Method and apparatus for emphasizing sharpness of image

Abstract: According to this invention, a sharpness emphasis is carried out by multiplying a sharpness emphasis signal of an original image by a function of a look-up table which is accessed based on an output from an edge detecting filter performing detections in accordance with only the edge portions of the original image. Since the edge detecting filter for detecting edges of the image is equipped in order to emphasize the sharpness only in the edge portions of the image, it is possible to prevent the image grains from being sharpness-emphasized. In addition, although the edge detecting filter may have a same size of a mask with that of the unsharpness mask, it is possible, if required, to modify freely both sizes of the masks, in order to perform a proper process in accordance with each image variation.

Description: BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for emphasizing sharpness of an image as indicated in the preamble portions of claims 1 and 5. Such a method and apparatus is to be used for a color scanner for mask-making, a laser color printer, a facsimile or the like. In the prior art of the image scanning recording apparatus such as a color scanner, there has been employed an electro-optical unsharpness masking or a differential masking method, an electrical crisping or a peaking method in order to improve the sharpness. These methods are effective to improve the sharpness and reproduce a recorded image sharply, however, at the same time, since the improvement of the sharpness is effected uniformly over the whole area of an original image, the grain clusters of the original film are disadvantageously emphasized also in an area having a flat tone on the original, thus. resulting in a poor quality of the recording image as having uneven, rough graininess. In order to solve the defect described above, there has been a method for sharpness emphasis disclosed in Japanese Patent Application Laid-open JP-A-55146451. In this method, a sharpness emphasis signal is formed based on a main signal obtained from a photoelectrically scanned original or, based on the main signal and an unsharp signal which includes he information on the peripheral area of the scanned area corresponding to the above mentioned main signal. This sharpness emphasis signal corrects or modifies the main signal so as to emphasize the image to be recorded. When this process is carried out, the variation level of the unsharp signal is measured or distinguished so that the quantity of the sharpness emphasis signal can be controlled on the basis of the measured variation level. In this method, no reference was made to the method of the auxiliary scanning, however, and the sharpness emphasis is carried out in accordance with only the main scanning. This disadvantageously causes the deterioration of the image quality. In addition, since the unsharp signal used for the sharpness emphasis and another unsharp signal to distinguish an edge are respectively formed of the same masks having a fixed size, this method exhibits a weakness in dealing with the image variation requirements such as a screen line number, edge width or the like. On the other hand, there are not such defects as described above in the other methods for the sharpness emphasis disclosed by the present applicant in Japanese Patent Application Laid-open JP-A-60 00 3269 and Japanese Patent Application Laid-open JP-A-600 54570. However, since it is impossible for the methods to distinguish on an original image (film: especially a reversal film) between the grain (coupler) and the edge, the sharpness in the flat area of the image is liable to be emphasized, and consequently leading to an unevenness of the image quality. An image sharpness emphasis method and apparatus as mentioned above and indicated in the preamble portions of claims 1 and 5 is already known from US Patent US-A-4,591,923. In this case, a sharp and an unsharp signal are subtracted so that a (S-u) signal is generated. A sharpness intensification signal S + K (S-u) is achieved by further processing said (S-u) signal in a function memory, a multiplier and an adder. From European Patent Application EP-A-0 153 167, it is generally known to use an edge detecting filter in order to achieve edge and contrast enhancement. SUMMARY OF THE INVENTION This invention has been achieved in consideration of the above discussed circumstances and an object of the present invention is to provide a method and its apparatus by which a sharpness emphasis can be sufficiently effected in proper manner without emphasizing the graininess of grains on the original image. This object is solved by an image sharpness emphasis method and apparatus having the features of claims 1 and 5. The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG.1 is a flow chart showing an example of the method of this invention; FIGa.2A and 2B are examples of the edge detecting filters used for this invention; FIG.3 is a plotted diagram showing an example of a look-up table (edge dependence table) for use in this invention; FIG.4 is a diagram for explaining the operation of this invention; FIG.5 is a blockdiagram showing an operational example of the apparatus according to this invention; FIG.6 is a block diagaram for partly illustrating another embodiment of this invention; and FIGs.7A and 7B are diagrams showing respective edge detecting filters for other examples. DESCRIPTION OF THE PREFERRED EMBODIMENT According to this invention, a sharpness emphasis is carried out by multiplying a sharpness emphasis signal of an original image by a function value of a look-up table (edge adusting table) which is accessed based on the output from an edge detecting filter performing detections in accordance with only the edge portions of the original image. FIG.1 shows a flow chart showing a method of the sharpness emphasis according to this invention. With reference to this flow chart, at first, an original image is read to generate a sharp signal S ij (Step S1). Extracted is an image data having a mask size of n × n in which the sharp signal S ij exists as a central data (Step S2). This process is followed by the Step S3 generating an edge detection level |E| as well as by the Step S5 generating an unsharp signal u . In the Step S3, the edge detection level |E| is calculated as following formula; In this expression, the constant k is k = (n-1)/2 and Mxy indicates an edge detecting filter. Here, when the mask size of the unsharp signal u is taken, for example, as 5x 5, the matrix for the edge detecting filter Mxy can be considered to be taken as the one shown in FIG.2A or 2B. It may be possible to make a matrix more weigted. It is also possible, for example to adopt the matrix of FIGs.2A and 2B as for the matrix of the main scanning direction and for the auxiliary direction, respectively. After the process Step S3 in which the edge detection level |E| is determined, an edge adjusting quantity f(|E|) is determined by using an edge adjusting table f( ) as shown in FIG.3 (Step S4). The edge detection level (|E|) is determined by using an edge adjusting table f( ) as shown in FIG.3 (Step S4). On the other hand, the unsharp signal u is calculated according to the following formula in the Step S5: Subsequently, the difference (S ij-u ) between the sharp signal Sij and the unsharp signal u is calculated to produce a sharpness emphasis signal (Step S6). Then, the multiplication of the sharpness emphasis signal (S ij-u ) by the edge adjusting quantity f(|E|) is carried out following the expression (3) shown below (Step S7). f(|E|) · (s ij -u)·K After this process, the sharp signal S ij is added to the above result so as to output the sharp signal S ij ' being sharpness-emphasized (Step S8). This is shown as the expression below; S ij ' = S ij + f(|E| · (S ij -u) ·K According to the method for sharpness emphasis of this invention as detailed above, since a function f(|E|) which does not effect any sharpness emphasis as long as the edge detection level |E| takes small value is stored in the edge adjusting table as shown in FIG.3, the sharpness emphasis is carried out as shown in (C) of FIG.4 in relation to the sharp signal as shown in (A) of FIG.4, only when the edge detection level |E| exceeds the threshold TH as shown in (B) of FIG.4, in other word, only when the edge is surely detected. FIG.5 is a block diagram showing an example of the apparatus for the sharpness emphasis according to this invention. A sharpness signal S is stored in a first means (memory) 1 as well as inputting to a subtracter 7 and an adder 6. Following to the input of the sharp signal S, the memory 1 outputs an unsharp signal u , and at the same time, outputs a sharp signal I in the form of the n × n matrix. The output M of an edge detecting filter 3 and the sharp signal I both of which are formed of the matrix like the ones shown in FIGs.2A and 2B, are inputted to a second means (e.g. a multiplication accumulator) 2 to be subjected to the multiplication accumulating operation as represented by the expression (1), in order to determine an edge detection level E. A look-up table which holds a function as shown in FIG.3, outputs a function in accordance with the edge detection level E. While, the unsharp signal u outputted from the memory 1 is inputted to the subtracter 7, in which the difference (S-u) between the sharp signal S and the unsharp signal u is calculated. Thus obtained result is inputted to a multiplier 5. The multiplier 5 to which is inputted a sharpness emphasis coefficient K as well as the functional output f(E) of the look-up talble 4, performs an operation as shown below; K · (S-u) · f(E) Then, the resultant of the above expression (5) is inputted to the adder 6 and added to the sharp signal S to output a sharpness-emphasized sharp signal S'. S' = S + K · (S-u) · f(E) In FIG.6 which shows another example of this invention, a plurality of the edge detecting filters shown in FIGs.2A and 2B are introduced respectively for the main scanning direction and the auxiliary scanning direction. A sharp signal X ij of matrix outputted from the memory 10 is inputted to a multiplication accumulator 11 for main scanning and to a multiplication accumulator 12 for auxiliary scanning. Then the output F ij of the edge detecting filter (FIG.2A) and the output E ij of the edge detecting filter (FIG.2B) are respectively inputted into the multiplication accumuator 11 for the main scannig and the multiplication accumulator 12 for the auxiliary scanning, to output the edge detection level F for the main scanning direction and the edge detection level E for the auxiliary sacanning direction with being multiplication-accumulated. The edge detection levels F and E are inputted into respective absolute value forming circuits 13 and 14 in which the values are transformed to respective absolute values. The resultant absolute value siganals |F| and |E| are added up in an adder 15 to be inputted to a look-up table 16. The look-up table 16 holds a function as shown in FIG.3 to output f(|E|+|F|) in accordance with "|F| +|E|". Accordingly, since this apparatus detects an edge in accordance with plural directions, i.e., by the main scanning direction and the auxiliary scanning direction, it is possible to effect the edge detection more precisely. It should be noted that although the absolute value forming circuit is employed in this configuration, the cicuit in which E 2 , F 2 are calculated can be used. Other examples of the edge detecting filters are shown in FIGs. 7A and 7B, which respectively example an edge detecting filter without weight and another one with weight. Also these edge detecting filters can achieve the edge detections as described above. According to the sharpness emphasis method and its apparatus of this invention, since the sharpness emphasis is to be carried out when the edge detection level has a low value, it is possible to avoid emphasizing the grains and the like on the original. In addition, since the size of the mask is variable, and edges are detected from the plural directions, it is possible to effect an optimum sharpness emphasis steadly in accordance only with the edges. It should be understood that many modifications and adaptations of the invention will become apparent to those skilled in the art and it is intended to encompass such obvious modifications and changes in the scope of the claims appended hereto.

Claims: An image sharpness emphasis method comprising the steps of: reading an original image data to generate a sharp signal (S), forming of an unsharp signal (u) from said original image data, calculating a difference between said sharp signal (S), and said unsharp signal (u) to produce a sharpness emphasis signal (S-u), multiplying said sharpness emphasis signal (S-u) by a function value (f(E)) output from a look-up table, characterized by multiplying the product of said sharpness emphasis signal (S-u) and said function value (f(E)), which is output from said look-up table, by a sharpness emphasis coefficient (K), wherein the function value (f(E)) output from said look-up table is based on an output of an edge detecting filter (M) for detecting only the edge portions of said image. The image sharpness emphasis method of claim 1, characterized in that the image data is extracted for a certain size of a masking matrix, said edge detecting filter is formed of the same masking matrix size and said unsharp signal is formed on the basis of the same masking matrix size. The image sharpness emphasis method of claim 2, characterized in that said edge detecting filter (M) comprises masking coefficients of different weights. The image sharpness emphasis method of claim 1, characterized in that the function stored in the look-up table is a thresholding function and is zero below a certain input. An image sharpness emphasis apparatus comprising: a subtracting means (7) for calculating a sharpness emphasis signal (S-u) as the difference between a sharp signal (S) and an unsharp signal (u), both obtained from an input image signal, a look-up table (4) for outputting a function value, a multiplication means (5) for multiplying said sharpness emphasis signal (S-u) and said function output by said look-up table, and an adder (6) for adding up said sharp signal and an output of said multiplication means (5) so as to be output, characterized by first means (1) for receiving said sharp signal and outputting said unsharp signal and a matrix signal, an edge detecting filter (3) for detecting edges of said image, a second means (2) for calculating the product of each coefficient of said matrix signal and each corresponding coefficient of a matrix of coefficients comprised in said edge detecting filter and for adding up said products, said look-up table (4) being accessed based on an output value of said second means (2), and said multiplication means (5) multiplying the product of said sharpness emphasis signal (S-u) and said output of said look-up table (f(E)),and a sharpness emphasis coefficient (K). The image sharpness emphasis apparatus of claim 5, characterized in that there are provided a plurality of said edge detecting filters and said second means (2) for a plurality of directions in which the original image is scanned, and said look-up table is to be accessed by adding up an output of each of said second means (2) of each of said directions.

Inventors: SHIMAZAKI OSAMU, TAKEMOTO FUMITO, SHIMAZAKI, OSAMU, TAKEMOTO, FUMITO
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1991
Main classification: H04N1
IPC codes: H04N1, G06T5