CN108174223B - Method and system for estimating quantization step size of JPEG (joint photographic experts group) compressed bitmap - Google Patents

Abstract

The invention relates to a method for estimating quantization step size of a JPEG compressed bitmap, which comprises the following steps: DCT transform is carried out on the input JPEG compressed bitmap to obtain pairsA corresponding DCT coefficient matrix; extracting the DCT coefficient matrix obtained according to different frequencies to obtain a DCT coefficient sequence; calculating a quantization error function for each obtained DCT coefficient sequence; calculating a function g (u; C) for each of said DCT coefficient sequences based on said quantization error function_ij) (ii) a Obtaining each g (u; C) by traversing all frequencies_ij) The position of the minimum to obtain an estimate of the quantization step. The invention also relates to a system for estimating the quantization step size of the JPEG compressed bitmap. The invention does not need to set the threshold value g (u; C) manually by experience_ij) Is calculated by using C_ijAll coefficients of the coefficient sequence have low calculation complexity and are easy to realize.

Description

Method and system for estimating quantization step size of JPEG (joint photographic experts group) compressed bitmap

Technical Field

The invention relates to a method and a system for estimating quantization step size of a JPEG (joint photographic experts group) compressed bitmap.

Background

The JPEG lossy compression technique can effectively remove redundant information of an image, has a high file compression rate and detail fidelity, and provides a mechanism for balancing the two (by selecting different compression quality factors), and thus is widely applied to image capturing devices. With the development of image editing technology, the content of a JPEG image is possibly tampered maliciously and used for illegal purposes, and the content is restored in a Bitmap (Bitmap) form after being tampered. These forged bitmaps, if not correctly discerned, can pose a serious social hazard: as seen in news reports that may mislead public opinion, use as forensic evidence may lead to false cases. It is inefficient and impractical to distinguish which ones have been tampered from the massive bitmaps by human eyes only. The currently available solution is a technique for automatic detection of tampering by computers.

Many scholars have conducted extensive and intensive studies on the tamper detection technique of bitmaps, and have proposed various methods. These methods usually need to know the compression history information of the whole bitmap or even each local bitmap to be detected, i.e. it needs to know whether the bitmap is compressed by JPEG or not, and if so, what quantization parameter is used in the compression. Whether the JPEG compression bitmap detection method and the JPEG compression bitmap detection system are fine and accurate directly influences the performance of the specific tampering detection method and the system. Therefore, reliable compression history information can be provided for the tamper detection of a Bitmap by detecting whether the Bitmap (Bitmap) is JPEG-compressed and estimating quantization parameters of the compressed Bitmap.

In the whole JPEG compression and decompression process, DCT and IDCT, encoding and decoding are two pairs of lossless operations, and only the loss of image information is mainly caused by quantization. In other words, the quantization operation leaves a quantization effect on the JPEG image. Quantization effects exist for JPEG compressed bitmaps, whereas non-JPEG compressed bitmaps do not. Therefore, by detecting the quantization effect, it can be identified whether a bitmap is JPEG-compressed, and estimating the quantization step size of the JPEG-compressed bitmap is a common method for detecting the quantization effect.

However, the existing method for estimating quantization step size of JPEG compressed bitmap has the following disadvantages: (1) only partial DCT coefficients are utilized instead of all coefficients, so that all available information is not fully utilized to estimate the quantization step size, and the estimation accuracy is not high; (2) the selection of the threshold value makes it difficult to determine an optimal threshold value suitable for different images and even different frequencies of the same image.

Disclosure of Invention

In view of the above, there is a need to provide a method and a system for estimating quantization step size of a JPEG-compressed bitmap, which can overcome the disadvantages of the existing estimation methods and provide a basis for detecting tampering of the bitmap.

The invention provides a method for estimating quantization step size of a JPEG compressed bitmap, which comprises the following steps: a. performing DCT (discrete cosine transformation) on the input JPEG (joint photographic experts group) compressed bitmap to obtain a corresponding DCT coefficient matrix; b. extracting the DCT coefficient matrix obtained according to different frequencies to obtain a DCT coefficient sequence; c. calculating a quantization error function for each obtained DCT coefficient sequence; d. calculating a function g (u; C) for each of said DCT coefficient sequences based on said quantization error function_ij) (ii) a e. Obtaining each g (u; C) by traversing all frequencies_ij) The position of the minimum to obtain an estimate of the quantization step.

Wherein, the step c specifically comprises: let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, and generally q is more than or equal to 1 and less than or equal to M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

The step c specifically comprises the following steps: let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, and generally q is more than or equal to 1 and less than or equal to M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

The step c specifically comprises the following steps: let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, and generally q is more than or equal to 1 and less than or equal to M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

The calculation function g (u) is: g (u; C)_ij)＝f(u；C_ij)-max_1≤x≤u-1f(x；C_ij)。

The invention also provides a system for estimating the quantization step size of the JPEG compressed bitmap, which comprises a transformation module, an extraction module, a calculation module and an estimation module, wherein: the transformation module is used for carrying out DCT transformation on the input JPEG compressed bitmap to obtain a corresponding DCT coefficient matrix; the extraction module is used for extracting the DCT coefficient matrix obtained according to different frequencies to obtainA sequence of DCT coefficients; the calculation module is used for calculating a quantization error function for each obtained DCT coefficient sequence; the calculation module is also used for calculating a function g (u; C) for each DCT coefficient sequence according to the quantization error function_ij) (ii) a The estimation module is used for traversing all frequencies to obtain each g (u; C)_ij) The position of the minimum to obtain an estimate of the quantization step.

Wherein the computing module is specifically configured to: let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, and generally q is more than or equal to 1 and less than or equal to M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

The calculation module is specifically configured to: let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, and generally q is more than or equal to 1 and less than or equal to M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

The calculation module is specifically configured to: let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible taking of the quantization step sizeThe value is generally 1. ltoreq. q. ltoreq.M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

The calculation function g (u) is: g (u; C)_ij)＝f(u；C_ij)-max_1≤x≤u-1f(x；C_ij)。

The invention can overcome the defects of the existing estimation method and provides a basis for the tampering detection of the bitmap. The beneficial effects of the invention include: (1) the threshold value is not required to be set manually by virtue of experience, and improper parameter setting is avoided. (2) The quantization step size estimation is performed using all DCT coefficients. (3) The method does not involve difficult mathematical operation, has low computational complexity and is easy to realize.

Drawings

FIG. 1 is a flow chart of a method of estimating quantization step size of a JPEG compressed bitmap according to the present invention;

FIG. 2 shows the function g (u; C) according to the invention_ij) A schematic of an example.

FIG. 3 is a diagram of the hardware architecture of the system for estimating quantization step size of a JPEG compressed bitmap according to the present invention.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

FIG. 1 is a flowchart illustrating the operation of the method for estimating quantization step size of JPEG compressed bitmap according to the preferred embodiment of the present invention.

Step S1, perform DCT transform on the input JPEG compressed bitmap to obtain a corresponding DCT coefficient matrix. Specifically, the method comprises the following steps:

and performing DCT (discrete cosine transformation) of 8x8 blocks on the input JPEG compressed bitmap to obtain a corresponding DCT coefficient matrix.

And step S2, extracting the DCT coefficient matrix obtained by the step S according to different frequencies to obtain a DCT coefficient sequence. The method specifically comprises the following steps:

since a total of 8 × 8 is 64 frequencies, all coefficients of the same frequency are extracted to form a coefficient sequence, so that a total of 64 coefficient sequences, denoted as C, can be obtained_ij，0≤i,j≤7。

In step S3, a quantization error function is calculated for each of the obtained DCT coefficient sequences. Specifically, the method comprises the following steps:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, and generally q is more than or equal to 1 and less than or equal to M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

In other embodiments of the present invention, the quantization error function may further be:

or

Step S4, calculating function g (u; C) for each DCT coefficient sequence according to the quantization error function_ij). Specifically, the method comprises the following steps:

this example calculates the function g (u) as follows:

if u is 1, let g (u; C)_ij)＝f(u；C_ij). The physical meaning of the above formula: f (u; C)_ij) Minus the maximum of all values before it. f (u; C)_ij) The general trend of (c) is larger as u increases, but a local minimum is taken at the quantization step size and its multiples or submultiples. Thus, the function g (u; C)_ij) A minimum value will appear at the position of the quantization step and by detecting this minimum value an estimate of the quantization step can be obtained.

g(u；C_ij) An example ofAs shown in fig. 2. g (u; C)_ij) The curve at frequency (i, j) — (2,2) is shown. When the curve takes the minimum value, the corresponding x coordinate is 12, i.e. 12 is the quantization step to be estimated for that frequency.

Step S5, go through all frequencies to get each g (u; C)_ij) The position of the minimum value is the estimated value of the quantization step. Specifically, the method comprises the following steps:

in this embodiment, all frequencies 0 ≦ i, j ≦ 7 are traversed, and g (u; C) is calculated_ij) Finding out g (u; c_ij) At the position corresponding to the minimum value under different frequencies, the quantization step length estimation value q of the JPEG compressed bitmap is obtained_ij. The quantization step estimate q_ijA total of 64, 8x8 matrices.

Referring now to FIG. 3, therein is shown a hardware architecture diagram of the system 10 for estimating quantization step size of a JPEG compressed bitmap in accordance with the present invention. The system comprises a transformation module 101, an extraction module 102, a calculation module 103 and an estimation module 104.

The transform module 101 is configured to perform DCT transform on an input JPEG compressed bitmap to obtain a corresponding DCT coefficient matrix. Specifically, the method comprises the following steps:

and performing DCT (discrete cosine transformation) of 8x8 blocks on the input JPEG compressed bitmap to obtain a corresponding DCT coefficient matrix.

The extraction module 102 is configured to extract the obtained DCT coefficient matrix according to different frequencies to obtain a DCT coefficient sequence. The method specifically comprises the following steps:

since a total of 8 × 8 is 64 frequencies, all coefficients of the same frequency are extracted to form a coefficient sequence, so that a total of 64 coefficient sequences, denoted as C, can be obtained_ij，0≤i,j≤7。

The calculation module 103 is configured to calculate a quantization error function for each obtained DCT coefficient sequence. Specifically, the method comprises the following steps:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

the upper typeWhere | represents taking the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, and generally q is more than or equal to 1 and less than or equal to M. f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

In other embodiments of the present invention, the quantization error function may further be:

or

The calculation module 103 is further configured to calculate a function g (u; C) for each of the above DCT coefficient sequences based on the quantization error function_ij). Specifically, the method comprises the following steps:

this example calculates the function g (u) as follows:

if u is 1, let g (u; C)_ij)＝f(u；C_ij). The physical meaning of the above formula: f (u; C)_ij) Minus the maximum of all values before it. f (u; C)_ij) The general trend of (c) is larger as u increases, but a local minimum is taken at the quantization step size and its multiples or submultiples. Thus, the function g (u; C)_ij) A minimum value will appear at the position of the quantization step and by detecting this minimum value an estimate of the quantization step can be obtained. g (u; C)_ij) An example of which is shown in figure 2.

The estimation module 104 is used to traverse all frequencies to get each g (u; C)_ij) The position of the minimum value is the estimated value of the quantization step. Specifically, the method comprises the following steps:

in this embodiment, all frequencies 0 ≦ i, j ≦ 7 are traversed, and g (u; C) is calculated_ij) Finding out g (u; c_ij) The position corresponding to the minimum value under different frequencies is obtainedJPEG compression bitmap quantization step length estimation value q_ij. The quantization step estimate q_ijA total of 64, 8x8 matrices.

Function g (u; C) of the invention_ij) Using f (u; c_ij) Information of the general trend, directly by finding g (u; c_ij) The quantization step can be estimated without setting the threshold value empirically and manually. At the same time, g (u; C)_ij) Is calculated by using C_ijAll coefficients of the coefficient sequence are more fully utilized from the aspect of information utilization rate.

Although the present invention has been described with reference to the presently preferred embodiments, it will be understood by those skilled in the art that the foregoing description is illustrative only and is not intended to limit the scope of the invention, as claimed.

Claims (8)

1. A method for estimating quantization step size of a JPEG-compressed bitmap, the method comprising the steps of:

a. performing DCT (discrete cosine transformation) on the input JPEG (joint photographic experts group) compressed bitmap to obtain a corresponding DCT coefficient matrix;

b. extracting the DCT coefficient matrix obtained according to different frequencies to obtain a DCT coefficient sequence;

c. calculating a quantization error function for each obtained DCT coefficient sequence;

d. calculating a function g (u; C) for each of said DCT coefficient sequences based on said quantization error function_ij)；

e. Obtaining each g (u; C) by traversing all frequencies_ij) The position of the minimum value to obtain an estimated value of the quantization step;

wherein a function g (u; C) is calculated_ij) Comprises the following steps: g (u; C)_ij)＝f(u；C_ij)-max_1≤x≤u-1f(x；C_ij)。

2. The method according to claim 1, wherein said step c specifically comprises:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, generally q is more than or equal to 1 and less than or equal to M, f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

3. The method according to claim 1, wherein said step c specifically comprises:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, generally q is more than or equal to 1 and less than or equal to M, f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

4. The method according to claim 1, wherein said step c specifically comprises:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, generally q is more than or equal to 1 and less than or equal to M, f (q; C)_ij) Is C_ijAs a parameter, inq is a function of the argument.

5. A system for estimating quantization step size of JPEG compressed bitmap is characterized in that the system comprises a transformation module, an extraction module, a calculation module and an estimation module, wherein:

the transformation module is used for carrying out DCT transformation on the input JPEG compressed bitmap to obtain a corresponding DCT coefficient matrix;

the extraction module is used for extracting the obtained DCT coefficient matrix according to different frequencies to obtain a DCT coefficient sequence;

the calculation module is used for calculating a quantization error function for each obtained DCT coefficient sequence;

the calculation module is also used for calculating a function g (u; C) for each DCT coefficient sequence according to the quantization error function_ij)；

The estimation module is used for traversing all frequencies to obtain each g (u; C)_ij) The position of the minimum value to obtain an estimated value of the quantization step;

wherein a function g (u; C) is calculated_ij) Comprises the following steps: g (u; C)_ij)＝f(u；C_ij)-max_1≤x≤u-1f(x；C_ij)。

6. The system of claim 5, wherein the computing module is specifically configured to:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, generally q is more than or equal to 1 and less than or equal to M, f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

7. The system of claim 5, wherein the computing module is specifically configured to:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, generally q is more than or equal to 1 and less than or equal to M, f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

8. The system of claim 5, wherein the computing module is specifically configured to:

let each coefficient sequence C_ijFor a total of N coefficients, the quantization error function is calculated as follows:

in the above formula, | | represents the absolute value, x_kIs a coefficient sequence C_ijQ is a possible value of the quantization step, generally q is more than or equal to 1 and less than or equal to M, f (q; C)_ij) Is C_ijIs a parameter, and q is a function of the argument.

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