JP4100597B2 - Image forming method, image processing apparatus, and storage medium - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to various apparatuses that handle multi-gradation image data such as a digital copying machine, a printer, a fax machine, and a display, and more particularly to an image forming method and an image processing apparatus that use an error diffusion method.
[0002]
[Prior art]
As a typical halftone processing method in an image processing apparatus related to image formation, there are a dither method and an error diffusion method.
[0003]
The dither method has an advantage that it has excellent graininess and can express a halftone image smoothly, but it also has a disadvantage. For example, resolution is deteriorated to obtain gradation. In addition, in the dither method for generating a periodic image, moire tends to occur on a printed image such as a halftone dot.
[0004]
On the other hand, the error diffusion method can obtain a resolution that is faithful to the original image and is suitable for reproducing a character image. However, in a halftone image such as a photograph, isolated dots are dispersed or irregularly connected to each other, so that granularity is poor and a peculiar texture may occur. In addition, in an electrophotographic printer, the image is unstable because an image is formed with isolated dots, and in error diffusion, the ratio of the small dots increases, so the stability further decreases, and graininess due to density unevenness Deterioration and banding are likely to occur.
[0005]
Regarding the error diffusion method, in order to improve the texture due to the irregular connection of dots, the dither threshold is used as the quantization threshold, and the following improvements including the method of improving the texture by disturbing the dot connection Technology has been proposed.
(1) For the purpose of eliminating the occurrence of pseudo contours and unique stripe patterns, a dither threshold is used, and the larger the edge amount, the larger the error diffusion amount (Japanese Patent No. 2733314).
(2) For the purpose of preventing white spots in a non-edge low density portion and preventing occurrence of a character notch, a fixed threshold is used in the edge portion of the image, a variation threshold is used in the non-edge portion, and the level of the variation threshold The lower the concentration, the lower the value (Japanese Patent No. 2755307).
(3) When using a multi-value printer of three or more values, a dither signal having a magnitude corresponding to the edge amount is added to the image data at the edge portion of the image for the purpose of preventing the occurrence of moire and pseudo contour. In the edge portion, a fixed value is added to the image data, and the image data after the addition is subjected to multi-value quantization using a fixed threshold value (Japanese Patent No. 2801195).
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide an image forming method and an image processing apparatus for compensating for the weak points of the error diffusion method and forming a high-quality image that is particularly excellent in stability. Another object of the present invention is to provide a high-quality image with excellent stability for a photograph or a halftone dot image portion having a high number of lines, and a resolution for a character or a halftone dot image portion having a relatively low number of lines. An object is to provide an image forming method and an image processing apparatus for forming a good image. Another object of the present invention is to provide an image forming method and an image processing apparatus capable of reproducing a high quality image with few missing dots (white spots) in a high density solid portion. Another object of the present invention is to provide an image processing apparatus capable of reducing registers or memories for generating a quantization threshold.
[0008]
  The invention described in claim 1
An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
In the dither threshold value matrix according to the edge amount, the threshold values are arranged in a spiral shape from the inside toward the outside in order from the smallest.
This is an image forming method.
As the density level of multi-tone image data rises, the output dots grow spirally from the inside to the outside, promoting dot concentration and preventing dot combination in adjacent periods at low and medium density areas. Enables image formation with excellent stability.
[0009]
  According to the second and third aspects of the present invention, the output dots are grown in a direction in consideration of the directionality of propagation of the quantization error to promote the concentration of dots having a uniform shape within the period.
That is, the invention described in claim 2
An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
The binary quantization process is performed from the upper left to the lower right in the image space,
In the dither threshold value matrix corresponding to the edge amount, the image forming method is characterized in that the threshold values are arranged in a clockwise spiral from the inside toward the outside in order from the smallest.
The invention according to claim 3
An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
The binary quantization process is performed from the upper right to the lower left in the image space,
In the dither threshold value matrix corresponding to the edge amount, the image forming method is characterized in that the threshold values are arranged in a counterclockwise spiral shape from the inner side to the outer side in order from the smallest.
[0010]
  The invention according to claim 4
An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
In the dither threshold value matrix corresponding to the edge amount, threshold values less than a predetermined value are arranged in a spiral shape from the inner side to the outer side in order from the smallest value, and threshold values greater than or equal to the predetermined value are arranged in a radial manner in order from the smallest. This is an image forming method.
  In the low / medium density level area of multi-tone image data, the output dots grow spirally from the inside to the outside, and in the high density level area, the output dots are dispersed and grown in a high density area. The white spot phenomenon can be alleviated.
[0011]
  The invention according to claim 5
An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
In the dither threshold matrix corresponding to the edge amount, threshold values less than a predetermined value are concentratedly arranged in the central portion, and threshold values equal to or more than the predetermined value are arranged radially and distributed in order from the smallest to the peripheral portion. An image forming method.
Output dots are intensively grown in the low density level area of multi-tone image data, and output dots are grown in the medium and high density level areas by radiating and growing from low density areas to high density areas. This suppresses fluctuations in the center position of the halftone dots and enables formation of a smooth and high-quality image that is not easily affected by density changes.
[0012]
  The invention described in claim 6
An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
In the dither threshold value matrix corresponding to the edge amount, the image forming method is characterized in that a threshold value less than a predetermined value is arranged in the center portion so as to be preferentially arranged in the sub-scanning direction in ascending order.
By growing output dots preferentially in the sub-scanning direction in the low density level area of multi-tone image data, the vertical tone is less susceptible to noise such as banding in electrophotographic printers in low density areas. It can be dot growth.
[0013]
  The invention described in claim 7
An image forming method for forming an image by performing multi-value quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the multi-value quantization processing,
Generating a plurality of dither threshold matrix thresholds according to the edge amount of the multi-tone image data, using the threshold values of the plurality of dither threshold matrices according to the edge amount as quantization thresholds of the multi-value quantization processing;
In each of the plurality of dither threshold matrixes according to the edge amount, the thresholds are arranged in a spiral shape from the inside to the outside in order from the smallest,
Among the plurality of dither threshold matrices corresponding to the edge amount, a threshold increase step in a dither threshold matrix used as a high-density quantization threshold is used as a dither threshold matrix used as a low / medium density quantization threshold. The image forming method is characterized in that the image forming method is smaller than the threshold value increasing step.
By reducing the concentration of output dots in the high density level region of multi-tone image data from the concentration of output dots in the low / medium density level region, it is possible to effectively eliminate dot loss (white spots) in high density solid areas. Can be suppressed.
[0015]
  The invention described in claim 8
An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
In the image processing apparatus, the dither threshold value matrix corresponding to the edge amount is arranged in a spiral shape from the inner side to the outer side in order from the smallest.
[0016]
  The invention according to claim 9
An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
The binary quantization process is performed from the upper left to the lower right in the image space,
In the image processing apparatus, the dither threshold value matrix corresponding to the edge amount is arranged in a clockwise spiral from the inner side to the outer side in order from the smallest.
[0017]
  The invention according to claim 10 is:
An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
The binary quantization process is performed from the upper right to the lower left in the image space,
In the dither threshold value matrix corresponding to the edge amount, the threshold value is arranged in a counterclockwise spiral from the inner side to the outer side in order from the smallest.
[0018]
  The invention according to claim 11
An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
In the dither threshold matrix according to the edge amount, threshold values less than a predetermined value are arranged in a spiral shape from the inside to the outside in order from the smallest value, and threshold values above the predetermined value are arranged in a radially distributed manner from the smallest value. An image processing apparatus characterized by this.
[0019]
  The invention according to claim 12
An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
In the dither threshold matrix corresponding to the edge amount, threshold values less than a predetermined value are concentratedly arranged in the central portion, and threshold values equal to or greater than the predetermined value are arranged in a radially distributed manner in order from the smallest to the peripheral portion. An image processing apparatus.
[0020]
  The invention according to claim 13
An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
In the dither threshold value matrix corresponding to the edge amount, an image processing apparatus is arranged such that a threshold value less than a predetermined value is arranged in the center portion in order from the smallest one in order in the sub-scanning direction.
[0021]
  The invention according to claim 14 is the image processing apparatus according to any one of claims 8 to 13,
Means for generating a threshold value of a dither threshold value matrix according to the edge amount,
Edge detection means for detecting an edge amount from the multi-tone image data, quantizing the detected edge amount into a plurality of edge levels from a maximum edge degree to a non-edge, and output,
Area expansion processing means for outputting, as an edge level for the target pixel, an edge level indicating the maximum edge degree among the edge levels output from the edge detection means for the target pixel and its surrounding pixels,
A value obtained by adding a predetermined fixed value to a value obtained by multiplying a threshold value of a predetermined dither threshold matrix stored in the storage unit by a coefficient corresponding to the edge level output from the region expansion processing unit, according to the edge amount. Output as the threshold of the dither threshold matrix
It is characterized by that.
[0022]
  According to a fifteenth aspect of the present invention, in the image processing apparatus according to any one of the eighth to thirteenth aspects,
Means for generating a threshold value of a dither threshold value matrix according to the edge amount,
Edge detection means for detecting an edge amount from the multi-tone image data, quantizing the detected edge amount into a plurality of edge levels from a maximum edge degree to a non-edge, and output,
Area expansion processing means for outputting, as an edge level for the target pixel, an edge level indicating the maximum edge degree among the edge levels output from the edge detection means for the target pixel and its surrounding pixels,
A dither threshold matrix selected from a plurality of predetermined dither threshold matrices stored in the storage unit based on the edge level output from the area expansion processing unit is used as a dither threshold matrix corresponding to the edge amount. Output as the threshold of
It is characterized by that.
[0023]
  The invention according to claim 16
An image processing apparatus that performs multi-level quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
Means for generating threshold values of a plurality of dither threshold matrixes corresponding to the edge amounts of the multi-tone image data, and quantizing the threshold values of the plurality of dither threshold matrixes corresponding to the edge amounts in the multi-level quantization processing; Used as a threshold,
In each of the plurality of dither threshold matrixes according to the edge amount, the thresholds are arranged in a spiral shape from the inside to the outside in order from the smallest,
Among the plurality of dither threshold matrices corresponding to the edge amount, a threshold increase step in a dither threshold matrix used as a high-density quantization threshold is used as a dither threshold matrix used as a low / medium density quantization threshold. The image processing apparatus is characterized in that it is smaller than the threshold increase step in FIG.
[0024]
  According to a seventeenth aspect of the present invention, in the image processing apparatus according to the sixteenth aspect, the high-density quantization threshold used for the multi-value quantization process is a fixed value regardless of the edge amount.
[0025]
  The invention according to claim 18 is the image processing apparatus according to claim 16 or 17,
Means for generating threshold values of a plurality of dither threshold value matrixes according to the edge amount,
Edge detection means for detecting an edge amount from the multi-tone image data, quantizing the detected edge amount into a plurality of edge levels from a maximum edge degree to a non-edge, and output,
Area expansion processing means for outputting, as an edge level for the target pixel, an edge level indicating the maximum edge degree among the edge levels output from the edge detection means for the target pixel and its surrounding pixels,
A threshold of a dither threshold matrix selected from a plurality of predetermined dither threshold matrices stored in the storage means based on the edge level output from the area expansion processing means is converted into a plurality of dithers according to the edge amount. Output as threshold matrix thresholds,
It is characterized by that.
[0026]
  A nineteenth aspect of the invention is a computer-readable storage medium in which a program for causing a computer to function as the image processing apparatus according to any one of the eighth to eighteenth aspects is recorded.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings. In addition, in order to avoid duplication of description, the same reference number is used for the same part or a corresponding part in several drawings in the attached drawings.
[0029]
Example 1
FIG. 1 shows a block diagram of an image processing apparatus according to Embodiment 1 of the present invention. This image processing apparatus receives multi-gradation image data 100 and outputs quantized data 101. The image processing apparatus includes a quantization processing unit 120, an image feature extraction unit 130, a quantization threshold generation unit 140, and a quantization processing unit. 120 and a signal delay unit 150 for timing adjustment between the image feature extraction unit 130. The signal delay unit 150 is provided as necessary, and is a line memory having a required number of lines, for example. The input image data 100 is 8-bit / pixel data read by a scanner at 600 dpi, for example. In general, such image data 100 is input after passing through a smoothing filter in order to smoothly express a halftone. Usually, since smoothing is performed from an image period of about 150 Lpi, the periodic component of a high-line number halftone dot image of 175 Lpi or more used in gravure printing or the like does not remain in the image data 100.
[0030]
The quantization processing unit 120 quantizes the multi-tone image data by the error diffusion method using the quantization threshold generated by the quantization threshold generation unit 140. In the present embodiment, as illustrated in FIG. , A quantizer (comparator) 121, an error calculation unit 122, an error storage unit 123, an error diffusion matrix unit 124, and an error addition unit 125. The image data 100 is adjusted in timing by the signal delay unit 150 and input to the error addition unit 125. The image data added with the diffusion error by the error adder 115 is input to the quantizer 121. The quantizer 121 quantizes the input image data using the quantization threshold given from the quantization threshold generator 140 and outputs the quantization result as the quantized data 101.
[0031]
In order to simplify the explanation, in this embodiment and each embodiment described later, the quantization threshold value generator 140 generates only one quantization threshold value, and the quantizer 121 has the input image data equal to or greater than the quantization threshold value. In the following description, it is assumed that 1-bit quantized data 101 having a value of “1” and “0” is output otherwise. However, the present invention is not limited to this. For example, the quantization threshold generation unit 140 generates three quantization thresholds, and the quantizer 121 quantizes the image data into four levels using the quantization thresholds, and outputs 2-bit quantization data 101. It can also be set as such a structure.
[0032]
The error calculation unit 122 calculates the quantization error of the quantizer 121. Since 8-bit image data is handled here, in this error calculation, for example, “1” of the quantized data 101 is treated as 255 (decimal) and “0” is treated as 0 (decimal). The calculated quantization error is temporarily stored in the error storage unit 123. The error storage unit 123 is for storing a quantization error related to a processed pixel around the target pixel. In this embodiment, as will be described below, for example, a three-line line memory is used as the error storage unit 123 in order to diffuse the quantization error to peripheral pixels ahead of two lines.
[0033]
The error diffusion matrix unit 124 calculates a diffusion error to be added to the next pixel of interest from the quantization error data stored in the error storage unit 123. In this embodiment, the error diffusion matrix unit 125 calculates diffusion error data using an error diffusion matrix having a size of 3 pixels in the sub-scanning direction and 5 pixels in the main scanning direction as shown in FIG. In FIG. 2, the mark * corresponds to the position of the next pixel of interest, and a, b,. . . , K, l are coefficients corresponding to the positions of the 12 processed pixels in the vicinity (the sum is 32). In the error diffusion matrix unit 125, a value obtained by dividing the product sum of the quantization error for the 12 processed pixels and the corresponding coefficients a to l by 32 is sent to the error addition unit 125 as a diffusion error for the next pixel of interest. give.
[0034]
The image feature extraction unit 130 includes an edge detection unit 131 and a region expansion processing unit 132. The edge detection unit 131 performs edge detection of the image data 100, and outputs 4-bit edge data representing edge levels from level 0 (maximum edge degree) to level 8 (non-edge) in this embodiment. More specifically, for example, the four types of 5 × 5 differential filters shown in FIG. 3 are used to detect the edge amount in the four directions of the main scanning direction, the sub-scanning direction, and the direction inclined by ± 45 ° from the main scanning direction. Then, the edge amount having the maximum absolute value is selected, and the absolute value of the edge amount is quantized to 9 level edge levels from level 0 to level 8 and output. The region expansion processing unit 132 performs a region expansion process with a width of 7 pixels on the edge detected by the edge detection unit 131. The region expansion processing unit 132 refers to the edge data output from the edge detection unit 131 and generates 7 pixels around the target pixel. The minimum edge level (maximum edge degree) in the 7 pixel area (range of 3 pixels before and after in the main scanning direction and 3 pixels before and after in the sub-scanning direction) Output as edge data. This edge data is given to the quantization threshold value generator 140.
[0035]
The quantization threshold generation unit 140 generates a quantization threshold that periodically oscillates in the image space with a vibration width corresponding to the edge level represented by the edge data output from the region expansion processing unit 132. This is given to the quantizer 121 of the quantization processing unit 120, and a dither threshold value generation unit 141 and a coefficient (0 to 8) corresponding to the edge level indicated by the edge data are output to the dither threshold value generation unit 141. The multiplication unit 142 and the addition unit 143 that adds a fixed value to the output value of the multiplication unit 142 are configured.
[0036]
In this embodiment, the dither threshold value generator 141 spirals clockwise from the inside to the outside in order from the smallest threshold values from −7 to +8 as shown in FIG. 4 (−7 is minimum and +8 is maximum). Using the arranged 4 × 4 dither threshold matrix, a dither threshold that oscillates periodically from −7 to +8 in the image space is output. The dither threshold period is 4 pixels, which corresponds to 150 Lpi in the case of 600 dpi image formation. Such a dither threshold value generation unit 141 can be easily realized by a ROM storing the dither threshold value matrix, a counter that counts main and sub-scan timing signals of image data, and generates a read address of the ROM. .
[0037]
The multiplication unit 142 sets the coefficient 8 when the edge level indicated by the edge data from the image feature extraction unit 130 is level 8 (non-edge), the coefficient 7 when it is level 7, the coefficient 6 when it is level 6, and the level 5 Factor 5 at level 4, factor 4 at level 4, factor 3 at level 3, factor 2 at level 2, factor 1 at level 1, factor 0 at level 0 (maximum edge degree) The output value of the dither threshold value generator 141 is multiplied. Therefore, the output value of the multiplication unit 142 vibrates with the maximum vibration width from +64 to −56 at the edge level 8 (non-edge). The fixed value added by the adding unit 143 is selected as the median value of the image data width +128 (decimal). Therefore, the quantization threshold given to the quantizer 121 vibrates around +128, and the maximum vibration width is 120 (from +192 to 72).
[0038]
If the quantized data 101 of the image processing apparatus configured as described above is given to, for example, an electrophotographic printer, the character, the changing point of the image, the halftone dot image portion having a relatively low number of lines, and the like can be resolved. In addition, a photograph, a portion with little change in the image, a halftone dot image with a high number of lines, etc. are smooth and stable, and it is possible to form a high-quality image in which these regions are aligned without a sense of incongruity. This will be described below.
[0039]
The quantization threshold generated by the quantization threshold generation unit 140 is fixed to +128 in a portion where the change in the edge portion of the character or line drawing in the image is steep and the edge level is level 0 (edge degree is the highest). Therefore, the quantization processing unit 120 performs a quantization process by a pure error diffusion method using a fixed threshold value, and thus an image with high resolution can be formed.
[0040]
In a portion where the edge degree is low (the edge level is high) such as a flat portion of a photograph or image, the oscillation width of the quantization threshold generated by the quantization threshold generation unit 140 becomes large. The dithering process is a dithering process, and the image data is halftoned at a dither threshold period. Since a dither threshold matrix having a threshold arrangement as shown in FIG. 4 is used to generate the quantization threshold, the output dots grow spirally from the center within the dither threshold period as the density level of the image data increases. become.
[0041]
When scanning a document image with a scanner, etc., as shown by thin arrows in FIG. 10, the main scanning is usually performed from left to right, the sub scanning is performed from top to bottom, and the image data is input in the order of reading. Therefore, the quantization processing direction of the quantization processing unit 120, that is, the propagation direction of the quantization error is the direction from the upper left to the lower right as indicated by the thick arrow in FIG. In this case, since the quantization threshold as shown in FIG. 6 is generated within the 4 pixel × 4 pixel dither threshold period in the flat image portion, as the density level increases from the low level, as shown in FIG. Output dots are generated in order. That is, output dots grow from the inside toward the outside in a clockwise spiral shape. The appearance of output dots in the low density part, medium density part, and high density part of the image is shown in FIGS. 7, 8, and 9, respectively. As seen in FIGS. 7 and 8, the output dots are concentrated in the low / medium density portion, and the output dots of the adjacent dither threshold periods are not coupled to each other. Accordingly, it is possible to form an image with good granularity and excellent stability in the flat portion of the low / medium density image. Further, since the threshold values are arranged in the center of the dither threshold matrix in order of increasing priority in the sub-scanning direction, the output dots grow upward in the low density portion, as understood from FIG. And then grow downward. That is, in the low density portion, the vertical dot growth grows preferentially in the sub-scanning direction. With this vertical tone dot growth, especially when an electrophotographic printer is used for image formation, it is less susceptible to noise from factors other than image processing such as banding, and more stable than horizontal dot growth. High-quality images with excellent properties can be formed.
[0042]
When the original image is read by performing main scanning from right to left and sub-scanning from top to bottom as shown by thin arrows in FIG. 11, the image data is input in the order of reading. The direction of the quantization processing of the processing unit 120, that is, the propagation direction of the quantization error is the direction from the upper right to the lower left as shown by the thick arrow in FIG. In this case, since the quantization threshold value as shown in FIG. 13 is generated within the dither threshold period of 4 pixels × 4 pixels in the flat image portion, output dots are generated in the order shown in FIG. That is, the output dots grow in a counterclockwise spiral shape.
[0043]
In the quantization process by the error diffusion method, output dots fluctuate due to the diffusion effect of the quantization error. Therefore, the output dots do not always grow in the low / medium density portion as described above. When the quantization process is performed from the upper left to the lower right, the output dot grows clockwise, and when the quantization process is performed from the upper right to the lower left, the output dot grows counterclockwise, the quantization error propagates. From the relationship with the directional characteristics, more concentrated dot concentration is promoted within the dither threshold period, so that the stability and graininess of the low / medium density portion are improved.
[0044]
In the image processing apparatus of the present embodiment, the vibration width of the quantization threshold is gradually increased or decreased according to the edge degree at the boundary portion between the area with a large edge degree and the area with a small edge degree. The characteristics of the quantization processing can be smoothly switched to the above processing or vice versa. Therefore, it is possible to form an image in which the boundary portion between both image regions is aligned without a sense of incongruity.
[0045]
The area expansion processing unit 132 of the image feature extracting unit 130 performs area expansion of 7 pixels on the edge data. In the case of 600 dpi image formation, the area expansion width of 7 pixels is about 0. At 3 mm, this corresponds to a halftone dot period of about 86 Lpi. Therefore, a halftone dot image portion having a line number higher than 86 Lpi is evaluated as an edge portion, and an error diffusion main process using a quantization threshold fixed in the quantization processing unit 120 or a quantization threshold having a small vibration width is performed. Therefore, halftone dots can be faithfully reproduced with high resolution, and moire is not generated.
[0046]
In addition, as described above, a dot component having a high line number of 175 Lpi or more is smoothed and does not remain in the image data 100. Therefore, such a high dot number halftone dot image portion has an edge level of level 8 or a high level. As in the case of the flat image portion, the image is re-dotted with the dither threshold period (150 Lpi) by the dither tone processing using the quantization threshold having a large vibration width, and an image having excellent graininess and stability can be formed. Further, since the halftone dot component is lost from the image data 100, moire does not occur.
[0047]
In a halftone dot image having a line number lower than 86 Lpi, a halftone dot boundary portion evaluated as an edge is subjected to error diffusion-based processing using a quantization threshold with a fixed or small vibration width, so that the halftone dot is reproduced faithfully. In addition, the generation of moire can be prevented, and a halftone dot center portion that is not evaluated as an edge is subjected to dither tone processing using a quantization threshold having a large vibration width, so that an image having good stability and granularity can be formed. .
[0048]
Example 2
According to the second embodiment of the present invention, in the image processing apparatus having the configuration shown in FIG. 1, the dither threshold value generator 141 of the quantization threshold value generator 140 generates an 8 × 8 dither threshold matrix as shown in FIG. Used to generate a dither threshold that oscillates periodically from -7 to +8 in the image space. The other configuration is the same as that of the first embodiment.
[0049]
The dither threshold matrix shown in FIG. 14 is a combination of four 4 × 4 basic dither threshold values (same as the dither threshold matrix shown in FIG. 4) surrounded by a thick frame and expanded to 8 × 8. The basic dither threshold matrix adjacent in the main scanning direction is relatively half-phase shifted in the sub-scanning direction.
[0050]
If such a dither threshold matrix is used, halftone dots are arranged as shown in FIGS. 15, 16, and 17 in each of the low density, medium density, and high density areas in an image flat portion that is a dither tone process. Since the screen angle is approximately 63.5 °, the compatibility with the error diffusion process is good, the halftone dot is hardly broken, and the graininess is good.
[0051]
Note that the dither threshold value generation unit 141 does not necessarily have the ROM storing the 8 × 8 dither threshold matrix shown in FIG. 14, and has the ROM storing the 4 × 4 basic dither threshold matrix, and has the read address of the read address. By the control, it is possible to generate a quantization threshold by the dither threshold matrix of FIG.
[0052]
Example 3
According to the third embodiment of the present invention, in the image processing apparatus having the configuration shown in FIG. 1, the dither threshold value generation unit 141 of the quantization threshold value generation unit 140 sets threshold values from −7 to +2 as shown in FIG. Dither that periodically oscillates from -7 to +8 in the image space using a 4x4 dither threshold matrix that is arranged in a clockwise spiral in order from the smallest and radially distributed with thresholds greater than +3 Generate a threshold. The other configuration is the same as that of the first embodiment.
[0053]
Since such a dither threshold matrix is used, a quantization threshold as shown in FIG. 19 is generated within the dither threshold period in an image flat portion or the like that is a process of dither keynote. Therefore, the growth of output dots in the low density portion, medium density portion, and high density portion is as shown in FIG. As can be seen from FIG. 20 (c), the output dots are dispersed and grow in the peripheral portion within the dither threshold period. As is apparent from comparison with FIG. 9, the white spot phenomenon occurs in the solid portion having a high density. Is alleviated. Further, the concentration of dots is promoted in the low / medium density portion. Therefore, it is possible to form a high-quality image that is excellent in stability and in which white spots are not noticeable.
[0054]
Example 4
According to the fourth embodiment of the present invention, in the image processing apparatus having the configuration shown in FIG. 1, the dither threshold value generation unit 141 of the quantization threshold value generation unit 140 uses threshold values from −7 to −4 as shown in FIG. Oscillate periodically from -7 to +8 in the image space using a 4x4 dither threshold matrix in which the thresholds from -3 to +8 are distributed radially around the periphery. A dither threshold is generated. The other configuration is the same as that of the first embodiment.
[0055]
Since such a dither threshold matrix is used, output dots are generated in the order as shown in FIG. That is, output dots grow intensively at the center of the dither threshold period in the low density portion, and output dots grow radially in the middle and high density portions. Due to such output dot growth, the center position of the halftone dots to be formed is located substantially at the center of the dither threshold period from the low density portion to the high density portion, and the fluctuation of the halftone dots is reduced. Therefore, it is possible to form a smooth and high-quality image that is hardly affected by the density change.
[0056]
Example 5
According to the fifth embodiment of the present invention, in the image processing apparatus having the overall configuration shown in FIG. 1, the quantization threshold value generator 140 is configured as shown in FIG. The edge detection unit 131 of the image feature extraction unit 130 (FIG. 1) quantizes the edge amount into four edge levels from level 0 (maximum degree of edge) to level 3 (non-edge), and converts it into a 2-bit edge. Changed to output as data.
[0057]
As shown in FIG. 23, the quantization threshold value generation unit 140 in this embodiment includes a threshold value generation unit 145_0 corresponding to the edge level 0, a threshold value generation unit 145_1 corresponding to the edge level 1, and a threshold value generation corresponding to the edge level 2. 145_2, a threshold generation unit 145_3 corresponding to edge level 3, and a threshold generated by any one of threshold generation units 145_0 to 145_3 according to the edge level indicated by the edge data output from the image feature extraction unit 130 , And a threshold selection unit 146 that gives the quantization threshold to the quantizer 121 of the quantization processing unit 120 (FIG. 1).
[0058]
The threshold generation unit 145_3 corresponding to the edge level 3 (non-edge), for example, multiplies each threshold of the dither threshold matrix used in the first, second, third, or fourth embodiment by 8 and then adds 128 to the dither threshold matrix. Is used to generate a threshold value that vibrates at the maximum vibration width. The threshold value generation unit 145_2 corresponding to the edge level 2 uses the dither threshold value matrix obtained by multiplying each threshold value of the dither threshold value matrix of the above-described embodiment by 5 and then adding 128 to the threshold value that vibrates with a smaller vibration width. Is generated. The threshold value generation unit 145_1 corresponding to the edge level 1 uses the dither threshold value matrix obtained by multiplying each threshold value of the dither threshold value matrix of the above-described embodiment by 2 and then adding 128 to the threshold value that vibrates with a smaller vibration width. Is generated. The threshold value generator 145_0 corresponding to the edge level 0 (edge degree maximum) generates a fixed value (+128).
[0059]
Therefore, it is apparent that a high-quality image can be formed in this embodiment by performing the same quantization process as in the first, second, third, or fourth embodiment.
[0060]
According to the configuration of the quantization threshold value generation unit 140 of the present embodiment, a means for multiplication that is generally disadvantageous in terms of cost or processing time (multiplication in FIG. 1) is realized in either hardware or software. Can be eliminated. The area expansion processing unit 132 needs to temporarily store edge data for a plurality of lines corresponding to the area expansion width. However, the edge data is temporarily stored by the amount corresponding to the compression of the edge data to 2 bits. The capacity of the line memory can be reduced. In addition, since the number of edge levels is as small as four, the threshold generation units 145_0 to 145_2 need a small amount of memory for storing the dither threshold matrix.
[0061]
Although the memory of the area expansion processing unit 132 is not reduced, a configuration in which the edge detection unit 131 outputs 9 levels of the edge level and the area expansion unit 132 converts the edge level to 4 levels and outputs it is also possible.
[0062]
Each of the above embodiments is an example of quantizing multi-valued image data into binary values. However, (n-1) quantization threshold values are generated, and the multi-valued image is changed to n values (n ≧ 3). The present invention can also be applied to an image processing apparatus that performs quantization. An embodiment for quantizing multivalued image data into three values will be described below.
[0063]
Example 6
The overall configuration of the image processing apparatus according to the sixth embodiment of the present invention is as shown in FIG. 1, but since it is ternary quantization, the quantization threshold generator 140 has two quantization thresholds as shown in FIG. The configuration is changed to generate th1 and th2 (th1 <th2).
[0064]
Further, the quantizer 121 of the quantization processing unit 120 quantizes the multi-valued image data using the quantization threshold values th1 and th2 supplied from the quantization threshold value generation unit 140, and generates the 2-bit quantized data 101. Is changed to a configuration that outputs. For example, “00” (no dot) is set when the level of the multilevel image data is lower than the quantization threshold th1, and “01” (no dot) when the level of the multilevel image data is equal to or higher than the quantization threshold th1 and lower than the quantization threshold th2. When the level of the multi-valued image data is equal to or higher than the quantization threshold th2, “11” (large dot) is output as the quantized data 101.
[0065]
Further, the error calculation unit 122 (FIG. 1) sets “00” of the quantized data 101 to 0 (decimal), “01” to 127 (decimal), for example, and “11” to 255 (decimal). The configuration is changed to handle and calculate the quantization error.
[0066]
The image feature extraction unit 130 (FIG. 1) is configured to output 4-level edge data from level 0 (maximum edge degree) to level 3 (non-edge), as in the fifth embodiment.
[0067]
As shown in FIG. 24, the quantization threshold value generation unit 140 includes threshold value storage units 200, 201, 202, and 203 that store four dither threshold value matrices for generating the quantization threshold value th1, and the edge data. The threshold generation unit 204 that selects one of the threshold storage units 200 to 203 and generates the quantization threshold th1 using the dither threshold matrix stored therein, and four dithers for generating the quantization threshold th2 Threshold storage units 210, 211, 212, and 213 storing threshold matrixes and one of threshold storage units 210 to 213 are selected according to edge data, and quantization is performed using the dither threshold matrix stored in the threshold storage units 210, 211, 212, and 213 The threshold generation unit 214 generates the threshold th2.
[0068]
In this embodiment, dither threshold value matrices 220, 221, 222, and 223 shown in FIG. 25 are stored in the threshold value storage units 200, 201, 202, and 203, respectively. The threshold value generation unit 204 stores the threshold value storage unit 200 when the edge level indicated by the edge data input from the image feature extraction unit 130 is level 0 (edge degree maximum), the threshold value storage unit 201 when the level is level 1, and the threshold value storage when the edge level is level 2. When the unit 202 is level 3 (non-edge), the threshold storage unit 203 is selected, and the quantization threshold th1 is generated using the dither threshold matrix stored in the selected threshold storage unit.
[0069]
In addition, dither threshold value matrices 230, 231, 232, and 233 shown in FIG. 25 are stored in the threshold value generation units 210, 211, 212, and 213, respectively. The threshold generation unit 214 includes a threshold storage unit 210 when the edge level indicated by the edge data is level 0, a threshold storage unit 211 when the level is 1, level threshold storage unit 212 when the level is 2, and a threshold storage unit 213 when the level is 3. A quantization threshold th2 is generated by using the dither threshold matrix stored in the selected threshold storage unit.
[0070]
Referring to FIG. 25, the dither threshold value matrixes 223 and 233 at the edge level 3 are arranged in a spiral shape from the smallest threshold value, and the threshold value is increased by 6 steps. Therefore, in a continuous tone image portion where the density change is small and a halftone dot image portion where the number of lines is high, the dots are strongly concentrated from low density to high density, and the image has excellent banding, graininess, stability and gradation. It can be reproduced.
[0071]
On the other hand, the dither threshold value matrices 220 and 230 at the edge level 0 are arranged in a spiral shape from the smallest threshold value, but the threshold value is increased by two steps. Therefore, in an image portion having a large density change such as a character or a line drawing, or a halftone dot image portion having a relatively low number of lines, the amplitudes of the quantization thresholds th1 and th2 are small, and the quantization is close to error diffusion using a fixed threshold. Image reproduction with high resolution becomes possible.
[0072]
Since the threshold levels of the dither threshold value matrices 222 and 232 at the edge level 2 increase by 4 steps, the amplitudes of the quantization threshold values th2 and th2 in the image part at the edge level 2 are slightly smaller than those at the image part at the edge level 3, and the dot concentration is reduced. Slightly weaken. In the edge level 1 dither threshold matrixes 221 and 231, the threshold value is increased by 3 steps, so that the amplitudes of the quantization thresholds th 2 and th 2 in the edge level 1 image portion are slightly smaller than those in the edge level 0 image portion, and the dot concentration is further increased. Weaken. Therefore, it is possible to reproduce a high-quality image with no sense of incongruity from a flat portion to an edge portion of the image, from a halftone dot image portion having a low number of lines to a halftone dot image portion having a high number of lines.
[0073]
Although the present embodiment is ternary quantization, it is apparent that high-quality image reproduction can be achieved by the same configuration as the present embodiment even in the case of multi-value quantization of four or more values.
[0074]
Note that a configuration in which the image feature extraction unit 130 does not perform area expansion processing is also possible.
[0075]
Example 7
The image processing apparatus according to the seventh embodiment of the present invention includes an overall configuration, a configuration of the quantization threshold value generation unit 140, a configuration of the quantizer 121 and the error calculation unit 122 of the quantization processing unit 120, and a configuration of the image feature extraction unit 130. These are the same as in the sixth embodiment. However, dither threshold value matrices 240 to 243 and 250 to 253 as shown in FIG. 26 are stored in the error storage units 200 to 203 and 210 to 213 of the quantization threshold value generation unit 140.
[0076]
As can be seen from FIG. 26, in this embodiment, compared to the dither threshold matrix 240 to 243 for generating the low / medium density quantization threshold th1 at each edge level, the high density quantization threshold th2 is used. The dither threshold matrixes 250 to 253 are set so that the threshold increase step is small, and therefore, at each edge level, the amplitude of the high-density quantization threshold th2 is smaller than the amplitude of the low-density quantization threshold th1. Become.
[0077]
In the sixth embodiment, the threshold increase step of the dither threshold matrix at each edge level is the same on the low density side and the high density side, and the high density side threshold has the same strong dither shape as the low density side. Since a dot at a position corresponding to the largest threshold is always missing in an image portion that is not completely black, dot omission (whiteout) tends to be conspicuous particularly in a high-density solid portion of an image flat region that is a dither tone process. In this embodiment, by increasing the threshold increase step on the high density side to be smaller than the threshold increase step on the low density side to alleviate dot concentration, dot omission (whiteout) in such a high density solid image portion is achieved. ) Can be effectively reduced. In addition, since the threshold value on the low density side is the same as in the sixth embodiment, the anti-banding property, graininess, and gradation property in the low density part are not impaired.
[0078]
Although the present embodiment is ternary quantization, even in the case of multi-value quantization of four or more values, the present embodiment can be realized by setting the dither threshold matrix so that the increase step is smaller as the threshold value is higher. As in the example, an effect of reducing dot omission (white omission) in the high density solid region can be obtained.
[0079]
Although there is a method of setting a dither threshold matrix in which dots in a high density region are dispersed by simple multi-value dither processing that does not use error diffusion, since this method does not preserve density, gradation expression is not possible. It may be insufficient. In the present invention, by combining error diffusion and a dither threshold, and by reducing the concentration of dots in the high density portion, it is possible to achieve both a reduction in missing dots (white spots) in the high density solid portion and good gradation expression. Can do.
[0080]
Note that a configuration in which the image feature extraction unit 130 does not perform area expansion processing is also possible.
[0081]
Example 8
The image processing apparatus according to the eighth embodiment of the present invention includes an overall configuration, a configuration of the quantization threshold value generation unit 140, a configuration of the quantizer 121 and the error calculation unit 122 of the quantization processing unit 120, and a configuration of the image feature extraction unit 130. Is the same as that of the sixth embodiment, but the dither threshold matrix stored in the error storage units 200 to 203 and 210 to 213 of the quantization threshold generation unit 140 is changed as shown in FIG. The low-density dither threshold matrix 260 to 263 stored in the threshold storage units 200 to 203 is the same dot concentration type matrix as the dither threshold matrix 220 to 223 or 240 to 243 of the sixth or seventh embodiment. On the other hand, the dither threshold value matrixes 270 to 273 on the high density side stored in the respective threshold value storage units 210 to 213 are all fixed values having the same threshold value (the threshold increase step is 0).
[0082]
In this way, by setting the threshold values on the high density side at the respective edge levels to the same fixed value, it is sufficient to store one threshold value in each of the threshold value storage units 210 to 213. The capacity of the registers or memories as the storage units 210 to 213 can be reduced to one threshold value, and the cost of the apparatus can be reduced. Further, the threshold value on the high density side is changed depending on the edge level, such as 200 at the edge level 3 (non-edge portion) and 160 at the edge level 0 (edge portion). The frequency of occurrence is controlled. At edge level 0 (edge portion), the threshold on the high density side is lowered to suppress the generation of small dots and the generation of large dots is promoted, thereby improving the visibility of black characters or the like existing in a background area of uniform density. An effect is obtained.
[0083]
Although this embodiment is an example of ternary quantization, even in the case of quaternary quantization, for example, by fixing the high density side threshold value at each edge level to a different value, large dots (high density) can be obtained. By promoting the generation and suppressing the frequency of occurrence of medium dots (medium density) and small dots (low density), the visibility of black characters and the like in the background area of uniform density can be improved. Note that a configuration in which the image feature extraction unit 130 does not perform area expansion processing is also possible.
[0084]
In the method of setting a dither threshold matrix in which dots in a high density region are dispersed by a simple multi-value dither process that does not use error diffusion as described above, the threshold value on the high density side is set to the same value as in this embodiment. If fixed to, gradation expression in the high density portion becomes very poor. In this embodiment, since error diffusion is performed, such inconvenience can be avoided.
[0085]
Example 9
The image processing apparatus according to the ninth embodiment of the present invention includes an overall configuration, a configuration of the quantization threshold value generation unit 140, a configuration of the quantizer 121 and the error calculation unit 122 of the quantization processing unit 120, and a configuration of the image feature extraction unit 130. Is the same as in the sixth embodiment, but the dither threshold value matrix stored in the error storage units 200 to 203 and 210 to 213 of the quantization threshold value generation unit 140 is changed as shown in FIG. The low-density dither threshold matrix 280 to 283 stored in the threshold storage units 200 to 203 is the same dot concentration as the dither threshold matrix 220 to 223, 240 to 243, or 260 to 263 of the sixth, seventh, or eighth embodiment. A matrix of types. On the other hand, all the threshold values of the dither threshold value matrix 290 to 293 on the high density side stored in the threshold value storage units 210 to 213 are 200. In this way, regardless of the edge level, the threshold values on the high density side are set to the same value, so that the threshold value storage units 210 to 213 are integrated into one threshold value storage unit, and it has a capacity for one threshold value. Since it can be realized by one register or memory, the cost of the apparatus can be further reduced than in the eighth embodiment.
[0086]
Note that a configuration in which the image feature extraction unit 130 does not perform area expansion processing is also possible.
[0087]
The image processing apparatus according to each embodiment described above can also be realized by software using a general computer. In this case, a program for realizing the functions of each unit of the image processing apparatus on a computer is read from various storage media such as a magnetic disk, an optical disk, a magneto-optical disk, and a semiconductor storage element on which the program is recorded, or The image processing apparatus of the present invention can be realized on a computer by receiving it from an external computer or the like via a network, loading it into the main memory of the computer, and causing the CPU to execute it. For example, a main memory is used as a storage area such as a line memory necessary for storage of various data and signal delay. Various computer-readable storage media in which such a program is recorded are also included in the present invention.
[0088]
The image processing apparatus according to each of the embodiments can be incorporated in a device such as a printer or a display, which is related to image formation, or a device such as a digital copying machine or a fax device related to both image reading and image formation. As an example of such an embodiment, an example of a digital copying machine to which the present invention is applied will be described below.
[0089]
Example 10
FIG. 24 is a schematic cross-sectional view showing a configuration example of an image reading mechanism and an image forming mechanism of a digital copying machine. This digital copying machine has a scanner unit 400 that optically scans and reads a document, a laser printer unit 411 as an image forming unit, and a circuit unit 550 (not shown) (FIG. 25).
[0090]
The scanner unit 400 illuminates a document placed on a flat document table 403 with an illumination lamp 502 and connects the reflected light image to an image sensor 507 such as a CCD via mirrors 503, 504 and 505 and a lens 506. At the same time, the image information of the original is read by sub-scanning the original by moving the illumination lamp 502 and the mirrors 503 to 505. The analog image signal output from the image sensor 507 is input to the circuit unit 550 (FIG. 25) and processed. Image data output from the circuit unit 550 is input to the laser printer unit 411.
[0091]
In the laser printer unit 411, the writing optical unit 508 converts the image data input from the circuit unit 550 into an optical signal, and exposes an image carrier made of a photoconductor, for example, a photoconductor drum 509, whereby an original image is obtained. An electrostatic latent image corresponding to is formed. For example, the writing optical unit 508 emits a laser beam whose intensity is modulated by driving a semiconductor laser by the light emission drive control unit with the image data, and deflecting and scanning the laser beam by the rotary polygon mirror 510, and an f / θ lens. Then, the photosensitive drum 509 is irradiated through the reflection mirror 511. The photosensitive drum 509 is driven to rotate by a drive unit and rotates clockwise as indicated by an arrow, and is uniformly charged by a charger 512 and then exposed by a writing optical unit 508 to form an electrostatic latent image. Is done. The electrostatic latent image on the photosensitive drum 509 is developed by the developing device 513 to become a toner image. Further, the paper is fed to the registration roller 520 from any of the plurality of paper feeding units 514 to 518 and the manual paper feeding unit 519. The registration roller 520 sends a sheet to the toner image on the photosensitive drum 509 in time. The transfer belt 521 is applied with a transfer bias from a transfer power source, transfers the toner image on the photosensitive drum 509 to a sheet, and conveys the sheet. The sheet onto which the toner image has been transferred is conveyed to the fixing unit 522 by the transfer belt 521 and the toner image is fixed, and then is discharged to the paper discharge tray 523. Further, the photosensitive drum 509 is cleaned by the cleaning device 524 after the toner image is transferred, and is further discharged by the charge eliminator 525 to prepare for the next image forming operation.
[0092]
FIG. 25 is a simplified block diagram showing an example of the circuit unit 550 of the digital copying machine. The input of the circuit unit 550 is an analog image signal read by the image sensor 507 of the scanner unit 400 at, for example, 600 dpi. The level of the analog image signal is adjusted by the AGC circuit 551, and then converted into 8-bit digital image data per pixel by the A / D conversion circuit 552. Further, the shading correction circuit 553 further converts the analog image signal for each pixel of the image sensor 507. Variations in sensitivity and illuminance are corrected.
[0093]
The image data after the shading correction is sent to the filter processing circuit 556 and subjected to filter processing for MTF correction and smoothing. As described above, this filtering process smoothes halftone dot components with a line number higher than about 150 Lpi, and removes halftone dot components with a high line number higher than 175 Lpi almost completely. The filtered image data is sent to the gamma correction circuit 555 and subjected to gamma correction for conversion to writing density.
[0094]
Reference numeral 560 denotes a halftone processing unit. The halftone processing unit 560 is composed of the image processing apparatus of each of the embodiments. The image data after the filter processing is input to the image feature extraction unit 130, and the image data after the gamma correction is input to the quantization processing unit 120 via the signal delay unit 150. The quantized data output from the quantization processing unit 120 is sent to the light emission drive control unit of the semiconductor laser in the writing optical unit 508.
[0095]
In the filter processing circuit 556, the signal delay unit 150 can be omitted by adjusting the timing of the signal output to the image feature extraction unit 130. In the digital copying machine, the scaling process of the image data in the main scanning direction is performed, for example, before the gamma correction circuit 555, or the background removal process or the flare removal process is performed, for example, with the gamma correction unit 555. The processing may be performed in the middle of the processing unit 560, or the 90 ° rotation processing may be performed, for example, before the filter processing circuit 554 or after the halftone processing unit 560, but the description thereof is omitted.
[0096]
【The invention's effect】
  Claim1 to 18According to the described invention, the dot concentration is promoted, and the combination of dots between adjacent periods in the low / medium density portion is prevented to form an image excellent in anti-banding property, graininess, and stability. be able to.
[0097]
  Claim2, 3, 9, 10According to the described invention, it is possible to form an image having excellent stability and graininess in the low and medium density portions by promoting the concentration of dots having a uniform shape.
[0098]
  Claim4, 11According to the described invention, the white spot phenomenon in the high density part is alleviated, and the dot concentration in the low / medium density part is promoted so that the white spot is not noticeable and has excellent stability. High-quality images can be formed.
[0099]
  Claim5, 12According to the described invention, by suppressing the fluctuation of the center position of the halftone dot from the low density portion to the high density portion, it is possible to form a smooth and high quality image that is hardly affected by the density change.
[0100]
  Claim6, 13According to the described invention, the low-density portion has a high-quality, excellent stability in the low-density portion by making the dot growth of the vertical tone that is not easily affected by noise such as banding in an electrophotographic printer or the like. An image can be formed.
[0101]
  ClaimOn 7, 16According to the described invention, white spots in the high density solid portion can be reduced.
[0102]
  Claim1 to 18According to the described invention, the change points of characters and images have good resolution, the portions with little change of photographs and images are smooth and stable, and both areas are aligned without a sense of incongruity. Can form imagesThe
[0103]
  Claim14, 15, 18According to the described invention, the change point of characters and images and the halftone dot image portion with a relatively low number of lines have good resolution, and the portion with little change in the photograph and image and the halftone dot image portion with a high number of lines are smooth. Therefore, it is possible to form a high-quality image with good stability and in which both areas are aligned without a sense of incongruity.
[0104]
  ClaimTo 17According to the described invention, it is possible to reduce the capacity of a register or a memory for storing a dither threshold matrix for generating a high-density quantization threshold.
[0105]
  Claim19According to the described invention,Claims 18 to 18The image processing apparatus can be realized using a general computer.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an example of a block configuration of an image processing apparatus according to the present invention.
FIG. 2 is a diagram illustrating an example of an error diffusion matrix.
FIG. 3 is a diagram illustrating an example of a differential filter for edge detection.
FIG. 4 is a diagram illustrating a dither threshold matrix used for quantization threshold generation in the first embodiment.
FIG. 5 is a diagram illustrating the generation order of output dots in an image flat portion according to the first exemplary embodiment.
FIG. 6 is a diagram illustrating a quantization threshold generated in an image flat portion in the first embodiment.
7 is a diagram showing a state of output dot growth in a low density portion in Example 1. FIG.
FIG. 8 is a diagram showing a state of output dot growth at a medium density portion in Example 1.
9 is a diagram showing a state of output dot growth in a high density portion in Example 1. FIG.
FIG. 10 is a diagram for explaining a normal processing direction.
FIG. 11 is a diagram for explaining another processing direction;
12 is a diagram showing the generation order of output dots in the case of the processing direction shown in FIG.
13 is a diagram illustrating a quantization threshold generated in an image flat portion in the case of the processing direction illustrated in FIG.
FIG. 14 is a diagram illustrating a dither threshold matrix used for generating quantization thresholds in the second embodiment.
15 is a diagram showing a state of output dot growth in a low density portion in Example 2. FIG.
FIG. 16 is a diagram showing a state of output dot growth at a medium density portion in Example 2.
FIG. 17 is a diagram showing a state of output dot growth at a high density portion in Example 2.
FIG. 18 is a diagram illustrating a dither threshold matrix used for generating a quantization threshold in the third embodiment.
FIG. 19 is a diagram illustrating a quantization threshold generated in an image flat portion in the third embodiment.
FIG. 20 is a diagram illustrating a state of output dot growth in a low density part, a medium density part, and a high density part in Example 3.
FIG. 21 is a diagram illustrating a dither threshold matrix used for generating quantization thresholds in the fourth embodiment.
FIG. 22 is a diagram illustrating a quantization threshold generated in an image flat portion in the fourth embodiment.
FIG. 23 is a block diagram of a quantization threshold value generator in the fifth embodiment.
FIG. 24 is a block diagram for explaining a configuration of a sixth embodiment.
FIG. 25 is a diagram illustrating a dither threshold matrix used for generating a quantization threshold in the sixth embodiment.
FIG. 26 is a diagram illustrating a dither threshold matrix used for generating a quantization threshold in the seventh embodiment.
27 is a diagram showing a dither threshold matrix used for quantization threshold generation in Embodiment 8. FIG.
FIG. 28 is a diagram illustrating a dither threshold matrix used for quantization threshold generation according to the ninth embodiment.
FIG. 29 is a schematic cross-sectional view showing a configuration example of a mechanism related to image reading and image formation of a digital copying machine according to Embodiment 10.
30 is a block diagram of a circuit unit of a digital copying machine according to Embodiment 10. FIG.
[Explanation of symbols]
100 Multi-tone image data
101 Quantized data
120 Quantization processor
121 Quantizer
122 Error calculator
123 Error storage
124 Error diffusion matrix part
125 Error adder
130 Image feature extraction unit
131 Edge detector
132 Area expansion processing unit
140 Quantization threshold value generator
141 Dither threshold generator
142 Multiplier
143 Adder
145_0 to 145_3 threshold generation unit
146 Threshold selection unit
150 Signal delay unit
200 to 203 Threshold storage unit
204 Threshold generation unit
210 to 213 Threshold storage unit
214 Threshold generation unit
400 Scanner unit
411 Laser printer
551 AGC circuit
552 A / D conversion circuit
553 Shading correction circuit
556 Filter processing circuit
557 Gamma correction circuit
560 Halftone processing unit

Claims (19)

An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
  Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
  In the dither threshold value matrix according to the edge amount, the threshold values are arranged in a spiral shape from the inside toward the outside in order from the smallest.
An image forming method. An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
  Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
  The binary quantization process is performed from the upper left to the lower right in the image space,
  In the dither threshold matrix according to the edge amount, the thresholds are arranged in a clockwise spiral from the inside to the outside in order from the smallest.
An image forming method. An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
  Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
  The binary quantization process is performed from the upper right to the lower left in the image space,
  In the dither threshold matrix according to the edge amount, the threshold values are arranged in a counterclockwise spiral shape from the inside toward the outside in order from the smallest.
An image forming method. An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
  Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
  In the dither threshold value matrix corresponding to the edge amount, threshold values less than a predetermined value are arranged in a spiral shape from the inner side to the outer side in order from the smallest value, and threshold values greater than or equal to the predetermined value are arranged in a radial manner in order from the smallest. ,
An image forming method. An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
  Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
  In the dither threshold matrix according to the edge amount, threshold values less than a predetermined value are concentrated in the central portion, and threshold values greater than or equal to the predetermined value are radially distributed in order from the smallest to the peripheral portion,
An image forming method. An image forming method for forming an image by performing binary quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the binary quantization processing,
  Generating a threshold of a dither threshold matrix according to the edge amount of the multi-tone image data, using the threshold of the dither threshold matrix according to the edge amount as a quantization threshold of the binary quantization process;
  In the dither threshold value matrix according to the edge amount, a threshold value less than a predetermined value is arranged at the center so as to be preferentially arranged in the sub-scanning direction in ascending order.
An image forming method. An image forming method for forming an image by performing multi-value quantization processing by an error diffusion method on multi-tone image data and outputting dots according to the quantized data by the multi-value quantization processing,
  Generating a plurality of dither threshold matrix thresholds according to the edge amount of the multi-tone image data, using the threshold values of the plurality of dither threshold matrices according to the edge amount as quantization thresholds of the multi-value quantization processing;
  In each of the plurality of dither threshold matrixes according to the edge amount, the thresholds are arranged in a spiral shape from the inside to the outside in order from the smallest,
  Among the plurality of dither threshold matrices corresponding to the edge amount, a threshold increase step in a dither threshold matrix used as a high-density quantization threshold is used as a dither threshold matrix used as a low / medium density quantization threshold. Smaller than the threshold increase step in
An image forming method. An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
  Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
  An image processing apparatus, wherein a dither threshold value matrix corresponding to the edge amount is arranged in a spiral shape from the inner side to the outer side in order from the smallest. An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
  Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
  The binary quantization process is performed from the upper left to the lower right in the image space,
  An image processing apparatus, wherein a dither threshold value matrix corresponding to the edge amount is arranged in a clockwise spiral from the inner side to the outer side in order from the smallest. An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
  Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
  The binary quantization process is performed from the upper right to the lower left in the image space,
  An image processing apparatus comprising: a dither threshold matrix corresponding to the edge amount, wherein threshold values are arranged in a counterclockwise spiral shape from the inside toward the outside in order from the smallest. An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
  Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
  In the dither threshold value matrix corresponding to the edge amount, threshold values less than a predetermined value are arranged in a spiral shape from the inner side to the outer side in order from the smallest value, and threshold values greater than or equal to the predetermined value are arranged in a radial manner in order from the smallest. An image processing apparatus. An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
  Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
  In the dither threshold matrix corresponding to the edge amount, threshold values less than a predetermined value are concentratedly arranged in the central portion, and threshold values equal to or more than the predetermined value are arranged radially and distributed in order from the smallest to the peripheral portion. Image processing device. An image processing apparatus that performs binary quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
  Means for generating a threshold value of a dither threshold value matrix corresponding to the edge amount of the multi-tone image data, and using a threshold value of the dither threshold value matrix corresponding to the edge amount as a quantization threshold value of the binary quantization process;
  In the dither threshold value matrix corresponding to the edge amount, an image processing apparatus is arranged such that threshold values less than a predetermined value are arranged preferentially in the sub-scanning direction in order from the smallest in the center. Means for generating a threshold value of a dither threshold value matrix according to the edge amount,
  Edge detection means for detecting an edge amount from the multi-tone image data, quantizing the detected edge amount into a plurality of edge levels from a maximum edge degree to a non-edge, and output,
  Area expansion processing means for outputting, as an edge level for the target pixel, an edge level indicating the maximum edge degree among the edge levels output from the edge detection means for the target pixel and its surrounding pixels,
  A value obtained by adding a predetermined fixed value to a value obtained by multiplying a threshold value of a predetermined dither threshold value matrix stored in the storage unit by a coefficient corresponding to the edge level output from the region expansion processing unit, according to the edge amount. Output as the threshold of the dither threshold matrix
The image processing apparatus according to claim 8, wherein the image processing apparatus is an image processing apparatus. Means for generating a threshold value of a dither threshold value matrix according to the edge amount,
  Edge detection means for detecting an edge amount from the multi-tone image data, quantizing the detected edge amount into a plurality of edge levels from a maximum edge degree to a non-edge, and output,
  Area expansion processing means for outputting, as an edge level for the target pixel, an edge level indicating the maximum edge degree among the edge levels output from the edge detection means for the target pixel and its surrounding pixels,
  A dither threshold matrix selected from a plurality of predetermined dither threshold matrices stored in the storage unit based on the edge level output from the area expansion processing unit is used as a dither threshold matrix corresponding to the edge amount. Output as the threshold of
The image processing apparatus according to claim 8, wherein the image processing apparatus is an image processing apparatus. An image processing apparatus that performs multi-level quantization processing by an error diffusion method on multi-tone image data and outputs quantized data,
  Means for generating threshold values of a plurality of dither threshold matrixes corresponding to the edge amounts of the multi-tone image data, and quantizing the threshold values of the plurality of dither threshold matrixes corresponding to the edge amounts in the multi-level quantization processing; Used as a threshold,
  In each of the plurality of dither threshold matrixes according to the edge amount, the thresholds are arranged in a spiral shape from the inside to the outside in order from the smallest,
  Among the plurality of dither threshold matrices corresponding to the edge amount, a threshold increasing step in a dither threshold matrix used as a quantization threshold having a high threshold value An image processing apparatus characterized in that it is smaller than a threshold increase step in a dither threshold matrix used as a quantization threshold of the image. The image processing apparatus according to claim 16, wherein a high-density quantization threshold used for the multi-value quantization process is a fixed value regardless of an edge amount. Means for generating threshold values of a plurality of dither threshold value matrixes according to the edge amount,
  Edge detection means for detecting an edge amount from the multi-tone image data, quantizing the detected edge amount into a plurality of edge levels from a maximum edge degree to a non-edge, and output,
  Area expansion processing means for outputting, as an edge level for the target pixel, an edge level indicating the maximum edge degree among the edge levels output from the edge detection means for the target pixel and its surrounding pixels,
  A threshold of a dither threshold matrix selected from a plurality of predetermined dither threshold matrices stored in the storage means based on the edge level output from the area expansion processing means is converted into a plurality of dithers according to the edge amount. Output as threshold matrix thresholds,
The image processing apparatus according to claim 16 or 17, characterized in that: 19. A computer-readable storage medium in which a program for causing a computer to function as the image processing apparatus according to claim 8 is recorded.

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