JPH0533869B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an image processing method used in facsimile and the like, in which an image is divided into pixels and processed, and in particular, it is used to read and process pseudo-halftone images such as halftone photographs. The present invention relates to an image processing method for converting an image back into a binary image with pseudo halftone display using a systematic dither method. (Prior Art) A systematic dither method has been known as a method for expressing halftone images pseudo-binary. When this dither image processing method processes images that include periodic images such as lattice patterns or halftone photographs, moiré may occur due to interference between the period of the periodic image and the period of the dither matrix. This has the disadvantage that the image quality deteriorates due to this. As a method for reducing the occurrence of moire, there is a method disclosed in Japanese Patent Application Laid-Open No. 111471/1983. This method detects the average period of the input image from the image signal sequence of the input image signal, selects a dither matrix according to the average period, and reduces moiré even for input images with strong periodicity. This is the method of trying. Another method is disclosed in US Pat. No. 4,194,221. This method detects whether the input image is a periodic image or not. For periodic images, a low-pass filter is used to remove signals above a certain frequency in the image, and then the periodicity is removed. This is a method of reshading. (Problems to be Solved by the Invention) However, the method disclosed in Japanese Unexamined Patent Publication No. 59-11471 requires calculating the average period of the input image, and also Since the dither matrix pattern ROM and the like must be prepared in advance, there is a problem in that the hardware configuration becomes complicated. Furthermore, the dither matrix size, which should normally be determined based on the required number of gradations at the design stage of the image processing device, is changed depending on the cycle of the input image. There was a problem in that the number of gradations changed as the number of gradations increased. In addition, in the method disclosed in U.S. Pat. No. 4,194,221, the fundamental waves of multiple types of halftone dots and their harmonics are removed by one type of low-pass filter, which results in a large cutoff area. Put it away. 9th
The figure shows an example of blocking 3 lines/mm or more at the halftone dot frequency. If the cutoff area is large like this, for example, the resolution of the input image is 85 lines/inch (however, 1
An inch is approximately 2.54cm. The same low-pass filter is used to process both images obtained with a coarse mesh such as (the same applies hereafter) and images obtained with a fine mesh such as 150 lines/inch, so the image after this processing is a highly blurred image. There was a problem that it became . An object of the present invention is to solve the above-mentioned problems and provide an image processing method capable of obtaining an image with excellent gradation, less blur, and less noticeable moiré even when using the dither method. There is a particular thing. (Means for Solving the Problems) In order to achieve this object, according to the present invention, a halftone image is read pixel by pixel by a reading means, and an image signal obtained is subjected to pseudo-halftone conversion. In the processing method, a pixel position exhibiting an extreme value is detected from the multilevel image signal level of each pixel, and a pixel position exhibiting an extreme value is detected between the pixel position exhibiting an extreme value and another pixel position exhibiting an extreme value other than the pixel position. is determined as a processing section, the periodicity of the processing section is determined by comparing the length of the processing section with the length of other processing sections near the processing section, and the processing section is determined to have periodicity. If determined, calculate the average image signal level of each pixel within the processing section,
The average image signal level is used as the image signal level of each pixel in the processing section, each image signal at the average image signal level is used for pseudo halftoning processing, and when the processing section is determined to have no periodicity, It is characterized in that the image signal of each pixel in the processing section is used for pseudo halftone processing. In carrying out the present invention, it is preferable to detect the position of a pixel exhibiting an extreme value by changing the sign of the difference between the image signal levels of adjacent pixels. In implementing the present invention, it is preferable that the periodicity be determined by comparing the length of a processing section of interest with the length of a processing section in the vicinity of the processing section of interest. Furthermore, in implementing the present invention, it is preferable to remove noise contained in the image signal and then detect the pixel position exhibiting an extreme value. (Function) According to such an image processing method, the image signal level of an image portion having periodicity caused by the halftone dot period in the image is replaced with the average image signal level of the image signal of that image portion. and periodicity is reduced. Therefore, since the image signal appears to be one continuous image signal having the same image signal level, periodicity is reduced. At this time, to determine whether or not there is an image part with periodicity in the image, a processing section is determined based on the extreme value of the multilevel image signal level of each pixel, and the length of the processing section (number of pixels) is determined. ) and the length (number of pixels) of other processing sections in the vicinity of the processing section. This is because in the halftone image part, the length of the processing section (number of pixels) has a specific value depending on the halftone dot period (for example, number of lines/inch) used to create the image, and This is based on the fact that a processing section having the above-mentioned specific length is easily extracted from an image signal corresponding to a halftone image portion. In other words, this means that even when reading various images created using various halftone dots with different halftone dot periods (for example, number of lines/inch), according to the present invention, the periodicity is determined for each image. This means that periodicity can be automatically determined without having to prepare a special reference value for determination. Furthermore, since the periodicity determination is performed by comparing the lengths of processing sections, it is simpler than, for example, using autocorrelation. Furthermore, since periodicity is removed by replacing the image signals of each pixel within the processing section with the average value of these image signals, descreening (halftone dot removal) itself can be easily performed. In addition, since the processing interval is determined using a multivalued image signal, the synchronization of images created using halftone dots with a high number of lines (fine halftone dots) can be detected more accurately than when using a binary image signal. can. Furthermore, even if the periodicity is reduced, the image signal of a non-periodic portion such as an edge portion of the image portion is preserved. (Embodiments) Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that in these figures, the same components are designated by the same reference numerals. FIG. 1 is a block diagram showing an example of an image processing apparatus suitable for use in the image processing method of the present invention. 1st
In the figure, reference numeral 11 denotes a reading device as a reading means, which reads the image by decomposing the image into pixels at a pixel density of, for example, 16 lines/mm, and further quantizes the read image into multi-values and outputs the peak (peak) signal as a digital image signal 13. Value) position detection circuit 15 and line memory 17
Output to. The line memory 17 is configured to be able to store one line of images. Reference numeral 19 indicates a peak position memory in which the peak pixel position detected by the peak position detection circuit 15 is written. Reference numeral 21 denotes a first microprocessor (hereinafter also referred to as the first microcomputer 21), which reads out the image signal 13 from the line memory 17 and the signal 23 carrying the peak pixel position from the peak position memory 19, respectively.
Performs smoothing processing on the halftone dot image (details will be described later).
The signal 25 that has undergone the smoothing process is outputted to a pseudo halftoning means constituted by, for example, a dither processing circuit 27, and then recorded by a recording device 31. Next, the functions of the peak position detection circuit 15, an example of which is shown in a block diagram in FIG. 2, will be explained. One-pixel delay circuits 41 and 43, adders 45 and 47, multiplier 49 having coefficient A, and coefficient B
The digital image signal 13 output from the reading device 11 is processed by the one-dimensional transversal filter 53 in the main scanning direction, which includes a multiplier 51 having a
Remove noise. If the respective coefficients of the multipliers 49 and 51 are set to A=0.5 and B=0.25, the frequency characteristics of the filter 53 become the characteristics shown in FIG. 3, and high frequency components such as noise can be removed. Fourth
FIG. 4A shows the image signal waveform before processing by the filter 53, and FIG. 4B shows the image signal waveform after the image signal is processed by the filter 53. Furthermore, Figure 4A
and B, the horizontal axis represents the individual pixel, the vertical axis represents the image signal level, and the image signal level of each pixel is plotted. As is clear from comparing both figures, in the waveform of Fig. 4(A) there are a maximum value 81 caused by the dot period T and a maximum value 83 caused by noise etc. In the waveform shown in Figure B, the maximum value is 83 due to noise due to the filter 53.
It can be seen that has been removed. Therefore, it is possible to prevent detection of a maximum value other than the maximum value 81 due to the halftone period. The signal 55 from which the noise component has been removed is sent to a difference circuit 6 comprising a one-pixel delay circuit 57 and a subtracter 59.
1, the difference 63 between adjacent pixels is calculated and this value is output to the difference value memory 65. Reference numeral 69 denotes a second microprocessor (hereinafter also referred to as a second microcomputer), which reads the difference from the difference value memory 65. This second microcomputer 69 has functions as shown in the operation flowchart of FIG. The details of this function will be explained below using an example. In the embodiment described below, the main scanning direction refers to scanning from left to right of an image to be read, and the sub-scanning direction refers to scanning from top to bottom of the image. First, after reading the line to be read (line of interest), the leftmost pixel (leftmost pixel) of the line of interest is set as the pixel position corresponding to the first extreme value, and this position is set as the peak. It writes to the position memory 19 and initializes it (step 101). Next, the zero change identification flag F ₀ is reset to initialize the zero change identification flag F ₀ (step 102). Next, J is set as a parameter indicating a pixel position for detecting an extreme value from the image signal levels of individual pixels lined up in the main scanning direction, and J is set to 1 (step 103). Next, the J-th and J+1-th differences D(J) and D are stored in the difference value memory 65 (see FIG. 2).
(J+1) is read out (step 104). Find the product MD of consecutive differences from equation (1) below (step
105). MD=D(J)×D(J+1)...(1) Next, the extreme value is detected from this MD value. Detection of this extreme value is performed by determining the sign of the difference between the pixel signal levels of this pixel and the pixel one position to the left of this pixel in the main scanning direction, and the sign of the difference between the pixel signal levels before and after the pixel that is the extreme value, and The fact that the difference in sign is different from that of the pixel to the right in the scanning direction is utilized. If “YES” (MD<0) in step 106, the peak position memory 19 is set to pixel position J+1 as the peak position.
(Step 107). “NO” at step 106
In this case, in step 108, it is determined whether MD=0 or not. If MD=0, jump to step 121. MD=0 means that the difference is 0, that is, the image signal level of each adjacent pixel is the same level, so check at which position the difference is 0 using the zero change identification flag _F0 . (Step 121). When F ₀ =0, it means that the difference that was positive or negative while progressing in the main scanning direction becomes zero when MD=0. In other words, since this is the left-hand change point, the process jumps to step 122. Here, if the first difference is 0 (MD=D(J)×D(J+1)),
D(J)=0) is set to 0 without setting the zero change identification flag.
Leave as is. Furthermore, if the first difference is not 0 (“NO” in step 122), the zero change identification flag F ₀ is set to F ₀ =1 (step 123).
At the same time, the pixel position J+1 is stored in a register DL called a zero change difference side position register, and the difference D(J) is stored in a register IL called a pre-zero change left register (steps 123 and 124). Note that for the registers DL and IL, for example, registers of the first microcomputer may be used, or a work memory may be provided separately. Subsequently, the sign of the next difference is checked (steps 109, 110 and 104). Also, in step 121, "NO" (zero change identification flag
If F ₀ = 1), it is determined whether the difference = 0 continues (step 126), and if it is "YES", the sign of the next difference is subsequently checked (steps 109 _, 110).
and 104). If "NO" in step 126, it means that the state where the difference is 0, that is, the state where the image signal level is the same level, has ended, so the zero change identification flag is reset (step 127) and the difference is set to 0. It is determined whether the sign of each difference before and after becomes changed. This judgment is the difference before the difference = 0, in this case the difference stored in the register IL mentioned above, and the difference D (J + 1) after the difference = 0.
Judgment is made by the sign of the product MI. Here, if there is no sign change in the difference before and after the difference=0, MI>0. In this case, the extreme value has not yet appeared, so the sign of the difference is subsequently checked to find the extreme value (steps 128, 129,
109). Moreover, if there is a sign change in the difference before and after the difference=0, MI<0. In this case, it means an extreme value that has a continuous flat part at the same level of the image signal, so find the center value P of the extreme value that has a flat part from equation (2) below, and store that position in the peak position memory. 1
9 (steps 128, 129,
130). P=[(DL+J+1)/2]...(2) However, [ ] indicates a Gaussian symbol, and represents the largest integer that does not exceed the value in [ ]. Further, DL indicates the pixel position in the zero change left position register DL described above. Next, if MD>0 in step 108, this position is not an extreme value, so extreme values are sequentially searched for (steps 110 and 104). In addition, if it is determined in step 109 that the processing for one line has been completed (in the case of "YES"), the last peak position is written to a predetermined position in the peak position memory 19, one position to the right of the last pixel in that line. The processing for one line of the image is completed (steps 109 and 111). Hereinafter, with reference to FIG. 1 and FIG.
A method for performing image smoothing processing by the first microcomputer 21 using the image signal 13 read in the line memory 17 and stored in the line memory 17 will be described. First, let I be the parameter indicating the processing interval in which smoothing processing is performed before processing each row of the image, and I =
It is initialized to 1 (step 141). Next, from the peak position memory 19, the I-th and (I+1)
The th extreme value pixel positions P(I) and P(I+1) are read out (step 142), and the period between these extreme values is defined as a processing section. Next, the length D1 of this processing section is expressed as (3) below.
Determine from the formula (step 143). Here, when I=1, that is, when the processing section is between P1 and P2, from the line memory 17, from P(I)th to P(I
+1) - up to the first image signal (in this case, P1
(only the image signal) is read out, and this image signal is output to the dither processing circuit 27 (steps 145 and 146). If I≠1, a primary determination is made based on the following conditions as to whether or not the extreme value of the length D1 to P(I+1) of the processing section is an extreme value caused by the dot period. Attached Table 1 shows the halftone dots of 65 lines/inch to 150 lines/inch, which are usually used in the production of halftone photographs.
9 is a table showing the halftone dot periods L ₁ and L ₂ in the main scanning direction shown in FIGS. 8A and 8B in terms of the number of pixels when reading at a reading pixel density of 16 lines/mm and 16 lines/mm, respectively. The reading pixel density used in this example is 16
Since it is expressed as lines/mm, as is clear from Attached Table 1, the period at which the maximum value of the read halftone dots appears is the diagonal halftone dot of 45 degrees (see Figure 8B, details will be described later).
It can be seen that when the halftone dot is 65 lines/mm, it is the longest, and the period is about 9 pixels. Further, the interval between the local minimum value existing between the local maximum values and the local maximum value is about five pixels, which is half the period between the local maximum values. Therefore, since the interval between extreme values due to the halftone dot period is approximately five pixels at the longest, extreme values that appear at a longer period are not considered to be extreme values due to the halftone dot period.
In this embodiment, in consideration of errors, if D1 is longer than eight pixels, it is determined that the processing section corresponding to D1 has no periodicity, and the process jumps to step 145 (step 147). If D1≦8 in step 147, the length of the processing section in the vicinity of the processing section currently being smoothed (the processing section of interest), for example, the length of the previous processing section in the main scanning direction and the length of the processing section of interest. The absolute value of the difference AD is calculated from the following equation (4) (step 148). AD=|D1−D2| ...(4) If the value of AD is 0 or 1, the length of the processing section of interest is almost equal to the length of the processing section immediately before the processing section of interest, so It is determined that the image in the processing section of interest is periodic, and the next processing is performed (steps 149 and 150). If AD≧2, it is determined that there is no periodicity and the next process is performed (step 149,
145). The following processing is performed for the processing section determined to have periodicity. First, from line memory 17,
Read out image signals from P(I)th to P(I+1)-1 (step 150), then read out image signals from P(I) to P(I+1).
1) Average value S of the pixel signal level of each pixel between -1
is calculated from equation (5) below (step 151). S= _P(I+1)-1 〓 ^i=P (I)X(i)/{P(I+1)−P(I)} ...(5) However, X(i) is the i-th pixel Indicates the image signal level. Next, the pixel signal level of the previous pixel from P(I) to P(I+1)-1 in the processing section determined to have periodicity is set to the average value S, and then output to the dither processing circuit 27. (Step 152). In addition, for a processing section determined to have no periodicity, the image signals from P(I)th to P(I+1)-1th corresponding to that processing section are read out from the line memory 17, and this image signal is dithered. Processing circuit 27
(steps 145, 146). Next, the length D2 of the processing section of interest is set to D1 (step 153). Next, it is determined whether the smoothing process for one line of the image has been completed (step 154). If it has not been completed, the processing section is advanced in the main scanning direction and smoothing processing is performed cyclically (steps 155, 142).
Furthermore, when the smoothing process for one line is completed, the smoothing process for the next line is performed in the same way (step 141).
~). The image signal 25 in which the periodic part of the image signal is made into a smoothed image signal by the image processing method of the present invention, and the other part is made into the image signal as read from the line memory 17 is shown in FIG. The waveform will be as shown. This image signal 25 is input to a dither processing circuit 27 and processed by, for example, a systematic dither method, and a binary signal 29 is output to a recording device 31 and recorded. Since this dither processing and recording processing can be performed by a commonly used method, a description thereof will be omitted. FIGS. 8A and 8B are diagrams for explaining halftone dots. The 90 degree halftone dot described in the angle section of Attached Table 1 means the halftone dot shown in FIG. 8A, and the halftone period of this halftone dot in the main scanning direction is the distance indicated by L ₁ in the figure. Further, the 45 degree halftone dot means the halftone dot shown in FIG. 8B, and the halftone dot period of this halftone dot in the main scanning direction is the distance indicated by _L2 in the figure. Note that the peak position detection circuit, the algorithm and means for performing the smoothing process, etc. described in the above-mentioned embodiment are not limited to this embodiment, and can be changed depending on the content of the required image processing. Furthermore, the above-described embodiment detects the pixel position where the image signal level exhibits a maximum value from the input image signal, and detects the position of the pixel with the maximum value of interest in the input image signal and the next pixel that appears after this pixel in the main scanning direction. An example in which the processing interval is defined as the area between the maximum value pixel position and the maximum value pixel position has been described.
However, the present invention is not limited to this embodiment, and the processing interval may be defined as a processing interval between a pixel position exhibiting a minimum value and a pixel position exhibiting a minimum value that appears next in the main scanning direction from this pixel. Good and
Alternatively, the processing section may be set between a pixel position exhibiting three maximum values and a pixel position exhibiting a minimum value that appears next in the main scanning direction from this pixel. Furthermore, the pixel position of the extreme value of interest (maximum value or minimum value),
The processing section may be defined as a processing section between the pixel position and other extreme value pixel positions, for example, between the position of the maximum value pixel of interest and the nth maximum value pixel position proceeding from that pixel in the main scanning direction. (Effects of the Invention) As is clear from the above description, according to the image processing method of the present invention, the period of the input image is determined by detecting the polar region of the multilevel image signal level.
Therefore, the period can be determined with a simpler configuration than when using autocorrelation as in the conventional case. Furthermore, compared to the case where the period is determined using a binary image signal, the period of an image created using halftone dots (fine halftone dots) having a different number of lines can be detected with higher accuracy. Also, by using this extreme value and setting the area between the extreme value and other extreme values as an image processing interval, and comparing the length of that processing interval with the length of the neighboring processing interval,
It is determined whether the image within the processing section has periodicity. Therefore, the period of halftone dots (for example, the number of lines/
Even when reading various images created using various halftone dots with different sizes (inches), periodicity can be automatically determined without specially preparing a reference value for determining periodicity for each image. Furthermore, since the image signal of an image part with periodicity caused by the halftone period in the image is replaced with the average value of the image signal of that image part, the periodicity can be easily reduced, and the edge of the image can be easily reduced. By not averaging non-periodic parts such as parts, the resolution is preserved as is, and the image signal can be binarized by dither processing or the like. Therefore, even when an image including a periodic image such as a lattice pattern or halftone photograph is processed, moiré can be prevented from occurring. Therefore, it is possible to provide an image processing method that can obtain an image with excellent gradation, less blur, and less noticeable moiré even when using the dither method. 【table】

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of an image processing device suitable for use in the image processing method of the present invention, and FIG. 2 is a block diagram showing an example of a peak position detection circuit suitable for use in the image processing method of the present invention. 3, 4A and 4B are diagrams for explaining the image processing method of the present invention, FIG. 5 is an operation flowchart for explaining the functions of the second microcomputer included in the peak position detection circuit, and FIG. The figure is an operation flowchart for explaining the functions of the first microcomputer included in the image processing device, FIG. 7 is a waveform diagram showing the image signal obtained by the image processing method of the present invention and before dither processing, and FIG. 8 is a waveform diagram showing the image signal before dither processing. FIG. 3 is a diagram for explaining halftone dots. FIG. 9 is an explanatory diagram of the prior art. 11...Reading device, 13...Image signal, 15
... Peak position detection circuit, 17 ... Line memory, 19 ... Peak position memory, 21 ... First microprocessor, 23 ... Signal carrying extreme value pixel position, 25 ... Smoothing processing signal, 27 ... Dither processing circuit, 29 ... Binarized signal, 31 ... Recording device, 41, 43, 57 ... One pixel delay circuit,
45, 47... Adder, 49, 51... Multiplier,
53...One-dimensional transversal filter, 69
... second microprocessor, 81 ... maximum value due to halftone period, 83 ... maximum value due to noise.

Claims (1)

[Claims] 1. An image processing method that performs pseudo-halftoning on an image signal obtained by reading a halftone image pixel by pixel with a reading means, in which an extreme value is determined from the multi-valued image signal level of each pixel. detecting pixel positions exhibiting an extreme value, defining a processing interval between the pixel position exhibiting an extreme value and another pixel position exhibiting an extreme value other than the pixel position, and determining the periodicity of the processing interval as the processing interval. The determination is made by comparing the length of the processing interval with the length of other processing intervals in the vicinity of the processing interval, and if the processing interval is determined to have periodicity, the average image signal level of each pixel within the processing interval is determined. seek,
The average image signal level is used as the image signal level of each pixel in the processing section, each image signal at the average image signal level is used for pseudo halftoning processing, and when the processing section is determined to have no periodicity, An image processing method characterized in that an image signal of each pixel in a processing section is used for pseudo halftone processing. 2. The image processing method according to claim 1, wherein the detection of a pixel position exhibiting an extreme value is performed by changing the sign of a difference between image signal levels of adjacent pixels. 3. The image processing method according to claim 1, wherein the pixel position exhibiting an extreme value is detected after noise contained in the image signal is removed.