Method and apparatus for image processing using a signal of multiple scale values

An image processing apparatus includes an image data input mechanism which allows an input of image data and processes input image data in a unit of pixels. A determination mechanism analyzes and determines whether surrounding pixels close to an object pixel are part of a character area. A calculation mechanism calculates a distance value from the object pixel to the surrounding pixels determined as part of the character area by the determination mechanism. A signal output mechanism outputs a multiple value signal representing a feature of the object pixel as a character based on the distance value calculated by the calculation mechanism.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a method and apparatus for image processing, and
 more particularly to a method and apparatus for image processing that
 extracts characteristics of an image and generates an image signal having
 multiple scale values in accordance with the extracted characteristics.
 2. Discussion of the Background
 In general, an image processing apparatus in use for various image handling
 apparatuses, such as a digital copying machine, a facsimile machine, and
 so forth, has been provided with a data handling operation that separates
 an original image having a mixture of characters and patterns into a
 plurality of image areas in accordance with types of images as determined
 with detailed analysis of the original image. This background apparatus
 then switches to an appropriate image processing operation according to
 the image type to appropriately process each separated image area. For
 example, Japanese Laid-Open Patent Publication No. JPAP03-089677 (1991)
 describes a technique for emphasizing a contrast of characters in black,
 in particular, by switching a spatial filter, a gamma conversion, a
 screening, and so forth, to process each image area of a non-character
 (non-active) edge, a colored-character edge, an intermediate-chroma
 character edge, or a black character edge. As another example, Japanese
 Laid-Open Patent Publication Nos. JPAP07-108019 and JPAP06-018439 (1994)
 describe a technique that generates a specific signal having multiple
 scale values for representing an edge of a character, as illustrated in
 FIG. 8. FIG. 8 illustrates a relationship between density scales of an
 edge and an immediately adjacent background, wherein the letter A
 indicates an edge of a character and the letter X indicates an area of an
 immediately adjacent background which can be analyzed relative to the edge
 A. Accordingly, this technique switches an image processing operation at
 multiple levels in accordance with the multiple scale values of the
 generated specific signal.
 However, the technique of JPAP03-089677 has a drawback that a quality of an
 image deteriorates when the image has a sharply changed texture or when an
 erroneous image separation is made. This deterioration of image quality is
 due to a method of image separation using only binary scale values of 0
 and 1 which may be insufficient to judge each image area. Also, the
 techniques of JPAP07-108019 and JPAP06-018439 have drawbacks that
 factitious marks remain around edges in the image caused by switching of
 the image processing operations, although the switching is conducted at
 the multiple levels and deterioration of an image is therefore decreased
 to a certain extent. As illustrated in FIG. 8, the reason for this
 drawback is that since this technique can use a 5-by-5 pixel filter at
 best, an area X of an immediately adjacent background which can be
 analyzed relative to the edge is relatively narrow.
 SUMMARY OF THE INVENTION
 Accordingly, an object of the present invention is to provide a novel image
 processing apparatus which is capable of realizing reproducibility of both
 a resolution-oriented image area, such as edges of a character and a
 pattern, and a gray-scale-oriented image area, such as a pattern.
 Also, another object of the present invention is to provide a novel image
 processing method which is capable of realizing reproducibility of both a
 resolution-oriented image area, such as edges of a character and a
 pattern, and a gray-scale-oriented image area, such as a pattern.
 To achieve these and other objects, a novel image processing apparatus of
 the present invention includes an image data input mechanism which allows
 an input of image data and processes input image data in a unit of pixels.
 A determination mechanism analyzes and determines whether surrounding
 pixels close to an object pixel are part of a character area. A
 calculation mechanism calculates a distance value from the object pixel to
 the surrounding pixels determined as part of the character area by the
 determination mechanism. Further, a signal output mechanism outputs a
 multiple value signal for representing a feature of the object pixel as a
 character based on the distance value calculated by the calculation
 mechanism.
 A density decreasing mechanism may be added to decrease an original density
 of the input image data to a predetermined density immediately after the
 input image data is input through the image data input mechanism. Also, a
 density increasing mechanism may be added to increase the predetermined
 density of the output image data back to the original density.
 The signal output mechanism may calculate the distance value using a
 formula V=255-(255.times.D/6), wherein V is a density value of the object
 pixel and D is a distance from the object pixel to a peripheral pixel
 determined as part of the character area by the determination mechanism.
 The character area may include a part which is a cluster area of a black
 pixel, or which is an edge area, or which is a ridge area of a character.
 The novel image processing method of the present invention includes the
 steps of inputting image data, processing the input image data in a unit
 of pixels, analyzing and determining whether surrounding pixels close to
 an object pixel are part of a character area, calculating a distance value
 from the object pixel to the surrounding pixels determined as part of the
 character area by the determination step, and outputting a multiple value
 signal for representing a feature of the object pixel as a character based
 on the distance value calculated by the calculation step.
 Other objects, features, and advantages of the present invention will
 become apparent from the following detailed description when read in
 conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In describing preferred embodiments of the present invention illustrated in
 the drawings, specific terminology is employed for the sake of clarity.
 However, the present invention is not intended to be limited to the
 specific terminology so selected and it is to be understood that each
 specific element includes all technical equivalents which operate in a
 similar manner.
 Referring now to the drawings, wherein like reference numerals designate
 identical or corresponding parts throughout the several views, and more
 particularly to FIG. 1 thereof, an image processing apparatus 100
 according to an exemplary embodiment of the present invention is
 illustrated. The image processing apparatus 100 of FIG. 1 includes an
 entry unit 1, a character pixel area (CPA) detector 2, a distance
 calculator 3, and a feature amount (FA) converter 4. The entry unit 1
 includes a scanner (not shown) using a CCD (charge coupled device) that
 reads an original image and generates an analog signal which is then
 converted into an 8-bit digital image signal linearly expressing a density
 of each pixel with a value varying from 0 to 255.
 The CPA detector 2 analyzes the 8-bit digital image signal from the entry
 unit 1 and generates a signal that indicates whether an object pixel is
 part of a character area. The details of the CPA detector 2 are explained
 with reference to the following four variations illustrated in FIGS.
 2A-2D.
 A pixel cluster (PC) detector 2a of FIG. 2A is a first example of the CPA
 detector 2, and uses such a feature of characters that the pixels of high
 density exist in a form of a bar-shaped cluster so as to detect pixels of
 a character.
 More specifically, the PC detector 2a first performs, as a pre-treatment,
 an edge enhancing filtering for filtering pixels, including an object
 pixel and other pixels surrounding the object pixel, of an image signal
 from the entry unit 1 with an enhancing coefficient of 16, for example,
 and a 5-by-5 pixel filter, for example, as illustrated in FIG. 3. Then,
 the PC detector 2a compares the resultant values of the object pixel and
 the surrounding pixels with a predetermined threshold value so as to
 determine whether each of the object pixel and the surrounding pixels is a
 black pixel or not. A stroke formed by the black pixels including the
 object pixel and the surrounding pixels is subjected to a pattern matching
 with various strokes such as vertical, horizontal, slash-like, and
 reverse-slash-like strokes, as shown in FIGS. 4A-4D, respectively.
 Thereby, the PC detector 2a can detect a pixel cluster which forms a black
 bar.
 A second example of the CPA detector 2 is illustrated in FIG. 2B. This
 example is used when the image signal from the entry unit 1 is a color
 image signal, and includes a minimum value calculator 2b before the PC
 detector 2a. The minimum value calculator 2b calculates a minimum value of
 R (red), G (green), and B (blue) elements of the color image signal, and
 outputs an image signal based on the minimum value to the PC detector 2a.
 Then, the PC detector 2a analyzes the output image signal from the minimum
 value calculator 2b to detect a pixel cluster which forms a black bar in
 the ways as described above.
 A third example of the CPA detector 2 is illustrated in FIG. 2C as an edge
 pixel (EP) detector 2c. The EP detector 2c detects an edge of characters
 using a pattern matching for finding seriality of black pixels and white
 pixels on a character edge area. Specifically, the EP detector 2c uses
 such a feature of characters that in a character edge area there exist
 black pixels connected in series and white pixels connected in series so
 as to detect whether the object pixel is part of a character edge or not.
 The details of this detection are described in Japanese Laid-Open Patent
 Publication No. JPAP02-292957 (1990), the entire contents of which are
 hereby incorporated by reference. Other known methods than this technique
 may also be used to detect whether the pixels are part of a character.
 A fourth example of the CPA detector 2 is illustrated in FIG. 2D in which
 the CPA detector 2 is a character ridge pixel (CRP) detector 2d. The CRP
 detector 2d detects a ridge of characters using a pattern matching for
 finding a ridge-like form of pixels on a character edge area. That is, the
 CRP detector 2d uses such a feature of characters that in a middle of a
 character there exist black pixels in series having stepped values so as
 to detect whether the object pixel is part of a character or not. The
 details of this detection are described in Japanese Laid-Open Patent
 Publication No. JPAP02-123479 (1990), the entire contents of which are
 hereby incorporated by reference. Other known methods than this technique
 may also be used to detect whether the pixels are part of a character.
 In FIG. 1, the distance calculator 3 calculates and outputs a value
 indirectly proportional to a distance from the object pixel to a pixel
 which has been determined as part of a character based on the information
 detected by the CPA detector 2 as to whether the object pixel is part of a
 character (active pixel) or part of non-character (non-active pixel). In
 this operation, the output value from the distance calculator 3 becomes
 greater when an active pixel is located at a position closer to the object
 pixel.
 As shown in FIG. 5, a 9-by-9 pixel matrix is preferably used for this
 operation, for example. Assume that the object pixel indicated by letter A
 is located at the center, and active pixels indicated by letters B and C
 have their respective distances to the object pixel A. These distances can
 be calculated using a formula D=(x2+y2) wherein D is the distance, and x
 and y are the relative positions (x, y) of the active pixel B or C to the
 position of the object pixel as a point of origin (x=0, y=0).
 After calculating these distances, the distance calculator 3 selects the
 smallest value from among the calculated distances. The active pixel
 having the smallest distance value is the nearest active pixel relative to
 the object pixel A. In the case as shown in FIG. 5, the nearest active
 pixel is the active pixel B. With the resultant distance value D, the
 distance calculator 3 further calculates a value V of the object pixel A
 using a formula V=255-255.times.D/6. By this formula, the value V becomes
 approximately 0 when the nearest active pixel is located on a perimeter of
 the 9-by-9 pixel matrix, and increases as the active pixel becomes closer
 to the object pixel. The reason for dividing the distance D with 6 in the
 formula of V is as follows. A distance from the position of origin to a
 pixel on a perimeter of the matrix is a value of 5 pixels in the
 horizontal or vertical direction or a value of approximately 7 pixels in
 the diagonal direction. As a mean value of these 5 and 7 pixels, 6 is
 predetermined so as to set the value V to approximately 0 when an active
 pixel is located on a perimeter of the 9-by-9 pixel matrix.
 The FA converter 4 converts the value V of the object pixel, output from
 the distance calculator 3, into a value representing an amount of a
 feature of the object pixel, using a reference table (e.g., a gamma
 conversion table). By this operation, the image processing apparatus 100
 can provide a certain flexibility with respect to the relationships
 between the amount of the feature and each data processing operation that
 controls the amount of the feature. More specifically, such an operation
 allows the operator to select a preferable gamma from among a plurality of
 gamma conversion tables previously stored in the FA converter 4. For this
 purpose, the plurality of gamma conversion tables include an S-shaped
 gamma conversion table, for example, for performing a non-character
 deformation operation on a pixel which likely represents part of a
 character to some extent so as to perform a character deformation
 operation on a pixel truly representing part of a character when the image
 shows features of characters. In addition, the FA converter 4 may perform
 a quantization operation.
 Next, a modification of the image processing apparatus 100 is explained
 with reference to FIGS. 6 and 7. FIG. 6 shows a block diagram of a
 modified image processing apparatus 200. The modified image processing
 apparatus 200 includes the entry unit 1, the CPA detector 2, the distance
 calculator 3, and the FA converter 4, which perform the same functions as
 in the image processing apparatus 100. The modified image processing
 apparatus 200 further includes a first pixel density (PD) converter 5, a
 second pixel density (PD) converter 6, a gamma table memory 7, a gamma
 selector 8, and a gamma converter 9.
 The first PD converter 5 decreases the density of pixels received from the
 entry unit 1 and outputs the resultant signal to the CPA detector 2. Such
 a first PD converter 5 may be, as one example, configured with a simple
 circuit for thinning signals. The CPA detector 2 analyzes the pixels
 having the decreased density and generates a signal that indicates whether
 an object pixel is part of a character area. The distance calculator 3
 calculates and outputs the value V of each object pixel based on the
 signal from the CPA detector 2. The FA converter 4 converts the value V of
 each object pixel into feature amounts using the reference table. The
 second PD converter 6 returns the density of the pixel back to its
 original state and outputs the resultant signal to the gamma selector 8.
 Such a second PD converter 6 may, as an example, be configured with a
 simple circuit for automatically writing data twice.
 The reason for including an operation of reducing a density of pixels in
 the above operation is as follows. A method for judging an image based on
 pixel level information is prone to involve an erroneous judgement,
 resulting in an inferior quality of an image. In order to avoid this
 problem, a technique for performing the judgement in multiple levels has
 been developed. However, this technique has brought a relatively small
 effect with respect to the above problem since an area where the
 multiple-level judgement is introduced is basically narrow, or the pixel
 level. As a result, the deformation with such a multiple-level judgement
 appears to be factitious to the human eyes. Generally, this problem
 becomes distinguished as the integration of the memory device advances.
 Performing the multiple-level judgement based on a relatively large-sized
 matrix may be one solution. However, this solution has a disadvantage.
 This operation would cause the image processing apparatus 200 to increase
 a number of line buffers (not shown) for buffering the relatively
 large-number of pixel lines for forming the above large-sized matrix. As
 an alternative technique to avoid the above-mentioned problem and
 disadvantage, reduction of pixel density is adapted in the above
 operation. That is, in the present invention as shown in FIG. 6,
 operations are performed for detecting features of pixels in a density
 lower than the original density and returning to the original density
 after the judgement whether the object pixel is part of a character or not
 is completed.
 The gamma table memory 7 stores a plurality of gamma tables T0-T7, for
 example. These gamma tables T0-T7 represent gamma lines having different
 angles from each other, as illustrated in FIG. 7. The gamma table T7
 represents a gamma line having the greatest angle and is therefore
 suitable to be used for characters, while the gamma table T0 represents a
 gamma line having the smallest angle and is therefore suitable to be used
 for patterns. Accordingly, as the output signal from the second PD
 converter 6 becomes greater, a gamma table with the greater gamma angle is
 selected by the gamma selector 8. Then, the gamma converter 9 uses one of
 the gamma tables T0-T7 selected by the gamma selector 8 to convert the
 gamma of the image signal sent from the entry unit 1, as shown in FIG. 6.
 In the block diagram of FIG. 6, when the number of the prepared gamma
 tables is 8, as described above, the FA converter 4 performs the
 conversion by quantizing the amount of feature of a pixel into 3-bit-data.
 In addition, the gamma converter 9 may output an image signal in the same
 bit configuration when an image output machine such as a printer located
 at a stage following the image processing apparatus 200 outputs an image
 signal with a gray-scale in an 8-bit configuration. However, when the
 output gray-scale from the image output machine is smaller than 8, the
 gamma converter 9 may quantize 8-bit data of the image signal so as to fit
 to the bit number of the output gray-scale, or the gamma tables T0-T7 of
 the gamma table memory 7 may be configured to output data having a reduced
 number of bits so as to be suited to the output gray-scale.
 This invention may be conveniently implemented using a conventional general
 purpose digital computer programmed according to the teachings of the
 present specification, as will be apparent to those skilled in the
 computer art. Appropriate software coding can readily be prepared by
 skilled programmers based on the teachings of the present disclosure, as
 will be apparent to those skilled in the software art. The present
 invention may also be implemented by the preparation of application
 specific integrated circuits or by interconnecting an appropriate network
 of conventional component circuits, as will be readily apparent to those
 skilled in the art.
 Obviously, numerous additional modifications and variations of the present
 invention are possible in light of the above teachings. It is therefore to
 be understood that within the scope of the appended claims, the present
 invention may be practiced otherwise than as specifically described
 herein.
 This document is based on Japanese Patent Application Nos. JPAP 10-081228
 and JPAP11-048886 filed in the Japanese Patent Office on Mar. 27, 1999,
 and Feb. 25, 1999, respectively, the entire contents of which are hereby
 incorporated by reference.