WO2013179657A1 - Image processing apparatus and control method therefor - Google Patents

Image processing apparatus and control method therefor Download PDF

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Publication number
WO2013179657A1
WO2013179657A1 PCT/JP2013/003379 JP2013003379W WO2013179657A1 WO 2013179657 A1 WO2013179657 A1 WO 2013179657A1 JP 2013003379 W JP2013003379 W JP 2013003379W WO 2013179657 A1 WO2013179657 A1 WO 2013179657A1
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Prior art keywords
threshold value
dot
image data
tone
input
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PCT/JP2013/003379
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French (fr)
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Kyohei Kikuta
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Canon Kabushiki Kaisha
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/40—Picture signal circuits
    • H04N1/405—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels
    • H04N1/4051—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a dispersed dots halftone pattern, the dots having substantially the same size
    • H04N1/4052—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a dispersed dots halftone pattern, the dots having substantially the same size by error diffusion, i.e. transferring the binarising error to neighbouring dot decisions
    • H04N1/4053—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a dispersed dots halftone pattern, the dots having substantially the same size by error diffusion, i.e. transferring the binarising error to neighbouring dot decisions with threshold modulated relative to input image data or vice versa

Definitions

  • the present invention relates to a halftone processing for generating a dot pattern having high dispersibility based on image data.
  • An image forming apparatus for forming an image on a recording medium using dots is often used as an apparatus for outputting an image processed on a personal computer or an image captured by a digital camera.
  • a tone level which can be output from such an image forming apparatus is generally less than a number of tone level of image data handled by the personal computer or the like.
  • a dither method is known as one of the halftone processings.
  • the dither method is a method for determining an output value of each pixel by comparing a pixel value expressing input image data and a threshold value of a threshold value matrix on a pixel-to-pixel basis.
  • Such dither method using the threshold value matrix has a problem in that a sufficient dispersibility cannot be obtained. This is because, in the dither method, although a more suitable dot disposition is different depending on a tone (brightness), the dot disposition is influenced by dot dispositions of the other tones.
  • Japanese Patent Application Laid-Open No. 2003-46777 discusses a method for storing optimum dot patterns of all tones in advance.
  • a dot pattern is selected according to the tone of input image data.
  • the output of a pixel is determined by referring to whether the same pixel position in the selected dot pattern is an ON dot or an OFF dot.
  • this method since the dot patterns of all the tones are separately stored, a more highly dispersed dot disposition can be determined for each tone without being restricted by dot dispositions of the other tones.
  • Japanese Patent Application Laid-Open No. 2003-46777 also discusses an example of the dither method which uses a dispersed-type threshold value matrix to a portion of a continuous tone section. This method can reduce a storage capacity as compared to the method for storing the dot patterns of all the tones.
  • the method discussed in Japanese Patent Application Laid-Open No. 2003-46777 uses the stored dot patterns or threshold value matrixes to repetitively relate to the input image data in a tiled form. As a result, a non-uniform pattern in the dot pattern or threshold value matrix is generated as a texture (pattern) due to the repetition, thereby degrading the image quality.
  • the present invention is directed to reducing a texture generated by the repetition of dot patterns and generating a suitable dispersed-type dot pattern in a halftone processing for converting image data into a dot pattern.
  • an image processing apparatus configured to perform a halftone processing on input image data using a plurality of threshold value matrixes, includes a determination unit configured to, using a threshold value matrix according to an input value expressing a target pixel in the input image data, determine an output value of the target pixel by comparing the input value with a threshold value in the threshold value matrix.
  • a determination unit configured to, using a threshold value matrix according to an input value expressing a target pixel in the input image data, determine an output value of the target pixel by comparing the input value with a threshold value in the threshold value matrix.
  • Each of the plurality of threshold value matrixes is used for a different input value and has a size smaller than the input image data, and a size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined according to a dot pattern expressing a corresponding input value.
  • Fig. 1 is a diagram illustrating an image processing apparatus and an image forming apparatus.
  • Fig. 2 is a block diagram illustrating a detailed structure of a halftone processing unit.
  • Fig. 3 is a block diagram illustrating a detailed structure of the halftone processing unit.
  • Fig. 4 is a flowchart of a sequence of processes in the halftone processing unit.
  • Fig. 5 is an explanatory diagram of a suitable threshold value matrix applied to the present invention.
  • Fig. 6 is a graph illustrating suitable frequency characteristics in a dot-dispersed-type dot pattern.
  • Fig. 7 is an explanatory diagram in which a dot disposition is influenced by the size of a region.
  • Fig. 1 is a diagram illustrating an image processing apparatus and an image forming apparatus.
  • Fig. 2 is a block diagram illustrating a detailed structure of a halftone processing unit.
  • Fig. 3 is a block diagram illustrating a detailed structure of the halftone processing unit.
  • Fig. 8 is an explanatory diagram of a tone suitable for storage in a threshold value matrix of a certain size.
  • Fig. 9 is a flowchart illustrating a method for creating a plurality of threshold value matrixes.
  • Fig. 10 is an explanatory diagram of suitable dot patterns applied to the present invention.
  • Fig. 11 is a block diagram illustrating a detailed structure of a halftone processing unit.
  • Fig. 12 is a graph illustrating suitable frequency characteristics in a dot pattern in another exemplary embodiment.
  • Fig. 13 is a schematic diagram illustrating a suitable dot disposition in a dot pattern.
  • Fig. 1 is a block diagram illustrating structures of an image processing apparatus and an image forming apparatus which can be applied to the first exemplary embodiment.
  • an image processing apparatus 101 and an image forming apparatus 102 are connected by an interface or a circuit.
  • the image processing apparatus 101 is, for example, a printer driver installed in a general personal computer.
  • each unit within the image processing apparatus 101 described below is realized by a computer executing a predetermined program.
  • the image forming apparatus 102 may include the image processing apparatus 101.
  • the image processing apparatus 101 stores color image data to be printed, which has been input through an input terminal 103 (referred to as "input color image data" hereinafter), in an input image buffer 104.
  • the input color image data is formed by three color components of red (R), green (G), and blue (B).
  • a color separation processing unit 105 separates the stored input image data to image data corresponding to color material colors included in the image forming apparatus 102.
  • the color separation processing unit 105 refers to a look-up table for color separation (not illustrated).
  • the present exemplary embodiment will be described by using a monochrome of black (K) as an example.
  • a color separation process to a plurality of colors such as cyan (C), magenta (M), yellow (Y), and black (K)
  • C cyan
  • M magenta
  • Y yellow
  • K black
  • the color separated data is handled as 8 bits which express 256 tones of 0 to 255.
  • the data may be converted into the data having a number of tone level of more than 256.
  • a halftone processing unit 106 performs a halftone processing using a plurality of threshold value matrixes on the color separated data obtained from the color separation processing unit 105.
  • each of the threshold value matrixes is a dot-dispersed type.
  • a threshold value matrix to be used among the plurality of threshold value matrixes is determined according to an input value (tone level) expressing a pixel.
  • the halftone processing unit 106 converts the 8-bit color separated data into 1-bit (binary) data. Details of the conversion will be described below. Note that, in case of a color image, the halftone processing unit 106 separately processes each color using a plurality of threshold value matrixes prepared for respective colors.
  • Halftone image data obtained by the halftone process expresses a dot pattern which can be formed by the image forming apparatus 102.
  • the halftone processing unit 106 outputs the halftone image data to a halftone image buffer 107.
  • the stored halftone image data is output to the image forming apparatus 102 through an output terminal 108.
  • the image forming apparatus 102 Based on the halftone image data received from the image processing apparatus 101 through an input terminal 109, the image forming apparatus 102 forms an image on a recording medium by moving a recording head 111 sideways and lengthways relative to the recording medium.
  • the recording head 111 is of an ink-jet type and includes one or more recording elements (nozzles).
  • a head drive circuit 110 Based on the halftone image data, a head drive circuit 110 generates a driving signal for controlling the recording head 111. The recording head 111 actually records each ink dot onto the recording medium based on the driving signal.
  • Fig. 2 is a block diagram illustrating a detailed configuration of the halftone processing unit 106.
  • the halftone processing unit 106 converts the color separated data of 256 tones output from the color separation processing unit 105 into binary (1-bit) data. More specifically, the halftone processing unit 106 compares the input value expressing a target pixel 201 with a corresponding threshold value in the threshold value matrix and determines an output value.
  • the halftone processing unit 106 includes a memory 203, a threshold value matrix selector 205, and a comparator 206.
  • the memory 203 stores a plurality of threshold value matrixes 204 (M1, M2,..., MN) different from each other.
  • the threshold value matrix selector 205 determines, for each pixel, one threshold value matrix to be used from among the plurality of threshold value matrixes 204.
  • the comparator 206 compares the threshold value corresponding to the target pixel and a pixel value of the target pixel in the threshold value matrix selected by the threshold value matrix selector 205 and then determines an output value.
  • Fig. 4 illustrates a flowchart of a sequence of processes in the halftone processing unit 106.
  • a halftone processing method in the present exemplary embodiment will be described below.
  • step S402 as an initialization process, the image processing apparatus 101 reads N sheets of prepared threshold value matrixes.
  • step S403 the halftone processing unit 106 reads a pixel value g (x, y) expressing a target pixel (x, y) and stores the pixel value g (x, y) as a variable ("in") expressing an input value.
  • the threshold value matrix selector 205 determines a threshold value matrix to be used by the comparator 206 from among the plurality of threshold value matrixes (M1 to MN) stored in the memory.
  • a correspondence between the input value "in” and the threshold value matrix used is determined in advance and stored as a table.
  • the threshold value matrix selector 205 can determine a threshold value matrix by referring to the table. The correspondence between the input value "in” and the threshold value matrix will be further described below in detail.
  • the threshold value matrix selected by the threshold value matrix selector 205 is referred to as "Mk".
  • step S405 the comparator 206 reads a position, in the threshold value matrix "Mk" selected by the threshold value matrix selector 205, corresponding to a position (x, y) of the target pixel.
  • a method similar to the conventional dither method is used herein to read the position in the threshold value matrix corresponding to the target pixel.
  • the comparator 206 stores a threshold value stored at the position (i, j) in the threshold value matrix "Mk" in a variable "th".
  • step S406 the comparator 206 compares the input value "in” with the threshold value "th". If the input value "in” is less than or equal to the threshold value "th" (YES in step S406), the operation proceeds to step S407. If the input value "in” is more than the threshold value "th” (NO in step S406), the operation proceeds to step S408. In step S407, the comparator 206 stores an output value "out” of the target pixel as 0. On the other hand, in step S408, the comparator 206 stores the output value "out” of the target pixel as 1.
  • step S409 the halftone processing unit 106 outputs the output value "out" at a position (x, y) of an output image.
  • step S410 the above-described process is carried out on all the pixels in the input image.
  • step S411 if the process is finished on all the pixels (YES in step S410), the halftone processing ends.
  • FIG. 5 is a diagram illustrating a general dither method.
  • an ON dot black
  • an OFF dot white
  • an output image 503 is obtained.
  • an input image 504 has a single pixel value of 13 and is slightly brighter than the input image 501.
  • ON dots (black) in the output image 503 include all the ON dots in the output image 606 expressing a brighter tone.
  • Fig. 13 illustrates a dot pattern 1301 expressing a tone 8, and patterns 1302 and 1303 each expressing a tone 9.
  • the dot pattern 1301 has a good graininess.
  • the dot number of the tone 9 is more than the dot number of a dot pattern of the tone 8 by one.
  • the dots in the dot pattern 1302 include all the dots in the dot pattern 1301. In other words, the dot pattern 1302 is a dot disposition obtained by adding one dot to the dot pattern 1301.
  • the dot pattern 1302 has a high dot density locally, being non-uniform, thereby having a poor graininess.
  • the dot pattern 1303 has a good graininess, but does not include all the dots in the dot pattern 1301. In other words, it is found that the dot pattern of the tone 9 having a good graininess cannot be realized while holding all the dots in the tone 8.
  • the dot which has become an ON dot (black pixel) in a bright tone does not become an OFF dot (white pixel) in a tone darker than the bright tone.
  • the tone 8 and the tones 9 in Fig. 13 cannot obtain a dot pattern having high dispersibility using a common threshold value matrix.
  • respective tones having high dispersibility are expressed herein as "compatible".
  • to form respective tones into sufficiently highly dispersed dot patterns tones that are as compatible as possible with each other are related to a common threshold value matrix.
  • a highly dispersed dot pattern can be regarded to have a peak near this principal frequency fg and only have a frequency component higher than the principal frequency fg. For example, a dot pattern having frequency characteristics illustrated in Fig. 6 can be said to have a high dispersibility.
  • the position of each dot fluctuates and is not completely regular.
  • the dot pattern by separately disposing the dots at about a distance dg from each other, the dot pattern can be said to have a high dispersibility.
  • a suitable dot-to-dot distance dg is constant.
  • Fig. 7 illustrates dot patterns each expressing the tone g.
  • a dot pattern 701 expresses the tone g in a square region, L on a side.
  • the tone g is expressed by a dot number of 9.
  • a dot-to-dot distance dg of this tone g is L/3
  • the dot patterns similar to 701 in Fig. 7A are repetitively lined up from left to right or up and down, a visually characteristic texture is difficult to perceive therein.
  • a highly dispersed dot pattern is obtained by disposing dots in the region, 8L/9 on a side, in such a way that the dot-to-dot distance is substantially dg.
  • the dot pattern 702 disposes dots substantially periodically while maintaining the dot-to-dot distance dg.
  • the dot pattern 703 randomly disposes dots in the entire region while maintaining the dot-to-dot distance dg.
  • Each of the dot patterns 702 and 703 maintains the dot-to-dot distance dg in the region of the size 8L/9 * 8L/9.
  • the dot pattern when these dot patterns are repetitively lined up in a tiled form from left to right or up and down, the dot-to-dot distance dg cannot be maintained and a non-uniform dot disposition is generated.
  • the dot pattern may be perceived as a texture due to the repetition.
  • the present exemplary embodiment includes a plurality of threshold value matrixes and uses a common threshold value matrix for tones, of the tones forming input image data, which are as compatible as possible with each other. Moreover, the size of each threshold value matrix is determined according to a respectively corresponding tone. Consequently, the halftone processing unit 106 in the present exemplary embodiment includes a plurality of threshold value matrixes each having a size different from each other.
  • Fig. 8 illustrates an example of a tone group using a common threshold value matrix.
  • dots are disposed to maintain a dot-to-dot distance dg. Additionally, even if regions of the size L * L are lined up in a tiled form, a texture due to the repetition is difficult to perceive.
  • dots are disposed to include all dots having a brighter tone (tone of a smaller dot number).
  • the respective dots expressing a certain tone can be disposed to maintain about the dot-to-dot distance dg, with the positions slightly fluctuating.
  • a diagonal (2L) 1/2 is a value near the multiple of the dot-to-dot distance dg, the dots can be disposed on the diagonal.
  • n * L a square, L which satisfies L is nearly equal to n * dg or (2L) 1/2 is nearly equal to n * dg on a side is set as a threshold value matrix size (n is an arbitrary natural number).
  • Fig. 9 illustrates a flowchart for creating the plurality of threshold value matrixes used in the present exemplary embodiment.
  • a dot-to-dot distance dg necessary for a highly dispersed dot disposition is calculated for each tone and a combination of tones (tone group) using a common threshold value matrix is determined.
  • the tone group corresponding to a certain threshold value matrix i.e., handled by that threshold value matrix
  • "compatible" means tones which each can realize a highly dispersed dot pattern while holding dots expressing brighter tones (tones of a smaller dot number).
  • a size of the threshold value matrix is determined according to the tone group.
  • a square, L which satisfies L is nearly equal to n * dg or (2L) 1/2 is nearly equal to n * dg on a side, is determined as the size of the threshold value matrix.
  • steps S904 to S912 are repeated.
  • a threshold value to be stored in each threshold value matrix is determined and threshold value matrixes are created one by one.
  • a flow of creating a threshold value matrix M1 will be described as an example.
  • the other threshold value matrixes can be created likewise.
  • a matrix expressing the threshold value matrix M1 is initialized by first storing a threshold value which does not output a dot to any tone.
  • the brightest tone is selected from among a tone group corresponding to the threshold value matrix M1.
  • a dot pattern which has disposed the number of dots necessary to express the selected tone is determined in an input image having the same size as the threshold value matrix M1.
  • the dot pattern which does not form a non-uniform pattern and has a highly dispersed dot disposition can be used.
  • a conventional method may be used as a method for determining a highly dispersed dot pattern expressing a certain tone.
  • a threshold value to the tone expressed by this dot pattern is recorded at a position of the threshold value matrix M1 corresponding to the position of a dot in this dot pattern.
  • step S907 the next brightest tone is selected from among the tone group corresponding to the threshold value matrix M1.
  • a dot pattern having the number of dots necessary for the selected tone is determined.
  • a dot pattern expressing the selected tone is determined by adding a new dot to a dot pattern expressing a tone brighter than the selected tone.
  • the dot pattern which does not form a non-uniform pattern and has a highly dispersed dot disposition can be used.
  • dots should be disposed on grid points or on diagonals as centers in a square which is a threshold value matrix region.
  • the highly dispersed dot patterns are compatible with each other, and as determined in step S903, a square, L which satisfies L is nearly equal to n * dg or (2L) 1/2 is nearly equal to n * dg on a side, is determined as the size of a threshold value matrix.
  • a threshold value corresponding to the tone selected in step S907 is stored at a position of the threshold value matrix M1 corresponding to the position to which the new dot has been added in step S907.
  • a dot pattern having the next brightest tone is determined from among the tone group handled by the threshold value matrix M1, while storing the dots expressing the brighter tones. The determination of a dot pattern is repeated in order from the brightest tone level among the tone group handled by the threshold value matrix M1.
  • step S909 if the process has finished on the entire tone group handled by the threshold value matrix M1 (YES in step S909), then in step S911, the threshold value matrix M1 is stored.
  • the above process is repeated until all threshold value matrixes (M1, M2, and M3) are created.
  • the creation of a plurality of threshold value matrixes in different sizes used in the present exemplary embodiment is thereby completed.
  • the halftone process is carried out using a plurality of threshold value matrixes each having a different size.
  • the present exemplary embodiment has described an example in which the tones which are as compatible as possible are related to the common threshold value matrix.
  • the present invention is not limited to this. For example, even when a tone expressed by input image data is separated into some sections and a threshold value matrix is related to each of the sections, a more appropriate size of the threshold value matrix may be set to each section.
  • the halftone processing unit 106 includes one comparator.
  • one comparator is provided for each threshold value matrix and an output value is selected from the results of quantization at each comparator.
  • Fig. 3 illustrates a configuration of a halftone processing unit 106 which is applicable to the second exemplary embodiment. Configurations similar to those in the first exemplary embodiment are denoted by the same reference numerals and detailed descriptions thereof are omitted.
  • the halftone processing unit 106 includes comparators 206 in a number corresponding to threshold value matrixes M1 to MN.
  • each of the comparators 206 When an input value expressing a target pixel is input, each of the comparators 206 concurrently compares the input value and a threshold value in the threshold value matrix and quantizes the input value. Thereafter, according to the input value (tone level), an output determination unit 301 selects one as an output value from the results of quantization obtained by the plurality of comparators 206.
  • a creation method for each threshold value matrix and a correspondence relationship between the input value and the threshold value matrix are similar to those in the first exemplary embodiment.
  • a third exemplary embodiment will be described.
  • the above described exemplary embodiment illustrates the halftone process using a plurality of threshold value matrixes.
  • a similar method can be also applied to a halftone processing which determines a dot pattern by referring to dot patterns in the number of tones.
  • the third exemplary embodiment will describe a halftone processing which, by storing dot patterns of all tones, does not perform the comparison process using a threshold value matrix. Note that similarly to the above described exemplary embodiment, binary halftone image data illustrating ON and OFF of dot is generated.
  • Fig. 10 illustrates a scheme of the third exemplary embodiment.
  • a dot pattern corresponding to each tone has a sufficiently dispersed dot disposition. Additionally, even if the dot patterns are repetitively lined up in a tiled form, each of the dot patterns is optimized to a size in which a texture is not visible.
  • a dot pattern is selected according to a tone indicated by an input value of a target pixel. In the selected dot pattern, an output value is determined depending on whether a position corresponding to a position (x, y) of the target pixel is an ON dot (black pixel) or an OFF dot (white pixel). Here, the output value is 0 in case of an ON dot and 1 in case of an OFF dot.
  • Fig. 11 illustrates a halftone processing unit 106 which is applicable to the third exemplary embodiment.
  • 1-bit dot patterns of all tones (1004 in Fig. 11) each expressing ON or a OFF of dot are stored in a memory 203.
  • a dot pattern selector 1005 selects a dot pattern corresponding to a pixel value (tone g) expressing the target pixel (x, y).
  • a tone converter 1006 outputs a value stored at the position (i, j) in the dot pattern to a position (x, y) of an output image.
  • the aforementioned exemplary embodiments have described the case of frequency characteristics in Fig. 6 as an example of frequency characteristics of a highly dispersed dot pattern. Such characteristics are called "blue noise characteristics".
  • the blue noise characteristic is a dot pattern having a frequency component which has a peak near a principal frequency (fg) determined for tone and is biased toward a frequency higher than the fg.
  • the highly dispersed dot pattern is not limited to the one having the blue noise characteristics. More specifically, as illustrated by the curve B in Fig. 12, the frequency characteristics constraining a frequency component lower than the principal frequency can be mentioned.
  • the frequency characteristics do not have a peak of the frequency component present in the blue noise characteristics.
  • a threshold value matrix or dot pattern may be designed to have such frequency characteristics.
  • a suitable size may be selected from a plurality of trial results, or an approximation relationship in L is nearly equal to n * dg or (2L) 1/2 is nearly equal to n * dg may be moderately permitted.
  • a reciprocal of the minimum frequency value in which an amplitude exceeds a certain threshold value may be employed instead of dg defined in the above equation.
  • the halftone process may be a multivalued halftone processing.
  • T ij (r) is used when an input value x ij is within a range of: Due to such conversion, the present invention can be applied to a case of the multivalued halftone process.
  • the size L of the threshold value matrix or the dot pattern is set to satisfy any of L is nearly equal to n * dg and (2L) 1/2 is nearly equal to n * dg for each corresponding tone level.
  • a method for setting an appropriate size to the tone group is not limited to this.
  • the best-evaluated size may be selected by determining respectively suitable dispositions for dots in variously-sized regions and by effecting evaluation calculation or actually forming an image.
  • Various conventional methods can be used to determine and evaluate the dot disposition at this time.
  • the threshold value matrix or dot pattern does not necessarily have to be rectangular. Any form of the threshold value matrix or dot pattern can realize the effects similar to those in the aforementioned exemplary embodiments as long as the threshold value matrix or dot pattern is used to repetitively relate to the image data. Further, in the aforementioned exemplary embodiments, any of the plurality of threshold value matrixes or dot patterns optimizes the side of the square according to each tone. In other words, the plurality of threshold value matrixes has similar shapes to one another. However, all the threshold value matrixes or dot patterns do not necessarily have the same shape. For example, if the size is more suitable for the dot pattern expressing a corresponding tone level, a threshold value matrix or a dot pattern having a rectangle L * K (L is not equal to K) may be partly used.
  • the aforementioned exemplary embodiments have described an example of disposing the dots on the grid points or diagonals in the square threshold value matrix or dot pattern as centers.
  • the present invention is not limited to this. It is only necessary that the dot disposition has high dispersibility and is less likely to generate a visible texture even if the dot patterns are repetitively lined up.
  • the present invention is also applicable to a multi-pass recording system for forming an image on the same region of a recording medium by multiple times of recording.
  • the present invention may be implemented together with a conventional method for generating data corresponding to each scan.
  • the aforementioned exemplary embodiments have described the case of a monochrome.
  • the present invention is applicable to a case where the image forming apparatus 102 records a color image.
  • color separated data of each color is output from the color separation processing unit 105.
  • the color separated data of each color may be subjected to the halftone processing and output to the image forming apparatus 102.
  • the present invention may be applied to all the color material colors to generate each dot pattern, or may be applied to a portion of the color material highly requiring dot dispersibility.
  • the aforementioned exemplary embodiments have described, as an example, a case where the size optimization according to the corresponding tone level is performed on all the threshold value matrixes or dot patterns.
  • effects of the present invention can be also obtained by only performing size optimization on at least a portion of the threshold value matrixes or dot patterns.
  • a size optimization may be carried out repetitively on a tone level where a texture generated by repetitively using the threshold value matrixes or dot patterns is easily visible.
  • a halftone process similar to the conventional halftone process may be effected on a portion of the tone.
  • the present invention can also be realized by supplying a storage medium, which has stored a software program code realizing functions in the aforementioned exemplary embodiments, to a system or an apparatus.
  • a computer or a central processing unit (CPU) or a micro processing unit (MPU) of the system or the apparatus reads and executes a program code stored in the computer-readable storage medium. Accordingly, the functions in the aforementioned exemplary embodiments are realized. Effect of the Present Invention
  • the present invention can reduce a texture generated by repetition of the dot patterns and generate a dot pattern having high dispersibility.
  • Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment(s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s).
  • the computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors.
  • the computer executable instructions may be provided to the computer, for example, from a network or the storage medium.
  • the storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)(Trade Mark)), a flash memory device, a memory card, and the like.

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Abstract

An image processing apparatus configured to perform a halftone processing on input image data using a plurality of threshold value matrixes, includes a determination means configured to, using a threshold value matrix according to an input value expressing a target pixel in the input image data, determine an output value of the target pixel by comparing the input value with a threshold value in the threshold value matrix. Each of the plurality of threshold value matrixes is used for a different input value and has a size smaller than the input image data, and a size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined according to a dot pattern expressing a corresponding input value.

Description

IMAGE PROCESSING APPARATUS AND CONTROL METHOD THEREFOR
The present invention relates to a halftone processing for generating a dot pattern having high dispersibility based on image data.
An image forming apparatus for forming an image on a recording medium using dots is often used as an apparatus for outputting an image processed on a personal computer or an image captured by a digital camera. A tone level which can be output from such an image forming apparatus is generally less than a number of tone level of image data handled by the personal computer or the like. Thus, it is necessary to perform a halftone processing on the image data to have a number of tone level which can be output from the image forming apparatus. A dither method is known as one of the halftone processings. The dither method is a method for determining an output value of each pixel by comparing a pixel value expressing input image data and a threshold value of a threshold value matrix on a pixel-to-pixel basis. Such dither method using the threshold value matrix has a problem in that a sufficient dispersibility cannot be obtained. This is because, in the dither method, although a more suitable dot disposition is different depending on a tone (brightness), the dot disposition is influenced by dot dispositions of the other tones.
Therefore, Japanese Patent Application Laid-Open No. 2003-46777 discusses a method for storing optimum dot patterns of all tones in advance. A dot pattern is selected according to the tone of input image data. Then, the output of a pixel is determined by referring to whether the same pixel position in the selected dot pattern is an ON dot or an OFF dot. In this method, since the dot patterns of all the tones are separately stored, a more highly dispersed dot disposition can be determined for each tone without being restricted by dot dispositions of the other tones.
On the other hand, Japanese Patent Application Laid-Open No. 2003-46777 also discusses an example of the dither method which uses a dispersed-type threshold value matrix to a portion of a continuous tone section. This method can reduce a storage capacity as compared to the method for storing the dot patterns of all the tones.
The method discussed in Japanese Patent Application Laid-Open No. 2003-46777 uses the stored dot patterns or threshold value matrixes to repetitively relate to the input image data in a tiled form. As a result, a non-uniform pattern in the dot pattern or threshold value matrix is generated as a texture (pattern) due to the repetition, thereby degrading the image quality.
Japanese Patent Application Laid-Open 2003-46777
The present invention is directed to reducing a texture generated by the repetition of dot patterns and generating a suitable dispersed-type dot pattern in a halftone processing for converting image data into a dot pattern.
According to an aspect of the present invention, an image processing apparatus configured to perform a halftone processing on input image data using a plurality of threshold value matrixes, includes a determination unit configured to, using a threshold value matrix according to an input value expressing a target pixel in the input image data, determine an output value of the target pixel by comparing the input value with a threshold value in the threshold value matrix. Each of the plurality of threshold value matrixes is used for a different input value and has a size smaller than the input image data, and a size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined according to a dot pattern expressing a corresponding input value.
Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.
Fig. 1 is a diagram illustrating an image processing apparatus and an image forming apparatus. Fig. 2 is a block diagram illustrating a detailed structure of a halftone processing unit. Fig. 3 is a block diagram illustrating a detailed structure of the halftone processing unit. Fig. 4 is a flowchart of a sequence of processes in the halftone processing unit. Fig. 5 is an explanatory diagram of a suitable threshold value matrix applied to the present invention. Fig. 6 is a graph illustrating suitable frequency characteristics in a dot-dispersed-type dot pattern. Fig. 7 is an explanatory diagram in which a dot disposition is influenced by the size of a region. Fig. 8 is an explanatory diagram of a tone suitable for storage in a threshold value matrix of a certain size. Fig. 9 is a flowchart illustrating a method for creating a plurality of threshold value matrixes. Fig. 10 is an explanatory diagram of suitable dot patterns applied to the present invention. Fig. 11 is a block diagram illustrating a detailed structure of a halftone processing unit. Fig. 12 is a graph illustrating suitable frequency characteristics in a dot pattern in another exemplary embodiment. Fig. 13 is a schematic diagram illustrating a suitable dot disposition in a dot pattern.
Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. It should be understood that the arrangements described in the following exemplary embodiments are by way of example only and that the present invention is not limited to the illustrated arrangements.
A first exemplary embodiment will be described. Fig. 1 is a block diagram illustrating structures of an image processing apparatus and an image forming apparatus which can be applied to the first exemplary embodiment. In Fig. 1, an image processing apparatus 101 and an image forming apparatus 102 are connected by an interface or a circuit. The image processing apparatus 101 is, for example, a printer driver installed in a general personal computer. In this case, each unit within the image processing apparatus 101 described below is realized by a computer executing a predetermined program. The image forming apparatus 102 may include the image processing apparatus 101.
The image processing apparatus 101 stores color image data to be printed, which has been input through an input terminal 103 (referred to as "input color image data" hereinafter), in an input image buffer 104. The input color image data is formed by three color components of red (R), green (G), and blue (B). A color separation processing unit 105 separates the stored input image data to image data corresponding to color material colors included in the image forming apparatus 102. For this color separation process, the color separation processing unit 105 refers to a look-up table for color separation (not illustrated). The present exemplary embodiment will be described by using a monochrome of black (K) as an example. When a color image is formed on a recording medium, a color separation process to a plurality of colors, such as cyan (C), magenta (M), yellow (Y), and black (K), may be implemented. In the present exemplary embodiment, the color separated data is handled as 8 bits which express 256 tones of 0 to 255. However, the data may be converted into the data having a number of tone level of more than 256.
A halftone processing unit 106 performs a halftone processing using a plurality of threshold value matrixes on the color separated data obtained from the color separation processing unit 105. Note that each of the threshold value matrixes is a dot-dispersed type. A threshold value matrix to be used among the plurality of threshold value matrixes is determined according to an input value (tone level) expressing a pixel. Here, the halftone processing unit 106 converts the 8-bit color separated data into 1-bit (binary) data. Details of the conversion will be described below. Note that, in case of a color image, the halftone processing unit 106 separately processes each color using a plurality of threshold value matrixes prepared for respective colors. Halftone image data obtained by the halftone process expresses a dot pattern which can be formed by the image forming apparatus 102. The halftone processing unit 106 outputs the halftone image data to a halftone image buffer 107. The stored halftone image data is output to the image forming apparatus 102 through an output terminal 108.
Based on the halftone image data received from the image processing apparatus 101 through an input terminal 109, the image forming apparatus 102 forms an image on a recording medium by moving a recording head 111 sideways and lengthways relative to the recording medium. Here, the recording head 111 is of an ink-jet type and includes one or more recording elements (nozzles). Based on the halftone image data, a head drive circuit 110 generates a driving signal for controlling the recording head 111. The recording head 111 actually records each ink dot onto the recording medium based on the driving signal.
The halftone processing unit 106 in the present exemplary embodiment will be described below in detail. Fig. 2 is a block diagram illustrating a detailed configuration of the halftone processing unit 106. The halftone processing unit 106 converts the color separated data of 256 tones output from the color separation processing unit 105 into binary (1-bit) data. More specifically, the halftone processing unit 106 compares the input value expressing a target pixel 201 with a corresponding threshold value in the threshold value matrix and determines an output value. The halftone processing unit 106 includes a memory 203, a threshold value matrix selector 205, and a comparator 206. The memory 203 stores a plurality of threshold value matrixes 204 (M1, M2,..., MN) different from each other. N represents an integer of 2 or more. Further, based on the input value, the threshold value matrix selector 205 determines, for each pixel, one threshold value matrix to be used from among the plurality of threshold value matrixes 204. The comparator 206 compares the threshold value corresponding to the target pixel and a pixel value of the target pixel in the threshold value matrix selected by the threshold value matrix selector 205 and then determines an output value.
Fig. 4 illustrates a flowchart of a sequence of processes in the halftone processing unit 106. A halftone processing method in the present exemplary embodiment will be described below.
In step S402, as an initialization process, the image processing apparatus 101 reads N sheets of prepared threshold value matrixes. Next, in step S403, the halftone processing unit 106 reads a pixel value g (x, y) expressing a target pixel (x, y) and stores the pixel value g (x, y) as a variable ("in") expressing an input value.
In step S404, according to the input value "in", the threshold value matrix selector 205 determines a threshold value matrix to be used by the comparator 206 from among the plurality of threshold value matrixes (M1 to MN) stored in the memory. Here, a correspondence between the input value "in" and the threshold value matrix used is determined in advance and stored as a table. The threshold value matrix selector 205 can determine a threshold value matrix by referring to the table. The correspondence between the input value "in" and the threshold value matrix will be further described below in detail. The threshold value matrix selected by the threshold value matrix selector 205 is referred to as "Mk".
In step S405, the comparator 206 reads a position, in the threshold value matrix "Mk" selected by the threshold value matrix selector 205, corresponding to a position (x, y) of the target pixel. A method similar to the conventional dither method is used herein to read the position in the threshold value matrix corresponding to the target pixel.
The position (i, j) in the threshold value matrix corresponding to the target pixel is determined by:
i = x % Wk
j = y % Hk
where Wk is a width of the threshold value matrix "Mk", Hk is a height thereof, and % represents a residue.
The comparator 206 stores a threshold value stored at the position (i, j) in the threshold value matrix "Mk" in a variable "th".
In step S406, the comparator 206 compares the input value "in" with the threshold value "th". If the input value "in" is less than or equal to the threshold value "th" (YES in step S406), the operation proceeds to step S407. If the input value "in" is more than the threshold value "th" (NO in step S406), the operation proceeds to step S408. In step S407, the comparator 206 stores an output value "out" of the target pixel as 0. On the other hand, in step S408, the comparator 206 stores the output value "out" of the target pixel as 1. In other words, when the input value "in" is less than or equal to the threshold value "th" (YES in step S406), 0 (black) is output, and when the input value "in" is more than the threshold value "th" (NO in step S406), 1 (white) is output.
In step S409, the halftone processing unit 106 outputs the output value "out" at a position (x, y) of an output image.
In step S410, the above-described process is carried out on all the pixels in the input image. In step S411, if the process is finished on all the pixels (YES in step S410), the halftone processing ends.
The threshold value matrix used in the above-described halftone process is described herein. First, the correspondence between the input value (tone level) and the threshold value matrix used is described in detail. Fig. 5 is a diagram illustrating a general dither method. A threshold value matrix 502 has a width W = 4 and a height H = 4, and threshold values of "0 to 15" are stored in respective pixels one by one. Here, if an input pixel value is less than or equal to the threshold value, an ON dot (black) is output, and if the input pixel value is greater than the threshold value, an OFF dot (white) is output. Therefore, when this threshold value matrix 502 is used, a 16-tone dot pattern is obtained.
An input image 501 has a single pixel value of 12 and is an image having a width W = 4 and a height H = 4, which is the same as the threshold value matrix 502. When the threshold value matrix 502 is used on the input image 501 to determine an output value, an output image 503 is obtained. On the other hand, an input image 504 has a single pixel value of 13 and is slightly brighter than the input image 501. When the threshold value matrix 502 is used on the input image 504 to determine an output value, an output image 606 is obtained. At this time, ON dots (black) in the output image 503 include all the ON dots in the output image 606 expressing a brighter tone. Thus, in the conventional dither method, the dot which has become an ON dot (black) in a bright tone does not become an OFF dot (white) in a tone darker than the bright tone.
However, since an optimum dot pattern is different for each tone, when a dot is fixed in a bright tone, a tone that cannot set an optimum dot pattern is generated. Fig. 13 illustrates a dot pattern 1301 expressing a tone 8, and patterns 1302 and 1303 each expressing a tone 9. The dot pattern 1301 has a good graininess. The dot number of the tone 9 is more than the dot number of a dot pattern of the tone 8 by one. The dots in the dot pattern 1302 include all the dots in the dot pattern 1301. In other words, the dot pattern 1302 is a dot disposition obtained by adding one dot to the dot pattern 1301. However, the dot pattern 1302 has a high dot density locally, being non-uniform, thereby having a poor graininess. On the other hand, the dot pattern 1303 has a good graininess, but does not include all the dots in the dot pattern 1301. In other words, it is found that the dot pattern of the tone 9 having a good graininess cannot be realized while holding all the dots in the tone 8.
In this way, in the dither method using the threshold value matrix, the dot which has become an ON dot (black pixel) in a bright tone does not become an OFF dot (white pixel) in a tone darker than the bright tone. As a result, the tone 8 and the tones 9 in Fig. 13cannot obtain a dot pattern having high dispersibility using a common threshold value matrix. When a dot pattern having many dots includes all dots in a dot pattern having a few dots or has a dot disposition similar thereto, respective tones having high dispersibility are expressed herein as "compatible". In the present exemplary embodiment, to form respective tones into sufficiently highly dispersed dot patterns, tones that are as compatible as possible with each other are related to a common threshold value matrix.
Next, a suitable size of the threshold value matrix for a dot pattern expressing each tone will be described. A "principal frequency (= fg)" determined on a tone level is defined by the equation (1):
Figure JPOXMLDOC01-appb-M000001
 where the principal frequency fg is a value in a square grid, a tone g is normalized to 0 to 1, and R is a distance between pixels. Further, the principal frequency fg is a frequency of fewer dots in the tone g (i.e., black dots in case of a bright tone and white dots in case of a dark tone). A highly dispersed dot pattern can be regarded to have a peak near this principal frequency fg and only have a frequency component higher than the principal frequency fg. For example, a dot pattern having frequency characteristics illustrated in Fig. 6 can be said to have a high dispersibility.
Here, the reciprocal of the principal frequency fg is referred to as "principal wavelength (= dg)". In an actual dot pattern, the position of each dot fluctuates and is not completely regular. However, in the dot pattern expressing the tone g, by separately disposing the dots at about a distance dg from each other, the dot pattern can be said to have a high dispersibility. In the tone g expressed by this dot pattern, a suitable dot-to-dot distance dg is constant.
Fig. 7 illustrates dot patterns each expressing the tone g. A dot pattern 701 expresses the tone g in a square region, L on a side. In a size L * L, the tone g is expressed by a dot number of 9. When a dot-to-dot distance dg of this tone g is L/3, in the size L * L, the respective dots can be uniformly disposed at about a dot-to-dot distance dg = L/3, with the positions slightly fluctuating. Even if the dot patterns similar to 701 in Fig. 7A are repetitively lined up from left to right or up and down, a visually characteristic texture is difficult to perceive therein.
On the other hand, dot patterns 702 and 703 each express the same tone g as the dot pattern 701 in a square region, 8L/9 on a side. From the area ratio of the regions, if a dot number in the square region of the size 8L/9 * 8L/9 is 64/81 of the dot number in the size L * L, the same tone can be expressed in the square region of the size 8L/9 * 8L/9. In other words, from the dot number 9 * 64/81 = 7.111..., the dot number which expresses the same tone as the dot pattern 701 in the size 8L/9 * 8L/9 is 7. As described above, the suitable dot-to-dot distance dg is constant regardless of the size of the region. Similarly to the case of the dot pattern 701, the dot-to-dot distance dg = L/3 can be used for the dot patterns 702 and 703.
Accordingly, a highly dispersed dot pattern is obtained by disposing dots in the region, 8L/9 on a side, in such a way that the dot-to-dot distance is substantially dg. Thus, similarly to the dot pattern 701, the dot pattern 702 disposes dots substantially periodically while maintaining the dot-to-dot distance dg. Moreover, the dot pattern 703 randomly disposes dots in the entire region while maintaining the dot-to-dot distance dg. Each of the dot patterns 702 and 703 maintains the dot-to-dot distance dg in the region of the size 8L/9 * 8L/9. However, when these dot patterns are repetitively lined up in a tiled form from left to right or up and down, the dot-to-dot distance dg cannot be maintained and a non-uniform dot disposition is generated. The dot pattern may be perceived as a texture due to the repetition. In other words, there is a more suitable size for maintaining the highly dispersed dot pattern even if a highly dispersed dot pattern in a certain sized region is repetitively lined up. Therefore, when the halftone processing is carried out by repetitively relating to a threshold value matrix, it is desirable that the size of the threshold value matrix be determined according to each tone.
As described above, when the halftone processing is performed on the input image data using the threshold value matrix, if the output value is determined on all the pixel values (tones) using the common threshold value matrix, a highly dispersed dot pattern cannot be generated for each tone. Further, there is an appropriate repetitive size to maintain the dispersibility even if the dot patterns expressing each tone are repetitively lined up. Thus, the present exemplary embodiment includes a plurality of threshold value matrixes and uses a common threshold value matrix for tones, of the tones forming input image data, which are as compatible as possible with each other. Moreover, the size of each threshold value matrix is determined according to a respectively corresponding tone. Consequently, the halftone processing unit 106 in the present exemplary embodiment includes a plurality of threshold value matrixes each having a size different from each other.
Fig. 8 illustrates an example of a tone group using a common threshold value matrix. In any of the dot patterns 801, 802, and 803 illustrated in Fig. 8, dots are disposed to maintain a dot-to-dot distance dg. Additionally, even if regions of the size L * L are lined up in a tiled form, a texture due to the repetition is difficult to perceive. Furthermore, in dot patterns 802 and 803, dots are disposed to include all dots having a brighter tone (tone of a smaller dot number).
As illustrated in 701 in Fig. 7 as well, when the dots are disposed on grid points, if a side L in the square region is a value near a multiple of the dot-to-dot distance dg, the respective dots expressing a certain tone can be disposed to maintain about the dot-to-dot distance dg, with the positions slightly fluctuating. Alternatively, if a diagonal (2L)1/2 is a value near the multiple of the dot-to-dot distance dg, the dots can be disposed on the diagonal. Thus, in the size L * L, a square, L which satisfies L is nearly equal to n * dg or (2L)1/2 is nearly equal to n * dg on a side is set as a threshold value matrix size (n is an arbitrary natural number).
A method for creating a plurality of threshold value matrixes is described herein. Fig. 9 illustrates a flowchart for creating the plurality of threshold value matrixes used in the present exemplary embodiment. In step S902, a number N of the threshold value matrixes to be created is determined. As the number N of the threshold value matrixes becomes large, a corresponding tone is smaller, thereby increasing the freedom of a dot disposition. Additionally, as the corresponding tone is smaller, a more suitable size can be selected. However, the larger number N of the threshold value matrixes requires a larger storage capacity. Accordingly, a designer should properly select the number N of the threshold value matrixes according to conditions for implementation (e.g., N = 3).
Next, in step S903, a dot-to-dot distance dg necessary for a highly dispersed dot disposition is calculated for each tone and a combination of tones (tone group) using a common threshold value matrix is determined. The tone group corresponding to a certain threshold value matrix (i.e., handled by that threshold value matrix) is a combination of tones which are as compatible as possible with each other. As described above, "compatible" means tones which each can realize a highly dispersed dot pattern while holding dots expressing brighter tones (tones of a smaller dot number). Further, a size of the threshold value matrix is determined according to the tone group. Here, a square, L which satisfies L is nearly equal to n * dg or (2L)1/2 is nearly equal to n * dg on a side, is determined as the size of the threshold value matrix.
Next, steps S904 to S912 are repeated. A threshold value to be stored in each threshold value matrix is determined and threshold value matrixes are created one by one. Hereinafter, a flow of creating a threshold value matrix M1 will be described as an example. The other threshold value matrixes can be created likewise.
In step S905, a matrix expressing the threshold value matrix M1 is initialized by first storing a threshold value which does not output a dot to any tone. Next, in step S906, the brightest tone is selected from among a tone group corresponding to the threshold value matrix M1. Then, a dot pattern which has disposed the number of dots necessary to express the selected tone is determined in an input image having the same size as the threshold value matrix M1. At this time, the dot pattern which does not form a non-uniform pattern and has a highly dispersed dot disposition can be used. A conventional method may be used as a method for determining a highly dispersed dot pattern expressing a certain tone. A threshold value to the tone expressed by this dot pattern is recorded at a position of the threshold value matrix M1 corresponding to the position of a dot in this dot pattern.
In step S907, the next brightest tone is selected from among the tone group corresponding to the threshold value matrix M1. Under a condition that all the disposition-determined dots are stored, a dot pattern having the number of dots necessary for the selected tone is determined. In other words, among the tone group corresponding to the threshold value matrix M1, a dot pattern expressing the selected tone is determined by adding a new dot to a dot pattern expressing a tone brighter than the selected tone. Again, similarly to step S906, the dot pattern which does not form a non-uniform pattern and has a highly dispersed dot disposition can be used.
In the present exemplary embodiment, dots should be disposed on grid points or on diagonals as centers in a square which is a threshold value matrix region. In the tone group corresponding to the threshold value matrix M1, the highly dispersed dot patterns are compatible with each other, and as determined in step S903, a square, L which satisfies L is nearly equal to n * dg or (2L)1/2 is nearly equal to n * dg on a side, is determined as the size of a threshold value matrix. As a result, by adding the dot to the dot pattern expressing a brighter tone, the highly dispersed dot pattern can be generated without forming a non-uniform pattern.
In step S908, a threshold value corresponding to the tone selected in step S907 is stored at a position of the threshold value matrix M1 corresponding to the position to which the new dot has been added in step S907. As described above, a dot pattern having the next brightest tone is determined from among the tone group handled by the threshold value matrix M1, while storing the dots expressing the brighter tones. The determination of a dot pattern is repeated in order from the brightest tone level among the tone group handled by the threshold value matrix M1. In step S909, if the process has finished on the entire tone group handled by the threshold value matrix M1 (YES in step S909), then in step S911, the threshold value matrix M1 is stored.
The above process is repeated until all threshold value matrixes (M1, M2, and M3) are created. The creation of a plurality of threshold value matrixes in different sizes used in the present exemplary embodiment is thereby completed. According to the present exemplary embodiment, the halftone process is carried out using a plurality of threshold value matrixes each having a different size. As a result, the more highly dispersed dot pattern is realized in each tone, and even if the threshold value matrix is repetitively used to relate in a tiled form, a non-uniform pattern (texture) is less likely to be generated.
The present exemplary embodiment has described an example in which the tones which are as compatible as possible are related to the common threshold value matrix. However, the present invention is not limited to this. For example, even when a tone expressed by input image data is separated into some sections and a threshold value matrix is related to each of the sections, a more appropriate size of the threshold value matrix may be set to each section.
A second exemplary embodiment will be described. In the first exemplary embodiment, the halftone processing unit 106 includes one comparator. In the second exemplary embodiment, one comparator is provided for each threshold value matrix and an output value is selected from the results of quantization at each comparator. Fig. 3 illustrates a configuration of a halftone processing unit 106 which is applicable to the second exemplary embodiment. Configurations similar to those in the first exemplary embodiment are denoted by the same reference numerals and detailed descriptions thereof are omitted. In the present exemplary embodiment, the halftone processing unit 106 includes comparators 206 in a number corresponding to threshold value matrixes M1 to MN. When an input value expressing a target pixel is input, each of the comparators 206 concurrently compares the input value and a threshold value in the threshold value matrix and quantizes the input value. Thereafter, according to the input value (tone level), an output determination unit 301 selects one as an output value from the results of quantization obtained by the plurality of comparators 206. A creation method for each threshold value matrix and a correspondence relationship between the input value and the threshold value matrix are similar to those in the first exemplary embodiment.
A third exemplary embodiment will be described. The above described exemplary embodiment illustrates the halftone process using a plurality of threshold value matrixes. However, without performing a comparison process using a threshold value matrix, a similar method can be also applied to a halftone processing which determines a dot pattern by referring to dot patterns in the number of tones. The third exemplary embodiment will describe a halftone processing which, by storing dot patterns of all tones, does not perform the comparison process using a threshold value matrix. Note that similarly to the above described exemplary embodiment, binary halftone image data illustrating ON and OFF of dot is generated.
Fig. 10 illustrates a scheme of the third exemplary embodiment. A dot pattern corresponding to each tone has a sufficiently dispersed dot disposition. Additionally, even if the dot patterns are repetitively lined up in a tiled form, each of the dot patterns is optimized to a size in which a texture is not visible. A dot pattern is selected according to a tone indicated by an input value of a target pixel. In the selected dot pattern, an output value is determined depending on whether a position corresponding to a position (x, y) of the target pixel is an ON dot (black pixel) or an OFF dot (white pixel). Here, the output value is 0 in case of an ON dot and 1 in case of an OFF dot.
Fig. 11 illustrates a halftone processing unit 106 which is applicable to the third exemplary embodiment. 1-bit dot patterns of all tones (1004 in Fig. 11) each expressing ON or a OFF of dot are stored in a memory 203. A dot pattern selector 1005 selects a dot pattern corresponding to a pixel value (tone g) expressing the target pixel (x, y). In the selected dot pattern, a position corresponding to the target pixel (x, y) is determined by:
i = x % Wk
j = y % Hk
where Wk is a width of the selected dot pattern, Hk is a height thereof, and % represents a residue. A tone converter 1006 outputs a value stored at the position (i, j) in the dot pattern to a position (x, y) of an output image. By appropriately setting the size of this dot pattern for each tone level, effects similar to those in the first exemplary embodiment are obtained. Similarly to the above described exemplary embodiment, the size of each dot pattern may be determined according to the tone.
Other embodiments
The aforementioned exemplary embodiments have described the case of frequency characteristics in Fig. 6 as an example of frequency characteristics of a highly dispersed dot pattern. Such characteristics are called "blue noise characteristics". According to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 06-508007, the blue noise characteristic is a dot pattern having a frequency component which has a peak near a principal frequency (fg) determined for tone and is biased toward a frequency higher than the fg. However, the highly dispersed dot pattern is not limited to the one having the blue noise characteristics. More specifically, as illustrated by the curve B in Fig. 12, the frequency characteristics constraining a frequency component lower than the principal frequency can be mentioned. The frequency characteristics do not have a peak of the frequency component present in the blue noise characteristics. A threshold value matrix or dot pattern may be designed to have such frequency characteristics. In this case, a suitable size may be selected from a plurality of trial results, or an approximation relationship in L is nearly equal to n * dg or (2L)1/2 is nearly equal to n * dg may be moderately permitted. Alternatively, a reciprocal of the minimum frequency value in which an amplitude exceeds a certain threshold value may be employed instead of dg defined in the above equation.
Further, the aforementioned exemplary embodiments have described the example of conversion to binary data by the halftone processing. However, the halftone process may be a multivalued halftone processing. In general, in the multivalued halftone process, when an entire tone level of input is R, a multivalued level which can be output is m, and a stored threshold value of a threshold value matrix position (i, j) for a binary halftone process is Di,j, a threshold value matrix Tij (r) for a multivalued halftone process can be described as:
Figure JPOXMLDOC01-appb-M000002
 Where r = 0, 1,..., m - 2, int represents integerization. Tij (r) is used when an input value xij is within a range of:
Figure JPOXMLDOC01-appb-M000003
 Due to such conversion, the present invention can be applied to a case of the multivalued halftone process.
Moreover, in the aforementioned exemplary embodiments, using the appropriate dot-to-dot distance dg which is uniquely determined on each tone level, the size L of the threshold value matrix or the dot pattern is set to satisfy any of L is nearly equal to n * dg and (2L)1/2 is nearly equal to n * dg for each corresponding tone level. However, a method for setting an appropriate size to the tone group is not limited to this. For example, the best-evaluated size may be selected by determining respectively suitable dispositions for dots in variously-sized regions and by effecting evaluation calculation or actually forming an image. Various conventional methods can be used to determine and evaluate the dot disposition at this time.
The aforementioned exemplary embodiments have described the halftone processing using the rectangular threshold value matrix or dot pattern as an example. However, the threshold value matrix or dot pattern does not necessarily have to be rectangular. Any form of the threshold value matrix or dot pattern can realize the effects similar to those in the aforementioned exemplary embodiments as long as the threshold value matrix or dot pattern is used to repetitively relate to the image data. Further, in the aforementioned exemplary embodiments, any of the plurality of threshold value matrixes or dot patterns optimizes the side of the square according to each tone. In other words, the plurality of threshold value matrixes has similar shapes to one another. However, all the threshold value matrixes or dot patterns do not necessarily have the same shape. For example, if the size is more suitable for the dot pattern expressing a corresponding tone level, a threshold value matrix or a dot pattern having a rectangle L * K (L is not equal to K) may be partly used.
Additionally, the aforementioned exemplary embodiments have described an example of disposing the dots on the grid points or diagonals in the square threshold value matrix or dot pattern as centers. However, the present invention is not limited to this. It is only necessary that the dot disposition has high dispersibility and is less likely to generate a visible texture even if the dot patterns are repetitively lined up.
Further, in the aforementioned exemplary embodiments, with respect to the same region of the recording medium, the halftone image data for forming an image by a single recording is generated. However, the present invention is also applicable to a multi-pass recording system for forming an image on the same region of a recording medium by multiple times of recording. The present invention may be implemented together with a conventional method for generating data corresponding to each scan.
Moreover, the aforementioned exemplary embodiments have described the case of a monochrome. However, the present invention is applicable to a case where the image forming apparatus 102 records a color image. In this case, color separated data of each color is output from the color separation processing unit 105. The color separated data of each color may be subjected to the halftone processing and output to the image forming apparatus 102. Additionally, the present invention may be applied to all the color material colors to generate each dot pattern, or may be applied to a portion of the color material highly requiring dot dispersibility.
Furthermore, the aforementioned exemplary embodiments have described, as an example, a case where the size optimization according to the corresponding tone level is performed on all the threshold value matrixes or dot patterns. However, effects of the present invention can be also obtained by only performing size optimization on at least a portion of the threshold value matrixes or dot patterns. A size optimization may be carried out repetitively on a tone level where a texture generated by repetitively using the threshold value matrixes or dot patterns is easily visible. In short, a halftone process similar to the conventional halftone process may be effected on a portion of the tone.
The present invention can also be realized by supplying a storage medium, which has stored a software program code realizing functions in the aforementioned exemplary embodiments, to a system or an apparatus. In this case, a computer (or a central processing unit (CPU) or a micro processing unit (MPU)) of the system or the apparatus reads and executes a program code stored in the computer-readable storage medium. Accordingly, the functions in the aforementioned exemplary embodiments are realized.
Effect of the Present Invention
In the halftone processing for converting image data into a dot pattern, the present invention can reduce a texture generated by repetition of the dot patterns and generate a dot pattern having high dispersibility.
Other Embodiments
Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment(s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)(Trade Mark)), a flash memory device, a memory card, and the like.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2012-122915 filed May 30, 2012, which is hereby incorporated by reference herein in its entirety.

Claims (17)

  1. An image processing apparatus configured to perform a halftone processing on input image data using a plurality of threshold value matrixes, the image processing apparatus comprising:
    determination means configured to, using a threshold value matrix according to an input value expressing a target pixel in the input image data, determine an output value of the target pixel by comparing the input value with a threshold value in the threshold value matrix,
    wherein each of the plurality of threshold value matrixes is used for a different input value and has a size smaller than the input image data, and
    wherein a size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined according to the corresponding input value.
  2. The image processing apparatus according to claim 1, further comprising setting means configured to, according to the input value, set a threshold value matrix to be used from among the plurality of threshold value matrixes,
    wherein the determination means uses the threshold value matrix set by the setting means.
  3. The image processing apparatus according to claim 1, wherein from results of quantization using the input value of the target pixel and the plurality of threshold value matrixes, the determination means determines, according to the input value, the output value of the target pixel.
  4. The image processing apparatus according to any one of claims 1 to 3, wherein the size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined in such a manner that a texture is less likely to be generated even if the dot patterns each expressing the corresponding input value are repetitively lined up.
  5. The image processing apparatus according to any one of claims 1 to 3, wherein the size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined according to a dot-to-dot distance determined by the corresponding input value.
  6. The image processing apparatus according to claim 5, wherein the size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined in such a manner that a side length or a diagonal length substantially corresponds to a multiple of the dot-to-dot distance determined by one of the input values corresponding to the threshold value matrix.
  7. The image processing apparatus according to any one of claims 1 to 6, wherein the threshold value matrix is a threshold value matrix for generating a dispersed-type dot pattern.
  8. An image processing apparatus configured to convert input image data into halftone image data expressing a dot pattern, the image processing apparatus comprising:
    storage means configured to store a dot pattern corresponding to each tone expressed by the input image data; and
    determination means configured to, by referring to a dot pattern of the dot patterns stored in the storage means corresponding to a tone expressing a target pixel in the input image data, determine an output value of the target pixel,
    wherein each of the plurality of dot patterns is smaller than the input image data, and
    wherein a size of at least one dot pattern from among the plurality of dot patterns is determined according to the corresponding tone.
  9. The image processing apparatus according to claim 8, wherein the dot pattern is of a dispersed type.
  10. The image processing apparatus according to claim 8 or 9, wherein the size of at least one dot pattern from among the plurality of dot patterns is determined in such a manner that a texture is less likely to be generated even if the dot patterns are repetitively lined up.
  11. The image processing apparatus according to any one of claims 8 to 10, wherein the size of at least one dot pattern from among the plurality of dot patterns is determined according to a dot-to-dot distance determined by the corresponding tone.
  12. The image processing apparatus according to any one of claims 8 to 10, wherein the size of at least one dot pattern from among the plurality of dot patterns is determined in such a manner that a side length or a diagonal length substantially corresponds to a multiple of a dot-to-dot distance determined by the corresponding tone.
  13. The image processing apparatus according to any one of claims 1 to 12, wherein the plurality of threshold value matrixes or the plurality of dot patterns is similar to one another.
  14. An image processing apparatus which converts input image data formed by an M-value into halftone image data of an N-value (M > N) expressing a dispersed-type dot pattern, the image processing apparatus comprising:
    halftone processing means configured to perform a halftone processing on the input image data,
    wherein regarding halftone image data obtained by performing the halftone processing on each of the input image data expressing uniform tones p and q (p is not equal to q),
    the halftone image data corresponding to the tone p is expressed by a repetition of dot patterns expressing the tone p in a region of a first size which is smaller than the input image data, and the halftone image data corresponding to the tone q is expressed by a repetition of dot patterns expressing the tone q in a region of a second size which is smaller than the input image data and different from the first size, and
    the first size is determined according to the tone p and the second size is determined according to the tone q.
  15. A computer program to be read and executed by a computer to allow the computer to function as the image processing apparatus according to any one of claims 1 to 14.
  16. A control method for an image processing apparatus that includes determination means and performs a halftone processing on input image data using a plurality of threshold value matrixes, the control method comprising:
    causing the determination means to determine, using a threshold value matrix according to an input value expressing a target pixel in the input image data, an output value of the target pixel by comparing the input value with a threshold value in the threshold value matrix,
    wherein each of the plurality of threshold value matrixes is used for a different input value and has a size smaller than the input image data, and
    wherein the size of at least one threshold value matrix from among the plurality of threshold value matrixes is determined according to a dot pattern expressing a corresponding tone.
  17. A control method for an image processing apparatus that includes storage means and a determination means and converts input image data into halftone image data expressing a dot pattern, the control method comprising:
    causing the storage means to store a dot pattern corresponding to each tone expressed by the input image data; and
    causing the determination means to determine, by referring to a dot pattern of the dot patterns stored in the storage means corresponding to a tone expressing a target pixel in the input image data, an output value of the target pixel,
    wherein each of the plurality of dot patterns is smaller than the input image data, and
    wherein a size of at least one dot pattern from among the plurality of dot patterns is determined according to the corresponding tone.
PCT/JP2013/003379 2012-05-30 2013-05-29 Image processing apparatus and control method therefor WO2013179657A1 (en)

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JP2012122915A JP5950694B2 (en) 2012-05-30 2012-05-30 Image processing apparatus and control method thereof

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JP2001169095A (en) * 1999-12-06 2001-06-22 Kyocera Mita Corp Method for converting resolution of binarized picture data
JP2003046777A (en) * 2001-03-16 2003-02-14 Ricoh Co Ltd Mask preparation method, image processor, software program and mask data
JP2004133859A (en) * 2002-10-15 2004-04-30 Ricoh Co Ltd Control method of image processing processor

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JPH0919714A (en) * 1995-07-04 1997-01-21 Hitachi Ltd Devide for forming bent tube having special shaped cross section

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0918714A (en) * 1995-06-29 1997-01-17 Hitachi Ltd Image processing method and its device
JPH10191053A (en) * 1996-12-20 1998-07-21 Fuji Photo Film Co Ltd Image recording method
JP2001169095A (en) * 1999-12-06 2001-06-22 Kyocera Mita Corp Method for converting resolution of binarized picture data
JP2003046777A (en) * 2001-03-16 2003-02-14 Ricoh Co Ltd Mask preparation method, image processor, software program and mask data
JP2004133859A (en) * 2002-10-15 2004-04-30 Ricoh Co Ltd Control method of image processing processor

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