CN100514206C - Image forming method, screen set and image forming apparatus - Google Patents

Abstract

An image forming apparatus for forming a color image of CMK color plates by electrophotography system in which a K color plate having highest visibility among CMYK colors is composed of a rational number mesh. Profile of halftone dots (201Kh) becomes constant substantially and the factor for generating streak and unevenness is suppressed. Furthermore, C color plate and M color plate are formed of super cells and the K color plate, C color plate and M color plate are formed at an interval of 30 DEG . Consequently, a Rosetta formed of halftone dots (201Ch, 201Mh, 201Kh) of CMK is optimized and the factor for generating moire component is suppressed.

Description

Image forming method, screen group and image processing system

Technical field

The present invention relates to be applicable to image forming method, screen group and the image processing system of formation based on the image processing system of the coloured image of AM screen, duplicating machine or the printer etc. of described AM screen by the electrofax mode combines C (green grass or young crops) M (magenta) Y (Huang) K (deceiving) version of all kinds.

Background technology

Usually, in handle the site of color electronography mode, generate site threshold matrix (abbreviating threshold matrix as) ([0015] section of TOHKEMY 2002-112047 communique) in the little rational number screen mode of site cell size.

In the AM screen, but when for example having the pixel of 8 * 8,16 * 16 melanism (colorization), unit, site (Ha-Off ト-Application セ Le)) size is meant this size of 8 * 8,16 * 16, corresponding each pixel arranged threshold value threshold matrix size cable angle (

Angle, screenangle) equate with this site cell size when being 0 °.And, in the unit, site, the part of melanism (colorization) is called site (Ha-Off ト-Application De ッ ト).

In the rational number screen, unit, site (size) is identical.In addition, when the continuous grayscale image of uniform density relatively and threshold matrix generated the site, in each unit, site, the size of site was identical with shape.In this instructions, the rational number screen is meant with natural number m, n and uses the inverse function of trigonometric function (to be designated as arctangent; When arctan) cable angle being made as θ=arctan (m/n), the size of threshold matrix satisfies m ²+ n ²Positive integer grid doubly.

But, in the rational number screen, because site cell size little (size of threshold matrix is also little), so the net that can't carry out being considered in hectographic printing is spent (ロ ゼ _StarOptimization ) promptly can't correctly be set at 15 °, 45 °, 75 ° with 30 ° interval with CMK version of all kinds, thereby moire can take place.

For CMK correctly being set at 30 ° of intervals, can considering without rational number screen mode but form the site in big or small big ultra-fine born of the same parents (ス-パ-セ Le) mode of threshold matrix.

Summary of the invention

But, in the image processing system of electrofax mode, when in each look version of CMK, adopting the site of ultra-fine born of the same parents' mode, there is following problem as can be known, that is: in ultra-fine born of the same parents, the big young pathbreaker of each unit, site becomes inhomogeneous, and, when the Continuous Gray Scale image of uniform density relatively and threshold matrix generate the site, the size of site and the shape of site are also inconsistent, during the toner that therefore can take place to use in the electrofax mode and when adhering to and inadhering situation, and cause on coloured image, producing striped and inhomogeneous thus.

In addition, in the image processing system of electrofax mode also as can be known owing to use as particle size as big Powdered toner produced moire, be so-called piling height moire (pile heightmoire) ([0021] of above-mentioned TOHKEMY 2002-112047 communique section).

The present invention considers above-mentioned problem and makes, its purpose is, provides can be in the image processing system of electrofax mode the generation reason of moire composition and striped and uneven generation reason to be suppressed to minimal image forming method, screen group and image processing system.

The image forming method that the present invention relates to is a kind of image forming method that forms coloured image by the electrofax mode at least with CMK look version, it is characterized in that, form K look version with following rational number screen, described rational number screen is when being made as θ=arctan (m/n) with natural number m, n with cable angle, and the size of its threshold matrix satisfies m ²+ n ²Positive integer doubly, form C look version and M look version with ultra-fine born of the same parents, and form the cable angle of K look version, C look version, M look version with the interval of 30 ° (intermediate values) respectively.

The screen group that the present invention relates to is a kind of screen group of using when forming coloured image with CMK look version at least by the electrofax mode, it is characterized in that, comprise: with the K look version of following rational number screen formation, described rational number screen is when being made as θ=arctan (m/n) with natural number m, n with cable angle, and the size of its threshold matrix satisfies m ²+ n ²Positive integer doubly; And with super plastidogenetic C look version and M look version; Wherein, formed the cable angle of K look version, C look version, M look version respectively with the interval of 30 ° (intermediate values).

The image processing system that the present invention relates to is a kind of image processing system that forms coloured image by the electrofax mode at least with CMK look version, it is characterized in that, formed K look version with following rational number screen, described rational number screen is when being made as θ=arctan (m/n) with natural number m, n with cable angle, and the size of its threshold matrix satisfies m ²+ n ²Positive integer doubly, formed C look version and M look version with ultra-fine born of the same parents, and formed the cable angle of K look version, C look version, M look version with 30 ° interval respectively.

According to above-mentioned image forming method, screen group and image processing system, by forming the highest K look version of observability among the CMYK with the rational number screen, the constant shape of site, and inhibition produces striped and uneven reason, and, by forming C look version and M look version with ultra-fine born of the same parents, and form K look version, C look version and M look version with the interval of cable angle 30 ° (intermediate value/setting values), realize the optimization of net flower, and suppressed to produce the reason of moire composition.

In image processing system, how can on the direction of scanning, produce inhomogeneously, therefore when cable angle is 0 ° or 45 °, will disturb, thereby inhomogeneously become remarkable easily with the direction of scanning of picture shape device.Therefore, be made as angle except that 0 ° and 45 °, the inhomogeneous composition that can not highlight image processing system and had by cable angle with K look version.

In addition,, be 2400[DPI for example at output resolution ratio in the several sides of high line] and the line number be 200[LPI] situation under, the site diminishes, and becomes the adhering to of site unstable.Therefore, in order to increase the site, in each edition of CMYK, the value that preferably makes output resolution ratio ÷ line number is less than 12.

In addition, skew, the inaccurate colour cast amount (aberration) of so-called cover in order to reduce with respect to the site that overlaps each edition, in other words, in order to improve the color shift intensity inaccurate, to be so-called robustness with respect to cover, preferably the size with the threshold matrix of K look version is made as more than 80 * 80, and consider to preserve the memory span of threshold matrix, preferably be made as below 256 * 256.

When on CMK version of all kinds, also increasing Y look version, thereby when forming coloured image with CMYK look version, by using the rational number screen identical and form described Y look version with staggering phase place with described K look version with described K look version, the site coincidence each other of Y look version and K look version tails off, and has therefore suppressed owing to the moire that uses the big Powdered toner of particle size to produce, has been so-called piling height moire.Here, the phase shifting between the adjacent unit, site 180 ° (cycle of unit, site 1/2).

Here, if the phase deviation of Y look version and K look version just can suppress the piling height moire in 150 °～210 ° scope.

In addition, the deviation that makes versicolor actual line number becomes cable angle with K look version, C look version and M look version and is set at the condition of 30 ° of intervals with the reason that more effectively suppresses to produce the moire composition at interval in 10LPI.

In addition, preferably make cable angle 30 ° (intermediate values) at interval be spaced apart 30 ° ± 1.5 ° with interior scope as described K look version, described C look version and described M look version.

Description of drawings

Fig. 1 is a summary structural drawing of using the color copy machine of an embodiment of the present invention.

Fig. 2 is the block diagram that the structure of image processing part is shown.

Fig. 3 is the key diagram of the image of supracellular C version stipple pattern.

Fig. 4 is the key diagram of the image of supracellular M version stipple pattern.

Fig. 5 is the key diagram of image of the K version stipple pattern of rational number screen.

Fig. 6 is the key diagram of image of polymer fabric dot pattern of the stipple pattern of each edition of CMK.

Fig. 7 is with the image of the K version stipple pattern of the rational number screen of Fig. 5 example key diagram at the image of the Y version stipple pattern of the rational number screen after 180 ° that staggers on the phase place.

Fig. 8 is the key diagram of image of the polymer fabric dot pattern of the YK version that is synthesized into of the image with the image of the K version stipple pattern of rational number screen and Y version stipple pattern.

Fig. 9 is the key diagram of image of polymer fabric dot pattern of the stipple pattern of each edition of CMYK.

Figure 10 is the key diagram of binary conversion treatment.

Figure 11 is the key diagram of the threshold matrix size of rational number screen.

Figure 12 is the key diagram of supracellular threshold matrix size.

Figure 13 is the figure that the personal computer of using one embodiment of the present invention is shown.

Figure 14 is the figure that an embodiment of site of the present invention assessment process is shown.

Figure 15 is the functional block diagram of an embodiment of site of the present invention evaluating apparatus.

Figure 16 is the process flow diagram of an embodiment of site of the present invention evaluation method.

Figure 17 is the figure that is shown schematically in the site of CMY three looks that obtain among the step S103 of Figure 16.

Figure 18 A is the figure of the example of the lattice point structure of acquisition when being illustrated in side-play amount for " 0 ".

Figure 18 B is the table that is illustrated in the shared ratio of each color on the image.

Figure 19 A is the figure of the example of the lattice point structure that obtained when skew has taken place.

Figure 19 B is the table that is illustrated in the shared ratio of each color on the image.

Figure 20 is the figure of example that the spectrophotometric spectra of each color is shown.

Figure 21 is the figure that the example of the spectrophotometric spectra after synthesizing is shown.

Figure 22 is the figure that an example of light source light spectrum is shown.

Figure 23 is the figure of the color matching function of expression tristimulus value(s).

Figure 24 is the figure that illustrates as the screen group on the basis of shining upon.

Figure 25 A illustrates the figure that overlaps as the site in the screen group on the basis of shining upon, and is that aberration is 0 o'clock figure.

Figure 25 B illustrates the figure that overlaps as the site in the screen group on basis of mapping, is the figure of aberration when reaching maximum.

Figure 26 illustrates the figure of aberration with respect to an example of the mapping of screen group.

Figure 27 is the table that the example of screen group is shown.

Figure 28 is the key diagram of the image of supracellular C version stipple pattern.

Figure 29 is the key diagram of the image of supracellular M version stipple pattern.

Figure 30 is the key diagram of image of the K version stipple pattern of rational number screen.

Figure 31 is the key diagram of image of the Y version stipple pattern of rational number screen.

Figure 32 is the key diagram of image of the polymer fabric dot pattern of the YK version that is synthesized into of the image with the image of the K version stipple pattern of rational number screen and Y version stipple pattern.

Figure 33 is the key diagram of image of polymer fabric dot pattern of the stipple pattern of each edition of CMK.

Figure 34 is the key diagram of image of polymer fabric dot pattern of the stipple pattern of each edition of CMYK.

Embodiment

Below, with reference to the accompanying drawings embodiments of the present invention are described.

Fig. 1 shows the summary structure of color copy machine (image processing system) 10 of the digital and electronic photographic means of the AM screen that has adopted the image forming method of using an embodiment of the present invention.Image processing system is not limited to color copy machine, and also the coloured image applicable to all electrofax modes such as color printer, colour facsimile apparatus forms device.

Color copy machine 10 comprises scanner section 12, image processing part 14, ROS (Raster OpticalScanner, grating optical scanner) optical system 16, polygon mirror 18, photoreceptor 20, charged device 22, rotary developing machine 24, transfer drum 26 and fuser 28 etc.

Here, scanner section 12 reads the colored original image that places on the contact glass, and this picture signal that reads is shown 8 multivalue image signals (continuous toned image signal) of system and offers image processing part 14 as for example R (red) G (green) B (indigo plant).

Image processing part 14 is output as the continuous toned image signal scan image signal of width modulation that is adjusted to the two-value of the resolution that meets ROS optical system 16 by each sweep trace.

Fig. 2 shows the summary structure of image processing part 14.

In Fig. 2, in the 3 looks-4 look converter section 32 that comprises that color correct is handled, UCR (Under Colour Removal, undercolour removal) handles etc., be broken down into the picture signal of CMYK from the picture signal of the RGB of scanner section 12 input.The picture signal of this CMYK is provided for comparing section 34.

The picture signal of the CMYK of 34 pairs of inputs of comparing section compares with each threshold matrix that is stored in the CMYK in the threshold matrix preservation portion 36, and control melanism (colorization/the drawing)/non-melanism (retouching white) of each pixel according to comparative result, generate each stipple pattern (dot pattern) signal of CMYK thus, this stipple pattern signal is bianry image and is the middle tone picture signal with periodic structure of screen.

In threshold matrix preservation portion 36, store and be used for generating the threshold matrix of image and describing the information of the periodic structure of making by a plurality of threshold matrixes by stipple pattern with periodic structure.In threshold matrix preservation portion 36, stored that the size of threshold matrix satisfied m when cable angle was made as θ=arctan (m/n) with natural number m, n ²+ n ²Threshold matrix, supracellular C version, the threshold matrix of M version usefulness of K version (Y version) usefulness of positive integer rational number screen doubly.Threshold matrix by the threshold matrix of K version (Y version) usefulness of rational number screen, supracellular C version, M version usefulness has constituted the screen group.

The stipple pattern signal of the CMYK that generates in comparing section 34 is stored in the frame buffer 38 temporarily.The stipple pattern signal that sweep signal formation portion 40 will be stored in the CMYK in the frame buffer 38 temporarily synchronously is output as output image signal with the scanning commencing signal, this output image signal by width modulation the scan image signal of CMYK of two-value.

Return Fig. 1 once more, be provided for ROS optical system 16 from the output image signal of the CMYK of the two-value of image processing part 14 output.In ROS optical system 16, be provided with drive circuit for laser 70, as the laser light source 62 of exposure device, collimation lens 64, f θ lens 66 and cylindrical lens 68 as imaging device.

In this ROS optical system 16, drive circuit for laser 70 is according to the output image signal of the CMYK of the two-value that provides from image processing part 14, produces the Laser Drive signal of the laser that is used for a little going out, and laser light source 62 is put the driving of going out.And, make the imaging on photoreceptor 20 of this laser by optical systems such as collimation lens 64, f θ lens 66 and cylindrical lenses 68.

The laser of imaging scans at photoreceptor 20 upper edge main scanning directions (direction of advancing to the paper depths) by polygon mirror 18 on photoreceptor 20.Here, polygon mirror 18 generates the scanning commencing signal.Synchronous with this scanning commencing signal, from the scan image signal of the CMYK of the sweep signal formation portion 40 (with reference to figure 2) of image processing part 14 output two-value.

In order to form the electrostatic latent image of stipple pattern, drive photoreceptor 20 to sub scanning direction with drive units such as servo motors.By driving photoreceptor 20 to sub scanning direction and on main scanning direction, scanning this photoreceptor 20, on photoreceptor 20, form the electrostatic latent image of the stipple pattern of two dimension with laser.

Charged device 22 is to be used for making the charged device of photoreceptor 20 before the photoreceptor 20 in advance in exposure.

Rotary developing machine 24 is attached on the image of electrostatic latent image the toner (pigment) of these four kinds of colors of CMYK.

Transfer drum 26 looks like toner to be transferred on the white support.

Fuser 28 is to be used for device that pulverous toner-particle and described white support are glued together.

Then, simple explanation is carried out in the action of the color copy machine 10 that present embodiment is related to.

At first, read the colored original image with scanner section 12, and from the picture signal of scanner section 12 to image processing part 14 input RGB.Image processing part 14 is decomposed into the received image signal of RGB the picture signal of the CMYK of 4 looks.

In addition, also in the comparing section 34 of this image processing part 14, the threshold matrix that is stored in each look version usefulness of CMYK in the threshold matrix preservation portion 36 and each picture signal of CMYK are compared, generate the stipple pattern signal of periodic structure with screen.

This stipple pattern signal is stored in the frame buffer 38 temporarily, then as the scan image signal of width modulation that is adjusted to the two-value of the resolution that meets ROS optical system 16 by each sweep trace, synchronously be exported to the drive circuit for laser 70 of ROS optical system 16 with the scanning commencing signal.Drive circuit for laser 70 is put the control of going out according to scan image signal to laser light source 62.Then, make laser imaging on photoreceptor 20 of sending by optical systems such as collimation lens 64, f θ lens 66 and cylindrical lenses 68, and on photoreceptor 20, carry out main sweep by polygon mirror 18 from this laser light source 62.

Photoreceptor 20 in advance by charged device 22 and by charged be that surface potential is even.By to its irradiating laser, reduce the current potential of exposure portion, form electrostatic latent image.

This electrostatic latent image is developed by one in the bi-component developer that is disposed at four looks in the rotary developing machine 24.

The charge image developing toner picture that develops on the photoreceptor 20 from transfer drum 26 each a kind of colors be transferred on the white support.

Successively to the toner picture of white support transfer printing CMYK, and after the toner with four looks looks like to be transferred on the white support, peel off recording chart from transfer drum 26.Then, look like to make its fusion, thus it is adhered on the white support by fuser 28 heating toners.

So can be made into the hard copy that on white support, has formed the coloured image that bonding four colour toners look like to form.

Then, the characteristics of following coloured image are described, described coloured image is formed in the image of the stipple pattern from the hard copy of color copy machine 10 outputs.

The first, the formation based on the coloured image of CMK look version is described.

Fig. 3 shows in comparing section 34 the stipple pattern 201C of the result's that the C color image signal of uniform density and supracellular C version are compared with threshold matrix C look version.The stipple pattern 201C of this C look version is ultra-fine born of the same parents' mode, and cable angle θ is θ=15.255 ° (arctan (3/11)), and screen ruling LPI (Line Per Inch) is LPI=244.323.Owing to be ultra-fine born of the same parents' mode, thereby cable angle θ can be very near 15 ° angle, but all differences of the shape of site 201Ch.

Fig. 4 shows in comparing section 34 the stipple pattern 201M of the result's that the M color image signal of uniform density and supracellular M version are compared with threshold matrix M look version.The stipple pattern 201M of this M look version is ultra-fine born of the same parents' mode, and cable angle θ is θ=74.745 ° (arctan (11/3)), and screen ruling is 244.323LPI.Owing to be ultra-fine born of the same parents' mode, thereby with respect to the C version, cable angle θ be the angle at 60 ° at interval correctly, but all differences of the shape of site 201Mh.

Fig. 5 shows in comparing section 34 the stipple pattern 201K of the result's that the K version of the K color image signal of uniform density and rational number screen is compared with threshold matrix K look version.The stipple pattern 201K of this K look version is a rational number screen mode, and cable angle θ is θ=45 ° (arctan (1/1)), and screen ruling is 242.437LPI.The shape of site 201Kh is roughly the same.

As mentioned above, site 201Ch, the 201Mh of stipple pattern 201C, the 201M of ultra-fine born of the same parents' mode of pie graph 3, Fig. 4 is in pixel count and all differences in shape, but pixel count and the shape almost fixed of the site 201Kh of the stipple pattern 201K of the rational number screen mode of pie graph 5.

Fig. 6 shows each stipple pattern 201C of synthetic these CMK look versions, the polymer fabric dot pattern 201Scmk of CMK look version that 201M, 201K form.Promptly, forming K look version with the rational number screen, form C look version and M look version with ultra-fine born of the same parents, and form respectively at interval with 30 ° (intermediate value) among the polymer fabric dot pattern 201Scmk of CMK look version of K look version, C look version and M look version, can see the net flower, therefore suppress the generation of moire composition, and, because the site of the stipple pattern 201K of the K look version that observability is the highest has the shape of almost fixed, thereby has suppressed to result from toner and adhered to/inadhering striped and uneven generation reason.

The second, the characteristics by the coloured image of the CMYK look version formation of increase Y look version on the coloured image of CMK look version are described.

Fig. 7 shows in comparing section 34 the stipple pattern 201Y of the result's that the Y version of the Y color image signal of uniform density and rational number screen is compared with threshold matrix Y look version.The stipple pattern 201Y of this Y look version is a rational number screen mode, and cable angle θ is identical with the cable angle of above-mentioned K version, is θ=45 °, and screen ruling is also identical with the K version, is 242.437LPI.Hence one can see that, and the shape of site 201Yh is roughly consistent.That is the stipple pattern 201Y of this Y look version pattern that is to use the rational number screen identical and 180 ° of phase shiftings are formed, with the stipple pattern 201K of K look version shown in Figure 5.

In more detail, Fig. 8 shows the polymer fabric dot pattern 201Syk of the YK that the stipple pattern 201Y of the Y look version shown in Figure 7 of 180 ° of the stipple pattern 201K of K look version shown in Figure 5 and phase deviations is synthetic.As can be known on the polymer fabric dot pattern 201Syk of this YK, the site of representing with symbol Y, K on phase place, staggered 180 ° (not having overlapping).

The polymer fabric dot pattern 201Scmyk of the CMYK look version that the stipple pattern 201Y of synthetic again Y look version shown in Figure 7 forms on the polymer fabric dot pattern 201Scmk of the CMK look version that Fig. 9 shows at Fig. 6.Owing to be monochromatic the demonstration, be difficult to explanation, but as can be known on the polymer fabric dot pattern 201Scmyk of this CMYK look version, the site 201Yj of the site 201Kh of K look version and Y look version on phase place, staggered 180 ° (not having overlapping).

So, when adding that Y look version forms polymer fabric dot pattern 201Scmyk as the coloured image of CMYK look version, by similarly using the rational number screen and the phase place that staggers to form Y look version with K look version, reduced the overlapping of site, suppressed owing to the moire that uses size to produce, be so-called piling height moire as the Powdered toner of particle size.The skew of phase place just can suppress the piling height moire as long as in 150 °～210 ° scope.

In monochrome shown in Figure 9 shows, visually the density of the site 201Yh of Y look version is as equating with the density of the site 201Kh of K look version, but in fact, because Y look version is unshowy, thereby on coloured image, can see net flower as the characteristics of the polymer fabric dot pattern 201Scmk of CMK look version shown in Figure 6, therefore suppressed the generation of moire composition, and, because the site 201Kh of the stipple pattern 201K of the K look version that observability is the highest has the shape of almost fixed, thereby has similarly suppressed striped and uneven generation reason.

The 3rd, other the combination example with effect same as described above (the inhibition effect of moire composition and suppress striped and the effect of uneven generation reason) is described.The combination of above-mentioned CMYK look version is thought of as first combination (each look version of YK is the rational number screen, and each look version of CM is ultra-fine born of the same parents).

Second combination (each look version of YK is the rational number screen, and each look version of CM is ultra-fine born of the same parents)

K look version (Y look version is dual-purpose also): θ=77.471 °, LPI=260.317

C look version: θ=17.103 °, LPI=256.028

M look version: θ=46.469 °, LPI=259.635

The 3rd combination (each look version of YK is the rational number screen, and each look version of CM is ultra-fine born of the same parents)

K look version (Y look version is dual-purpose also): θ=77.471 °, LPI=260.317

C look version: θ=16.504 °, LPI=256.038

M look version: θ=47.862 °, LPI=266.538

The 4th combination (each look version of YK is the rational number screen, and each look version of CM is ultra-fine born of the same parents)

K look version (Y look version is dual-purpose also): θ=83.660 °, LPI=265.036

C look version: θ=23.749 °, LPI=266.468

M look version: θ=53.972 °, LPI=265.394

The 5th combination (each look version of YK is the rational number screen, and each look version of CM is ultra-fine born of the same parents)

K look version (Y look version is dual-purpose also): θ=84.289 °, LPI=238.8089

C look version: θ=23.962 °, LPI=234.032

M look version: θ=53.130 °, LPI=237.623

The 6th combination (each look version of YK is the rational number screen, and each look version of CM is ultra-fine born of the same parents)

K look version (Y look version is dual-purpose also): θ=66.038 °, LPI=243.683

C look version: θ=5.711 °, LPI=248.657

M look version: θ=36.870 °, LPI=247.423

If consider above-mentioned first to sextus combination, the deviation of the material line number by making each color becomes K look version, C look version and M look version is set at the condition of 30 ° of intervals with the generation reason that suppresses the moire composition in 10LPI.In addition we know C look version and M look version are set at 30 ° (setting values) at interval, in fact are preferably in 30 ° ± 1.5 ° with interior scope in ultra-fine born of the same parents' mode.

In color copy machine of the so-called multiple beam structure of arranging a plurality of laser light sources along 0 ° of direction etc., nominal series (CMYK) in the printing=(15 °, 45 °, 0 °, 75 °) cause interfering inhomogeneous easily, and therefore effectively way is that each edition equally angularly offseted 7.5 ° or 22.5 °.

Here, be used for the explanation of the effect of the present invention under the clearer and more definite electrofax mode, also other embodiments described in addition.

In the image processing system of traditional electrofax mode, example as shown in figure 10, with uniform density (in this example, each pixel value I of view data pixel value=14) and the threshold value TH of threshold matrix are mapped, and carry out binaryzation according to the magnitude relationship of pixel value I and threshold value TH, thereby retouch when obtaining site H{I ≦ H white, I〉melanism (drawing) during H.Here, in traditional image processing system, because the memory span of the threshold matrix when binaryzation is carried out in storage is little, do not use big or small big ultra-fine born of the same parents' threshold matrix of threshold matrix, so, fail the cable angle of CMK version is made as 30 ° of intervals, therefore be easy to generate two metamembers of the moire that causes by these three nets.Here, two metamembers of moire are meant the moire with following vector representation, when screen density is made as size, when angle was made as the frequency vector of the net on the direction vertical with the direction of netting arrangement, described vector was meant by the resultant vector of two nets wherein and forms the vector that the difference of vector of each net of the resultant vector of two nets generates respectively.

To this, if will all use ultra-fine born of the same parents' threshold matrix to each look version of CMK, then different with the rational number screen, it is inconsistent that dot shape can become, and because like this, easy absorption of noise in the electrofax mode, thus found out easily, therefore, will be made as the rational number screen as the K look version of the most showy color as mentioned above.

Figure 11, Figure 12 are respectively the figure of the size that is used to illustrate the threshold matrix of rational number screen, size of supracellular threshold matrix etc.

In rational number screen shown in Figure 11, when cable angle is made as θ=arctan (m/n) with natural number m, n, make the size of threshold matrix 202 satisfy m ²+ n ²Positive integer doubly, specifically, when n=3, m=1, arctan (m/n)=arctan (1/3), the size of threshold matrix 202 is m ²+ n ²=10 positive integer " 2 " doubly 20 * 20.At this moment, the dot shape of each site 204 equates, four angles of unit, site 206 respectively be positioned at these four angles on the angle of pixel consistent.

The size of supracellular threshold matrix 208 shown in Figure 12 is 21 * 21.At this moment, the dot shape difference of each site, four angles of each unit, site 210 are inconsistent with the angle that is positioned at the pixel on these four angles.

Then, to the change in color under the following situation, be that the evaluation of aberration (being also referred to as the colour cast amount) describes, wherein said situation is meant the desirable coincidence from designing when using the screen group, and the site skew takes place the inaccurate situation of so-called cover promptly takes place.This aberration proposed in inventor, the instructions of applying for identical with the application per capita application (special hope 2005-102898 number), accompanying drawing (figure that is described below).Here, reprint the described aberration of explanation.

Figure 13 is the figure that the personal computer that calculates aberration is shown.

As shown in Figure 13, personal computer 100 comprises: main body 110 is built-in with CPU, main storage means, hard disk, communication port etc.; Display 120, according to indication from this main body 110, display frame or character string in display frame 121; Keyboard 130 is used for indication or character information to this personal computer 100 input users; And mouse 140, by specifying the optional position in the above-mentioned display frame 121, import and when carrying out described appointment, be presented at corresponding indications such as this locational icon.

Have the CD loading port 111 of CD-ROM or CD-R of can freely packing into with taking out in main body 110, and be built-in with CD-ROM driver, this CD-ROM driver is used for reproducing the institute's canned data among the CD-ROM of this CD loading port 111 or the CD-R of packing into.In addition, the FD loading port 112 that in main body 110, also has the floppy disk (FD) of freely to pack into with taking out, and being built-in with the FD driver, this FD driver is used for floppy disk (FD) recorded information to this FD loading port 112 of packing into, or from this floppy disk (FD) information reproduction.

Here, by CD-ROM storage site assessment process and be loaded in the main body 110, the site assessment process that is stored among this CD-ROM is read in by CD-ROM driver, and is installed in the hard disk in the main body 110.

When starting and move the described site assessment process that is installed in the hard disk, personal computer 100 moves as the site evaluating apparatus.That is, here, by constituting the site evaluating apparatus in conjunction with personal computer 100 and site assessment process.And,, implement the site evaluation method here by the work of site evaluating apparatus.

Figure 14 is the figure that the site assessment process is shown.Here, this site assessment process 300 is stored among CD-ROM200.

The storage medium of storage site assessment process is not limited to CD-ROM, also can be storage mediums such as CD, MO, floppy disk (floppy (registered trademark)), tape in addition.In addition, the site assessment process also can be via storage medium but is directly offered personal computer via communication network.

This site assessment process 300 moves in personal computer shown in Figure 13 100, thereby this personal computer 100 is moved, as the site evaluating apparatus comprising configuration part 310, position determination portion 320, area occupation ratio calculating part 330, spectrum calculating part 340, Lab value calculating part 350 and evaluation portion 360.

Details about each key element of this site assessment process 300 will be explained hereinafter.

Figure 15 is the functional block diagram of site evaluating apparatus.

This site evaluating apparatus 400 is installed in the personal computer shown in Figure 13 100 by the site assessment process 300 with Figure 14 and operation constitutes.

This site evaluating apparatus 400 comprises configuration part 410, position determination portion 420, area occupation ratio calculating part 430, spectrum calculating part 440, Lab value calculating part 450 and evaluation portion 460, these configuration parts 410, position determination portion 420, area occupation ratio calculating part 430, spectrum calculating part 440, Lab value calculating part 450 and evaluation portion 460 are by the configuration part 310 that constitutes site assessment process 300 shown in Figure 14, position determination portion 320, area occupation ratio calculating part 330, spectrum calculating part 340, in Lab value calculating part 350 and the evaluation portion 360 each constitutes on personal computer 100.Like this, each key element of site evaluating apparatus 400 shown in Figure 15 is corresponding with each key element of site assessment process 300 shown in Figure 14 respectively, their difference is, each key element of Figure 15 is by the constituting of OS, the application program of the hardware of personal computer shown in Figure 13 100 and operation in this personal computer 100, and each key element shown in Figure 14 only is made of wherein application program.

These configuration parts 410, position determination portion 420, area occupation ratio calculating part 430, spectrum calculating part 440, Lab value calculating part 450 and evaluation portion 460 are equivalent to each the example of configuration part, position determination portion, area occupation ratio calculating part, spectrum calculating part, tristimulus value(s) calculating part and evaluation portion in the evaluating apparatus of site respectively.

Details about each key element of this site evaluating apparatus 400 also will be explained hereinafter.

Figure 16 is the process flow diagram of site evaluation method.

The site evaluating apparatus 400 of this site evaluation method by Figure 15 moves and is performed, comprising: assignment procedure (step S101～step S103), position deterministic process (step S104～step S105), area occupation ratio computation process (step S106～step S109), spectrum computation process (step S110～step S111), Lab value computation process (step S112～step S113) and evaluation procedure (step S114～step S116).These each processes are carried out by each key element of site evaluating apparatus 400 shown in Figure 15, and are stipulated the contents processing of each process by each key element of site assessment process 300 shown in Figure 14.In addition, these assignment procedures, position deterministic process, area occupation ratio computation process, spectrum computation process, Lab value computation process and evaluation procedure are equivalent to each the example of assignment procedure, position deterministic process, area occupation ratio computation process, spectrum computation process, tristimulus value(s) computation process and evaluation procedure in the evaluation method of site respectively.

Below, the details of each step shown in Figure 16 are described, each key element of the described site of Figure 15 evaluating apparatus 400 and each key element of site evaluating apparatus 300 shown in Figure 14 also are described thus in the lump.

When the evaluation method of beginning site shown in Figure 16, at first, set site screen (step S101).This site screen has defined the variation of the dot shape that the variation with the structure of jet nit point or dot area percentage takes place, and the design of site promptly refers to design that these site screens are carried out.The site screen is made of the arrangement of the threshold value that compares with pixel value, and to be the above part of threshold value drawn as the part of site pixel value, and pixel value becomes less than the part of threshold value retouches white part.That is, the site screen has the effect that the image transitions of representing with the pixel value of Continuous Gray Scale is served as reasons and drawn and retouch the bianry image of white formation.Every kind of color of the ink that such site screen uses when needing at the output coloured image is prepared, and therefore in the present embodiment, sets at each color in C (green grass or young crops), M (magenta), this three look of Y (Huang).So the group of the versicolor site screen of setting is equivalent to an example of screen group.

Then, for each color of CMY three looks, set dot area percentage (site %) (step S102).This dot area percentage (site %) is used for representing that the threshold value arrangement that constitutes the site screen has percent how much to become drawing position, if dot area percentage for example is 40%, then in threshold value is arranged, 40% the part that threshold value is little becomes the drawing part, and remaining 60% part becomes retouches white part.Obtain such drawing part and retouch white part at every kind of color in CMY three looks, determine the shape and the size (step S103) of the site of CMY three looks thus.

Figure 17 is the figure that is shown schematically in the site of CMY three looks that obtain among the step S103 of Figure 16.

In this Figure 17, beating hatched part is the drawing part, has constituted the site by it.Not beating hatched part is to retouch white part.Constitute the site or retouch the little square of white part corresponding one by one with the threshold value of formation site screen, and corresponding one by one with the pixel of composing images.

By to each color settings site screen and site %, as shown in figure 17, can obtain the CMY three looks site separately that on the aligning method of shape, size and site, differs from one another.

This CMY three looks site separately obtains in step S101～step S103 of Figure 16, in addition in this site evaluation method, set the maximum (maximum offset) (step S104) that the site is offset mutually because cover is inaccurate, and this maximum offset of automatic setting is with interior side-play amount (step S105).In the present embodiment, set the side-play amount of each site (C version, M version) of C look and M look with respect to the site (Y version) of Y look, for convenience of explanation, supposed that the offset direction of C version and M version is the above-below direction of Figure 17, and the following direction among the figure is a positive dirction.In addition, in the present embodiment, the unit of side-play amount has adopted the little square (pixel promptly) that constitutes site shown in Figure 17 or retouch white part.In the present invention, for example when wanting to improve the resolution of estimating the site, also can set side-play amount with the unit more tiny than pixel unit.

So, after the shape that obtains the site and size and site side-play amount each other, then, generate expression is overlapped the lattice point structure that forms by CMY three looks site separately image (step S106, step S107).Occasion (step S106) and side-play amount that correspondence has the side-play amount that sets are the occasion of " 0 ", blueness), 3 look (Gy: grey) calculate versicolor area occupation ratio (step S108, step S109) in the statistical distribution separately generate the image of this lattice point structure of expression respectively, and 1 look (pure color) of the white from the image of described generation, CMY three looks, 2 looks (R: red, Gn: green, B:.

Figure 18 A, Figure 18 B are the figure of the example of resulting lattice point structure when side-play amount is shown for " 0 ", and Figure 19 A, Figure 19 B are the figure that the example of resulting lattice point structure when skew takes place is shown.

Lattice point structure shown in these Figure 18 A, Figure 18 B and Figure 19 A, Figure 19 B is that the site by CMY shown in Figure 17 three looks overlaps and forms, in Figure 19 A, illustrated as benchmark the Y version, the C version is to the following direction skew 1 of figure, and the M version also is offset 2 o'clock state to the following direction of figure.

Shown in Figure 18 A and Figure 19 A, the site by CMY three looks overlaps, and on image, except CMY three looks 1 look separately, has also produced 2 looks of RGnB and 3 looks of Gy.Calculate these every kind color shared ratio in image in the mode of statistics, its result is indicated among Figure 18 B and Figure 19 B.Owing to be offset mutually because of overlapping the inaccurate site of waiting, the ratio that for example is shown in the white in the hurdle of " no pixel " becomes 49% from 44%, and the ratio of this pure color of magenta becomes 7% from 14%.

After the step S108 by Figure 16 and step S109 calculated such area occupation ratio, then, the spectrophotometric spectra by synthetic each color calculated all spectrophotometric spectras of this lattice point structure (step S110, step S111).

Figure 20 is the figure of example that the spectrophotometric spectra of each color is shown, and the transverse axis among the figure is represented wavelength, and the longitudinal axis is represented reflectivity.

Figure 20 illustrates the spectrophotometric spectra of 1 look of white, CMY, 2 looks and 3 looks reflectivity separately.This spectrophotometric spectra can obtain by each color that practical measurement outputs to the class's piece on the paper etc.By all spectrophotometric spectras of polymer fabric dot structure on the spectrophotometric spectra that above-mentioned area occupation ratio is added to these each colors as weight.

Figure 21 is the figure that the example of the spectrophotometric spectra after synthesizing is shown, and here, the transverse axis among the figure is represented wavelength, and the longitudinal axis is represented reflectivity.

Two spectrophotometric spectras have been shown, the spectrophotometric spectra the when spectrophotometric spectra the when spectrophotometric spectra that dots is the side-play amount shown in Figure 18 A, Figure 18 B for " 0 ", the spectrophotometric spectra of representing with solid line are generation skews shown in Figure 19 A, Figure 19 B in this Figure 21.Compare these spectrophotometric spectras as can be known, variation has taken place to the dependence of wavelength in reflectivity slightly before and after skew.

At the step S110 by Figure 16 and step S111 synthetic and calculate above-mentioned spectrophotometric spectra after, then, calculate the Lab value based on this spectrophotometric spectra, this Lab value is to be illustrated in the color (step S112, step S113) that produces in the lattice point structure such shown in Figure 18 A, Figure 18 B and Figure 19 A, Figure 19 B with colourity.When calculating this Lab value, at first use the spectrum of the light source of observing the site to obtain by the spectrum of the color of above-mentioned lattice point structure reflection.

Figure 22 is the figure that an example of light source light spectrum is shown.

As shown in Figure 22, in typical light source, there are mound shape part and some sharp-pointed crests of striding wide wavelength region may.The spectrum of spectrum by the described light source that multiplies each other and reflectivity shown in Figure 21 is obtained the spectrum of the light that is reflected by lattice point structure.

Then, by this catoptrical spectrum being multiply by the color matching function of tristimulus value(s) and carrying out integration and obtain tristimulus value(s) (XYZ value).

Figure 23 is the figure of the color matching function of expression tristimulus value(s).

Each color matching function of tristimulus value(s) (XYZ value) has been shown in this Figure 23,, then can have obtained the XYZ value if this color matching function be multiply by catoptrical spectrum and carries out integration in whole wavelength region may.

At last, the reduction formula by definite in the world regulation is converted into the Lab value with the above-mentioned XYZ value of obtaining.For example, the Lab value that finally obtains based on the spectrum of spectrophotometric spectra shown in Figure 21 and light source shown in Figure 22 when not being offset is

(L，a，b)＝(80.068058，1.8677918，14.667547)，

When skew has taken place be

(L，a，b)＝(82.740753，—1.0719514，14.274755)。

The step S112 by Figure 16 and step S113 such as above-mentioned calculate the Lab value after, then, obtain the colour cast amount not having the state that is offset and the state of skew is arranged, be aberration Δ E according to this Lab value and by following formula (1).

ΔE＝(ΔL ²+Δa ²+Δb ²) ^1/2 ...(1)

Wherein, the differential of Δ L:L value, the differential of Δ a:a value, the differential of Δ b:b value.

For example, can be regarded as out aberration Δ E=3.9924524 from above-mentioned illustrative Lab value.

Bit by bit changing side-play amount on one side in step S105 also sets automatically, repeat step S105, step S106, step S108, step S110, step S112, step S114 in each above-mentioned treatment step on one side, its result has made the mapping (step S115) of the corresponding relation between expression side-play amount and the aberration.An example of this mapping is shown below, the screen group as the mapping basis is shown earlier before this.

Figure 24 is the figure that illustrates as the screen group on mapping basis, and Figure 25 A, Figure 25 B illustrate the figure that the site in this screen group overlaps.

Figure 24 illustrates CMY three looks site separately, replace the site in Figure 17, schematically show and use these sites to obtain the mapping of aberration described later.

Lattice point structure when side-play amount has been shown in Figure 25 A for " 0 ", the lattice point structure when figure 25 illustrates in mapping described later aberration and reaching maximum side-play amount.In the side-play amount under state shown in this Figure 25 B, the C version has been offset 1 to the top of figure, and the M version has been offset 4 to the below of figure.

Figure 26 is the figure that illustrates with respect to an example of the mapping of the aberration of screen group.

The transverse axis of the mapping that illustrates is here represented the side-play amount of the relative Y version of C version, and the longitudinal axis is represented the side-play amount of the relative Y version of M version.In the figure as can be known, there is the big zone of aberration in the central authorities with respect to mapping to the right from downside.

In the step S115 of Figure 16, draw after such mapping, calculate the maximum aberration (step S116) in the mapping at last.In example shown in Figure 26, as describing among Figure 25 B, when C version skew+1, maximum aberration took place at-4 o'clock in the skew of M version.

The mapping of each aberration of calculating in above-mentioned steps S114, the aberration of making in step S115 and the maximum aberration of calculating in step S116 all are used for indicating the degree of the color shift of screen group, are the evaluations of estimate that is used to estimate the screen group.In addition, such evaluation of estimate is owing to directly estimate the color that obtains in the screen group, therefore can estimate the stability of actual color when the screen group is used to print etc. rightly.And, in the present embodiment,, therefore can estimate the impression of the color shift of generation rightly along with inaccurate grade of cover according to having estimated the screen group based on the aberration of Lab value.

In the above description, as an example of evaluation procedure and evaluation portion, show the example of estimating based on the Lab value that converts from tristimulus value(s), but evaluation procedure and evaluation portion also can directly use tristimulus value(s) evaluation.Such evaluation is the impression than the people, payes attention to itself the evaluation of change color of the coloured image that obtains by the site all the better.

In addition, in the above description, an example as tristimulus value(s) computation process and tristimulus value(s) calculating part, show the example that the spectrum of considering light source calculates tristimulus value(s), but for example under the situation of simply estimating or observing light source the unknown etc., also can ignore the spectrum of light source, only the spectrum according to lattice point structure calculates tristimulus value(s).

Above explanation is exactly to be willing in 2005-102898 number the instructions etc. with regard to the change in color under the following situation, to be the explanation that evaluation proposed of colour cast amount (being also referred to as aberration) the spy, wherein said situation is meant the desirable coincidence from designing when using the screen group, and the site skew takes place the inaccurate situation of so-called cover promptly takes place.

Here, be at (A) CMK version of all kinds the rational number screen the screen group situation and (B) CK version of all kinds compared the aberration Δ E that in above-mentioned (1) formula, calculates for ultra-fine born of the same parents, M look version are the situation of rational number screen.

The line number is 175[LPI], the site % of CMK look version is 30[%].

(A) Cyan:arctan (1/3)=18.4 °, the size 40 * 40 of threshold matrix, Magenta:arctan (1/1)=45 °, the size 20 * 20 of threshold matrix, Black:arctan (3/1)=71.6 °, the size 40 * 40 of threshold matrix.

Result of calculation: aberration Δ E=5.3

(B) Cyan:arctan (3/11)=15.3 °, the size 160 * 160 of threshold matrix, Magenta:arctan (1/1)=45 °, the size 20 * 20 of threshold matrix, Black:arctan (11/3)=74.7 °, the size 160 * 160 of threshold matrix.

Result of calculation: aberration Δ E=3.2

We can say from the result of calculation of aberration Δ E, be the situation of the screen group of rational number screen than (A) CMK version of all kinds, be that aberration Δ E is littler under the situation of rational number screen in (B) CK version of all kinds for ultra-fine born of the same parents, M look version, so, higher to the robustness of the color shift of being forbidden because of cover to cause.

Then, the restriction to the threshold matrix size of rational number screen describes.

When the threshold matrix size with the rational number screen is made as 80 * 80 when above, it is stronger that color shift will become.

That is,, also there is robustness if increase the size of threshold matrix even if K look version is the rational number screen, if but surpass 80 * 80 as the size of the threshold matrix of the K look version of rational number screen, then the value of aberration Δ E will be tending towards saturated.

In addition, if the size of threshold matrix surpasses 256 * 256, when then selecting to be used for preserving the storer of threshold matrix, the target of storer will be above 8[bit]=256, the therefore excessive memory span of needs, thus not ideal enough.

Aberration Δ E to concrete example (a) and (b), (c) describes.Wherein, in (a) and (b), (c) example, K look version and Y look version are the rational number screen, and C look version and M look version are ultra-fine born of the same parents.Following (a) example is the example that describes with reference to figure 3～Fig. 9.

(a) size of Black (Yellow): arctan (1/1), threshold matrix (Nc * Nc)=14 (=112/8) * 14 (in Fig. 5, Fig. 7, Fig. 9, drawing) with 14 * 8 times, the size 112 * 112 of Cyan:arctan (3/11), threshold matrix, the size 112 * 112 of Magenta:arctan (11/3), threshold matrix.

(b) size 85 * 85 of Black (Yellow): arctan (2/9), threshold matrix, the size 255 * 255 of Cyan:arctan (8/27), threshold matrix, the size 255 * 255 of Magenta:arctan (21/19), threshold matrix.

(c) Black (Yellow):

, threshold matrix size 82 * 82,

, threshold matrix size 246 * 246, Magenta:

, threshold matrix size 123 * 123.

(i) the site % of CMK look version is 30[%] aberration Δ E:(a when (being designated as CMK=30/30/30)) be 2.97, (b) be 2.47, (c) is 2.88.

The (ii) aberration Δ E:(a during CMK=40/40/30) be 3.18, (b) be 2.9, (c) is 2.88.

The (iii) aberration Δ E:(a during CMK=40/40/20) be 3.1, (b) be 2.68, (c) is 2.71.

The (iv) aberration Δ E:(a during CMK=70/70/70) be 2.59, (b) be 2.19, (c) is 2.18.

Hence one can see that, is (a) example below 80 * 80 than the size of threshold matrix, and aberration Δ E is littler under the situation of (b) more than 80 * 80 example, (c) example, and robustness is higher.

Each value of having expressed above-mentioned (b) example, (c) example of Figure 27.

Below, with reference to Figure 28～Figure 34, the stipple pattern routine to above-mentioned (c) describes.

Figure 28 shows the stipple pattern 501C of the C look version of ultra-fine born of the same parents' mode.

Figure 29 shows the stipple pattern 501M of the M look version of ultra-fine born of the same parents' mode.

Figure 30 shows the stipple pattern 501K of the K look version of rational number screen mode.

Figure 31 shows the stipple pattern 501K of the Y look version of rational number screen mode.The pattern that the stipple pattern 501Y of this Y look version is to use the rational number screen identical with the dot pattern 501K of K look version shown in Figure 30 and 180 ° of phase shiftings are formed.

The site of stipple pattern 501C, 501M of ultra-fine born of the same parents' mode that constitutes Figure 28, Figure 29 is in pixel count and all differences in shape, but constitutes the pixel count and the shape almost fixed of site of stipple pattern 501K, 501Y of the rational number screen mode of Figure 30, Figure 31.

Figure 32 shows the polymer fabric dot pattern 501Syk of the YK version that the stipple pattern 501Y with the Y look version shown in Figure 31 of 180 ° of the stipple pattern 501K of K look version shown in Figure 31 and phase deviations is synthesized into.As can be known on the polymer fabric dot pattern 501Syk of this YK, the site of K look version and Y look version on phase place, staggered 180 ° (not having overlapping).

Figure 33 shows the polymer fabric dot pattern 501Scmk of stipple pattern 501C, 501M, 501K, and Figure 34 shows the polymer fabric dot pattern 501Scmyk of stipple pattern 501C, 501M, 501K, 501Y.

In polymer fabric dot pattern 501Scmk, can see the net flower, therefore suppressed the generation of moire composition, and, because the site of the stipple pattern 501K of the K look version that observability is the highest has the shape of almost fixed, thereby has suppressed adhering to/non-cohesive striped that causes and uneven generation reason by toner.

In polymer fabric dot pattern 501Scmyk, by similarly using the rational number screen and the phase place that staggers to form Y look version with K look version, reduced the overlapping of site, suppressed owing to the moire that uses the big Powdered toner of particle size to cause, be so-called piling height moire.The skew of phase place just can suppress the piling height moire as long as in 150 °～210 ° scope.

In addition, the present invention is not limited to above-mentioned embodiment, much less can adopt various structures based on the record content of this instructions.

Industrial applicibility

According to the present invention, can will in the coloured image shape device of electrofax mode, mostly occur The reason that produces striped and inhomogeneous reason and produce moire at coloured image is suppressed at minimum Limit.

Claims (10)

1. image forming method, it forms coloured image with CMK look version at least by the electrofax mode, and this image forming method is characterised in that,

Form K look version with following rational number screen, described rational number screen is when being made as θ=arctan (m/n) with natural number m, n with cable angle, and the size of threshold matrix satisfies m ²+ n ²Positive integer doubly,

Form C look version and M look version with ultra-fine born of the same parents,

Interval with 30 ° of cable angles forms K look version, C look version, M look version respectively.

2. image forming method as claimed in claim 1 is characterized in that,

The cable angle of described K look version is the angle except that 0 ° and 45 °.

3. image forming method as claimed in claim 1 is characterized in that,

In above-mentioned C, M, K version of all kinds and Y look version, the value that makes output resolution ratio ÷ line number is less than 12.

4. image forming method as claimed in claim 1 is characterized in that,

Be made as the size of the threshold matrix of described K look version more than 80 * 80 and below 256 * 256.

5. image forming method as claimed in claim 1 is characterized in that,

When increasing Y look version again, thus when forming coloured image with CMYK look version, uses the rational number screen identical with described K look version and with the described K look version described Y look version of formation with staggering phase place.

6. image forming method as claimed in claim 5 is characterized in that,

With respect to the phase place of described K look version, in the phase place of 150 °～210 ° the described Y look of scope bias internal versions.

7. image forming method as claimed in claim 1 is characterized in that,

The deviation that makes versicolor actual line number is in 10LPI.

8. image forming method as claimed in claim 1 is characterized in that,

30 ° of intervals that make described K look version, described C look version and described M look version are in 30 ° ± 1.5 °.

9. screen group, it uses when forming coloured image with CMK look version at least by the electrofax mode, and this screen group is characterised in that, comprising:

With the K look version that following rational number screen forms, described rational number screen is when being made as θ=arctan (m/n) with natural number m, n with cable angle, and the size of threshold matrix satisfies m ²+ n ²Positive integer doubly; And

With super plastidogenetic C look version and M look version,

Wherein, the interval with 30 ° of cable angles forms K look version, C look version, M look version respectively.

10. image processing system, it forms coloured image with CMK look version at least by the electrofax mode, and this image processing system is characterised in that,

Form K look version with following rational number screen, described rational number screen is when being made as θ=arctan (m/n) with natural number m, n with cable angle, and the size of threshold matrix satisfies m ²+ n ²Positive integer doubly,

C look version and M look version form with ultra-fine born of the same parents, and

Interval with 30 ° of cable angles forms K look version, C look version, M look version respectively.

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