GB2165717A

GB2165717A - Image outline sharpness control

Info

Publication number: GB2165717A
Application number: GB08518279A
Authority: GB
Inventors: Hideaki Kitamura
Original assignee: Dainippon Screen Manufacturing Co Ltd
Current assignee: Dainippon Screen Manufacturing Co Ltd
Priority date: 1984-10-16
Filing date: 1985-07-19
Publication date: 1986-04-16
Also published as: DE3524852C2; DE3524852A1; JPS6195669A; GB8518279D0; GB2165717B

Abstract

Image outline sharpness control in an image reproducing system is established by slicing an outline signal obtained by processing a sharp signal sand an unsharp signal u. Positive and negative slice levels L+, L- are processed into first and second component signals Pa, Pb corresponding respectively to the inside of the range between the slice levels and, to the outside of the range. Sharpness deemphasises R2 is carried out on the image outline by using only the intra-range component signal n.Pb. Sharpness deemphasis and emphasis R1 are both carried out using the intra-range and the extra-range component signals n.Pb, m.Pa. <IMAGE>

Description

SPECIFICATION Image outline sharpness control Field of the Invention The present invention relates generally to controlling the sharpness ofthe outline of an image produced by an image reproducing system, and more particularly, to outline sharpness emphasis and deemphasis in such a system.

Background ofthe Invention It is well known in electronic image reproduction that an image of a specificobjectwithin an original must be relieved against a background. To satisfy this requirement, Japanese Patent Publication No. 4940088, U.S. Patent No. 4,319,268 and U.S. Patent Application Serial No. 573,967 provide a sharpness emphasis method as shown in Figure 1 herein, wherein an image signal D corresponds to an emphasized image outline obtained according to: D = S + k (S - U) ... (1) wherein S is a sharp signal, U is an unsharp or blurred signal and k is a coefficient.In this connection, the sharp signal S is a signal corresponding to the density of a pixel presently being processed, while the unsharp signal corresponds to the average density of a certain number of pixels surrounding the pixel presently being processed (whether or notthe sharp signal S is included in the average depends on particular cases). In other cases, object image outlines are required to bedeemphasizedatvarying degrees.

In addition, distortion due to the shape of a pixel which arises as a result of making an enlarged reproduction of an original image must be suppressed.

Summaryofthe Invention Accordingly, it is an object ofthe present invention to provide a method and apparatus free of the abovementioned drawbacksforcontrolling the sharpness of an object image.

Another object of the present invention isto provide a method of and apparatus for imposing sharpness deemphasis on image outlines.

Afurtherobjectofthe present invention isto provide a method of and apparatus for imposing sharpness emphasis as well as sharpness deemphasis on image outlines.

Afurtherobjectofthe present invention isto provide a method and apparatus as above, capable of varying the degrees of emphasis and deemphasis.

In order to achieve the above objects, the present invention employsthefollowing: First, an outline signal P is obtained by subtracting an unsharp signal U from a sharp signal S as expressed by: P = S - U ... (2) Then, a deemphasized image signal R1 is obtained according to: R1 = S - ...... (3) wherein K is a coefficientwhich controls the degree of sharpness deemphasis.

In another case, sharpness emphasis togetherwith sharpness deemphasis can be carried out as follows: First, the outline signal P is sliced into intra-range signals Pa and an extra-range signals Pb by positive and negative slice levels L+ and L-, respectively. Then a processed image signal R2 is obtained according to the equation: R, = S - m Pa + n Pb (4) wherein m and n are coefficients.

In this regard, the slice levels L+ and L- can be varied for each color separation or each portion of an image to control the degree of sharpness emphasis and deemphasis. The same effect can be achieved by varying the deemphasis coefficient m (constant) and the emphasis coefficient n (constant).

It must befurthermore notedthatthe degree of sharpness control can be varied positively or nega tivelyaccording to the magnitudes ofthe sharp and the unsharp signals. This degree of control can be attained first by obtaining a deemphasis coefficient C(u) and an emphasis coefficient ss(u) corresponding to the magnitude ofthe unsharp signal U, obtaining a deemphasis coefficient o:(5) and an emphasis signal (3s corresponding to the magnitude ofthe sharp signal S, and then respectively carrying out the following computations: R2 = S - oc(U) m Pa + (3unP... (5) and R1 = S - oc(S) m pa + (3(s)flPb... (6) Itshould be noted incidentallythat by carrying out sharpness deemphasis on the sharpness signal S with both the slice levels coverable over noise constituents attributable to the shapes of pixels of an image to be reproduced at a higher magnification ratio, noise constituents can be reduced.

The above and other objects and features ofthe present invention can be appreciated more fully from thefollowing detailed description when read with reference to the accompanying drawings.

BriefDescription ofthe Drawings Figure lisa diagram of waveforms produced in a preferred embodiment of sharpness deemphasis in accordance with the present invention.

Figure 2 is a diagram of waveforms produced in an embodiment of the present invention.

Figure 3 is a circuit diagram of an embodiment ofthe present invention.

Figure 4 is a diagram of waveforms produced in the embodiment of Figure 3.

Figures 5(a) and (b) show respectively a characteristic and simplified diagram of a table memory as an alternativeforthe latches shown in Figure3.

Figure 6 is a diagram of waveforms produced in the embodiment of Figure 3 in which slice levels are varied.

Figure 7(a) is a simplified diagram of an apparatus for varying the degrees of sharpness emphasis and deemphasis according to unsharp signal; Figures 7(b)-(e) are diagrams of the characteristics of devices employed therein.

Figure 8 is a diagram of waveforms produced in a combination of the apparatus shown in Figures 3 and 7.

DetailedDescription ofa Preferred Embodiment of the Invention Figure 1 shows a fundamental embodiment of a sharpness deemphasis method in accordance with the present invention, in which an outline signal P is initially obtained by subtracting an unsharp signal U fromasharpsignal Sasexpressed by the equation: P = S - U ... (2) Then, a deemphasized image signal R1 is obtained by subtracting a product of the outline signal P and an arbitrary coefficient K from the sharp signal S as expressed bythe equation: R, = S - K.P...(3) In this case, the degree of deemphasis can be varied according to the value ofthe coefficient K.

Figure 2 shows another embodiment of the present invention, in which, again, an outline signal P is initially obtained bysubtracting an unsharpsignal U from a sharp signal S as expressed by equation (2).

Then the outline signal P is sliced by a positive slice level L and negative slice level L- into a deemphasis signal P2 and an emphasis signal Pb. The deemphasis signal Pa has a range from the positive slice level L+ to the negative slice level L- whereas the emphasis signal Pb is outside the range. Next, a processed image signal R2 is obtained according to the equation: R2 = S - m-Pa + n.Pb... (4) wherein ill is a deemphasis coefficient and n is an emphasis coefficient.In equation (4),the emphasis signal Pb can be nullified by setting up the positive slice level L high enough from the positive peak p+ of the outline signal P (L+ ' P+) and the negative slice level L- low enough from the negative peak P ofthe same signal (L- sP-),wherebyequation (4) can be expressed as: R2' = S - m-Pa = S - rn-p -- - (5) Assuming that m = k, equation (5) can be reexpressed as R2' = S - k-p, which is the same as equation (3).

Consequently the same deemphasized signal as shown in Figure 1 can be obtained.

On the contrary, by setting up the slice levels L+ and L- both atzero,thedeemphasissignal P2 instead is nullified, whereby equation (4) can be expressed as: R3 = S + n-Pb = S + n-p = S + n (S - U) ...(6) Assuming again that n = k, equation (6) can be reexpressed as P3 = S + K (S - U), which is the same as equation (1), meaning that only sharpness emphasis is carried out. As can be conjectured from the above two examples showing the extremes, the degree ofthe emphasis/deemphasis can be varied by setting up the slice levels L+ and L- at desired levels when 0 < L+ < P+ and 0 > L+ > P [referto equation (4)3.

It is noteworthy in this regard thatthe degree of emphasis/deemphasis can also be controlled by varying the values of the coefficients m and n. In practice, the degree of only emphasis can be controlled by fixing the value of the coefficient mat zero and varying the value of n. Conversely, the degree of only deemphasis can be controlled by fixing the value of n atzero and varying the value of m.

Is should be noted incidentallythatan image of lower density is recorded by coarse halftone dots, and this coarseness tends to cause noise N, when the image is reproduced in a higher magnification ratio. In this case, the noise N can be reduced to a level N', shown in Figure2, by setting up the positive slice level L+ and the negative slice level L- above and below the corresponding peak PN of the outline signal P, respectively.

Figure 3 is a circuit diagram of an embodiment ofthe present invention, and Figure 4 is a timing chart thereof. In this embodiment, a sharp signal Sand an unsharp signal U are inputto a subtractor 1,which carries out computation of equation (2) to output an outline signal Pto the comparator4. The comparator4 compares the outline signal P with a signal representative of a positive slice level L+ which is set up by a controller, such as a CPU (notshown),to be inputvia a latch 2 synchronized to the controller a timing pulse generated bythe controller. When the comparison results in P < L+, a selection signal SEL 1 developed at the output ofthe comparator4 is "H" (high).When the comparison results in P < L+, on the other hand, the selection signal SEL 1 is "L" (low). The signal SEL1 is applied to a selector8.

In the meantime, the outline signal Pand the positive slice level signal L+ are also inputto a subtractor 5. Subtractor 5 carries out a subtraction P L+ to output a signal P'b+ to said selector 8. There is in addition azero level signal inputtotheselector8, which outputs the signal P'b+ when the selection signal SEL 1 is "L" (P > L+). A positive outline signal Pb+iS comprised of both these signals in series.

On the other hand, the outline signal P is inputto a comparator6which compares it with a signal representative of a negative slice level L- set up by the controller and to be input via latch 3thatissynchronized with the timing pulse t. When the comparison results in P' L~, a selection signal SEL 2 atthe output ofthe comparator 6 is "H", whereas when the comparison results in P,L-, the selection signal SEL2 is "L". The selection signal SEL 2 is applied to the selector 9.

In the meantime, the outline signal P and the negative slice level signal L- are also input to a subtractor 7 which carries out a subtraction P- L- to output a signal P'b- to said selector 9. There is, in addition, a zero level signal inputto the selector 9, which in turn outputs signal P'b- when the selection signal SEL 2 is "H" (P' L~), orthe zero level signal when the selection signal SEL2 is "L" (P > L-). A negative outline signal Pb- is comprised of both these signals in series.

Thethus-obtained positive and negative outline signals Pb+ and Pb- are inputto an adder 10 which sums them to obtain a synthesis outline signal Pb.

The synthesis outline signal Pb- togetherwith the outline signal P is then inputto a subtractor 11. The subtractor 11 carries out a computation P - Pbto output another synthesis outline signal Patio a multiplication table 12. The multiplication table 12 multiplies the synthesis outline signal P2 by a deemphasis coefficient m having a characteristic as shown in Figure 7(b) to output a deemphasis signal m-p2to asubtractorl4.

In the meantime, the synthesis outline signal Pb is inputto a multiplication table 13, which multiplies the synthesis outline signal Pb by an emphasis coefficient ntooutputan emphasis signal n.Pb.

The subtractor 14then subtracts the deemphasis signal m-Pafrom the sharp signal S, and then resultant is applied to an adder 15.

The adder 15 sums the emphasis signal n-Pb and the output of the subtractor 14 which is the signal obtained by subtracting the deemphasis signal P2 from the sharp signal S to finally obtain an output image signal R2 [as expressed bythe equation (4)].

It should be noted incidentally that several delay unitsseenin Figure3areemployedtosynchronizethe respective two inputs to the computation units 4 to 15.

As mentioned, the degree ofthe emphasis and deemphasis can be controlled byvarying the slice levels L+ and L-. In the circuit shown in Figure 3, the slice levels L+ and L- are latched in latches 2 and 3 by the operation ofthe CPU. In this connection, the slice levels L+ and L- can be madeto correspond to the magnitude ofthe sharp signal S or determined to bring forth a desired effect.

It is noteworthy thatthe slice levels L+ and L- need notbe maintained constantduring a process; rather they preferably are varied according to the magnitude ofthe sharp signal S. This is provided by replacing the latches 2 and 3 with a table memory 16.

More precisely, latches 2 and 3 are replaced by a table memory 16 in which data of L+ = f+(s) and L- = f-(s) as shown in Figure 5(a) are stored. Slice levels L+ and L~ of higher absolute valuesthereby are output when a sharp signal Sofa higher level (brighter) is inputthereto. Slice levels L+ and L- of lower absolute values are to be output when a sharp signal Sofa lower level (darker) is input, as shown by broken lines on the signal Pin Figure 6. Consequently, slice levels L+ and L- corresponding to the density variation of an image signal can be obtained to reproduce an image on which emphasis and deemphasis processes are performed. An example of output waveforms are shown in Figure 6.

It should be noted that image modification can be realized such that the outlines of brighter portions are emphasized or deemphasized less while those of darker portions are emphasized more.

Inerderto putthe above into practice,thernulti- plication tables 12 and 13 must be replaced by circuits as shown in Figure 7(a). In Figure 7(a), table memories 12~1 and 13i output respectively coefficients aand ss each of which varies according to the unsharp signal U as shown in Figures 7(d) and (e).In the particular case of Figures 7(d) and (e), the higher (brighter) the level of the unsharp signal U is,the smallerthe coefficients a and (3arse. Then, these coefficients or and P are multiplied respectively by the outputs m-Pa and n-P2 of the multiplication tables 12 and 13 by the operation of the multipliers 12~2 and 13~2, respectively. By these operations, the deemphasis component signal oc-m-Pa and the emphasis component signal (3-fl-Pb become signals which depend on the unsharp signal U.

According to the coefficient data produced by the tables 12-1 and 13-1 of Figure 7, the higherthe level of the unsharp signal U is, the smallerthe deemphasis and emphasis component signals are. This means that the brighterthe density level of an image to be processed is, the less effective the deemphasis (for portions whose density levels are lower than an established slice level) and emphasis are (for portions whose density levels are higherthan the slice level).

On the contrary, however, they are more effective for darker portions.

Figure 8 shows how the emphasis component signal ss-n-Pb varies in accordance with the unsharp signal U, wherein both the slice levels L+ and L- are zero,forsirnplification.lnthis particularcase,since the emphasis or brighter portions is reduced, the wavy portion of the positive side ofthe signal n-Pb is almost leveled when it is changed into the signal R1. Thus by using the circuit as shown in Figure 7, an emphasis deemphasis characteristic corresponding to any de density level of an image signal is provided.

In this respect, the reason the unsharp signal U is used as input image data to the table memories 12-1 and 13-1 is to fully impose the effect ofthe coefficient os or ss onto the negative side of the outline signal P.

The sharp signal Scan, of course, be used instead to be input to the table memories 12-1 and 13-1, when there is almost no effect on the negative side ofthe outline signal P.

Although the above description is made on the emphasis component signal ss-n-Pb,the samething can be said forthe deemphasis component signal a-rn-P2. Furthermore, both the components can coexist according to equation (4) provided that the values m-Pa and n-Pb are respectively replaced by the values a-rn-P2 and (3-n-Pb, whereby equation (4) is modified as:: R1 = S - -m-Pa +(3-fl-Pb... (7) It should be noted incidentally that the multiplicaton table 12 in Figure 3 can be constructed as seen in Figure7(b), wherein the positive outline signal P2+ and the negative outline signal Pa- are respectively multiplied by deemphasis coefficients m+ and m-.

In the same way, the multiplication table 13 in Figure 3 can be constructed as seen in Figure 7(c) wherein the positive outline signal P2+ and the negative outline signal Pa- are respectively multiplied by deemphasis coefficients n+ and n-.

It is noteworthy that by preparing the latches 2 and 3 (ortheir substitutes constructed as the table memory 16 shown in Figure 5) and the multiplication tables 12 and 13 (ortheir substitutes constructed as the circuit shown in Figure7) and by switching them for each color separation of an image, emphasis ordeemph- asis can be freely imposed on specific colors. For instance, by extracting the flesh color portions of a human being and imposing deemphasis thereon, the portions are smoothed when reproduced. For refer ence,the color extraction can be carried out by a method as disclosed in Japanese Patent Publication No.50-14845.

There is, of course, establishable a plurality of slice levels L+ and L- and coefficients m, and (3foreach portion or color of an original.

As mentioned above, the present invention is effective especially in deemphasing the outline of an object image against its background since it is capable of obtaining a reproduction image in which desired portions are deemphasized according to an outline signal obtained by processing a sharp signal and an unsharp signal. In addition, the degree ofthe sharp ness emphasis and deemphasis can be controlled by slicing an outline signal by positive and negative slice levels into intra-range and ultra-range signals and processing the same.

In anotherway, the magnitudes of emphasis and deemphasis signals can be varied by being multiplied by appropriate coefficients, respectively. Furthermore, by expanding the range between two slice levels ofthe outline signal to the extent that it covers the peak input density level, noise components attributabletotheshapes of pixeis can be reduced.

Claims

1. A method of controlling the sharpness ofthe outline of an object image in an image reproducing system, comprising the steps of: (a) obtaining a sharp signal S representative of the density of a pixel presently processed and an unsharp signal U representative of the average density of a specific number of pixels surrounding the pixel presently processed by scanning an original; (b) obtaining an outline signal P by carrying out a computation P = S - U; and (c) obtaining a deemphasized signal R1 by carrying out a computation R1 = S - kP, wherein kisa coefficient.

2. A method of controlling the sharpness of the outline of an object image in an image reproducing system, comprising the steps of: (a) obtaining a sharp signal S representative of the density of a pixel presently processed and an unsharp signal U representative of the average density of a specific number of pixels surrounding the pixel presently processed by scanning an original; (b) obtaining an outline signal P by carrying out a computation P = S - U; (c) slicing out an outline deemphasis signal P2 from the outline signal P, the outline deemphasis signal P2 being a signal corresponding to the inside of a range determined by a positive slice level L and negative slice level L- established forthe outline signal P;; (d) slicing out an outline emphasis signal Pb from the outline signal P, the outline emphasis signal Pb being a signal corresponding to the outside ofthe range determined by the positive slice level L+ and the negative slice level L- established forthe outline signal P; (e) multiplying the outline deemphasis signal P2 by a deemphasis coefficientf; (f) multiplying the outline emphasis signal Pb by and emphasis coefficientg; and (g) obtaining a sharpness-controlied signal R2 by carrying out a computatin R2 = S - f-P2 + g-Pb using the sharp signal S, the adjusted outline deemphasis signal f-P2 and the adjusted emphasis signal f-Pb.

3. A method as recited in claim 2 in which the sharpness deemphasis only is carried out according to an equation R2 = S - f-P2 by setting up the positive and the negative slice levels L+ and L- such that the range between the slice levels covers the outline signal P.

4. A method as recited in claim 2 in which the positive and the negative slice levels L+ and L- are determined according to the magnitude ofthe sharp signal S.

5. A method as recited in claim 2 in which the positive and the negative slice levels L+ and L- are determined according to the magnitude ofthe unsharp signal U.

6. A method as recited in claim 2 in which the deemphasis coefficientf is a constant m expressed as f = m, and the emphasis coefficientg is a constantn expressed as g = n.

7. A method as recited in claim 2 in which the deemphasiscoefficientfisthe product of a constant and a coefficient c(u) corresponding to the magnitude ofthe unsharp signal U expressed asf= a(u)-m, and the emphasis coefficientg is the product of a constant n and a coefficent ss(u) corresponding to the magnitude ofthe unsharp signal U expressed as g = ss(u) n-

8.A method as recited in claim 2 in which the deemphasis coefficientfisthe productof a constants and a coefficient Cc(s) corresponding to the magnitude ofthe sharp signal S expressed asf = a(s} m and the emphasis coefficient g is the product of a constant n and a coefficient (3s corresponding to the magnitude ofthe sharp signal Sexpressedasg = ss(s) n

9. A method as recited in claim 7 in which the slice levels L and L- and the coefficients m, n, or and ss are determined for each portion or color separation of an original.

10. An apparatus for controlling the sharpness of the outline of an object image in an image reproducing system, comprising: (a) meansforobtaining a sharp signal S representative of the density of a pixel presently processed and an unsharp signal U representative of average density of a specific number of pixels surrounding the pixel presently processed obtained by scanning an original; (b) first computation means for carrying outa computation S - U to obtain an outline signal P;; (c) outline deemphasis signal extraction means for slicing out an outline deemphasis signal P2 from the outline signal P, the outline deemphasis signal P2 being a signal corresponding to the inside of a range determined by a positive slice level L+ and a negative slice level L- established forthe outline signal P; (d) outline emphasis signal extraction means for slicing out an outline emphasis signal Pb from the outline signal P, the outline emphasis signal Pb being a signal corresponding to the outside ofthe range determined by the positive slice level L+ and the negative slice level L- established for the outline signal P; (e) deemphasis coefficient multiplication means for multiplying the outline deemphasis signal P2 by a deemphasis coefficientf;; (e) emphasis coefficient multiplication means for multiplying the outline emphasis signal Pb by an emphasis coefficient g; and (f) second computation meansforcarrying outa computation expressed by an equation R2 = S - f Pa + g-Pb, wherein a sharpness-controlled signal R2 is obtained using the sharp signal S, the adjusted outline deemphasis signal f-P2 and the adjusted emphasis signal f-Pb.

11. An apparatus as recited in claim 10 in which the outline deemphasis signal extraction means is a subtractorfor subtracting the outline emphasis signal Pb from the outline signal P.

12. An apparatus as recited in claim 10 in which the outline emphasis signal extraction means comprises: (a) first slicing means for slicing outthe component signal which is above the positive slice level L+ ofthe outline signal P; (b) second slicing meansfor slicing out the compo nent signal which is belowthe negative slice level Lofthe outline signal P; and (c) an adderfor summing the signals obtained by the first and the second slicing means.

13. An apparatus as recited in claim 12 in which the first slicing means comprises: (a) a subtractor for subtracting the positive slice level L+ from the outline signal P; and (b) output meansfor outputting the output of the subtractorto the adder only when the level of the outline signal P is higherthan the positive slice level L+.

14. An apparatus as recited in claim 12 in which the second slicing means comprises: (a) a subtractorfor subtracting the negative slice level L- from the outline signal P; and (b) an output means for outputting the output of the subtractorto the adder only when the level of the outline signal P is lower than the negative slice level L-.

15. An apparatus as recited in claim 10 in which the deemphasis coefficient multiplication means is a table memory for outputting a value m-Pa in conformity with the outline deemphasis signal P2 input thereto; and the emphasis coefficient multiplication means is a table memory for outputting a value n-Pb in conformity with the outline emphasis signal Pb input thereto.

16. An apparatus as recited in claim 10 in which the deemphasis coefficient multiplication means com prisesafirsttable rnemoryforoutputting avalue m-Pa inconfermitywiththeoutlinedeemphasissignal P2 input thereto, a second table memory for outputting a coefficient c(u) in conformity with the unsharp signal U inputthereto, and a first multiplication means for multiplying the output ofthefirsttable memory by the coefficient czu); ; and the emphasis coefficient multiplication means comprises a third table memory for outputting a value n-Pb in conformity with the outline emphasis signal Pb input thereto, a fourth table memory for outputting a coefficient au, in conformity with the unsharp U input thereto, and a second multiplication means for multiplying the output of the third table memory bythe coefficient ss(u)-