, into a linear digital video signal so that the signal may undergo the same degree of edge emphasis on its shadow and highlight sides later. Data necessary for such signal conversion are stored in the linear converting circuit 3. The linear digital video signal b appears on an output 10 of the circuit 3 which is connected to the sharpness processing circuit 4. 
     The sharpness processing circuit 4 extracts edge portions of the signal appearing on the output 10 of the linear converting section 3, then emphasizes those edges, and then produces the resulting signal c on its output 11. The output 11 of the sharpness processing section 4 is connected to the adding section, or adder, 5. Also connected to the adder 5 is the output 10 of the linear converting circuit 10, which supplies the linear digtal video signal 6. 
     The adder 5 adds the output 10 of the linear converting circuit 10 and the output 11 of the sharpness processing circuit 4 to produce a sum signal d on its output 12, which is connected to the non-linear converting circuit 6. 
     The non-linear converting circuit 6 stores data for applying non-linear conversion to the sum signal 12 which is inverse to the linear conversion as performed by the linear converting circuit 3. The non-linear converting section 6 produces on its output 13 a signal e which is equivalent to a gamma-corrected version of the sum output 12 of the adder 5. The output 13 of the converting circuit 6 is connected to the substracting circuit 8. 
     The other non-linear converting circuit 7 is connected to the previously stated linear converting circuit 3 and stores the same data as those stored in the non-linear converting circuit 6. A signal f appearing on an output 14 of the non-linear converting circuit 7 is equivalent to a gamma-corrected version of the output 10 of the linear converting circuit 3. The output 14 of the converting circuit 14 is also connected to the subtracting circuit 8. 
     The subtracting circuit, or subtractor, 8 subtracts the output 14 of the non-linear converting circuit 7 from the output 13 of the other non-linear converting circuit 6. The signal g developed at the output 15 of the subtractor 8 is delivered to the adding circuit, or adder, 9. By this procedure, only the edge-emphasized portions of the digital video signal are extracted so that the conversion errors between table data stored in the converting circuits 3 and 6 and a gamma-correction characteristic applied to the original signal are cancelled. 
     Connected to the input terminal A and the subtractor 8, the adder 9 adds the original signal coming in through the input terminal A and the output 15 of the subtractor 8. As a result, a digital video signal with its edges adequately emphasized appears on an output 16 of the adder 9. The sequence of processing steps described so far may advantageously be implemented with a digital processor system. 
     In operation, a gamma-corrected non-linear digital video signal having a waveform a is applied through the input terminal A to the linear converting circuit 3. Then, the linear converting circuit 3 transforms the input signal into a linear digital video signal having a waveform b. The output 10 of the converting section 3 is fed to the sharpness processing circuit 4, whereby its edge portions are extracted and, then, emphasized. As a result, a signal having a waveform c appears on the output 11 of the sharpness processing circuit 4. 
     The output 11 of the sharpness processing circuit 4 is routed to the adder 5 together with the output 10 of the linear converting circuit 3. The adder 5 adds the two input signal to produce on its output 12 a signal having a waveform d which is a linear digital video signal with emphasized edges. 
     The non-linear converting circuit 6 processes the output 12 of the adder 5 to produce on its output 13 a waveform e, i.e., a non-linear digital video signal which has undergone both of gamma-correction and edge emphasis. The output 10 of the linear converting circuit 3 is applied to the non-linear converting circuit 7 as well to be thereby subjected to gamma correction. Consequently, a waveform f which is a non-linear digital video signal having not undergone edge emphasis appears on the output 14 of the non-linear converting circuit 7 and is applied to the subtractor 8. 
     The subtractor 8 subtracts the output 14 of the non-linear converting circuit 7 from the output 13 of the non-linear converting circuit 6, resulting that a waveform g which is a digital video signal representative of the edge-emphasized portions only appears on the output 15 of the subtractor 8. Finally, the adding section 9 adds the digital video signal coming in through the input terminal A and the output 15 of the subracttor 8 to produce a waveform h, ie.., a gamma-corrected digital video signal with emphasized edges. The output 16 of the adder 9 is fed out through an output terminal B of the processing device. 
     As stated above the edge-emphasized digital video signal applied to the output terminal is the sum of the original signal coming in through the input terminal A and the output 15 of the subtractor 8 which is representative of the edge-emphasized portions only. Hence, the edges of the original signal are emphasized without entailing signal conversion errors otherwise brought about by the difference in conversion characteristic between the linear converting circuit 3 and non-linear converting circuits 6 and 7 and the original signal. 
     Referring to FIG. 2, another embodiment of the present invention is shown which is useful when conversion errors introduced in the gamma-corrected non-linear digital video signal by the linear converting circuit 3 and non-linear converting circuit 6 are negligible. In the figure, the blocks like those shown in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted for simplicity. 
     The processing apparatus shown in FIG. 2 lacks the non-linear converting circuit 7, subtractor 8, and adder 9 which are included in the processing apparatus of FIG. 1. The output 12 of the adder 5 is converted into a non-linear signal by a non-linear converting circuit 20 and, then, immediately fed out as the output 16. The non-linear converting circuit 20 included in the device of FIG. 2 corresponds to the non-linear converting circuit 6 of FIG. 1. In this particular embodiment, therefore, the previously stated conversion errors are not compensated for. 
     While the signal applied to the input terminal A has been shown and described as comprising a digital video signal in any of the foregoing embodiments, it will be apparent that the present invention is similarly applicable to an analog video signal to achieve the same effect. 
     Further, although the embodiments shown and described are applied to a gamma-corrected video signal, such is only illustrative and not restrictive. Specifically, the principle of the present invention is applicable even to video signals which are subjected to other kinds of non-linear conversion. 
     In summary, it will be seen that the present invention provides a processing apparatus capable of emphasizing the shadow and highlight sides of a non-linear video signal to the same degree without distorting the video signal, thereby producing a picture having sharp contours. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.