Image information secrecy method capable of being used for paper window envelope and image printing method based on secrecy method

Abstract

The invention discloses an image information secrecy method for a paper window envelope and an image printing method based on the secrecy method.The image information secrecy method comprises the steps that 1, a periodic base image is generated through a periodic function, and a secret key image IA is generated through the base image; 2, extracting components of RGB channels of the target color image, converting the components into a gray coding matrix, and normalizing pixel values; 3, embedding the components of the three channels of the target color image into the phase parameters of the substrate image to obtain an embedded image; 4, generating an information image IB containing target color image information according to the phase parameter of the embedded image; and 5, directly superposing the IA and the IB so as to display a decrypted image Ic approximate to the target color image. According to the method, high secrecy and clear display of the multi-tone target image are realized, and when the method is used for the window envelope, the information secrecy of the window envelope can be improved.

Description

Technical Field

[0001] The present invention relates to image information hiding and optical anti-counterfeiting technology, in particular to an image information confidentiality method that can be used for paper window envelopes and an image printing method based on the confidentiality method. Background Art

[0002] In today's digital age, the confidentiality and secure transmission of information have become crucial issues. With the rapid development of information technology, people's demand for information protection is growing. Traditional paper envelopes are gradually exposing their limitations in protecting the privacy of letter contents. If partial information needs to be displayed on the outside of the envelope or anti-counterfeiting verification is required, traditional envelope designs cannot meet modern information security requirements. Therefore, the development of image information confidentiality technology suitable for paper envelope windows is particularly urgent.

[0003] With the continuous advancement of visual encryption technology, such as the continuous development of Moiré gratings and visual cryptography, new ideas have been provided for solving this problem due to their convenient decryption methods. These technologies use visual encryption to divide information into two or more encrypted images to achieve information hiding, and display the target image information through physical superposition. Specifically, when two encrypted images are superimposed at a specific angle or spacing, the encrypted target image information can be quickly parsed with the help of the visual characteristics of the human eye. The entire process does not require complex equipment or calculations. With its unique advantages, this technology has shown broad application prospects in the field of information hiding.

[0004] Breakthroughs in visual encryption technology have also opened up a new dimension for the confidentiality of information in paper envelopes. By dispersing information into multiple unrecognizable pattern slices and leveraging the visual fusion characteristics of the human eye to achieve traceless decryption, it avoids the traditional encryption method's reliance on electronic devices and greatly enhances the anti-counterfeiting and privacy of physical letters. This encryption mode based on human-computer collaboration combines information fragmentation processing with the visual analysis capabilities of the naked eye. It not only breaks through the physical limitations of traditional envelope design, but also meets the personalized needs of different security levels through a multi-layer encryption structure, providing technical support for the survival of paper communications in the digital wave.

[0005] However, traditional visual encryption technology has obvious limitations when processing multi-tone images. This is due to its reliance on graphic overlay decryption, which has limited information carrying capacity and is unable to efficiently transmit large amounts of data, easily leading to information loss or blurring in the decrypted image. In addition, its color encryption mechanism is still imperfect. When processing multi-tone color images, color details are easily damaged, affecting the clarity and color reproduction of the decrypted image, making it difficult to meet high-quality display requirements and unable to achieve ideal hiding and display effects. Summary of the Invention

[0006] The purpose of the present invention is to provide an image information confidentiality method that can be used for paper window envelopes and an image printing method based on the confidentiality method, which achieves high confidentiality and clear display of multi-tone target images and improves the information confidentiality of the window envelope.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for keeping image information confidential that can be used on a paper window envelope comprises the following steps:

[0009] Step 1: Generate a periodic basis image G(x,y) through a periodic function, and directly generate the key image I from the basis image G(x,y) A , the basis image G(x,y) is expressed as:

[0010]

[0011] In the formula, the base image G(x,y) is a grating structure, A is the amplitude of the grating, is the initial phase, is the phase function of the grating, (x, y) is the coordinate of the hidden information pixel position, and the phase function of the linear grating is expressed as:

[0012]

[0013] Where x is the horizontal coordinate value and T is the grating period, which is expressed as:

[0014]

[0015] Where f is the frequency of the grating; T ranges from (0 to 1), and the grating shape is controlled by adjusting the grating period T;

[0016] Step 2: Process the target color image, extract the components of the three channels R, G, and B of the target image and convert them into a grayscale encoding matrix M, and then normalize the pixel values ​​of the target color image to the range of [0, 1], which can be expressed as:

[0017] M=[RGB]

[0018] R,G,B=I r (x,y),I g (x,y),I b (x,y)

[0019] Step 3: embed the R, G, and B channel components of the target color image into the phase parameters of the base image to hide the image and obtain the embedded image. Expressed as:

[0020]

[0021] Step 4: According to the phase parameters of the embedded image Generate hidden image raster G hidden (x, y), that is, the information image I containing the target color image information B , expressed as:

[0022]

[0023] Step 5: Key image I A and information image I B Directly superimpose to display the decrypted image I that is similar to the target color image c .

[0024] Furthermore, the specific process of extracting the components of the three channels R, G, and B of the target color image and converting them into the grayscale coding matrix M in step 2 is expressed as follows:

[0025] I r (x,y),I g (x,y),I b (x,y)=extract_rgb(I(x,y))

[0026] Where, I r (x,y),I g (x,y),I b (x, y) are the grayscale encoding matrices of the three channels R, G, and B respectively, I(x, y) is the target color image, extract_rgb() is the function for extracting the grayscale encoding matrix of the color channel. In Python, it is expressed as:

[0027] extract_rgb(I(x,y))=[:,:,X],X∈{0,1,2}.

[0028] Furthermore, the process of normalizing the pixel values ​​of the target color image to the range of [0, 1] in step 2 is expressed as:

[0029]

[0030] Where, I z is the grayscale encoding matrix of the R, G, and B channels, and r, g, and b are the components of the R, G, and B channels respectively.

[0031] Furthermore, in step 5, the key image I A and information image I B Directly superimpose to display the decrypted image I that is similar to the target color image c The principle is expressed as:

[0032]

[0033] Where G overlay (x,y) is the decrypted image, and Δφ is the phase offset.

[0034] Furthermore, the key image I of step 1 A , the target color image in step 2, the information image I in step 4 B and the decrypted image I in step 5 c The formats are JPG, PNG or TIFF, and the resolution is 300 to 2540 dpi.

[0035] A method for printing images on paper window envelopes, comprising the following steps:

[0036] Step 1: Give the key image I A Add positioning lines and key image I A Binarization processing, the process of binarization processing is expressed as:

[0037]

[0038] Where, Binarization() is the binarization function, Threshold is the threshold parameter for image binarization processing;

[0039] Step 2: After processing A and positioning lines are printed on the lower surface of the transparent film by gravure printing, wherein: the key image I A The size of the transparent film is not less than the size of the envelope window, the size of the transparent film is larger than the size of the envelope window, and the four sides of the transparent film are bonded to the inner surface of the envelope;

[0040] Step 3: Give information image I B Add positioning lines and place the information image I B The conversion process from RGB image to CMYK image can be expressed as:

[0041] d min =min(I r (x,y),I g (x,y),I b (x,y))

[0042] e max =max(I r (x,y),I g (x,y),I b (x,y))

[0043]

[0044] C=ir (x,y)-K

[0045] M=I g (x,y)-K

[0046] Y=I b (x,y)-K

[0047] Where, d min is the minimum value among the normalized RGB values, e max is the maximum value of the normalized RGB value, C, M, Y, K are cyan, magenta, yellow and black color plate images respectively, and c is the adjustment coefficient;

[0048] Step 4: Halftone the C, M, Y, and K color plate images obtained in step 3, and print the halftone-processed color plates and positioning lines on the envelope paper on the bottom of the envelope by offset printing. B The valid information is hidden in the corresponding position of the envelope window and can be fully displayed in the envelope window;

[0049] Step 5: After the contents and air in the envelope are exhausted, align the positioning lines and superimpose the decrypted image I that is close to the target image. C .

[0050] Furthermore, the transparent film in step 2 is a PVC film or a PET film.

[0051] Furthermore, in step 2, a black plate K is used for gravure printing, the screen line number is 150 to 175 L / inch, the screen angle is 45°, and the cell shape is diamond.

[0052] Furthermore, in the offset printing process of step 4, the screen line count is the same as that of step 2, and the screen angles of the C, M, Y, and K color plates are 15°, 75°, 0°, and 45°, respectively, and the dot shape is circular or square.

[0053] Furthermore, the envelope paper in step 4 is coated paper, kraft paper or cardboard.

[0054] The present invention has the following beneficial technical effects:

[0055] 1) Based on the moiré effect, the present invention utilizes the nonlinear coupling characteristics of the grating phase parameters and combines it with the color space separation technology to hide a multi-tone target color image in a linear grating, thereby realizing the hiding and display of the color multi-tone image. When the hidden image is correctly decrypted, it can display image information with a high similarity index with the target color image. When the hidden image is illegally decrypted, the effective color information cannot be restored due to spectrum aliasing caused by phase mismatch. It can be seen that the present invention enhances the confidentiality of information and is suitable for anti-counterfeiting identification, information hiding and other fields.

[0056] 2) The present invention designs two layers of information encryption images, one of which is printed on the envelope window, processed by a periodic function, and not combined with the target color image, and is used to provide a basic moiré fringe structure and a key image; the other layer of information encryption image is printed inside the envelope, and the target color image is subjected to channel separation and normalized grayscale processing to generate an information hiding image containing multi-tone color image information. When the contents of the envelope are taken out, the two layers of images are superimposed to obtain a decrypted image that approximates the target image information, thereby improving the information confidentiality of the paper window envelope.

[0057] 3) The target color image of the present invention can be associated with the content of the letter. For example, the hidden information can be the initial password of a bank card. Only by opening the envelope, taking out its contents and superimposing the two layers of gratings can the initial password information be obtained. The hidden information can also be the official seal of a company. By opening the envelope, the authenticity can be determined by checking whether it is consistent with the official seal on the letter content. In short, by virtue of the characteristic of associating the image information confidentiality method of the present invention with the content of the letter, not only can a better confidentiality effect be achieved, but also the application range of paper window envelopes can be broadened. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 The decrypted image obtained in Application Example 1 of the present invention for decryption;

[0059] Figure 2 The information image containing target color image information obtained in Application Example 1 of the present invention;

[0060] Figure 3 The target image is displayed after the two layers of gratings are correctly superimposed in Application Example 2 of the present invention;

[0061] Figure 4 The target image displayed after the two layers of gratings are incorrectly superimposed in Application Example 2 of the present invention;

[0062] Figure 5 This is Application Example 3 of the present invention, which is applied to a window envelope and displays the contents without removing them;

[0063] Figure 6 This is Application Example 3 of the present invention, which is applied to an envelope and displays the contents thereof having been taken out. DETAILED DESCRIPTION

[0064] The present invention will be further described in detail below with reference to specific embodiments, which are intended to explain the present invention rather than to limit it.

[0065] A method for keeping image information confidential that can be used on a paper window envelope comprises the following steps:

[0066] Step 1: Generate a periodic basis image G(x,y) through a periodic function, and directly generate the key image I from the basis image G(x,y) A , key image I A The grating structure directly generated by the base image G(x,y) is not combined with the target color image. When the periodic function is a sine function, it is expressed as:

[0067]

[0068] In the formula, the base image G(x,y) is a grating structure, A is the amplitude of the grating, is the initial phase, is the phase function of the grating, (x, y) is the coordinate of the hidden information pixel position, and the phase function of the linear grating is expressed as:

[0069]

[0070] Where x is the horizontal coordinate value and T is the grating period, which is expressed as:

[0071]

[0072] Where f is the frequency of the grating; T ranges from (0 to 1). By adjusting the grating period T, the grating period is controlled, thereby controlling the grating shape, that is, the spacing and direction of the grating stripes.

[0073] The periodic basis image G(x,y) and the key image I A The formats are JPG, PNG or TIFF, with a resolution of 300 to 2540 dpi;

[0074] Step 2: Process the target color image, extract the components of the three channels R, G, and B of the target image and convert them into a grayscale encoding matrix M, and then normalize the pixel values ​​of the target color image to the range of [0, 1], which can be expressed as:

[0075] M=[RGB]

[0076] R,G,B=I r (x,y),I g (x,y),I b (x,y)

[0077] The specific process of extracting the components of the three channels R, G, and B of the target color image and converting them into the grayscale coding matrix M is expressed as follows:

[0078] I r (x,y),I g (x,y),I b (x,y)=extract_rgb(I(x,y))

[0079] Where, I r (x,y),I g (x,y),I b (x, y) are the grayscale encoding matrices of the three channels R, G, and B respectively, I(x, y) is the target color image, extract_rgb() is the function for extracting the grayscale encoding matrix of the color channel. In Python, it is expressed as:

[0080] extract_rgb(I(x,y))=[:,:,X],X∈{0,1,2}

[0081] The process of normalizing the pixel values ​​of the target color image to the range of [0, 1] is expressed as:

[0082]

[0083] Where, I z is the grayscale encoding matrix of the three channels R, G, and B, r, g, and b are the components of the three channels R, G, and B respectively;

[0084] The target color image is in the format of JPG, PNG or TIFF, with a resolution of 300 to 2540 dpi. The target color image information can be associated with the envelope contents to achieve better confidentiality. For example, the hidden information is the initial password of a bank card, and the initial password information can only be obtained by opening the envelope and taking out the contents. The hidden information can also be the official seal of a company, and the authenticity can be determined by checking whether the seal on the envelope contents is consistent with the seal on the envelope contents.

[0085] Step 3: embed the R, G, and B channel components of the target color image into the phase parameters of the base image to hide the image and obtain the embedded image. Expressed as:

[0086]

[0087] Step 4: According to the phase parameters of the embedded image Generate hidden image raster G hidden (x, y), that is, the information image I containing the target color image information B, expressed as:

[0088]

[0089] The information image I B The formats are JPG, PNG or TIFF, with a resolution of 300 to 2540 dpi;

[0090] Step 5: Key image I A and information image I B Direct overlay to display the decrypted image I that is similar to the target color image c .

[0091] A method for printing images on paper window envelopes, comprising the following steps:

[0092] Step 1: Give the key image I A Add positioning lines and key image I A Binarization processing, the process of binarization processing is expressed as:

[0093]

[0094] Where, Binarization() is the binarization function, Threshold is the threshold parameter for image binarization processing;

[0095] Step 2: After processing A The key image I and the positioning line are printed on the lower surface of the transparent film (ie the surface sealed inside the envelope) by gravure printing, wherein: A The size of the transparent film is not less than the size of the envelope window, the size of the transparent film is larger than the size of the envelope window, and the four sides of the transparent film are bonded to the inner surface of the envelope;

[0096] The gravure printing adopts black plate K printing, the screen number is 150-175L / inch, the screen angle is 45°, and the cell shape is diamond;

[0097] The transparent film is a PVC film or a PET film, or other transparent films suitable for gravure printing;

[0098] The envelope window is rectangular, and can also be designed to be circular, polygonal, or other shapes according to actual usage scenarios, so long as it can fully display the effective information of the hidden image;

[0099] Step 3: Give information image I B Add positioning lines and place the information image I B The conversion process from RGB image to CMYK image can be expressed as:

[0100] dmin =min(I r (x,y),I g (x,y),I b (x,y))

[0101] e max =max(I r (x,y),I g (x,y),I b (x,y))

[0102]

[0103] C=I r (x,y)-K

[0104] M=I g (x,y)-K

[0105] Y=I b (x,y)-K

[0106] Where, d min is the minimum value among the normalized RGB values, e max is the maximum value of the normalized RGB value, C, M, Y, K are cyan, magenta, yellow and black color plate images respectively, and c is the adjustment coefficient;

[0107] Step 4: Halftone the C, M, Y, and K color plate images obtained in step 3, and print the halftone-processed color plates and positioning lines on the envelope paper on the inside of the bottom surface of the envelope by offset printing. B The size of the envelope is the same as that of the envelope, but the effective information should be hidden in the corresponding position of the envelope window and can be fully displayed in the envelope window, thereby improving the hiding effect of the information;

[0108] The offset printing screen count is 150 to 175 L / inch, and the offset printing screen count is consistent with the screen count in step 2. The screen angles of the C, M, Y, and K color plates are 15°, 75°, 0°, and 45°, respectively, and the dot shape is circular or square.

[0109] The printing paper is coated paper or kraft paper;

[0110] Step 5: After the contents of the envelope are taken out, align the positioning line and key image I A and information image I B Directly superimpose to display the decrypted image I that is similar to the target color image c , the principle is expressed as:

[0111]

[0112] Where G overlay (x,y) is the decrypted image, is the phase offset.

[0113] Whether it is applied to window envelope or other fields, as long as the appropriate phase offset is selected, the key image I A and information image I B Directly superimpose to obtain the decrypted image I that is close to the target image information c .

[0114] Application Example 1

[0115] The audrey image with a size of 1000 pixels × 1000 pixels, a resolution of 1120 dpi, and a JPG format is used as the target color image to hide and display confidential information:

[0116] Step 1: Generate Figure 1 The key image shown is in TIFF format and has a resolution of 1120 dpi. A , the amplitude of the key image grating structure is 0.5, the initial phase is 0, and the period is 0.025;

[0117] Step 2: Process the target color image I(x,y) in TIFF format and with a resolution of 1120 dpi, extract the components of its R, G, and B channels and convert them into a grayscale encoding matrix, and then normalize the pixel values ​​to the range of [0,1].

[0118] Step 3: embed the R, G, and B channels of the image into the phase parameters of the grating respectively to hide the image and obtain the phase parameters of the embedded grating.

[0119] Step 4: Based on the embedded phase parameters Generates a hidden image raster G in TIFF format with a resolution of 1120 dpi hidden (x,y), that is, Figure 2 The information image I shown B ;

[0120] Step 5: Key image I A and information image I B Use computer simulation to superpose and take the phase offset Get the decrypted image I of the target color image in JPG format and 1120dpi resolution c .

[0121] from Figure 1 and Figure 2 It can be seen that the generated key image I AWith information image I B None of them displays the effective original target color image information, and has an excellent hiding effect.

[0122] Application Example 2

[0123] The target color image is a lena image with a size of 1000 pixels × 1000 pixels, a resolution of 720 dpi, and a JPG format, to hide and display confidential information:

[0124] Step 1: Generate a key image I in TIFF format with a resolution of 720dpi A , key image I A The amplitude of the grating structure is 0.8 and the initial phase is The period is 0.02;

[0125] Step 2: Process the target color image I(x,y) in TIFF format and with a resolution of 720 dpi, extract the components of its R, G, and B channels and convert them into a grayscale encoding matrix M, and then normalize the pixel values ​​to the range of [0,1].

[0126] Step 3: embed the R, G, and B channels of the image into the phase parameters of the grating respectively to hide the image and obtain the phase parameters of the embedded grating.

[0127] Step 4: Based on the embedded phase parameters Generates a hidden image raster G in TIFF format with a resolution of 720dpi hidden (x,y), that is, information image I B ;

[0128] Step 5: Key image I A and information image I B Directly perform superposition, and the phase offset is Get as Figure 3 The decrypted image I of the target image is shown in JPG format with a resolution of 720dpi. C .

[0129] Figure 4 When the phase offset is π, the key image I A and information image I B Directly superimpose and obtain the target image display, by comparison Figure 3 and Figure 4 It can be seen that only by selecting the correct phase offset can the decrypted image I similar to the target color image be obtained. c ,Otherwise, the displayed decrypted image will be obviously distorted compared to the original target color image.

[0130] Application Example 3

[0131] The target color image is a number image with a size of 612 pixels × 612 pixels, a resolution of 960 dpi, and a JPG format. The confidential information is hidden and displayed:

[0132] Step 1: Generate a key image in TIFF format with a resolution of 960dpi. A , key image I A The amplitude of the grating structure is 0.8 and the initial phase is The period is 0.02;

[0133] Step 2: Process the target color image I(x,y) in TIFF format and with a resolution of 960dpi, extract the components of its R, G, and B channels and convert them into a grayscale encoding matrix M, and then normalize the pixel values ​​to the range of [0,1].

[0134] Step 3: embed the R, G, and B channels of the image into the phase parameters of the grating respectively to hide the image and obtain the phase parameters of the embedded grating.

[0135] Step 4: Based on the embedded phase parameters Generates a hidden image raster G in TIFF format with a resolution of 960 dpi hidden (x,y), that is, information image I B ;

[0136] Step 5: Key image I A Printed on the envelope window, the specific process is: first the key image I A Add positioning lines and perform binarization processing. Select a transparent film with a size of 812 pixels × 392 pixels. Take the center of the transparent film as the center of the key image. Use a black plate K to convert the processed I A and positioning lines are printed on the lower surface of the PET film by gravure printing, and the envelope window is rectangular;

[0137] The gravure printing has a screen line count of 175 L / inch, a screen angle of 45°, and is printed using a black plate (K), with a diamond-shaped cell shape.

[0138] The portion of the PET film that is larger than the envelope window is bonded to the envelope, so that the positioning line and the effective information of the hidden image are completely displayed in the envelope window;

[0139] Step 6: Transform the information image I B Printed on the envelope paper on the inside of the bottom of the envelope, the specific process is: first, the information image I BAdd registration lines and convert the RGB image into a CMYK image. Set the adjustment coefficient c to 15. Then, halftone the C, M, Y, and K color plates in sequence. Print the halftoned color plates and registration lines on the white cardboard on the inside of the bottom of the envelope using offset printing. Use a screen ruling of 175 L / inch, screen angles of 15°, 75°, 0°, and 45° for the C, M, Y, and K color plates, respectively, and a circular dot shape.

[0140] Step 7. Take out the contents of the envelope and expel the air. Move it to align the positioning line so that the key image I A With information image I B Superposition occurs to obtain the decrypted image I that approximates the target image information C .

[0141] Figure 5 The display results when there is a letter in the envelope are shown. It can be seen that when there is a letter in the envelope, the hidden image is not displayed; Figure 6 The display results when the letter is taken out of the envelope are shown. It can be seen that when there is no letter in the envelope, the window shows a hidden image similar to the target color image.

Claims

1. A method for keeping image information confidential that can be used for paper window envelopes, characterized in that: The steps include: Step 1: Generate a periodic basis image G(x,y) through a periodic function, and directly generate the key image I from the basis image G(x,y) A , the basis image G(x,y) is expressed as: In the formula, the base image G(x,y) is a grating structure, A is the amplitude of the grating, is the initial phase, is the phase function of the grating, (x, y) is the coordinate of the hidden information pixel position, and the phase function of the linear grating is expressed as: Where x is the horizontal coordinate value and T is the grating period, which is expressed as: Where f is the frequency of the grating; T ranges from (0 to 1), and the grating shape is controlled by adjusting the grating period T; Step 2: Process the target color image, extract the components of the three channels R, G, and B of the target image and convert them into a grayscale encoding matrix M, and then normalize the pixel values ​​of the target color image to the range of [0, 1], which can be expressed as: M=[RGB] R,G,B=I r (x,y),I g (x,y),I b (x,y) Step 3: embed the R, G, and B channel components of the target color image into the phase parameters of the base image to hide the image and obtain the embedded image. Expressed as: Step 4: According to the phase parameters of the embedded image Generate hidden image raster G hidden (x, y), that is, the information image I containing the target color image information B , expressed as: Step 5: Key image I A and information image I B Direct overlay to display the decrypted image I that is similar to the target color image c .

2. The image information confidentiality method applicable to paper window envelopes according to claim 1, characterized in that: The specific process of extracting the components of the three channels R, G, and B of the target color image and converting them into the grayscale coding matrix M in step 2 is expressed as follows: I r (x,y),I g (x,y),I b (x,y)=extract_rgb(I(x,y)) Where, I r (x,y),I g (x,y),I b (x, y) are the grayscale encoding matrices of the three channels R, G, and B respectively, I(x, y) is the target color image, extract_rgb() is the function for extracting the grayscale encoding matrix of the color channel. In Python, it is expressed as: extract_rgb(I(x,y))=[:,:,X],X∈{0,1,2}.

3. The image information confidentiality method applicable to paper window envelopes according to claim 1, characterized in that: The process of normalizing the pixel values ​​of the target color image to the range of [0, 1] in step 2 is expressed as: Where, I z is the grayscale encoding matrix of the R, G, and B channels, and r, g, and b are the components of the R, G, and B channels respectively.

4. The image information confidentiality method applicable to paper window envelopes according to claim 1, characterized in that: In step 5, the grating I A and grating I B Directly superimpose to display the decrypted image I that is similar to the target color image c The principle is expressed as: Where G overlay (x,y) is the decrypted image, and Δφ is the phase offset.

5. The image information confidentiality method applicable to paper window envelopes according to claim 1, characterized in that: The key image I of step 1 A , the target color image in step 2, the information image I in step 4 B and the decrypted image I in step 5 c The formats are JPG, PNG or TIFF, and the resolution is 300 to 2540 dpi.

6. A method for printing images on paper window envelopes based on the image information confidentiality method of claim 1, characterized in that: The steps include: Step 1: Give the key image I A Add positioning lines and key image I A Binarization processing, the process of binarization processing is expressed as: Where, Binarization() is the binarization function, Threshold is the threshold parameter for image binarization processing; Step 2: After processing A and positioning lines are printed on the lower surface of the transparent film by gravure printing, wherein: the key image I A The size of the transparent film is not less than the size of the envelope window, the size of the transparent film is larger than the size of the envelope window, and the four sides of the transparent film are bonded to the inner surface of the envelope; Step 3: Give information image I B Add positioning lines and place the information image I B The conversion process from RGB image to CMYK image is expressed as: d min =min(I r (x,y),I g (x,y),I b (x,y)) e max =max(I r (x,y),I g (x,y),I b (x,y)) C=i r (x,y)-K M=I g (x,y)-K Y=I b (x,y)-K Where, d min is the minimum value among the normalized RGB values, e max is the maximum value of the normalized RGB value, C, M, Y, K are cyan, magenta, yellow and black color plate images respectively, and c is the adjustment coefficient; Step 4: Halftone the C, M, Y, and K color plate images obtained in step 3, and print the halftone-processed color plates and positioning lines on the envelope paper on the bottom of the envelope by offset printing. B The valid information is hidden in the corresponding position of the envelope window and can be fully displayed in the envelope window; Step 5: After the contents and air in the envelope are exhausted, align the positioning lines and superimpose the decrypted image I that is close to the target image. C .

7. The printing method for paper window envelope images according to claim 6, characterized in that: The transparent film in step 2 is a PVC film or a PET film.

8. The printing method for paper window envelope images according to claim 6, characterized in that: In the step 2, a black plate K is used for gravure printing, the screen line number is 150 to 175 L / inch, the screen angle is 45°, and the cell shape is diamond.

9. The printing method for paper window envelope images according to claim 8, characterized in that: During the offset printing process of step 4, the screen ruling is the same as that of step 2, and the screen angles of the C, M, Y, and K color plates are 15°, 75°, 0°, and 45°, respectively, and the dot shape is circular or square.

10. The printing method for paper window envelope images according to claim 6, characterized in that: The envelope paper in step 4 is coated paper, kraft paper or cardboard.

Images