Copymove forgery detection for image forensics using the superpixel segmentation and the Helmert transformation
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Abstract
The increasing popularity of the internet suggests that digital multimedia has become easier to transmit and acquire more rapidly. This also means that this multimedia has become more susceptible to tampering through forgery. One type of forgery, known as copymove duplication, is a specified type that usually involves image tampering. In this study, a keypointbased image forensics approach based on a superpixel segmentation algorithm and Helmert transformation has been proposed. The purpose of this approach is to detect copymove forgery images and to obtain forensic information. The procedure of the proposed approach consists of the following phases. First, we extract the keypoints and their descriptors by using a scaleinvariant feature transform (SIFT) algorithm. Then, based on the descriptor, matching pairs will be obtained by calculating the similarity between keypoints. Next, we will group these matching pairs based on spatial distance and geometric constraints via Helmert transformation to obtain the coarse forgery regions. Then, we refine these coarse forgery regions and remove mistakes or isolated areas. Finally, the forgery regions can be localized more precisely. Our proposed approach is a more robust solution for scaling, rotation, and compression forgeries. The experimental results obtained from testing different datasets demonstrate that the proposed method can obtain impressive precision/recall rates in comparison to stateoftheart methods.
Keywords
Image forensics Copymove forgery detection Tampering detection Region duplication SuperpixelsAbbreviations
 CIELAB
International Commission on Illumination, L for the lightness and A and B for the green–red and blue–yellow color components
 DCT
Discrete cosine transform
 DWT
Discrete wavelet transform
 FPR
False positive rate
 HAC
Hierarchical agglomerative clustering
 HOG
Histogram of oriented gradient
 LPM
logpolar maps
 NNDR
Nearest neighbor distance ratio
 SIFT
Scaleinvariant feature transform
 SLIC
Simple linear iterative clustering
 SURF
Speedup robust features
 TPR
True positive rate
 ZNCC
Zero mean normalized crosscorrelation
1 Introduction
As a result of technological advances and the convenience of the internet, human beings are now able to easily access interesting multimedia from the internet and remake or tamper with it as they see fit. Copymove forgery imaging is a special type of forgery that involves copying parts of an image and then pasting the copied parts into the same image. Hence, image forensics associated with copymove forgery detection have become increasingly important in our networked society. The technology used in image forensics can be categorized into passive detection or active detection [1]. The active detection method requires prior information derived from an image to identify the image authenticity, such as watermarking. Contrary to active detection methods, passive detection methods are not required to obtain previous information on an image. Passive detection methods can utilize the advantages of the detective strategy to find the tampering regions. Hence, a large majority of image forgery detection methods adopt a passivebased strategy to perform the type of tampering identification discussed in the present study. Passive detection technology can be categorized into blockbased methods [2, 3, 4, 5, 6, 7, 8, 9, 10] and keypointbased methods [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. In the present study, we focus on the keypointbased approach.
Blockbased methods segment an image into overlapping blocks and then extract features from those blocks. The forgery regions are determined by computing the similarity between block features. Wang et al. [2] proposed blockbased forensics to detect region duplication for an image. The method mainly used the mean intensities of a circle with different radii around the center of the block to represent the features of the block. Ryu et al. [3, 4] used Zernike moments as block features. The method can identify the forged region by copyrotatemove forgery. Huang et al. [5] proposed a discrete cosine transform (DCT)based forgery detection method. The image is first divided into overlapping blocks and the DCT is applied, thus the DCT coefficients for each block are quantized by fixed stepsize q and then rounded to the nearest integer. A row vector as block feature can then be obtained by using a zigzag scan. The duplicated image blocks are compared in the matching step. This method can detect JPEG compression, but the DCTbased feature vector cannot resist geometrical tampering.
Wang et al. [6] proposed a forgery method that combines the discrete wavelet transform (DWT) and the DCT. The DWT and DCT are applied to each image block to extract features. The coefficients obtained by the DWT and DCT are multiplied to form the eigenvectors. Then, the similarity of two blocks is estimated, along with the mean and variance distances between the eigenvalues in their corresponding eigenvectors. This method can resist JPEG compression but not image processing operations.
BravoSolorio and Nandi [7] proposed a polarbased forgery detection method to detect copymove attacks for an image. This method subdivided an image into overlapping blocks of pixels. The pixels within the block are first transformed into logpolar maps (LPM), and then summed along the angle axis, to generate onedimensional descriptors. Subsequently, they will compute the Fourier coefficient magnitude after Fourier transformation. The descriptors are invariant to reflection and rotation. The descriptor of each block is used to compute the information entropy as block features. By computing the entropy difference between blocks, the similar regions are found. However, a significant amount of smooth duplication regions may arise during mistake detection.
Davarazni et al. [8] used multiresolution local binary patterns (MLBP) for forgery detection. This method used LBP operations to extract feature vectors for each block, and then sorted these vectors based on lexicographical order. The duplicated image blocks are detected in the block matching step using a ktree. This method is time consuming and does not detect any rotation angles for duplication regions. Lee et al. [9] used a histogram of oriented gradients (HOG) of each block as features; these features are ordered by using lexicographical sorting. The duplicated image blocks are detected by measuring similar block pairs. Li et al. [10] used a polar harmonic transform to extract the rotation and scaling invariant features as block features (similar to the method of Lee et al. [9]). These feature vectors are lexicographically sorted, and the forged regions are detected by finding similar block pairs.
In keypointbased methods, image features are extracted and matched with the entire image to identify the regions that were tampered with. Common and wellknown feature points have scaleinvariant feature transform (SIFT) [22] features and speedup robust features (SURF) [23]. These feature points have been widely used for image retrieval and object recognition because of their robustness in geometrical transformations (e.g., scaling and rotation). Based on these advantages, these features have been applied to digital forensics. In [11, 12], these methods applied a SIFT to the host image to extract keypoints, which were then matched to one another. When the value of the displacement vector exceeded the threshold, the sets of corresponding SIFT keypoints are labeled as the tampered regions. The method used for combining the SIFT keypoints and Jlinkage algorithm to localize the forgery regions has been reported [13].
In [14, 15], the SURFs were applied to extract the keypoint features, which makes it possible to detect duplicated regions of various sizes. Additionally, Mishra et al. [15] also used hierarchical agglomerative clustering (HAC) to group the matched keypoints from these sets of keypoints. Several different technologies based on SURF and SIFT and Harris were applied in [16, 17]. Pun et al. [18] integrated both the blockbased and keypointbased methods to detect the forged regions. Several keypointbased methods involved with segmentation methods have been reported in the following references: [19, 20, 21, 24, 25, 26, 27]. Christlein et al. [28] evaluated the performance of feature sets in existing copymove forgery detection algorithms.
Many methods from the literature deal only with simple copymove forgery scenarios, while other approaches present relevant contributions toward the detection of sophisticated tampering. However, these approaches still have major limitations. Most of the current blockbased methods use a similar framework; the main differences between frameworks are that they use different feature extraction methods to extract the block features. The blockbased detection of forgery regions can be timeconsuming because the host image is divided into overlapping blocks, and they cannot detect geometrical transformations of the forgery regions. In contrast, the keypointbased forgery detection methods can detect geometrical transformations and require less computational resources; however, they do not have good localization power. Thus, there is room for improving true positive rate (TPR) results. Based on the above reasons, we propose conducting image forensics based on an simple linear iterative clustering (SLIC) algorithm [29] and Helmert transformation [30] to achieve copymove forgeries with rotations, resizings, and combinations of the two. This proposed scheme uses the SIFT algorithm to extract the keypoints from an image and then designs our algorithm. Our approach can efficiently resist geometrical transformations and JPEG compressions, and localize the forgery regions more precisely at a reasonable computational cost.
The rest of this study is organized as follows. Section 2 presents the related techniques. The proposed method is described in Section 3. In Section 4, we present the experimental results to verify the robustness of the proposed algorithm. Finally, Section 5 concludes this study.
2 Related techniques
In this section, we briefly describe the related methods that apply to our proposed approach.
2.1 Superpixel segmentation
One type of image segmentation method is called the superpixel segmentation method. It groups the pixels of an image into perceptually meaningful atomic regions that can be used to replace the rigid structure of the pixel grid. A simple linear iterative clustering (SLIC)based superpixel algorithm is proposed by Achanta et al. [29]. It uses a kmeans clustering approach to efficiently generate superpixels, and it can adhere to the boundaries very well. The only parameter (k) in the SLIC algorithm is to assign the desired number of approximately equally sized superpixels. The algorithm is briefly described in the following paragraph. Details of the procedures have been reported in [29].
This SLIC algorithm is adopted in CIELAB color space. The SLIC algorithm adapts a kmeans clustering approach to efficiently generate the superpixels, and it adheres to the boundaries very well. First, the clustering procedure begins with an initialization step where the k initial cluster centers, where (l, a, b) are the three color components of a pixel, and (x, y) are its two spatial coordinates, are sampled on a regular grid (called a superpixel), spaced S pixels apart. The S interval is \( \sqrt{\raisebox{1ex}{$N$}\!\left/ \!\raisebox{1ex}{$k$}\right.} \), in which N represents the number of pixels for an image. In order to avoid centering a superpixel on an edge or on a noisy pixel, the centers are moved to seed locations corresponding to the lowest gradient position in a n × n neighbC_{i} = [l_{i}, a_{i}, b_{i}, x_{i}, y_{i}]^{T}, i = 1, 2, ⋯, k,orhood. As is known to us, the edge or noisy pixel is often positioned on a pixel point that has the largest gradient variation. Therefore, selecting the lowest gradient pixel point to position the center for a superpixel can efficiently reduce the chance of seeding a superpixel with an edge or a noisy pixel.
2.2 Helmert transformation
3 Proposed method
3.1 Keypoint extraction and matching
Based on the SIFT algorithm [22], we can obtain all candidates of keypoints and the corresponding descriptors for an image. Using these candidates, we will search for the best matching pairs to perform additional grouping.
After computing the distances for all keypoints, we can obtain all matching pairs in an image. In order to avoid incorrect matching pairs, if the distance between matched pairs is less than T_{NNDR}, they will be ignored and deleted.
3.2 Clustering and group merging

Spatial adjacency: consider that we have a match pair between keypoints A and B belonging to group G. Keypoint A might belong to the G_{source} subgroup, and keypoint B might belong to the G_{target} subgroup, or vice versa. For a subgroup to admit a paired keypoint as a new member, the spatial distance between the keypoint and its nearest keypoint in such a subgroup needs to be smaller than a predefined threshold, T_{c}. Moreover, it is necessary to analyze both matched keypoints, since they have to be in the same group, but in different subgroups.

The angle consistency: the angle in the range of [0^{∘}, 360^{∘}] with a 15^{∘} step is used to determine the angle consistence. It can obtain 24 range partitions. As described above, a new keypoint A candidate to be included into G_{source} will be included in G_{source}, only if the angle of the line that connects the candidate point A and its matching point B stays in the same range of the other points in G_{source}.
After performing coarse clustering, we will further merge these clusters based on the Helmert transformation and spatial adjacent relationship between clusters. Therefore, the transformation can efficiently merge some clusters with a high correlation into a compact cluster. A Helmert transformation is used to describe the relationships between two different coordinate systems without distortion. In 2D space, the Helmert transformation is defined as Eq. (1). We use the Helmet transformation to analyze the geometric relationships between matching pairs. Assuming that the number of keypoints in a cluster is greater than one, we will compute the Helmert parameters of the cluster (source and target subgroup); otherwise, this cluster will be discarded. For instance, given any two matching pairs, by assuming that (X_{p}, Y_{p}) are target coordinates and (x_{p}, y_{p}) are source coordinates, the transformation can easily compute and obtain four Helmert parameters by Eq. (1).
 1.
The number of the extension (N_{e}) has reached a value of five, and there is no cluster that can be combined. Here, the range of each extension (R_{e}) is multiplied the rectangle searching region by a factor of 1.25.
 2.
The rectangle search region (S_{r}) is greater than 0.125 times of size of a host image.
Repeat the above steps until no clusters can be combined. Finally, we remove the invalid clusters that involve less than five keypoints.
3.3 Forgery regions localization and refining
We use zero mean normalized crosscorrelation (ZNCC) [32] to measure the similarity between source regions and target regions. Assuming that a Helmert transform matrix, H, exists, the relationships between source group and target group are expressed as [13]. Let a = [x_{1} y_{1}]^{T} be a point in the source cluster and \( b={\left[{x}_2\kern0.5em {y}_2\right]}^T \) be a point in the target cluster, then a = H × b, and since H is invertible, b = H^{−1} × a. Combining these relationships and the ZNCC measurement, the forgery region can be further localized.
First, if the number of keypoints in a group is less than a threshold (T_{k}), we will regard this group as unimportant and it will be discarded. The source subgroup with the matching points for each cluster is labeled as image I, and all the pixel points (x, y) in image I are converted to the new locations (x', y') in image W by using the Helmert parameters defined as Eq. (1). Therefore, a new image W of the same size is produced. Then, we create a ZNCC binary map using Eq. (3).
Then, we apply a Gaussian filter to the correlation map in order to reduce the noisy pixels, and a binary correlation map is given by means of a threshold (T_{b}). If the ZNCC value for point (x, y) is greater than a threshold, this point (x, y) is assigned as true; otherwise, this point is assigned as false. Next, we will perform connectedcomponent labeling on this binary map. This threshold, T_{b}, is set to 0.55, which is a value obtained through experimentation.
The host image is segmented into many subregions by the SLIC algorithm. In the SLIC algorithm, the smaller the size of a superpixel (S), the greater the number of superpixels present. Moreover, very few true edges are missed. In contrast to increasing size, the number of superpixels is reduced, and many true edges will be missed. Therefore, in our approach, the size of a superpixel (S) is assigned to 300 pixels by experiments. For each subregion, we will count the number of pixels that are considered true in the ZNCC binary map. If this number (N_{d}) is greater than a threshold in the relative subregion, all the pixels in this subregion are labeled as a detection map that serves as a part of forgery regions, as shown in green color areas of Fig. 8d. Afterwards, we label the connected components as the detection map, and delete the regions that have an area less than 0.1%. Finally, each of the remaining regions will use the convexhull morphologic method to connect together in the binary detection map. Figure 8 illustrates the profile of the detection map. After performing our proposed method, we can efficiently detect and localize the forgery regions more precisely.
4 Experimental results and discussion
To verify the performance of the proposed image forensics, the experimental results are compared to Amerini et al. [13], Silva et al. [16], Pun et al. [18], and Li et al. [19] to perform the forgeries, including copying and translations, scaling, rotation, and compression.
4.1 Experimental setup and datasets
Setup for the parameters
Parameter  Value 

T_{NNDR} [31]: a threshold for nearest neighbor distance  0.6 
T_{c} [16]: a threshold for making a subgroup  50 
T_{h}: a threshold for making a group  10n, n: number of keypoints in group. 
N_{e}: the number of the extension  5 
R_{e}: the range of each extension  0.25 
S_{r}: the rectangle search region  0.125 
T_{k}: a threshold for the number of keypoints in a group  5 
m, n: the size of computing ZNCC value  7 
T_{b}: a threshold  0.55 
E_{r}: the expanded range  0.25 
S: a superpixel size  300 
N_{d}: the number of pixels with true in the ZNCC binary map  0.5 

CMH1: 23 images that were only copied and then translated.

CMH2: 25 images with a rotation of the duplicated region, the orientations are in the range of [−90^{∘}, 180^{∘}].

CHM3: 26 images with resizing of the duplicated region; the scaling range is between 80 and 154%.

CHM4: 34 images with rotation and resizing entirely.

CMH5: 108 images that are derived from 36 randomly selected images from the CMH1–4 datasets and compressed with a quality factor of 70%, 80%, 90%.

D0 dataset: 50 images that are copied and translated.

D1 dataset: 600 images with a rotation of the duplicated region. This dataset is further subdivided into subsets. The first subset, D1.1, is created by rotating the copies with 11 different types of rotation around the angle zero in the range of [−25^{∘}, 25^{∘}] with a step of 5^{∘}. The second subset, D1.2, is created by rotating the copies with 12 different types in the range of [0^{∘}, 360^{∘}] with a step of 30^{∘}. The third subset, D1.3, is created by rotating the copies with 11 different types in the range of [−5^{∘}, 5^{∘}] with a step of 1^{∘}

D2 dataset: 320 images with resizing of the copied region. This dataset is subdivided into two subsets. The D2.1 subset is obtained by scaling the copies with 8 different scaling factors in the range of [0.25, 2] with a step of 0.25. The D2.2 subset is scaled by 11 scaling factors in the range of [0.75, 1.25] with a step of 0.05.

D3 dataset: 50 original images without tampering to verify the forensic ability between tampered and untampered images.
Every image in every dataset has its own binary ground truth displaying the original and duplicated regions in white color. And the tampered region within the datasets is of a single region copied one time and stayed in the same image.
4.2 Performance evaluation

Precision: represents the probability that the detected regions are truly the forgery regions, as expressed in (4).

Recall: represents the probability that the forgery regions are detected, as expressed in (5).

F_{1}: this score combines both the precision and recall into a signal value. It is calculated by (6).

FPR: indicates the percentage of incorrectly located tampering regions. It is defined as
Because the datasets have been tampered with in different ways, they are not consistent in our experiments, and therefore we compute the average values for these evaluation criteria in the dataset to verify the performance. As indicated above, the precision is the probability that a detected forgery is truly a forgery, and the recall is the probability that a forgery image is detected. Generally, a higher precision and a higher recall represent better performance.
5 Results
Regarding the different forgery images created by copying and translation, scaling, rotation, and compression, the experimental results are presented and discussed in the following section.
5.1 Detection results for copying and translation
Detected results for CMH1 dataset under simple copies
Detected results for D0 dataset under simple copies
5.2 Detection results for rotation
Detected results for CMHD2 dataset under rotation
As presented above, our proposed method can achieve much better detection results under rotation forgery.
5.3 Detection results for resizing/scaling and rotation
Detected results for CMH3 dataset under resizing
Detected results for D2 dataset under resizing
Detected results for CMH4 dataset under resizing and rotation
5.4 Detection results for compression
From Table 9, it is obvious that our proposed results can achieve the best precision and recall results, and are superior than that of the methods of Amerini et al., Silva et al., Pun et al., and Li et al.
5.5 Comparison of execution time
In summary, the proposed method can achieve impressive accuracy in a reasonable time.
6 Discussions
The experimental results have been presented above. In summary, for CMH1–4 datasets including copying and translations, rotation, resizing/scaling, resizing, and rotation, the precision values are greater than 97.1%, and the recall values achieved at least 86.21%. The global performance of the precision/recall values is superior to the methods used by Amerini et al., Silva et al., Pun et al., and Li et al., as shown in Tables 2, 4, 6, and 8. For the D0–2 datasets, the precision values can achieve 97.36% (the highest) and 86.76% (the lowest) and the recall values can achieve 92.18% (the highest) and 79.09% (the lowest), as shown in Tables 3, 5, and 7. It is clear that the detection results for the D0–2 datasets obtained by our approach are superior to that obtained by the methods of Amerini et al., Silva et al., Pun et al., and Li et al. At the same time, for compression forgery, Fig. 15 and Table 9 show the results compared to the other methods. For robustness of false positive detection, our approach has sufficient robustness to resist the false detection, and Fig. 16 and Table 10 present the results. For execution time, although our approach is not the fastest, it is acceptable. For rotation tampering, our proposed method can detect copymove forgery regions with a large rotation. Evidently, our proposed system works well for image forensics under rotation, scaling, and compression forgery attacks, compared to other methods.
7 Conclusions
In this study, the major strategy of our proposed algorithm focuses on a single tampered region detection. And we have proposed keypointbased image forensics for copymove forgery images based on a Helmert transformation and SLIC superpixel segmentation. Compared to the sliding window approach, the keypointbased technique can be applied at a lower computational cost because of the significantly reduced number of points required. In addition, we use the Helmert transformation to estimate the geometric relationships between matching pairs and to work the merging clusters. On the other hand, we use an SLIC algorithm to localize the tampering regions more precisely. Based on these strategies, we can keep much more important information to conduct image forensics.
As previously presented in the experiments, it is clear that the proposed method is highly robust against many kinds of forged images, such as geometric transformations (scaling, rotation) and JPEG compression. However, the current method is not robust against symmetric, recurring, and smooth patterns for tampering region. Progress in detecting symmetric, recurring, smooth forgery images, and tampering region copied multiple times will be a major focus in the future.
Notes
Acknowledgements
Not applicable.
Authors’ contributions
For corresponding author: Dr. Huang is major to design the framework of the system and theorem base and writing.
Author 2: Miss Ciou is to code and implement the experimental results.
Funding
There are not any funding for my affiliation, but I will pay an article publication fee by myself if my manuscript is accepted.
Competing interests
The authors declare that they have no competing interests in this work.
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