JPH0974483A - Image processing method and its device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the image processing unit and its method in which a dot image can be generated without the use of a memory of a large capacity. SOLUTION: A screen conversion section 202 specified a location in a prescribed block as to plural picture elements forming one dot of a dot image by a row address counter 204 and a column address counter 205 and obtains an offset in a register table 207, based on the specified position. Then a characteristic of a picture element is identified from a register table 206 based on the offset and density conversion information is obtained from a register table 208, based on the identified characteristic, to generate the dot image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus and a method thereof, and more particularly, to an image processing apparatus such as a digital copying machine for generating a halftone image from a multi-valued image and a method thereof.

[0002]

2. Description of the Related Art In recent years, various output systems of images have been devised in digital office equipment.

In a conventional digital copying machine or the like, there is a PWM (pulse width modulation) method for expressing gradation on a pixel-by-pixel basis,
There is a halftone dot process for expressing a gradation of one dot using a plurality of pixels. The halftone dot processing is used especially when importance is attached to the stability of gradation and the preservation of gradation after copying.

FIG. 6 shows an example of dots in halftone dot processing. In FIG. 6, each cell represents one pixel.
6A shows an example of gradation expression with 8 dots per dot, and FIG. 6B shows an example of gradation expression with 13 pixels per dot.
The numbers in each square represent the order numbers for filling the densities, that is, the gradations are expressed by, for example, black pixels in order of the numbers according to the densities.

In each graph of FIG. 7, the graph of FIG.
The state of density conversion in the dot example shown in FIG. In FIG. 7, the number shown on the left side of each graph is the number of each pixel shown in FIG. 6B, and the horizontal axis shows the input 1
The density value of the dot, and the vertical axis represents the density value of the pixel output as a result of the halftone dot processing. For example, the uppermost graph of FIG. 7 shows “0” for the input density in (b) of FIG.
The output densities of the pixels indicated by No. According to FIG. 7, it can be seen that each pixel is binarized by the threshold value for each pixel.

Similarly, the graph of FIG. 8 shows the state of density conversion by halftone in the dot example shown in FIG. According to FIG. 8, each pixel has two thresholds, an upper limit and a lower limit. When the input density is above the upper limit, it is represented by black, when it is below the lower limit, it is represented by white, and when it is between the upper and lower limits, halftone is used. As a result, the number of gradations that can be represented by halftone dots is increased.

Next, with reference to FIG. 9, halftone dot processing by dot arrangement will be described.

9 (a) and 9 (b) are the same as FIG. 6 described above.
1 dot of 8 pixel expression shown in (a) of
An example in which different arrangements are made at 5 degrees and about 72 degrees is shown. In FIGS. 9A and 9B, the squares of 4 × 4 pixels and 8 × 8 pixels surrounded by thick lines are used as a unit, and the pixel arrangement has periodicity.

In the conventional image processing apparatus, a structure for converting a multivalued image into a halftone image is realized by utilizing the periodicity as shown in FIG. FIG. 10 is a block diagram showing a configuration example for generating a halftone image in a conventional image processing apparatus.

In FIG. 10, reference numeral 51 is an input terminal for inputting an original image, 52 is a row address counter for counting addresses in the row direction, 53 is a column address counter for counting addresses in the column direction, and 54 is halftone dot processing. A screen conversion unit, 55 is a memory, 56 is a register table, and 57 is an output terminal for outputting a halftone image.

For example, if the dot arrangement shown in FIG. 9A is handled, the 4 × 4 mask shown in FIG. 11A is stored in the memory 55 in advance. The row address counter 52 and the column address counter 53 are controlled by a CPU (not shown) for each counter value so as to indicate the position of the pixel of interest within the square when the entire image is divided into square units of 4 × 4 pixels. To be done. Row address counter 5
2 counts lines, and column address counter 53
Counts the pixels in one line. The row address counter 52 is reset at the beginning of the image and every four lines, and the column address counter 53 is reset at the beginning of the line and every four pixels.

The pixel number within one dot of the pixel currently being processed is the pixel number of the pixel indicated by each counter in the 4 × 4 mask ((a) of FIG. 11) stored in advance in the memory 55. It is obtained by retrieving the position value corresponding to the position. For example, FIG. 11B
As shown in FIG. 11, the pixel indicated by the address (2, 1) marked with a circle corresponds to the position indicating “3” in (a) of FIG. 11, so that the pixel number is “3”. I understand.

The register table 56 stores in advance a threshold value relating to the density corresponding to each pixel number in the dot, which gives the above-mentioned input / output density conversion characteristic shown in FIG. The screen conversion unit 54 determines the density of each pixel based on the density value of the input image and the threshold value stored in the register table 56 to generate a halftone image.

As described above, in the conventional image processing apparatus, information on the density of each pixel is extracted from the rectangular mask data, and the density of each pixel is converted based on the information to output a halftone dot image. Was.

[0015]

However, in the above-mentioned conventional example, since one dot of the halftone dot is expressed by 8 pixels or 13 pixels, the dot size is large, and therefore the pixel number within 1 dot is The size of the mask for obtaining the mask also becomes large, and the memory capacity for storing the mask also becomes large. For this reason, the hardware configuration for halftone dot conversion has been increased to increase the number of internal registers such as an IC, or an external memory has been required.

On the other hand, if the dot size of the halftone dot is increased, the gradation is improved, and if it is decreased, the resolution is improved. Therefore, conventionally, there has been a demand for changing the dot size according to the image. However, in the conventional halftone dot processing, it is difficult to rewrite the mask data and to save a plurality of sets of data for rewriting because of the large capacity of the mask.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an image processing apparatus and method capable of generating a halftone image without using a large capacity memory. And

It is another object of the present invention to provide an image processing apparatus and method capable of generating a halftone dot image with a variable dot size.

[0019]

In order to achieve the above-mentioned object, the present invention has the following arrangement.

That is, an image processing apparatus for generating a halftone image by inputting a multi-valued image and constructing one dot by a plurality of pixels of the multi-valued image, according to the arrangement of the dots of the target pixel. Identifying means for identifying the position in the periodic pattern obtained by the above, pixel identifying means for identifying the characteristics of the pixel at the location identified by the identifying means based on a plurality of tables, and identified by the pixel identifying means. A density value determining unit that determines the density value of the pixel of interest by referring to the density conversion information according to the characteristics of the pixel.

Further, first holding means for holding a first table holding the characteristics of all the pixels in the pattern, and among the pixels in the first table, at the positions specified by the specifying means. A second holding a second table for determining an offset for identifying a pixel
And a holding means for storing the characteristic of the pixel of interest based on the first table and the second table.

Further, there is provided third holding means for holding the density conversion information as a third table, and the density value determining means refers to the third table based on the characteristics of the pixel. The density value of the pixel of interest is determined.

For example, the third table is characterized in that the threshold of the density level is stored according to the position of each pixel forming the dot.

For example, the third table stores a plurality of density level threshold values for each position of each pixel forming a dot, and the density value determining means determines the density value of the pixel of interest as a multivalue. It is characterized by

For example, the first to third tables are rewritable.

Further, there is provided size setting means for setting the number of pixels constituting one dot of the halftone dot image, and the first to third
The holding means holds the first to third tables by the number corresponding to the kind of the number of pixels that can be set by the size setting means.

For example, the size setting means includes means for increasing the number of pixels forming one dot of a halftone image and means for decreasing the number of pixels.

Further, it has array setting means for setting the type of dot array of the halftone image, and the first to third holding means can set the first to third tables by the array setting means. It is characterized by holding according to the number of different dot arrangements.

With the above-mentioned structure, it is possible to obtain a peculiar function and effect that the density conversion characteristic of each pixel forming one dot of a halftone image can be determined by referring to a small capacity table.

Further, there is a peculiar effect that the number of pixels constituting one dot of a halftone dot image or the dot arrangement is set, the table is selected according to the setting, and an appropriate halftone dot image can be generated. can get.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an image processing configuration in a digital copying machine to which this embodiment is applied. FIG.
1, 101 is a CCD, 102 is a sample / hold (S / H) unit, 103 is an A / D conversion unit, 104 is a correction unit, 105 to 107 are image processing units, 108 is a D / A conversion unit, and 109 is a pulse. A width modulation (PWM) unit 110 is a laser unit.

A document placed on a document table (not shown) is illuminated by a halogen lamp, and the reflected light is photoelectrically converted by the line type CCD 101 via an optical system to be read. At this time, the halogen lamp is scanned in the direction perpendicular to the longitudinal direction of the original, and the original image is sequentially read by the CCD 101 line by line.

The image signal input from the CCD 101 is sampled / held by the S / H unit 102, and A / H
The D-converter 103 converts the sampled and held signal into an 8-bit digital signal. Next, the correction unit 104 corrects variations in gain and offset for each pixel due to the halogen lamp, the optical system, sensitivity unevenness in the CCD 101, and the like for the digital signal.

The output signal of the correction unit 104 is the image processing unit A
105 is input. In the image processing unit A105,
For digital signals after correction, scaling, negative / positive inversion,
In addition to movement, known image editing processing such as trimming, masking, shading, shadowing, contour extraction, gradation, etc. is performed.

The output signal of the image processing unit A105 is input to the image processing unit B106 which is a feature of this embodiment. The image processing unit B106 converts the input multivalued image into a halftone dot image and outputs it. The halftone image generation processing in the image processing unit B106 will be described later. Image processing unit C1
In 07, the output signal of the image processing unit B106 is subjected to non-linear processing such as logarithmic conversion and output. Image processing unit C107
The digital signal output from is converted into an analog signal by the D / A converter 108, pulse width modulated by the PWM unit 109, and then supplied to the laser unit 110. In the laser section 110, a so-called electrophotographic process irradiates a photosensitive drum (not shown) with laser light to generate a latent image, and after development, the latent image is transferred to a recording medium such as recording paper.

The halftone image generation processing in the image processing unit B106 described above will be described in detail below.

In the present embodiment, the dot size of the halftone dot is input by the operator from the operation unit (not shown). Of course, the operator may select a desired size from a plurality of dot sizes prepared in advance. The input dot size information is stored in the CPU (not shown).
Is transmitted to the image processing unit B106 via.

FIG. 2 shows an example of the operation unit for inputting the dot size in this embodiment. FIG. 2A shows a display example of the dot size selection on the touch panel. When the right arrow portion is touched, the dot size of the halftone dot is set large,
Touch the left arrow to set it smaller. In this embodiment, the touch panel shown in FIG. 2A also serves as an operation unit for selecting density. As a result, the amount of display memory used by the CPU for the touch panel can be saved.

A display unit such as an LCD is provided on the upper portion of the touch panel shown in FIG. 2A. For example, "normal" near the center of the display unit, "fine" at the right end, "coarse" at the left end, etc. Characters may be displayed. Further, in the present embodiment, the dot size selection is described as being operated by the touch panel, but of course it may be selected by the operation key. Alternatively, the selection may be performed by using a pointing device such as a mouse or a trackball, for example, by clicking an arrow portion shown in FIG.
Further, a plurality of operation means may be combined and selectable, such as operation with either a touch panel or a track ball. Further, as shown in FIG. 2B, for example, a switch (a portion indicated by “A”) for automatically measuring dots of a document image in an apparatus and automatically selecting a dot size of halftone dots to be output. ) May be provided together with other selection means.

As described above, since the operator can select the dot size with the two operation keys, the cost and operability of the entire apparatus can be reduced.

Next, the detailed configuration of the image processing unit B106 will be described.
This is shown in the block diagram of FIG. In FIG. 3, 201 is an input terminal for inputting an original image, 204 is a row address counter for counting addresses in the row direction, 205 is a column address counter for counting addresses in the column direction, and 202.
Is a screen conversion unit that performs halftone dot processing, 206 to 208 are register tables, and 203 is an output terminal that outputs a halftone dot image.

In comparison with the configuration of FIG. 10 shown in the above-mentioned conventional example, FIG. 3 does not include a memory that stores a dot mask for generating a halftone image, and instead has a plurality of register tables. It is characterized by including.

The row address counter 204 counts the lines of the input image, and the column address counter 205 counts the pixels in one line. Further, the row address counter 204 is reset for each line number set as the beginning of an image, and the column address counter 205 is reset for each pixel number set as the beginning of a line. The set value of the reset cycle of each counter is given by a CPU (not shown) based on the information from the touch panel described above. For example, from the touch panel shown in FIG.
When 8 pixels are set as the dot size of the halftone dot, this means that the dot shape of 1 dot 8 pixel expression shown in (a) of FIG. 6 is selected. The dot shape selected in this embodiment is not limited to the example shown in FIG. 6 as a matter of course, and may be another shape such as a rectangular dot of 2 × 4 pixels.
Further, not only the dot size but also the dot shape may be edited or selected.

In the dot shape shown in FIG. 6A,
As described in the above-mentioned conventional example, (a) of FIG.
As shown in (b), there are a plurality of different dot arrangement methods. In the present embodiment, the arrangement method closest to 45 degrees is selected as the default value, but the arrangement method at other angles can also be selected by an operation on the touch panel shown in FIG. In this case, the display on the touch panel is the same as that shown in FIG. 2A, but for example, "45 degrees" near the center of the LCD, "90 degrees" at the right end, and "0 degrees" at the left end.
It is also possible to display the characters and set it in the range of 0 to 90 degrees. Then, if there is no dot arrangement method suitable for the specified angle, the closest arrangement is selected.

Hereinafter, description will be given assuming that the dot arrangement method shown in FIG. 9A is selected. First, a CPU (not shown) sets "4", which is the number of pixels in the vertical and horizontal directions of the mask, as the set value of the reset period for the row address counter 204 and the column address counter 205, respectively. The two counter values represent the position within the square mask. For example, when the value of the column address counter 204 is "2" and the value of the row address counter is "1", the position of the pixel in the mask is expressed as (2,1), and the position is shown in FIG. It is indicated by a circle in (b) of.

Register tables A to C of 206 to 208
Is the first, second, third corresponding to the dot information of the halftone dot.
Table is stored.

In the pattern in the square surrounded by the thick frame in FIG. 9A, the numbers in the first row and the rows in the second row are regularly shifted to form the third row, respectively. It is equal to the number sequence in the fourth row. In this case, the first row and the second row of the number sequences are connected according to a predetermined rule to form a first table, and the amount by which the first row number sequence in each row is shifted is set as the second table. 4A and 4B show examples of the first and second tables stored in the register tables A206 and B207, respectively.

Here, consider the case where the pixel position in the mask is (m, n).

The value of the row address counter 204 is n and the counter is 0 origin, which means that the pixel currently being processed is in the (n + 1) th row. Therefore, first, from the second table shown in FIG.
The 1) th data T2 is taken out. And T2 is the first
Column address counter 2 as table offset
(T2 + m + 1) th data T added to the value of 05
Get one.

For example, if the pixel position is (2,1),
First, the second data “6” is taken out as T2 from the second table shown in FIG. The column address counter 205 uses this value as the offset of the first table.
The ninth data "3" is obtained by adding to the value "2". This value is the pixel number in the dot mask shown in FIG. 6A, and the information regarding the density conversion of the corresponding pixel is retrieved from the register table C208. The register table C208 stores a threshold value for the density corresponding to each pixel number in the dot mask, and the screen conversion unit 202 binarizes each pixel based on the threshold value. , Halftone conversion is performed.

Next, the case where the dot arrangement method shown in FIG. 9B is selected will be described.

In the square pattern surrounded by a thick frame in FIG. 9B, it is necessary to shift the number sequence in the first row to the left or to the right, and move the number at the left end to the right end or vice versa. By repeating, it matches the sequence of each other row. That is, each table is set with the first row number sequence as the first table and the amount by which the first row number sequence in each row is shifted as the second table. As a result, the data shown in FIGS. 5A and 5B are written in the first and second tables, respectively. Note that the procedure for obtaining the pixel number in the dot mask is the same as in the above case, so description will be omitted.

As described above, the multi-valued image input from the input terminal 201 in FIG.
According to the procedure described above in 02, halftone dot conversion is performed based on the threshold value information regarding density conversion extracted for each pixel, and output from the output terminal 203.

As described above, according to the present embodiment, the dot mask for generating the halftone image is not provided and only the first and second tables are provided in the register. Can be obtained. First and second
Since the capacity of the table is much smaller than the capacity required for the conventional dot mask, the memory capacity can be reduced according to this embodiment.

In the present embodiment, an example in which each pixel of the output halftone dot image is represented by a binary value has been described. However, the present embodiment is not limited to this example. For example, as shown in FIG. 7B in the above-mentioned conventional example, each pixel has two thresholds, an upper limit and a lower limit, and if the upper limit is black, It is also possible to perform pixel expression using halftones, such as white if the lower limit or less, and halftone between the upper and lower limits. Of course, the number of thresholds for each pixel may be two or more. In addition, in FIG.
Although only one pixel is expressed in halftone in a dot, a plurality of pixels may be simultaneously expressed in halftone in consideration of the conditions of the output system.

Although the present embodiment has been described by taking a digital copying machine as an example, the method of generating a halftone image in this embodiment may be a system such as a facsimile or a scanner device which expresses pixels in multi-value. For example, the halftone dot processing of the present invention can be applied.

The present invention may be applied to either a system composed of a plurality of devices or an apparatus composed of a single device. Needless to say, the present invention can be applied to a case where the present invention is achieved by supplying a program to a system or an apparatus. In this case, the storage medium storing the program according to the present invention constitutes the present invention. Then, by reading the program from the storage medium to the system or device, the system or device operates in a predetermined manner.

[0059]

As described above, according to the present invention, even if the dot size of a halftone dot is large, it is possible to convert to a halftone dot image by using a conversion table having a small capacity, so that the device configuration is simplified. It is possible to reduce the cost while reducing the cost.

Further, when it is desired to make the dot size variable according to the image, the conversion table can have a small capacity.
It becomes easy to save the conversion table according to the dot size and rewrite it. Therefore, it is possible to realize a system in which the operator can select a dot size of a halftone dot suitable for an image and output an appropriate halftone dot image with a simple configuration.

[0061]

[Brief description of drawings]

FIG. 1 is a block diagram showing an image processing configuration of a digital copying machine according to an embodiment of the present invention.

FIG. 2 is a diagram showing a display example of an operation unit in the present embodiment.

FIG. 3 is a block diagram showing a detailed configuration of an image processing unit B in this embodiment.

FIG. 4 is a diagram showing an example of a conversion table in the present embodiment.

FIG. 5 is a diagram showing an example of a conversion table in this embodiment.

FIG. 6 is a diagram showing a pixel group that represents one dot in conventional halftone dot processing.

FIG. 7 is a graph showing an example of density conversion of each pixel in conventional halftone dot processing.

FIG. 8 is a graph showing an example of density conversion of each pixel in conventional halftone dot processing.

FIG. 9 is a diagram showing an example of dot distribution values in conventional halftone dot processing.

FIG. 10 is a block diagram showing a configuration for performing conventional halftone dot processing.

FIG. 11 is a diagram showing an example of a dot mask in conventional halftone dot processing.

[Explanation of symbols]

 101 CCD 102 Sample / Hold Unit 103 A / D Converter 104 Shading Correction Unit 105, 106, 107 Image Processing Unit 108 D / A Converter 109 PWM Modulator 110 Laser Unit 201 Input Terminal 202 Screen Conversion 203 Output Terminal 204, 205 Counter 206, 207, 208 register

Claims (17)

[Claims] 1. An image processing apparatus for generating a halftone dot image by inputting a multi-valued image and constructing one dot by a plurality of pixels of the multi-valued image, the image processing device according to an arrangement of the dots of a target pixel. Identifying means for identifying the position in the periodic pattern obtained by the above, pixel identifying means for identifying the characteristics of the pixel at the location identified by the identifying means based on a plurality of tables, and identified by the pixel identifying means. An image processing apparatus comprising: a density value determining unit that determines the density value of the pixel of interest by referring to the density conversion information according to the characteristics of the pixel. 2. A first holding unit that holds a first table that holds the characteristics of all the pixels in the pattern; and a pixel specified by the specifying unit among the pixels in the first table. Second holding means for holding a second table for determining an offset for specifying a pixel at a position, wherein the pixel identifying means includes the first table and the second table.
The image processing apparatus according to claim 1, wherein the characteristic of the pixel of interest is identified based on the table. 3. A third holding unit that holds the density conversion information as a third table, wherein the density value determining unit refers to the third table based on the characteristics of the pixel. The image processing apparatus according to claim 2, wherein the density value of the pixel of interest is determined by this. 4. The image processing apparatus according to claim 3, wherein the third table stores a density level threshold value according to a position of each pixel forming a dot. 5. The third table stores a plurality of density level threshold values for each position of each pixel forming a dot, and the density value determination means determines the density value of the pixel of interest as a multivalued value. The image processing apparatus according to claim 4, wherein 6. The image processing apparatus according to claim 3, wherein the first to third tables are rewritable. 7. A size setting means for setting the number of pixels forming one dot of a halftone image, wherein the first to third holding means respectively store the first to third tables. 4. The image processing apparatus according to claim 3, wherein the number of pixels is held by the size setting unit according to the number of pixels that can be set. 8. The image processing apparatus according to claim 7, wherein the size setting means includes means for increasing the number of pixels forming one dot of a halftone image and means for decreasing the number of pixels. 9. The apparatus further comprises array setting means for setting the type of dot array of the halftone image, wherein the first to third holding means stores the first to third tables by the array setting means. The image processing apparatus according to claim 3, wherein the image is held according to the number of dot arrays that can be set. 10. An image processing method in an image processing apparatus for generating a halftone image by inputting a multi-valued image and forming one dot by a plurality of pixels of the multi-valued image, wherein the dot of the pixel of interest is The position in the periodic pattern obtained according to the array of is identified, the characteristics of the pixel at the specified position are identified based on a plurality of tables, and the density conversion information is referred to by the characteristics of the identified pixel. Then
An image processing method, characterized in that the density value of the pixel of interest is determined. 11. A first table holding characteristics of all pixels in the pattern, and an offset for specifying a pixel at a position specified by the specifying means among pixels in the first table. 11. The characteristic of the pixel of interest is identified based on the determined second table.
The described image processing method. 12. The density conversion information is held as a third table, and the density value of the pixel of interest is determined by referring to the third table based on the characteristics of the pixel. The image processing method according to claim 11. 13. The image processing method according to claim 12, wherein the third table stores a threshold value of a density level according to a position of each pixel forming a dot. 14. The third table stores a plurality of density level threshold values for each position of each pixel forming a dot, and determines the density value of the pixel of interest as a multivalue. 13. The image processing method according to item 13. 15. The image processing method according to claim 12, wherein the first to third tables are rewritable. 16. The method according to claim 12, further comprising setting the number of pixels forming one dot of a halftone dot image and selecting the first to third tables according to the settable number of pixels. The described image processing method. 17. The image according to claim 12, further comprising setting a type of dot arrangement of the halftone dot image and selecting the first to third tables according to the settable dot arrangement. Processing method.