DE10229975B4 - Method for compressing and decompressing image data - Google Patents

Abstract

A method of compressing image data consisting of an array of individual pixels, each pixel (0-419) having a pixel value describing color or brightness information of the pixel, comprising the steps of: a) determining a priority value for each pixel setting the pixel used as a reference pixel (P0) and calculating a pixel difference value from the respective pixel value of the reference pixel (P0) with respect to the pixel values of a predetermined group of adjacent pixels (P1-P4); b) combining the pixels (P0-P4) used for the calculation of the priority value into a pixel group, c) sorting the pixel groups of the image array based on the priority value of the assigned reference pixel (P0); and d) storing and / or transmitting the pixel groups according to their priority, wherein depending on the desired compression factor only a part of the pixel groups is stored and / or transmitted, according to the main patent DE 101 52 612 B4, characterized in that at least one key is applied with which optionally the position value and / or the pixel value (s) of a pixel group are encrypted.

Description

• a) Ermitteln eines Prioritätswertes für jedes Pixel des Arrays durch Festlegen des verwendeten Pixels als Bezugspixel (P0) und Berechnen eines Pixeldifferenzwertes anhand des jeweiligen Pixelwerts des Bezugspixels (P0) in Bezug auf die Pixelwerte einer zuvor festgelegten Gruppe von benachbarten Pixeln (P1–P4);
• b) Zusammenfassen der für die Berechnung des Prioritätswertes hinzugezogenen Pixel (P0–P4) zu einer Pixelgruppe,
• c) Sortieren der Pixelgruppen des Bildarrays anhand des Prioritätswertes des zugeordneten Bezugspixels (P0); und
• d) Abspeichern und/oder Übertragen der Pixelgruppen entsprechend ihrer Priorität, wobei je nach dem angestrebten Kompressionsfaktor nur ein Teil der Pixelgruppen abgespeichert und/oder übertragen wird,

nach dem Hauptpatent DE 101 52 612 B4 .The invention relates to a method for compression and image data consisting of an array of individual pixels, each pixel having a pixel value describing color or brightness information of the pixel, comprising the steps of:

• a) Determining a priority value for each pixel of the array by specifying the pixel used as a reference pixel (P0) and calculating a pixel difference value from the respective pixel value of the reference pixel (P0) with respect to the pixel values of a predetermined group of neighboring pixels (P1-P4) ;
• b) combining the pixels (P0-P4) used for the calculation of the priority value into a pixel group,
• c) sorting the pixel groups of the image array based on the priority value of the associated reference pixel (P0); and
• d) storing and / or transmitting the pixel groups according to their priority, wherein, depending on the desired compression factor, only a part of the pixel groups is stored and / or transmitted,

after the main patent DE 101 52 612 B4 ,

• a) Generieren eines leeren Bildarrays aus den eingelesenen Kenngrößen des komprimierten Bildes,
• b) Einlesen der Eck-Pixelgruppen und Einfügen an den vier Bildecken des Bildarrays,
• c) Bilden von Dreiecken durch Verbinden von jeweils drei unmittelbar benachbarten Pixelgruppen durch mindestens eine Linie,
• d) Ausfüllen der die Fläche der Dreiecke bildenden Pixel durch einen aus den das Dreieck bildenden Pixelgruppen berechneten Farb- und/oder Helligkeitsverlauf,
• e) Einlesen und Einfügen der nächsten Pixelgruppe in das Bildarray;
• f) Wiederholen der Schritte c) bis f).

nach dem Hauptpatent DE 101 52 612 B4 .Similarly, the invention relates to a method for decompressing image data compressed by the corresponding compression method, comprising the steps of:

• a) generating an empty image array from the imported characteristics of the compressed image,
• b) reading in the corner pixel groups and inserting them on the four corners of the image array,
• c) forming triangles by connecting each of three immediately adjacent pixel groups by at least one line,
• d) filling in the pixels forming the surface of the triangles by a color and / or brightness curve calculated from the pixel groups forming the triangle
• e) reading and inserting the next pixel group into the image array;
• f) repeating steps c) to f).

after the main patent DE 101 52 612 B4 ,

The creation of information content z. As pictures, videos, audio data and documents is very expensive. In the transmission and storage of such information content, it is useful and necessary in many applications to encrypt the information content in order to protect it against unauthorized access. There are a number of encryption methods and applications that fulfill this task.

• – Direkt in einer Anwendung, z. B. durch Kennwortschutz bei Personal Computern oder für einen Programmzugriff
• – Unabhängig von einer Anwendung, z. B. mittels des bekannten PGP Verschlüsselungsverfahrens bei E-Mail Anwendungen
• – Bei der Übertragung der Informationen, z. B. Informationsübertragung über das Internet mittels IPSec (Internet Protocol Security)

In this case, the information content can be encrypted at various levels.

• - Directly in an application, eg. By password protection on personal computers or for program access
• - Regardless of an application, eg. By using the well-known PGP encryption method in e-mail applications
• - When transmitting the information, eg. B. Information transmission over the Internet by means of IPSec (Internet Protocol Security)

The previously known encryption method lacks the possibility of encrypting the information content in a flexible and scalable manner. That is, there are no flexible settings, for example, depending on the information content and the specific application to perform an adapted encryption can. However, this can be useful if you z. B. video on demand (video on demand) wants to offer, for different video qualities, eg. B. depending on the image resolution, different fees should be calculated.

Various methods for compressing and decompressing image data are also known with additional encryption. The Essay by Simon, Serge: Generalized Run-Length Coding for SNR-scalable Image Compression; Signal Processing VII, Theories and Applications, Edinburgh, UK, 1994, pages 560-563, describes methods for encoding digital images. The methods are based on the so-called run length encoding. Run-length coding is a method of data compression used in source coding. It is based on the fact that in some file often certain characters appear several times in a row. This string is then replaced by the character itself and a number for its frequency. The storage of N contiguous pixels, which all have the identical pixel value Px, takes place as a value pair (N, Px), whereby for N> 2 a Information loss-free and uniquely reversible data reduction is effected. An encryption of the encoded data or parts thereof with an additional key does not occur.

The publication JP 11-88857 A deals with an encryption of a compressed image signal. The compression of the image signal is performed by dividing the image signal into blocks, each block undergoing a discrete cosine transformation (DCT). This results in a signal consisting of individual frequency components that are at least partially subjected to encryption.

The object of the invention is a method according to the main patent DE 101 52 612 B4 be supplemented so that a flexible and scalable encryption and decryption of the information content is possible.

This object is achieved by the characterizing features of claims 1 and 2.

According to the main patent z. Digital image or video data consisting of an array of individual pixels (pixels), each pixel having a time-varying pixel value describing color or brightness information of the pixel. According to the invention, a priority is assigned to each pixel or each pixel group and the pixels are stored in a priority array in accordance with their prioritization. At any given time, this array contains the pixel values sorted after prioritization. According to the prioritization, these pixels, and the pixel values used for the calculation of the prioritization, are transferred or stored. A pixel gets a high priority if the differences to its neighboring pixels are very large. For reconstruction, the current pixel values are shown on the display. The not yet transmitted pixels are calculated from the already transmitted pixels.

According to the invention, the transmission or storage of the prioritized pixel groups takes place in the form of data packets, wherein the data packets can contain not only image data in the form of pixels, but any type of digital data that can be stored in an array. In this case, a data packet consists of a data value that describes the position of the pixel group in the array and the values of the individual pixels of the pixel groups. By encrypting the position value of the pixel groups and / or the pixel values of the pixel groups, it is possible to protect the data content against unauthorized access. Depending on the keys used and which parts of the information content are encrypted, e.g. As position values and / or pixel group values, the most diverse needs in the encryption can be considered. The data packets are transmitted in their order of importance in outgoing order and / or stored. As a result, according to the invention, at least in the case of static non-changing n-dimensional arrays of temporal duration, encryption and decryption of the pixel groups based on their importance are also possible.

The advantages of the invention over the current state of the art are the scalable handling of the encryption method. In contrast to conventional methods, the separate encryption of the position values and / or pixel group values for different requirements offers the advantage that in the corresponding applications and devices only this method has to be implemented. Once this procedure has been implemented, the most diverse requirements can use a common procedure. This reduces the number of implementations, which, among other things, saves storage space that is only limitedly available, in particular for mobile terminals. The reduction in the number of implementations results from the ability to encrypt audio, video and video data using the same procedure.

Advantageous embodiments and further developments of the invention are specified in the subclaims. In the following some embodiments of the invention will be explained.

It is assumed that the information content is present as a 2-dimensional image file (image array). Each pixel of the image array is z. B. represented by a 32 bit value (pixel value). The 32 bits are z. B. divided into 4 values (transparent, red, green, blue) with 8 bits each. The pixels of the image array are counted, the position of each pixel being determined by an integer. Pixel groups are formed which consist of a reference pixel which indicates the position of the pixel group within the array and further pixels surrounding the reference pixel. Each pixel group is assigned a priority according to their "image importance", with the highest priority pixel groups first being stored or transmitted.

The pixel groups can now be transmitted or stored according to the invention in various encryption levels.

Without encryption:

There is free access to the entire information content, ie. H. the pixel groups are transmitted unencrypted.

Using a simple key:

A single key is used for encryption and decryption, i. H. a symmetric encryption method applied. This z. B. the position values of the reference pixels of a pixel group are encrypted, so that without the right key a positionally correct positioning of the pixel group in the image array is no longer possible. The key can be transmitted via a second transmission path, z. By e-mail or post. No additional infrastructure is needed. A symmetric encryption method is faster than an asymmetric method, such as PGP.

Using an asymmetric encryption method: A private and a public key are used to encrypt and decrypt the information content. Encryption is more expensive than the symmetric method and is limited to a point-to-point relationship. However, there is no need for a second transmission path for transmitting the key.

Using a multiple key:

• – Zeitliche Komponente im Schlüssel: Der Informationsinhalt lässt sich nur ab/bis zu einen bestimmten Zeitpunkt entschlüsseln
• – Schlüssel abhängig vom Übertragungsmedium: Der Informationsinhalt lässt sich nur entschlüsseln, wenn das Übertragungsmedium eine bestimmte Identifikation besitzt
• – Schlüssel abhängig von der Urheberquelle: Der Informationsinhalt lässt sich nur auf dem Gerät entschlüsseln, auf dem er aufgenommen wurde, z. B. als Missbrauchsschutz bei der Erstellung von Sicherheitskopien

For a multiple key, the key is composed of a combination of individual keys. The keys may have dependencies on the content of the content, the time, the source of the author, the transmission medium or other features. In this way, the reproduction possibilities of the information content can be restricted as required, and the information content can thus be displayed as appropriate to the situation. Here are some examples:

• - Time component in the key: The information content can only be decrypted from / to a certain point in time
• - Key depends on the transmission medium: The information content can only be decrypted if the transmission medium has a specific identification
• - Key depending on the originator: The information content can only be decrypted on the device on which it was recorded, eg. B. as abuse protection when creating backup copies

Use of cascaded keys:

Cascaded keys can be used to perform partial encryption of the information content. This may be appropriate, for example, in the same data stream, the normal quality in encrypted form, and poor quality, e.g. B. for a picture preview, in unencrypted form, without causing redundancy. In this case, for example, the resolution of an image can be lowered. By "resolution" in this case is not meant the "image height × image width" since it remains unchanged when the method is used. Rather, with lowered resolution, a deviation from the original image is meant, which can arise during reconstruction by not yet transmitted and / or decrypted pixel groups. The process of cascaded keys works on the principle of onion skins. When using the method of prioritized pixel transmission z. For example, a reduction in pixel group size may serve to form a cascaded encryption. A pixel group consists of a (reference) pixel, which is uniquely determined by its position value, and a number of other pixels. Is there a pixel group z. B. from a total of 9 pixels, so for example 5 pixels can be transmitted unencrypted and 4 pixels encrypted. The outer shell, which includes the 5 unencrypted pixels, contains no encryption and would allow, for example, to view a stamp-sized video without a key. In the next shell, one or more of the encrypted pixels are transmitted. For each additional bowl another key is used. The type of shells are agreed upon before transmission between sender and receiver. In this way, the one who has all the keys and can decrypt all the shells can watch the video in the best quality.

To reduce interference, the z. For example, due to dependencies between the data of the individual switches, in this type of encryption, in addition to the position value and the values of the pixel groups, a hash value can be transmitted which is calculated from the position value and the values of the pixel groups. If the hash value calculated in the receiver does not match the transmitted hash value, that pixel group is not decrypted. This ensures that no interference from other shells occur.

A combination of different keys and procedures is possible.

Of course, the encryption method according to the invention is applicable not only to image and video data, but to all types of digital data that can be subdivided into data blocks, similar to the data blocks of pixels.

The invention will be explained in more detail below with reference to a simple example.

Table 1 shows a part of a data stream that has been prepared according to the method of prioritizing pixel transmission. The value "Pos x" indicates the respective position of the pixel group, the values "Px_n" the individual pixel values of the pixels contained in the pixel group. For example, each pixel group consists of 5 pixels. Table 1:

Table 2 shows the encryption of only the position values. Advantage: Only a part of the data stream needs to be encrypted, which results in a significant increase in performance compared to a complete encryption of all data. A reconstruction of the data thus encrypted without knowledge of the key is not possible or only with great computational effort. Table 2:

Table 3 shows the encryption of a part of the pixel group. Advantage: Due to a different encryption of the pixel values, the same data stream allows different qualities in the reconstruction of the image data. In the example below, the receiver may use the position value and pixel values Px_0 to Px_2 without a key. For decrypting the pixel values Px_3 to Px_5, the appropriate key is required in each case. If the receiver does not have the key (s) for the pixel values Px_3 to Px_5, then the application must reconstruct these pixel values from the freely available values Px_0 to Px_2. However, since the receiver lacks a multiplicity of pixel values, the quality of the reconstruction (resolution) is significantly reduced. Table 3:

In the examples given, encrypted and non-encrypted data is transmitted in the same data stream. To detect transmission errors and to see if the decryption was successful, each part of the pixel group (position value and Px_n) in the decrypted form may contain a CRC check. If a transmission error occurs and the CRC check fails, the corresponding pixel value is not used for reconstruction. The other part of the pixel group can still be used. In this way, the robustness of the transmission method against transmission errors increases at the same time. Instead of a CRC check, hash functions can also be used. These offer better protection but require more computing power.

Claims (7)

A method of compressing image data consisting of an array of individual pixels, each pixel (0-419) having a pixel value describing color or brightness information of the pixel, comprising the steps of: a) Determining a priority value for each pixel of the array by specifying the pixel used as a reference pixel (P0) and calculating a pixel difference value from the respective pixel value of the reference pixel (P0) with respect to the pixel values of a predetermined group of neighboring pixels (P1-P4) ; b) combining the pixels (P0-P4) used for the calculation of the priority value into a pixel group, c) sorting the pixel groups of the image array based on the priority value of the assigned reference pixel (P0); and d) storing and / or transmitting the pixel groups according to their priority, wherein depending on the desired compression factor only a part of the pixel groups is stored and / or transmitted, according to the main patent DE 101 52 612 B4, characterized in that at least one key is applied with which optionally the position value and / or the pixel value (s) of a pixel group are encrypted. A method for decompressing image data compressed by the method according to claim 1, characterized by the steps of: a) generating an empty image array from the read-in characteristics of the compressed image, b) reading in the corner pixel groups and inserting them at the four corners of the image array , c) forming triangles by connecting in each case three immediately adjacent pixel groups by at least one line, d) filling in the color and / or brightness curve calculated from the pixel groups forming the triangle, d) reading in and inserting the pixels forming the surface of the triangles the next pixel group in the image array; f) repeating steps c) to f). according to the main patent DE 101 52 612 B4, characterized in that at least one key is used with which optionally the position value and / or the pixel value (s) of a pixel group are decrypted. A method according to claim 1 or 2, characterized in that the key is optionally associated with the type of information content to be encrypted and / or with the author source, and / or with the transmission medium used or has a temporal dependence. Method according to one of claims 1 to 3, characterized in that each pixel value or one or more selected pixel values are each encrypted or decrypted with a separate key. Method according to one of claims 1 to 4, characterized in that a symmetric encryption method is performed. Method according to one of claims 1 to 5, characterized in that an asymmetric encryption method is performed. Method according to one of Claims 1 to 6, characterized in that the color depth of the pixel values is encrypted or decrypted in increments with a separate key.