Ink-jet printing stripe defect compensation method

Abstract

The invention discloses an ink-jet printing stripe defect compensation method, and belongs to the technical field of digital ink-jet printing. The method comprises the following steps: S1, acquiring halftone image data to be printed; s2, determining pixel columns with stripe defects in the image; s3, for each pixel with defects, calculating an average gray value M in an n * n pixel grid taking the pixel as a center; s4, selecting a corresponding compensation mode according to the range of the average gray value M; s5, according to the selected compensation mode, dynamic point compensation is carried out on the defective pixel and the surrounding pixels of the defective pixel; s6, performing chromatic aberration compensation on the compensated image; and S7, outputting the compensated halftone image data. According to the method, the most suitable compensation mode can be adopted for defect areas in different density ranges, and the visual effect of stripe defects is effectively improved; the perception characteristic of human eyes on colors is fully considered, so that the compensated printing effect is closer to an ideal state; and multi-channel cooperative compensation is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of digital inkjet printing, and in particular relates to an inkjet printing stripe defect compensation method. Background Art

[0002] In the field of digital inkjet printing, single-channel inkjet printing technology is widely used in the production of books, newspapers, magazines, advertisements and packaging due to its advantages of high speed, high precision, high flexibility and low cost. However, technical problems such as print head clogging, jet direction deviation and failure have always been a continuous challenge facing the single-channel inkjet printing industry.

[0003] In some cases, damage to the printhead may result in the loss of consecutive columns of pixels in the image, resulting in noticeable streaks on the printed surface. These streaks are usually perpendicular to the printhead arrangement direction and are mainly caused by the lack of ink from the damaged nozzles in the corresponding color channel. Faced with this defect, the traditional solution is to replace the printhead. However, this measure is not only costly, but also significantly reduces production efficiency. Therefore, exploring a method that can effectively reduce the impact of streak defects without the need for additional hardware investment is of great significance for improving the quality of printed products, extending the service life of printheads, and improving production efficiency.

[0004] In the prior art, there are some compensation methods for inkjet printing streak defects. For example, some studies have proposed to improve the printing quality by detecting print head defects and adjusting the size and position of adjacent dots. However, these methods often only consider the compensation of a single color channel, fail to fully utilize the visual perception characteristics of the human eye, and fail to achieve multi-channel coordinated compensation.

[0005] Therefore, there is an urgent need for an inkjet printing stripe defect compensation method that can fully utilize the visual perception characteristics of the human eye and achieve multi-channel collaborative compensation, so as to further improve the compensation effect and improve the printing quality. Summary of the invention

[0006] In view of the above problems existing in the prior art, the object of the present invention is to provide a method for compensating inkjet printing stripe defects.

[0007] The present invention provides the following technical solution: a method for compensating for inkjet printing stripe defects, comprising the following steps: S1: Acquire halftone image data with stripe defects to be printed; S2: Determine the pixel columns with streak defects in the image; S3: For each defective pixel, calculate the average gray value M in the n×n pixel grid centered on the defective pixel, where n is the number of pixels; S4: Select a corresponding compensation mode according to the range of the average gray value M; S5: Perform dynamic point compensation on the defective pixel and its surrounding pixels according to the selected compensation mode; S6: performing chromatic aberration compensation on the compensated image; S7: Output the compensated halftone image data.

[0008] Furthermore, the specific steps of S4 are as follows: When M is in the range of 198-255, high-density compensation mode is used: Add a number of adjacent pixels on both sides of the defective pixel and increase the volume of ink droplets to enhance the printing density and concentration; When M is in the range of 142-198, medium and high density compensation mode is used: Add b adjacent pixels on both sides of the defective pixel and increase the ink density to effectively repair the defective pixel, where 1<b<a; When M is in the range of 85-142, the medium and low density compensation mode is adopted: Add a dot to the left or right of the defective pixel in an adjacent position without ink to adjust the brightness; When M is in the range of 0-85, low-density compensation mode is used: Randomly add a dot to the left, right, or pixel position of the defective pixel to minimize the probability of adding a dot and prevent overcompensation.

[0009] Furthermore, the specific process of S6 is as follows: S6.1: Simulating local image data under ideal printing conditions on a computer: Using an image processing algorithm, simulating the diffusion and coloring effects of ink droplets under ideal conditions, so as to provide a reference image for subsequent color difference calculation and compensation processes; the ideal printing state is a state in which the generated image data is not affected by nozzle failure during the printing process, and the distribution, size and position of the ink droplets are in accordance with expectations; S6.2: local printing image data after simulation compensation; S6.3: Convert CMYK color space to LAB color space; S6.4: Calculate the color difference between the compensated image and the ideal image using the color difference formula; S6.5: According to the color difference results, the dot structure of the remaining channels is adjusted so that the printing effect after compensation is closest to the ideal state.

[0010] Furthermore, in S6.2, a simplified ink droplet diffusion model is used to simulate the printing effect, as follows: Define an N×N matrix G, where N=(diffusion diameter) / (pixel size). For each element G[i, j] in G, we have: ; in, ; Then G is normalized so that its sum is 1: .

[0011] Furthermore, the CIEDE2000 color difference formula is used in S6.4 to calculate the color difference between the compensated image and the ideal image; the expression of the CIEDE2000 color difference formula is: ; in: are the differences in brightness, chroma, and hue, respectively; are the scaling factors for brightness, chroma, and hue; is the interaction term between chroma and hue; are weighting factors for brightness, chroma, and hue.

[0012] By adopting the above technology, compared with the prior art, the beneficial effects of the present invention are as follows: 1) The present invention adopts a dynamic point compensation strategy based on the average gray value, which can adopt the most suitable compensation mode for defect areas in different density ranges and effectively improve the visual effect of stripe defects; 2) The present invention adopts a color difference compensation algorithm based on visual perception, which fully considers the color perception characteristics of the human eye, so that the printing effect after compensation is closer to the ideal state; 3) The present invention realizes multi-channel coordinated compensation, and compensates for the deficiency of defective channels by adjusting the point structure of the remaining channels, thereby further improving the compensation effect; 4) The present invention does not require additional hardware investment and can be easily integrated into the existing digital printing workflow, with good practicality and economy; 5) The present invention can extend the service life of the print head, reduce maintenance costs and improve production efficiency through effective defect compensation. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is an overall flow chart of the method of the present invention; The figure shows the complete process from acquiring halftone image data to outputting compensated image data, including key steps such as stripe defect recognition, dynamic dot compensation, and color difference compensation; Figure 2 It is a schematic diagram of the dynamic point compensation strategy; The figure illustrates the specific implementation methods of four different compensation modes (high density, medium-high density, medium-low density, and low density) based on the average gray value M. The figure shows how the point distribution around the defective pixel is adjusted in each mode; Figure 3 Schematic diagram of the chromatic aberration compensation process; The figure describes the detailed steps of color difference compensation, including the ideal printing state simulation, compensated state simulation, color space conversion, CIEDE2000 color difference calculation, and the process of fine-tuning the remaining channel point structure; Figure 4 This is a comparison chart of the compensation effect of the CMYK four-color gradient image, which shows the following: (1) Stripe defect state of CMYK four-color gradient image caused by nozzle failure; (2) The state of the image after being processed by the compensation method of the present invention; (3) The juxtaposition effect of the two states; The improvement effect of the compensation method of the present invention on stripe defects can be intuitively observed through the comparison chart; among which, the continuity and color uniformity of the gradual transition area are significantly improved. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments of the specification. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0015] On the contrary, the present invention covers any substitution, modification, equivalent method and scheme made on the essence and scope of the present invention as defined by the claims. Further, in order to make the public have a better understanding of the present invention, some specific details are described in detail in the detailed description of the present invention below. Those skilled in the art can fully understand the present invention without the description of these details. DETAILED DESCRIPTION

[0016] See also Figure 1-4 This embodiment proposes a method for compensating inkjet printing stripe defects based on a visual perception model, which effectively compensates for stripe defects by combining dynamic dot compensation and color difference compensation. The specific implementation steps of the method are as follows: Step 1: Obtain halftone image data to be printed.

[0017] Step 2: Identify the pixel columns in the image that have streak defects. This can be done by analyzing the print head status or performing image analysis on the print results.

[0018] Step 3: For each defective pixel, calculate the average grayscale value M in the 3×3 pixel grid centered on it. The calculation formula is: ; in, is a single pixel value, Indicates centered Neighborhood.

[0019] Step 4: According to the average gray value M, select the corresponding compensation mode: When M is in the range of 198-255, the high-density compensation mode is adopted: 6 dots of adjacent pixels are added on both sides, and the ink drop volume is increased.

[0020] When M is in the range of 142-198, the medium-high density compensation mode is adopted: 2 dots of adjacent pixels are added on both sides to moderately increase the ink density.

[0021] When M is in the range of 85-142, the medium and low density compensation mode is adopted: one dot is added to the adjacent position without ink on the left or right side.

[0022] When M is in the range of 0-85, low-density compensation mode is used: a point is randomly added on the left, right or pixel position.

[0023] Step 5: According to the selected compensation mode, dynamic point compensation is performed on the defective pixel and its surrounding pixels.

[0024] Step 6: Perform chromatic aberration compensation on the compensated image: (1) Simulate local image data under ideal printing conditions.

[0025] (2) Simulate the local printed image data after compensation. Here, a simplified ink droplet diffusion model is used to simulate the printing effect: Define an N×N matrix G, where N=(diffusion diameter) / (pixel size). For each element G[i, j] in G, we have: ; in, ; Then G is normalized so that its sum is 1: .

[0026] (3) Convert the CMYK color space to the LAB color space. This conversion is performed using a device-specific ICC profile and absolute colorimetric rendering intent.

[0027] (4) Use the CIEDE2000 color difference formula to calculate the color difference between the compensated image and the ideal image. The expression of the CIEDE2000 color difference formula is: ; in: are the differences in brightness, chroma, and hue respectively.

[0028] is the scaling factor for brightness, chroma, and hue.

[0029] is the interaction term between chroma and hue.

[0030] are weighting factors for brightness, chroma, and hue.

[0031] (5) Based on the color difference results, fine-tune the dot structure of the remaining channels to make the compensated printing effect closest to the ideal state.

[0032] Step 7: Output the compensated halftone image data.

[0033] In order to verify the effectiveness of the method of the present invention, the following experimental platform and experimental group were designed: Experimental platform: The experimental platform is based on KGT-L-press 330s inkjet printer, and the experiment is carried out under ideal conditions for this model of inkjet printer. The image acquisition device uses TCX6ML620-221220A contact image sensor.

[0034] Experimental design: Four groups of experimental images corresponding to four color gradient levels of CMYK were designed. Two intentionally manufactured defects were set for control experiments in each group of images, namely, the uncompensated part and the part compensated by the method of the present invention.

[0035] Experimental steps: 1. Obtain halftone image data for each set of images to be printed.

[0036] 2. Determine the x-coordinates of the pixel columns where the two intentionally created stripe defects are located in each set of images.

[0037] 3. Get the positions of the pixels in the two pixel columns where ink should be sprayed.

[0038] 4. Calculate the average gray value M of the surrounding 3×3 pixels at each pixel location: ; in, is a single pixel value, Indicates centered Neighborhood.

[0039] 5. Dynamically select adjacent ink dot compensation mode according to M: High-density compensation mode: When M is in the range of 198-255, 6 adjacent pixels are added on both sides of the defective pixel, and the ink drop volume is increased to a certain extent.

[0040] Medium and high density compensation mode: When M is in the range of 142-198, two adjacent pixels are added on both sides of the defective pixel to moderately increase the ink density.

[0041] Medium and low density compensation mode: M is in the range of 85-142, and a dot is added to the adjacent position with relatively less ink on the left or right side of the defective pixel.

[0042] Low-density compensation mode: M is in the range of 0-85, and a point is randomly added to the left, right or pixel position of the defective pixel.

[0043] 6. Dynamic dot compensation: Perform dynamic dot compensation on the defective pixel and its surrounding pixels according to the selected compensation mode.

[0044] 7. Chromatic aberration compensation: 1) Simulate local image data under ideal printing conditions: Use device-specific ICC profiles and absolute colorimetric rendering intent to convert the halftone images of each set of ideal images from CMYK color space to LAB color space to simulate local image data under ideal printing conditions.

[0045] 2) Simulate the local printing image data after compensation: Use a simplified ink drop diffusion model to simulate the printing effect. Define an N×N matrix G, where N=(diffusion diameter) / (pixel size). For each element G[i, j] in G, we have: ; where σ = N / 6. G is then normalized so that it sums to 1: .

[0046] 3) Apply this ink droplet diffusion model to the 3×3 area around each defective pixel of the LAB image in the ideal printing state to obtain the result after simulated printing.

[0047] 4) After simulating the halftone image of the ideal printing state, the remaining channel color combinations are applied to each pixel position of the CMYK defect. The combinations are shown in Table 1. 1 represents that one of the remaining channels is inkjetted, and 0 represents that one of the remaining channels is not inkjetted.

[0048] Table 1: Channel color combinations Channel 1 Channel 2 Channel 3 1 1 1 0 1 1 1 0 1 1 1 0 0 0 1 1 0 0 0 1 0 0 0 0 For the remaining channels of all defective pixels, the above 8 colors are used to match 8 different color combinations. The 3×3 area of ​​the pixel position of each color combination after matching is converted using the device-specific ICC profile and absolute colorimetric rendering intent, from CMYK to LAB space, and then the ink droplet diffusion model is applied to generate the LAB map. Each group of 8 combinations of 3×3 is compared with the 3×3 area of ​​the corresponding position of the simulated ideal printed image for color difference, and the color difference between the compensated image and the ideal image is calculated using the CIEDE2000 color difference formula: ; in: are the differences in brightness, chroma, and hue respectively.

[0049] is the scaling factor for brightness, chroma, and hue.

[0050] is the interaction term between chroma and hue.

[0051] are weighting factors for brightness, chroma, and hue.

[0052] 5) Select the best color ink dot combination to replace the original channel combination arrangement of defective pixels.

[0053] 6) Apply this method to all pixels.

[0054] Outputs compensated halftone image data for actual printing.

[0055] Experimental data and results: In order to verify the effectiveness of the method of the present invention, we conducted the following specific experiments: Set the CMYK four-group color structure image to 1200dpi and the inkjet ink volume to 01.

[0056] Experimental data analysis: By comparing the experimental results, the following conclusions can be drawn: 1. In the high density range (198-255), the method of the present invention can significantly reduce stripe defects, reduce stripe color difference, and improve printing quality.

[0057] 2. In the medium and high density range (142-198), the method of the present invention can effectively repair the density effect around the defective pixels, match the color with the minimum color difference, and improve the printing effect.

[0058] 3. In the low to medium density range (85-142), the method of the present invention matches the color with the smallest color difference and reduces the visual impact of the stripe defect.

[0059] 4. In the low density range (0-85), the method of the present invention can minimize the probability of adding points, prevent over-compensation, and the color difference compensation algorithm adjusts the defect position to yellow or colorless.

[0060] In summary, the method of the present invention can effectively reduce stripe defects and improve printing quality within different density ranges, which verifies the effectiveness and superiority of the fusion compensation algorithm of the present invention.

[0061] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for compensating inkjet printing stripe defects, characterized in that: The following steps are involved: S1: Acquire halftone image data with stripe defects to be printed; S2: Determine the pixel columns with streak defects in the image; S3: For each defective pixel, calculate the average gray value M in the n×n pixel grid centered on the defective pixel, where n is the number of pixels; S4: Select a corresponding compensation mode according to the range of the average gray value M; S5: Perform dynamic point compensation on the defective pixel and its surrounding pixels according to the selected compensation mode; S6: performing chromatic aberration compensation on the compensated image; S7: Output the compensated halftone image data.

2. The inkjet printing stripe defect compensation method according to claim 1, characterized in that: The specific steps of S4 are as follows: When M is in the range of 198-255, high-density compensation mode is used: Add a number of adjacent pixels on both sides of the defective pixel and increase the volume of ink droplets to enhance the printing density and concentration; When M is in the range of 142-198, medium and high density compensation mode is used: Add b adjacent pixels on both sides of the defective pixel and increase the ink density to effectively repair the defective pixel, where 1<b<a; When M is in the range of 85-142, the medium and low density compensation mode is adopted: Add a dot to the left or right of the defective pixel in an adjacent position without ink to adjust the brightness; When M is in the range of 0-85, low-density compensation mode is used: Randomly add a dot to the left, right, or pixel position of the defective pixel to minimize the probability of adding a dot and prevent overcompensation.

3. The inkjet printing stripe defect compensation method according to claim 1, characterized in that: The specific process of S6 is as follows: S6.1: Simulating local image data under ideal printing conditions on a computer: Using an image processing algorithm, simulating the diffusion and coloring effects of ink droplets under ideal conditions, so as to provide a reference image for subsequent color difference calculation and compensation processes; the ideal printing state is a state in which the generated image data is not affected by nozzle failure during the printing process, and the distribution, size and position of the ink droplets are in accordance with expectations; S6.2: simulating the compensated local printing image data on a computer; S6.3: Convert CMYK color space to LAB color space; S6.4: Calculate the color difference between the compensated image and the ideal image using the color difference formula; S6.5: According to the color difference results, the dot structure of the remaining channels is adjusted so that the printing effect after compensation is closest to the ideal state.

4. The inkjet printing stripe defect compensation method according to claim 3, characterized in that: In S6.1 and S6.2, a simplified ink droplet diffusion model is used to simulate the printing effect, as follows: Define an N×N matrix G, where N=(diffusion diameter) / (pixel size). For each element G[i, j] in G, we have: ； in: i represents the row index in the matrix G, and the value of i ranges from 0 to N-1; j represents the column index in the matrix G, and the value of j ranges from 0 to N-1; e represents the base of natural logarithm, which is approximately 2.71828; σ represents the standard deviation of Gaussian distribution, and its value is equal to N / 6; N represents the dimension of the matrix, and its value is equal to the quotient of the diffusion diameter divided by the pixel size; G is then normalized so that its sum is 1: 。 5. The inkjet printing stripe defect compensation method according to claim 3, characterized in that: In S6.4, the CIEDE2000 color difference formula is used to calculate the color difference between the compensated image and the ideal image; the expression of the CIEDE2000 color difference formula is: ； in: are the differences in brightness, chroma, and hue, respectively; are the scaling factors for brightness, chroma, and hue; is the interaction term between chroma and hue; are weighting factors for brightness, chroma, and hue.

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