The present invention relates to a device for emphasizing an edge of a video signal.
Sharpness is one important factor that determines image quality. An edge emphasis circuit is used to improve the sharpness.
FIG. 6 shows a conventional edge emphasis circuit. Referring to FIG. 6, reference numeral 1001 denotes a video input terminal; 1002 and 1003, delay circuits; 1004, 1005, and 1006, multipliers; 1007, an adder; 1008, a multiplier; 1009, an adder; and 1010, a video output terminal. Also, reference numerals s1 to s6 denote signals at respective circuits. FIG. 7 shows the waveforms of these signals.
An input video signal s1 is input to the multiplier 1004 and first delay circuit 1002. The first delay circuit 1002 delays the input signal one pixel, and outputs a signal s2. The signal s2 is input to the multiplier 1005, second delay circuit 1003, and adder 1009. The signal input to the second delay circuit 1003 is further delayed one pixel, and is output as a signal s3. The waveforms of these three signals s1, s2, and s3 are as shown in FIG. 7. That is, these signals are delayed one pixel each.
These signals s1, s2, and s3 are respectively input to the multipliers 1004, 1005, and 1006, and are respectively multiplied by xc2xd, 1, and xc2xd. The signs of the signals s1 and s3 are inverted, and the inverted signals s1 and s3, and non-inverted signal s2 are input to the adder 1007. The adder 1007 adds these signals, and outputs a sum signal s4. That is, the signal s4 assumes a value obtained by subtracting the average value of the previous and next image data from an image data value at a given point, i.e., a second derivative value at that point. The waveform of the signal s4 is as shown in FIG. 7.
Furthermore, the multiplier 1008 amplifies the signal s4 to obtain a signal s5. The signal s5 is an edge-emphasized signal. The adder 1009 adds the signal s2 output from the delay circuit 1002 to the edge-emphasized signal s5, thus generating an edge-emphasized video signal s6. A signal indicated by the solid curve in the signal s6 in FIG. 7 is the edge-emphasized signal, and a signal indicated by the broken curve is the original signal s2.
However, in the conventional edge emphasis circuit described above, as indicated by the solid curve in the signal s6 in FIG. 7, the leading edge of an edge portion of the signal can be improved, while overshot and undershot portions are produced. If such overshoot and undershoot are considerable, the image quality deteriorates due to a fringe formed at the boundary. When the coefficient of the multiplier 1008 is decreased to eliminate such overshoot and undershoot, the edge emphasis amount decreases in turn, and the improvement effect of the leading edge of the edge portion becomes small.
As described above, in the conventional edge emphasis circuit, the leading or trailing edge of the edge portion can be improved to obtain a sufficient edge emphasis effect, while overshoot and undershoot tend to increase, resulting in deterioration of image quality.
The present invention has been made in consideration of the above situation, and has as its object to provide an edge emphasis device which can maintain the improvement effect of the leading edge of an edge portion while suppressing overshoot and undershoot.
The present invention also provides a device which can set the overshoot/undershoot suppression level.
In order to achieve the above object, an edge emphasis device of the present invention comprises the following arrangement. That is, there is provided an edge emphasis device comprising:
extraction means for extracting signals for three pixels including a pixel of interest and two neighboring pixels, from a time-serially input video signal;
first computation means for computing absolute values of differences between the extracted pixel of interest and the two neighboring pixels;
means for selecting a smaller one of the two absolute values obtained by the first computation means, and outputting the selected value as a first signal;
second computation means for computing a second derivative on the basis of the signal for three pixels extracted by the extraction means, and outputting the second derivative signal as a second signal;
third computation means for computing an absolute value of the second signal, comparing the absolute value with a predetermined threshold value, selecting a smaller one of the absolute value and threshold value, and outputting the selected value as a third signal;
sign checking means for checking a sign of the second signal, and outputting a checking result as a fourth signal;
fourth computation means for generating an edge-emphasizing signal on the basis of the first, third and fourth signals; and
addition means for adding the edge-emphasizing signal obtained by the fourth computation means to the signal of the pixel of interest, and outputting the sum signal as an edge-emphasized signal.
Also, according to a preferred embodiment of the present invention, the edge emphasis device comprises the following arrangement.
That is, an edge emphasis device comprises delay means for delaying an input first signal a predetermined period time to generate a second signal, delay means for further delaying the second signal the predetermined time to generate a third signal, three multiplication means for respectively multiplying the first, second, and third signals by predetermined coefficients to generate fourth, fifth, and sixth signals, computation means for computing the fourth, fifth, and sixth signals to output a seventh signal as a second derivative signal, minimum value selection means for comparing the absolute value of the seventh signal and a given adjustable value, selecting a smaller one, and outputting the selected signal as an eighth signal, sign checking means for checking the sign of the seventh signal, and outputting a ninth signal, minimum value selection means for comparing the absolute value of the difference between the first and second signals, and that of the difference between the second and third signals, selecting a smaller one, and outputting the selected signals as a tenth signal, multiplication means for multiplying the eighth and tenth signals, and determining the sign of the product based on the ninth signal to generate an eleventh signal, and addition means for adding the eleventh signal and a signal obtained by delaying the second signal the predetermined time.
Alternatively, the edge emphasis device the following arrangement.
That is, an edge emphasis device comprises delay means for delaying an input first signal a predetermined period time to generate a second signal, delay means for further delaying the second signal the predetermined time to generate a third signal, three multiplication means for respectively multiplying the first, second, and third signals by predetermined coefficients to generate fourth, fifth, and sixth signals, computation means for computing the fourth, fifth, and sixth signals to output a seventh signal as a second derivative signal, means for suppressing the seventh signal to have a value within a given range, and outputting the suppressed signal as an eighth signal, minimum value selection means for comparing the absolute value of the difference between the first and second signals, and that of the difference between the second and third signals, selecting a smaller one, and outputting the selected signals as a tenth signal, multiplication means for multiplying the eighth and tenth signals to generate an eleventh signal, and addition means for adding the eleventh signal and a signal obtained by delaying the second signal the predetermined time.
The present application includes the following invention.
That is, one of the inventions of an edge emphasis device associated with the present application is an edge emphasis device for emphasizing an edge of an image signal, comprising:
a preliminary signal generation circuit for generating, on the basis of a target signal, a lead signal that leads ahead of the target signal a predetermined time, and a lag signal that lags behind the target signal a predetermined time, a preliminary signal as a second derivative signal of the target signal;
a limiting circuit for limiting a value of the preliminary signal using a predetermined upper limit value when the value of the preliminary signal is larger than the upper limit value, limiting the value of the preliminary signal using a predetermined lower limit value when the value of the preliminary signal is smaller than the lower limit value, or imposing both limitations using the upper and lower limit values; and
an edge-emphasizing signal generation circuit for generating an edge-emphasizing signal by computing a product of a minimum value of an absolute value of a difference between the target signal and the lead signal and an absolute value of a difference between the target signal and the lag signal, and an output value from the limiting circuit.
Note that the predetermined time the lead signal leads ahead of the target signal is preferably equal to the predetermined time the lag signal lags behind the target signal. Also, the predetermined time the lead signal leads ahead of the target signal and/or the predetermined time the lag signal lags behind the target signal is preferably equal to the interval between signals corresponding to neighboring pixels in the input signal upon image information.
The respective circuits can have various patterns. Also, the respective circuits may be integrated to build an integrated circuit.
In the present invention, the edge-emphasizing signal becomes the one corresponding to the target signal.
Therefore, a circuit for generating an edge-emphasized signal using the edge-emphasizing signal as a compensation signal corresponding to the target signal can obtain the edge-emphasized signal. More specifically, by adding the edge-emphasizing signal to a signal to be emphasized, the edge-emphasized signal can be obtained. Especially, the edge-emphasizing signal often has a delay that cannot be ignored with respect to the signal to which the edge-emphasizing signal is added. In such case, these signals can be added in correspondence with each other. More specifically, as will be described later in the following embodiment, an arrangement for delaying the target signal in correspondence with the timing of the edge-emphasizing signal corresponding to the target signal is preferably used.
Note that the second derivative signal means xe2x80x9ca signal that represents the difference between the image signal amplitude (or image data value) of a given point (or pixel) and those of surrounding points (pixels)xe2x80x9d. More specifically, a value obtained by subtracting the average value of the lead signal that leads ahead of the target signal the predetermined time, and the lag signal that lags behind the target signal the predetermined time from the target signal can be preferably used.
The present application includes the invention of an image forming apparatus which has the aforementioned edge emphasis device, and an image forming unit for forming an image on the basis of a signal, the edge of which is emphasized by the edge emphasis device. Note that the image forming unit can preferably use, for example, a display for displaying an image.
The present application includes the following invention as an image signal processing method.
An image signal processing method comprises the steps of:
generating, on the basis of a target signal, a lead signal that leads ahead of the target signal a predetermined time, and a lag signal that lags behind the target signal a predetermined time, a preliminary signal as a second derivative signal of the target signal;
limiting a value of the preliminary signal using a predetermined upper limit value when the value of the preliminary signal is larger than the upper limit value, limiting the value of the preliminary signal using a predetermined lower limit value when the value of the preliminary signal is smaller than the lower limit value, or imposing both limitations using the upper and lower limit values; and
generating an edge-emphasizing signal by computing a product of a minimum value of an absolute value of a difference between the target signal and the lead signal and an absolute value of a difference between the target signal and the lag signal, and the limited value.
Especially, the method preferably further comprising the step of generating an edge-emphasized signal using the edge-emphasizing signal as a compensation signal corresponding to the target signal.
The present application includes, as an invention of an image forming method, that of an image forming method for forming an image on the basis of a signal obtained by the aforementioned image signal processing method.
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.
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