JP5281561B2 - Image processing apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent an unexpected pseudo-grayscale image from being obtained when a part of a multi-gray-scale image interferes with a dither matrix. <P>SOLUTION: A specific pattern detecting section S2a detects a region that is included in a multi-gray-scale image 100T, in which pixel values have an element period of a dither matrix 101 and do not include a pattern of only minimum and maximum values of possible values, and a specific pattern smoothing section S2b replaces the pixel values in the region with an average value in the region, and converts the multi-gray-scale image 100T into pseudo-grayscale. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an image processing apparatus that converts a multi-gradation image into a pseudo gradation image by a dither method (halftone screen method), and an image forming apparatus using the image processing apparatus.

  When printing a multi-tone image with an image forming apparatus, the multi-tone image is usually converted into a pseudo-tone image by a dither method. At this time, the multi-tone image is divided into blocks having the same size as the dither matrix, and each block is converted into a pseudo-tone image using the dither matrix.

  For example, as shown in FIG. 8A, when a 4 × 4 pixel multi-tone image block 100 is converted into a pseudo-tone image block 104 by a dither matrix 101, each element of the multi-tone image block 100 is converted to a dither matrix. 101 is added to the corresponding element to obtain the added image 102 and each element is compared with a threshold value “15”. If it is equal to or greater than the threshold value, “1” (white), otherwise “0” (black) As a result, the pseudo gradation image block 104 is generated.

  As the dither matrix 101, a Bayer pattern is usually used, and a group having a low value of 0 to 7 and a group having a high value of 8 to 14 have a period of 3 pixels. If the multi-tone image block 100 similarly has a period with respect to the dither matrix 101 having this period, both patterns interfere with each other and moire occurs.

  Patent Document 1 below discloses a configuration in which a plurality of dither matrices having different periods are provided and a dither matrix having a period closest to the period of a multi-tone image is selectively used to prevent moire.

Japanese Patent Laid-Open No. 2004-200746

  However, if the periods of both patterns are the same, even if the tone values of the multi-tone image are the same, the pseudo values after the conversion are different between when the phases of the two patterns match and when the phases differ by 180 °. The pseudo gradation values of the gradation image are greatly different.

  For example, as shown in FIG. 8B, in the case of a multi-tone image block 100A having the same period and phase as the dither matrix 101 and having an average of 25% gray, an added image 102A is obtained, and the average becomes 50% gray. On the other hand, as shown in FIG. 8C, in the case of a multi-tone image block 100B having the same period as that of the dither matrix 101 and having a phase difference of 180 ° and an average of 25% gray, an added image 102B is obtained. % White.

  That is, in such a region, a print image in which the pattern of the multi-tone image has completely disappeared is obtained, or a print image having a double density is obtained, resulting in an unexpected result. A similar problem occurs when the size of the dither matrix is larger than 4 × 4 pixels and the period of the multi-tone image is smaller than that of the dither matrix.

  In view of such problems, an object of the present invention is to provide an image processing apparatus and an image forming apparatus capable of preventing a part of a multi-tone image from interfering with a dither matrix and obtaining an unexpected result. It is to provide.

In the first aspect of the present invention, an image processing apparatus that converts a multi-tone image into a pseudo-tone image by a dither method includes a pre-processing unit that processes the multi-tone image before the conversion. The processing unit
An area (for example, a possible value) in which the distribution of pixel values included in the multi-tone image has a period equal to or less than the element period of the dither matrix used in the dither method and is expected to interfere with the dither matrix And a specific pattern smoothing unit that replaces the pixel values of the region with the average value of the region.

  In the second aspect of the image processing apparatus according to the present invention, in the first aspect, the period equal to or less than the period of the dither matrix is 3 pixels.

In the third aspect of the image processing apparatus according to the present invention, in the second aspect, the region is expressed as a pixel value c (x, y) at the coordinates (x, y) of the sample point of the multi-tone image, Conditional expression c (x, y) = c (x−2, y) where cmax is the difference between the maximum value and the minimum value of the range that the pixel value can take
c (x, y) ≠ c (x-1, y)
| C (x, y) -c (x-1, y) | ≠ cmax
Is equal to the area of the logical sum of the line areas from coordinates (xd, y) to coordinates (x, y) that satisfy all of the above.

  In a fourth aspect of the image processing apparatus and the image forming apparatus according to the present invention, in the first aspect, the average value is an average value of two adjacent pixels in the region.

According to a fifth aspect of the image processing apparatus and the image forming apparatus of the present invention, in any one of the first to fourth aspects, the multivalued image as the first multivalued image is processed before the processing by the specific pattern detection unit. Is further converted to a second multi-value image having a lower gradation,
The specific pattern detection unit and the specific pattern smoothing unit perform processing on the second multi-value image and the first multi-value image, respectively.

  According to the configuration of the first aspect, in the specific pattern detection unit, the distribution of pixel values included in the multi-tone image has a period equal to or less than the element period of the dither matrix used in the dither method, and the dither matrix After detecting a region that is predicted to interfere with the image and replacing the pixel value of the region with the average value of the region by the specific pattern smoothing unit, the multi-gradation image is converted into a pseudo gradation. In this way, it is possible to prevent a part of the multi-tone image from interfering with the dither matrix to obtain an unexpected result.

  According to the configuration of the second aspect, the specific pattern with a period of 3 pixels included in the multi-tone image is smoothed, so that the specific pattern included in the multi-tone image is lost by the pseudo-tone conversion. As a result, it is possible to prevent a peculiar phenomenon that the gradation value is doubled.

  According to the structure of the said 3rd aspect, since a specific pattern can be detected by the one-dimensional scan with respect to a multi-tone image, there exists an effect that a process is easy.

  According to the configuration of the fourth aspect, there is an effect that the smoothing process is simple.

  According to the structure of the said 5th aspect, there exists an effect that the specific pattern similar to a dither matrix can be detected easily by requantization of a multi-value image.

  Other objects, configurations and effects of the present invention will become apparent from the following description.

FIG. 3 is a schematic functional block diagram illustrating a configuration of a printer driver and its peripheral software according to the first exemplary embodiment of the present invention. It is a schematic explanatory drawing of the process of the specific pattern detection / smoothing part S2 and pseudo gradation conversion part S3 in FIG. It is a detailed flowchart of the specific pattern detection and smoothing part S2 in FIG. It is explanatory drawing of the process of FIG. It is pixel value smoothing explanatory drawing of the detected smoothing object area | region. 1 is a schematic block diagram illustrating a hardware configuration of an image forming system according to Embodiment 1 of the present invention. FIG. 9 is a schematic functional block diagram illustrating a configuration of a printer driver and peripheral software according to a second embodiment of the invention. It is explanatory drawing of the problem of a prior art.

  FIG. 6 is a schematic block diagram illustrating a hardware configuration of the image forming system according to the first embodiment of the present invention.

  In this system, an image forming apparatus 10 and a host computer 20 are connected by a LAN 30.

  In the host computer 20, a CPU 21 is coupled to a PROM 23, a DRAM 24, a hard disk drive 25, a network interface 26, and an interactive input / output device 27 via an interface 22. In FIG. 6, a plurality of types of interfaces are represented by one block 22 for simplification.

  The PROM 23 is a flash memory, for example, and stores a BIOS. The DRAM 24 is used as a main storage device. The hard disk drive 25 stores a virtual storage OS (operating system), various drivers including a printer driver, applications, and data. Network interface 26 is coupled to LAN 30. The interactive input / output device 27 includes, for example, a keyboard, a pointing device, and a display device.

  FIG. 1 is a schematic functional block diagram showing the configuration of the printer driver 52 and its peripheral software provided in the host computer 20 according to the first embodiment of the present invention.

  The user gives a print instruction via the interactive input / output device 27 in order to print a document created by the application application 50. As a result, the content of this document is converted into a drawing command common to the display device and the printing device via the drawing library of the OS 51, and this is supplied to the printer driver 52. In the printer driver 52, the drawing command is converted into print data described in PDL (page description language) that can be interpreted by the image forming apparatus 10, and is transmitted to the image forming apparatus 10 via the communication unit 26A. The communication unit 26A includes the network interface 26 in FIG. 6 and a communication program.

  The CPU 21 performs processing in the following processing units S0 to S3 in accordance with instructions in the printer driver 52.

  (S0) The drawing data received from the OS 51 is converted into PDL data.

  (S1) The PDL data is developed as a bitmap to generate a multi-tone image 100T.

  (S2) For this multi-tone image 100T, there is a possibility that a pseudo-tone image having a pseudo-tone value greatly different from the tone value of the multi-tone image 100T may be obtained due to the interference with the pattern of the dither matrix 101. The pattern is detected and smoothed so that the difference between the two values is reduced.

  (S3) This partially smoothed multi-tone image is converted into a pseudo-tone image by a dither method, and further converted into a PDL image object that can be interpreted by the image forming apparatus 10.

  FIG. 2 is a schematic explanatory diagram of the processing of the specific pattern detection / smoothing unit S2 and the pseudo gradation conversion unit S3.

  The specific pattern detection / smoothing unit S2 includes a specific pattern detection unit S2a and a specific pattern smoothing unit S2b.

  The specific pattern detection unit S2a detects a specific pattern included in the multi-tone image 100T, and writes information indicating that the pattern has been detected in a corresponding portion of the smoothing target area storage unit 103T. This specific pattern is a pattern having the same cycle as that of the dither matrix 101, in this example, a cycle of 3 pixels. In the smoothing target area storage unit 103T, “1” indicates a pixel included in the specific pattern, and “0” indicates a pixel not included in the specific pattern.

  The specific pattern smoothing unit S2b smoothes the pixel values in the region in the multi-tone image 100T corresponding to this region for each region in which '1' in the smoothing target region storage unit 103T is continuous. A gradation image 100TA is obtained. This smoothing is performed by obtaining an average value for each region and replacing each pixel value in the region with the average value, or by smoothing with a moving average filter (smoothing filter).

  The pseudo gradation converting unit S3 includes an adding unit S3a and a comparing unit S3b.

  The adding unit S3a divides the multi-tone image 100T into 4 × 4 pixel blocks having the same size as the dither matrix 101, and adds the value of the corresponding element of the dither matrix 101 to the pixel value of each block. The addition image 102T is generated.

  The comparison unit S3b generates a pseudo gradation image 104T by comparing each pixel value in the addition image 102T with a threshold value “15” and binarizing it.

  FIG. 3 is a detailed flowchart of the specific pattern detection / smoothing unit S2 in FIG. FIG. 4 is an explanatory diagram of FIG.

  As shown in FIG. 4A, it is assumed that the multi-tone image 100T is a rectangular region having a point (0, 0) and a point (xmax, ymax) on the xy coordinate system as diagonal points. For simplicity of explanation, it is assumed that the multi-gradation image 100T has 16 gradations and the range of pixel values that can be taken is 0 to 15, and the dither matrix 101 is the same as that in FIG. Suppose that it is a Bayer matrix of 4 rows. The pixel value at the sample point (x, y) of the multi-tone image 100T is c (x, y), and the difference between the maximum value and the minimum value that the pixel value can take is cmax (in the above case, cmax = 15). Is written.

  FIG. 4B shows the smoothing target area storage unit 103T. One bit of the smoothing target area storage unit 103T corresponds to each pixel of the multi-tone image 100T. The bit at the position (x, y) on the smoothing target area storage unit 103T is expressed as d (x, y). It is assumed that the smoothing target area storage unit 103T is cleared to zero as an initial state.

  The processes in steps S20 to S24 below are configuration examples of the specific pattern detection unit S2a, and steps S25 to S28 are configuration examples of the specific pattern smoothing unit S2b.

  (S20) The initial value (2, 0) is substituted into the scanning point (x, y) on the multi-tone image 100T.

  (S21) It is determined whether or not all of the following conditional expressions are satisfied.

c (x, y) = c (x-2, y)
c (x, y) ≠ c (x-1, y)
| C (x, y) -c (x-1, y) | ≠ cmax
If satisfied, the process proceeds to step S22, and if not, the process proceeds to step S23. This last conditional expression is for excluding the line regions (x−2, y) to (x, y) where interference with the dither matrix 101 does not occur.

  (S22) The smoothing target line regions (x−2, y) to (x, y) detected in step S21 are stored in the smoothing target region storage unit 103T. That is, d (x−2, y) = “1”, d (x−1, y) = “1”, d (x, y) = “1”.

  (S23) The scanning point (x, y) is updated. That is, x is incremented by 1, and when x exceeds the maximum value xmax, x = 2 and y is incremented by 1.

  (S24) If y> ymax, the process proceeds to step S25; otherwise, the process returns to step S21.

  (S25) The initial value 0 is substituted for y.

  (S26) For the line areas (y, 0) to (y, xmax) in the x direction on the smoothing target area storage unit 103T, for each area (one continuous area) where “1” continues, the multi-tone image 100T. Replace the pixel value in the corresponding region above with its average value. Due to the periodicity, this average value is equal to the average value of two adjacent pixels in one continuous area. In calculating the average value, the decimal part is rounded down.

  (S27) The value of y is incremented by 1.

  (S28) If y> ymax, the process of FIG. 3 is terminated, otherwise the process returns to step S26.

  In FIG. 5, hatched areas on the multi-tone images 100T1 and 100T2 indicate specific patterns detected by the specific pattern detection unit S2a. According to the method of FIG. 3, even if a single line indicated by a dotted line on a multi-tone image includes two line regions in the same specific pattern as shown in FIG. Even when line areas having different specific patterns are included as shown in FIG. 5B, the pixel value of the area is replaced with the average value of any two adjacent pixels in each continuous line area. Thus, smoothing for each specific pattern can be performed accurately.

  According to the first embodiment, in the specific pattern detection unit S2a, the pixel value included in the multi-tone image 100T has the element period of the dither matrix 101 and does not include the minimum and maximum possible values. Is detected, and the specific pattern smoothing unit S2b replaces the pixel value of the region with the average value of the region, and then converts the multi-tone image 100T into a pseudo-tone, so a part of the multi-tone image 100T Can be prevented from interfering with the dither matrix 101 to obtain an unexpected result.

  In addition, since the specific pattern with a period of 3 pixels included in the multi-tone image 100T is smoothed, the specific pattern included in the multi-tone image 100T disappears by pseudo-tone conversion, or the tone value is 2 This has the effect of preventing the unique phenomenon of doubling.

  Furthermore, since the specific pattern can be detected by one-dimensional scanning on the multi-tone image 100T, there is an effect that the processing is simple.

  Further, since the smoothing process is performed by using the average value of two adjacent pixels in the specific pattern, there is an effect that the smoothing process is simple.

  FIG. 7 is a schematic functional block diagram showing the configuration of the printer driver 52A and its peripheral software provided in the host computer 20 according to the second embodiment of the present invention.

  In the second embodiment, the printer driver 52A detects and smoothes a specific pattern only for the image object among the patterns received from the OS 51 before the PDL conversion unit S0, as in the first embodiment.

  Other points are the same as those in the first embodiment.

  In the above, preferred embodiments of the present invention have been described. However, the present invention includes various modifications, and other combinations of the components described in the above-described embodiments, and functions of the components. Those using other configurations for realizing the above, and other configurations that would be conceived by those skilled in the art from these configurations or functions are also included in the present invention.

  For example, the process of step S26 may be performed every time the process of steps S21 to S24 is repeated for one line of the image. In this case, the smoothing target area storage unit 103T may be one line. Moreover, the structure which performs the same process for every line area | region which further divided | segmented this 1 line may be sufficient.

  In the present invention, the processing in the specific pattern detection / smoothing unit S2 may be performed before the processing in the pseudo gradation conversion unit S3. The image forming apparatus 10 includes the specific pattern detection / smoothing unit S2. There may be.

  Further, immediately before step S25 in FIG. 3, for each continuous region in the smoothing target region storage unit 103T, for a corresponding region in the multi-tone image 100T, c (x, y) = c (x + 1, Alternatively, the region where y + 1) is established may be detected, and only this region, that is, only the region where the direction of the wave pattern (wave number direction) matches that of the dither matrix 101 may be smoothed.

Furthermore, the conditional expression of step S21 is changed to the following conditional expression | c (x, y) −c (x−2, y) | ≦ Δ1
| C (x, y) -c (x-1, y) |> Δ2
| C (x, y) -c (x-1, y) |> C
A configuration may be adopted in which it is determined whether or not the pattern is similar to the specific pattern depending on whether or not all the conditional expressions after the replacement are satisfied. Here, Δ1 and C are positive integers, Δ2 is an integer greater than or equal to 0, Δ1 and Δ2 are determined empirically, and C is determined so that a region that does not necessarily interfere with the dither matrix 101 is excluded. It is done. Also in this case, as described above, the configuration may be such that the smoothing is performed only in the region where the direction of the wave pattern substantially coincides with that of the dither matrix 101 immediately before step S25.

These similar and substantially coincidence determination processes can be replaced with identical and coincidence determinations by requantizing the multi-tone image. That is, after the multi-gradation image 100T is re-quantized by the re-quantization unit to reduce the gradation level, the same processing as in the first embodiment is performed, so that substantially similar and substantially matching determination processing can be achieved. . According to this configuration, there is an effect that a specific pattern similar to a dither matrix can be easily detected.

10 Image forming apparatus 20 Host computer 21 CPU
22 Interface 23 PROM
24 DRAM
25 Hard Disk Drive 26 Network Interface 26A Communication Unit 27 Interactive Input / Output Device 30 Network 50 Application 51 OS
52, 52A Printer driver 100, 100A, 100B, 100T Multi-gradation image 101 Dither matrix 102, 102A, 102B Addition image 103T Smoothing target area storage unit 104, 104T Pseudo gradation image S0 PDL conversion unit S1 Bitmap development unit S2 Specific pattern detection / smoothing unit S2a Specific pattern detection unit S2b Specific pattern smoothing unit S3 Pseudo gradation conversion unit S3a Addition unit S3b Comparison unit

Claims (5)

In an image processing apparatus that converts a multi-tone image into a pseudo-tone image by a dither method, the image processing apparatus includes a pre-processing unit that processes the multi-tone image before the conversion, and the pre-processing unit includes the multi-tone image. A specific pattern detection unit for detecting a region in which a distribution of pixel values included in an image has a period equal to or less than an element period of a dither matrix used in the dither method and is predicted to interfere with the dither matrix; possess a specific pattern smoothing unit that replaces the pixel values of the region with an average value of the region, a region is the pixel value at the sample point of the multi-gradation image coordinates (x, y) c ( x, y), and the difference between the maximum value and the minimum value of the pixel value range is expressed as cmax, the conditional expression c (x, y) = c (x−2, y) c (x, y) y) ≠ c (x-1 , y) | c (x, y) -c (x-1, y) | satisfies all ≠ cmax Mark (x-d, y) image processing apparatus, characterized in that equal to the area of the logical sum of the line area from to the coordinates (x, y). The image processing apparatus according to claim 1, wherein a period equal to or less than an element period of the dither matrix is 3 pixels. The average image processing apparatus according to claim 1 or 2, characterized in that an average value of two pixels adjacent to each other within the region. A requantization unit that converts the multi-valued image as the first multi-valued image into a second multi-valued image having a lower gradation before processing by the specific pattern detecting unit; each parts and the specific pattern smoothing unit, an image according to the second processing is performed on multivalued image and the first multivalued image, any one of claims 1 to 3, characterized in that Processing equipment. Multi-tone image input means, the image processing apparatus according to any one of claims 1 to 4 , multi-tone image input means for supplying a multi-tone image to the image processing apparatus, and the image processing apparatus An image forming apparatus comprising: a pseudo gradation image output unit that outputs the pseudo gradation image obtained in (1).

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