Method for modulating multi-channel threshold matrix in multi-channel printing

Abstract

The invention discloses a method for modulating a multichannel threshold matrix in multichannel printing, and relates to the technical field of digital printing. Based on the original threshold matrix, different threshold matrixes are modulated for each printing channel, so that different printing channels output different halftone images; the original threshold matrix is an image, and the value range of each pixel is a subset of the value range of the color component of the input continuous tone image; different threshold matrixes generated for different channels are used for halftone processing of the input continuous tone image, so that the effects of uniform printing effect and printing ink saving can be realized.

Description

Technical Field

[0001] The invention relates to the technical field of digital printing, and in particular to a modulation method of a threshold matrix in a multi-channel digital printing technology. Background Art

[0002] In printing and reproduction, multi-channel printing technology is often used in order to pursue better color expression (for example, to make the blackness of pure black text better), or for some special purposes (such as reducing the white line phenomenon caused by blockage of individual nozzles in inkjet printing technology).

[0003] Multi-channel printing technology refers to the use of the same color ink or ink in different color channels, so that the same color ink has multiple inking processes in the same printing area, thereby achieving the effect of enhancing color expression or reducing white line pulling.

[0004] Multiple channels using the same color ink or inks are often considered symmetrical, meaning that the continuous-tone images input to these channels before halftoning are identical.

[0005] The bit depth of continuous tone images is generally not less than 8 bits. Taking an image with a bit depth of 8 bits as an example, each color component of each pixel has a value of 0--255, a total of 256 cases. However, the bit depth supported by most printing devices is much smaller than the bit depth of continuous tone images. Most printing devices support only 1 bit, that is, there are only two states: inked and uninked. Since the bit depth of printing devices is much smaller than the bit depth of continuous tone images, the images that printing devices can accept are called halftone images. The most common way to convert continuous tone images into halftone images is to use a threshold matrix. The threshold matrix is ​​a grayscale image in which the value range of each pixel is a subset of the value range of the color component of the input continuous tone image.

[0006] In the existing multi-channel printing technology, different channels of the same color ink or ink use the same threshold matrix. The continuous tone images before halftone processing of multiple channels are the same, and the threshold matrices of multiple channels in the prior art are also the same, so the halftone images generated by different channels are also the same.

[0007] However, outputting the same halftone image from different channels makes the ink points too concentrated, which will not only lead to uneven printing effects, but also affect the ink or the color components in the ink, which will cause ink waste. The prior art does not modulate different threshold matrices for different printing channels, making it difficult to obtain good printing effects. Summary of the invention

[0008] To overcome the deficiencies of the above-mentioned prior art, the present invention provides a method for modulating a multi-channel threshold matrix in multi-channel printing. By using this method, different threshold matrices can be modulated for each printing channel based on an initial or existing threshold matrix, so that different printing channels output different halftone images, thereby obtaining a more uniform printing effect and at the same time saving printing ink.

[0009] The different threshold matrices modulated by the present invention for different printing channels have the following three characteristics:

[0010] First, the intersection of the sets of pixel points with the same value in the threshold matrices of any two different printing channels is an empty set;

[0011] Second, let the number of printing channels be K, and the pixel value range of the input continuous-tone image be 0 - D; for the coordinate points in the threshold matrix of the printing channel, the pixel value at this coordinate point in the threshold matrix is a; if a×K is less than or equal to D, then for any two different printing channels (0 <= i, j < K and i ≠ j), the intersection of the sets of all coordinate points with pixel values less than or equal to a in the threshold matrices used by these two different printing channels (denoted as Φ i (a) and Φ j (a)) is an empty set;

[0012] Third, the threshold matrices of multiple printing channels still maintain the properties of the original threshold matrix. Assume the original threshold matrix is T, and the threshold matrices generated from T for K channels are T0...T K-1 , Num(T0,d)+...+Num(T k-1 ,d)≈(K×d) / D; the distribution of the union of all pixels with pixel values less than or equal to d in the threshold matrices T0...T K-1 in the M×N image maintains the characteristics of the original threshold matrix T. Among them, Num(T0,d) represents the number of all pixels with pixel values less than or equal to d in the threshold matrix T0, where 0 <= d <= D; Num(T,d)≈d / D.

[0013] To achieve the above object, an embodiment of the present invention provides a method for modulating a multi-channel threshold matrix in multi-channel printing. This method will modulate different threshold matrices for each printing channel based on the existing threshold matrix. The steps are as follows:

[0014] 1) Copy K identical threshold matrices from the original threshold matrix: Th0..., Th k ,..., Th K-1 ;

[0015] 2) Add D×k to the values of all pixel points of the threshold matrix Th k , where 0 <= k < K;

[0016] 3) Calculate the pixel point coordinate set sequence Ω Th (1),...,Ω Th (kd),...,Ω Th (K×D), where Ω Th (kd) represents the threshold matrix Th0...,Th k ,...,Th K-1 The set of pixel coordinates of all pixels in the set whose values ​​are kd, where 1 <kd<=K×D;

[0017] 4) Set Ω Th All elements in (kd) are assigned to the threshold matrix T0...T K-1 (each element is assigned once and only once), and the value of the pixel point at the corresponding coordinate is assigned to kd / K, where it represents rounding up. Any threshold matrix T k Any pixel can only be assigned once.

[0018] In specific implementation, in order to enhance the effect of the present invention, the present invention also proposes a method for completing the above step 4), which can make the number of ink points generated by each channel as consistent as possible for any identical input image. The method includes the following steps:

[0019] a) initializing processing variables, where the processing variables include the pixel value currently processed in the original threshold matrix;

[0020] b) generating each channel threshold matrix in sequence according to the order of printing channels from 0 to K-1 and pixel values ​​from 1 to D based on the original threshold matrix;

[0021] c) All pixel values ​​in the threshold matrix are processed according to b) to generate a threshold matrix for each channel, and the processing is completed.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] Compared with the existing technology that uses the same threshold matrix for different channels, this method can make the distribution of ink points in the multi-channel printing process more reasonable, thereby significantly improving the uniformity of the printed product and giving full play to the role of color components to save ink. DETAILED DESCRIPTION

[0024] The present invention is further described below by way of examples, but the scope of the present invention is not limited in any way.

[0025] The present invention provides a method for modulating a multi-channel threshold matrix in digital printing. Using this method, based on an initial or existing threshold matrix, different threshold matrices are modulated for each printing channel, so that different printing channels output different halftone images, thereby obtaining a uniform printing effect and saving printing ink at the same time.

[0026] In digital printing technology, the threshold matrix used is a grayscale image, where the value range of each pixel is a subset of the value range of the color components of the input continuous-tone image. Assume that the size of the input continuous-tone image is X×Y, and I(x,y) represents the value of the pixel in the x-th row and y-th column, where 0 <= x < X and 0 <= y < Y. T(m,n) represents the value of the pixel in the m-th row and n-th column of the threshold matrix, where 0 <= m < M and 0 <= n < N; M and N are the width and height of the threshold matrix respectively. Then the 1-bit halftone image H(x,y) after processing with the threshold matrix is:

[0027] If I(x,y) >= T(x mod M, y mod N), then H(x,y) = 1; (in the case of >= judgment, the pixel value in the threshold matrix is generally not 0); otherwise H(x,y) = 0.

[0028] The threshold matrix usually needs to have the following two properties:

[0029] Assume that the value range of each pixel of the continuous-tone image is 0--D, and Num(T,d) represents the number of pixels in the threshold matrix T whose pixel values are less than or equal to d, where 0 <= d <= D. Then Num(T,d) ≈ d / D;

[0030] The distribution of all pixels in the threshold matrix T whose pixel values are less than or equal to d in the M×N image satisfies certain characteristics, such as blue noise characteristics, or is closely connected together with the center point of the threshold matrix as the core.

[0031] The method for modulating a multi-channel threshold matrix in multi-channel printing provided by the present invention modulates different threshold matrices for each printing channel based on the existing threshold matrix, and includes the following steps:

[0032] 1) Copy K identical threshold matrices from the original threshold matrix: Th0..., Th k ,..., Th K-1 ;

[0033] 2) Add the value of D×k to all pixel points of the threshold matrix Th k , where 0 <= k < K;

[0034] 3) Calculate the sequence of pixel point coordinate sets Ω Th (1),..., Ω Th (kd),..., ΩTh (K × D), where Ω Th (kd) represents the set of pixel coordinates where all pixel values in the threshold matrices Th0..., Th k ,..., Th K-1 are kd, where 1 < kd <= K × D;

[0035] 4) Assign all elements in the set Ω Th (kd) to the threshold matrices T0... T K-1 (each element is assigned and only assigned once), and assign the pixel value of the corresponding coordinate pixel to kd / K, where represents rounding up. Any pixel of any threshold matrix T k can only be assigned a value once.

[0036] To enhance the effect of the present invention, the present invention also proposes a method for completing step 4) above, which can make the number of ink dots generated in each channel as consistent as possible for any same input image. The method includes the following steps:

[0037] 5) Initialize the processing variables;

[0038] 6) Generate each channel threshold matrix in sequence according to the order from channel 0 to K - 1 and pixel values from 1 to D;

[0039] 7) Judge the end of processing.

[0040] The above steps realize modulating different threshold matrices for each printing channel based on the existing threshold matrix, and the modulated different threshold matrices have the following three characteristics:

[0041] First, the intersection of the sets of pixel points with the same value in the threshold matrices of any two different printing channels is an empty set;

[0042] Second, record the number of printing channels as K, and the pixel value range of the input continuous tone image is 0 - D; for the coordinate points in the threshold matrix of the printing channel, the pixel value of the coordinate point in the threshold matrix is a; if a × K is less than or equal to D, then for any two different printing channels (0 <= i, j < K and i ≠ j), the intersection of the sets of coordinate points where all pixel values in the threshold matrices used by these two different printing channels are less than or equal to a (denoted as Φ i (a) and Φ j (a)) is an empty set;

[0043] Third, the threshold matrices of multiple printing channels still maintain the properties of the original threshold matrix. Assume the original threshold matrix is T, and the threshold matrices generated from T for K channels are T0... T K-1 , Num(T0, d) +... + Num(T k-1, d)≈(K×d) / D; threshold matrix T0...T K-1 The distribution of the union of all pixels whose pixel values ​​are less than or equal to d in the M×N image maintains the characteristics of the original threshold matrix T.

[0044] By using different threshold matrices generated for different channels for halftone processing of the input continuous tone image, a uniform printing effect can be achieved while saving printing ink.

[0045] In the following embodiments, the number of printing channels is K, and the value range of the continuous tone image color component of the input printing channel is 0--D. D is the upper limit of the value of the continuous tone image color component. The value of the pixel point in the original threshold matrix T is 1-D, and the threshold matrix generated for each printing channel is T0...T K-1 .

[0046] The threshold matrix is ​​an M×N image in which the value range of each pixel is a subset of the value range of the input continuous tone image color component.

[0047] The original threshold matrix in the embodiment is exemplified as follows:

[0048] 1 32 8 40 2 34 10 42 48 16 56 24 50 18 58 26 12 14 4 36 14 46 6 38 60 28 52 20 62 30 54 22 3 35 11 43 1 33 9 41 51 19 59 27 49 17 57 25 15 47 7 39 13 45 5 37 63 31 55 23 61 29 53 21

[0049] The width (M) and height (N) of the above original threshold matrix are both 8; the bit depth of the continuous tone image processed by halftone using the threshold matrix is ​​6 bits, that is, D is 63, and the pixel value range is 0-63.

[0050] The method for modulating a multi-channel threshold matrix provided by the present invention comprises the following steps:

[0051] Step S1: Copy K identical threshold matrices Th0...Th from the original threshold matrix of the printing channel K-1 ;

[0052] Step S2: Set the threshold matrix Th k The values ​​of all pixels of D×k are added, where 0<=k <K;

[0053] Step S3: Calculate Ω Th (1)...Ω Th (K×D), where Ω Th (kd) represents the threshold matrix Th0...Th K-1 The set of pixel coordinates of all pixels in the set whose values ​​are kd, where 1 <kd<=K×D;

[0054] Step S4: Set Ω Th All elements in (kd) are assigned to T0...T K-1(each element is assigned once and only once), and the value of the pixel point at the corresponding coordinate is assigned to kd / K, where it represents rounding up. k Any pixel can only be assigned once.

[0055] Step S4 can be implemented in a variety of ways, for example, the assignment method can be used: k (m,n)=Th k (m,n) / K. However, the disadvantage of this method is that for any identical input image, the number of ink dots generated by each channel decreases as the channel index increases. This makes the channels with smaller indexes (especially channel 0) overloaded, causing the corresponding imaging units to be damaged too quickly.

[0056] In order to enhance the effect of the present invention, the present invention proposes a method for completing step S4, which can make the number of ink dots generated by each channel as consistent as possible for any identical input image.

[0057] Assume that the number of different pixel values ​​appearing in the original threshold matrix T is D'. The least common multiple of K and D' is Z.

[0058] Taking the above original threshold matrix as an example, since the pixel value that does not appear in the threshold matrix is ​​only 44, D'=62. When the number of printing channels K is 2, Z is equal to 62.

[0059] The method for completing step S4 provided by the present invention comprises:

[0060] Step S41: Initialize processing variables,

[0061] kd=1; (kd represents the pixel value currently processed in the original threshold matrix)

[0062] k=0; (k represents the subscript of the multi-channel threshold matrix that needs to be processed currently)

[0063] j=0; (a control variable to prevent a pixel point in the multi-channel threshold matrix from being repeatedly assigned a value)

[0064] Step S42: Generate each channel threshold matrix in sequence from channel 0 to K-1 and pixel values ​​from 1 to D;

[0065] If Ω Th (kd) is not an empty set, then:

[0066] T (k mod K) Medium T (kd) All corresponding pixels in the set are assigned a value of kd / K;

[0067] k = k + 1;

[0068] j=j+1;

[0069] If (j mod Z) is equal to 0, k=k+1.

[0070] Step S43: If all pixel values ​​in the threshold matrix have been processed, the process ends. At this time, all pixels in all threshold matrices of the K channels are assigned values ​​and are assigned values ​​only once.

[0071] kd=kd+1;

[0072] If kd = K × D + 1, the threshold matrix T0 ... T of the printing channel K-1 Already generated, processing completed.

[0073] Repeat step S42.

[0074] By using different threshold matrices generated for different channels for halftone processing of the input continuous tone image, a uniform printing effect can be achieved while saving printing ink.

[0075] In the case of dual printing channels, the threshold matrices of the two channels obtained by processing the original threshold matrix using the method of the present invention (steps S1-S7) are respectively expressed as follows:

[0076] 1 48 36 52 33 49 37 53 24 40 28 44 25 41 29 45 38 39 34 50 39 23 35 51 30 46 26 42 31 47 27 43 2 18 6 22 1 17 5 21 57 10 61 14 56 9 60 13 8 55 4 20 7 54 3 19 63 16 59 12 62 15 58 11

[0077] and

[0078] 32 16 4 20 1 17 5 21 56 8 60 12 57 9 61 13 6 7 2 18 7 55 3 19 62 14 58 10 63 15 59 11 33 49 37 53 32 48 36 52 26 41 30 45 25 40 29 44 39 24 35 51 38 23 34 50 32 47 28 43 31 46 27 42

[0079] It can be seen that the sets of pixels with the same value in these two threshold matrices do not overlap at all. The former is the threshold matrix for channel 0, and the latter is the threshold matrix for channel 1.

[0080] The halftone images obtained by processing the given image using the threshold matrices of the above two channels are:

[0081] 1 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 0 0 0 0 0 1 0 0 1 0 1 0 1 0 1 0 1 1 1 1 1 1 1 1 0 1 0 1 0 1 0 1 1 0 1 1 1 0 1 1 0 1 0 1 0 1 0 1

[0082] and

[0083] 0 1 1 1 1 1 1 1 0 1 0 1 0 1 0 1 1 1 1 1 1 0 1 1 0 1 0 1 0 1 0 1 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 1 0 1 0

[0084] The former is the halftone image output for a given image channel 0; the latter is the halftone image output for a given image channel 1. The threshold matrices of the two printing channels are used to process the given image, respectively, to obtain halftone images of the two channels. The size of the given image is 8×8, the bit depth is 6 bits, and each pixel value is 31 (close to 50% gray value). The output halftone image bit depth is 1, where 1 means inking and 0 means no inking. It can be seen that the inking points of the halftone images generated by the two channels do not overlap at all.

[0085] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for modulating a multi-channel threshold matrix in multi-channel printing, characterized in that: Based on the original threshold matrix, a different threshold matrix is ​​modulated for each printing channel, so that different printing channels output different halftone images; The original threshold matrix is ​​an image, and the value range of each pixel in the image is a subset of the value range of the color component of the input continuous tone image; Modulate a different threshold matrix for each printing channel: First, the intersection of the sets of pixels with the same value in the threshold matrices of any two different printing channels is an empty set; Second, the number of printing channels is recorded as K, and the pixel value range of the input continuous tone image is 0-D; for the coordinate point in the threshold matrix of the printing channel, the pixel value of the coordinate point in the threshold matrix is ​​a; If a×K is less than or equal to D, then the intersection of the sets of coordinate points whose pixel values ​​are less than or equal to a in the threshold matrices used by any two different printing channels is an empty set; Third, the different threshold matrices of multiple printing channels still maintain the properties of the original threshold matrix.

2. The method for modulating a multi-channel threshold matrix in multi-channel printing as claimed in claim 1, characterized in that: The process of modulating different threshold matrices for different printing channels includes the following steps: Step S1: Copy K identical threshold matrices from the original threshold matrix of the printing channel, expressed as: Th0...,Th k ,...,Th K-1 ; Step S2: Add D×k to the values of all pixel points of each obtained threshold matrix Th k , where 1 <= k < K; D is the upper bound of the pixel value range of the input continuous tone image; K is the number of printing channels; Step S3: Calculate the pixel point coordinate set sequence Ω Th (1),...,Ω Th (kd),...,Ω Th (K×D), where Ω Th (kd) represents the threshold matrix Th0...Th K-1 The set of pixel coordinates of all pixels whose values ​​are kd; 1 <kd<=K×D; Step S4: Set the set Ω Th All elements in are assigned to the threshold matrix, and each element is assigned only once; and the value of the pixel point of the corresponding coordinate is assigned to kd / K, where it represents rounding up; any pixel point of any threshold matrix is ​​assigned only once; thereby, K different threshold matrices are obtained, which are used for each printing channel respectively.

3. The method for modulating a multi-channel threshold matrix in multi-channel printing as claimed in claim 2, characterized in that: The method adopted in step S4 is: generating each channel threshold matrix in sequence from channel 0 to K-1 and pixel values ​​from 1 to D; Specifically: when Ω Th When (kd) is not an empty set, T (kmodK) Medium T All corresponding pixel points in the (kd) set are assigned a value of kd / K; where kd represents the pixel value currently being processed in the original threshold matrix.

4. The method for modulating a multi-channel threshold matrix in multi-channel printing according to claim 1, characterized in that: Different threshold matrices for multiple printing channels still maintain the properties of the original threshold matrix, that is, the distribution of the union of all pixels whose pixel values ​​are less than or equal to a certain pixel value in the threshold matrix generated by the original threshold matrix for multiple channels in the image maintains the characteristics of the original threshold matrix.