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A multiframe superresolution method based on the variableexponent nonlinear diffusion regularizer
EURASIP Journal on Image and Video Processing volume 2015, Article number: 22 (2015)
Abstract
In this work, the authors have proposed a multiframe superresolution method that is based on the diffusiondriven regularization functional. The new regularizer contains a variable exponent that adaptively regulates its diffusion mechanism depending upon the local image features. In smooth regions, the method favors linear isotropic diffusion, which removes noise more effectively and avoids unwanted artifacts (blocking and staircasing). Near edges and contours, diffusion adaptively and significantly diminishes, and since noise is hardly visible in these regions, an image becomes sharper and resolute—with noise being largely reduced in flat regions. Empirical results from both simulated and real experiments demonstrate that our method outperforms some of the stateoftheart classical methods based on the total variation framework.
Introduction
Overview of the superresolution methods
Superresolution image reconstruction refers to a collection of various imageprocessing tools used to reconstruct highresolution images from their corresponding degraded versions ([1–5], and references therein). The term resolution, as used in this work, is the number of pixels per unit area. Therefore, a highresolution image with both appreciable subjective and objective qualities has a larger pixel count compared with a lowresolution image.
Humans are naturally inclined to vouch for highquality images. Perhaps the reason for this inclination is the fact that such images contain more information and, hence, are easier to comprehend and interpret. In machine learning and computer vision tasks, detailed images are useful to accurately and robustly highlight and extract critical features, such as edges and contours. These typical examples make superresolution a vital technology.
Superresolution methods attempt to address the hardware limitations for improving image qualities. The advantage of these methods is that they discourage hardware modifications to achieve highresolution images and, thus, are cheap and promote portability. The hardware approach, on the contrary, prompts for changes in the sensor, chip size, and device’s internal circuitry to meet the same goal of the superresolution process. These hardware changes come at the following costs: (1) the current technology disallows further reduction of the sensor’s pixel size (which increases pixel count and, therefore, improves resolution) as the process may introduce unwanted shot noise that degrades images; (2) increasing the chip size, which improves the spatial resolution, raises capacitance that slows down the charge transfer rate; and (3) additional weight due to hardware change may limit particular applications, such as remote sensing or unmanned aerial surveillance.
The methods for solving the superresolution problem can be put into two groups, namely multiframe and singleframe. The former group uses a single image to generate its corresponding highresolution version [6–9]. Despite the extensive review, singleframebased superresolution approaches—interpolation (nearest neighbor, bilinear, and bicubic) and examplebased [10–13]—tend to generate unpleasing results due to insufficient amount of input images. For example, interpolation methods are known for producing blurry and aliased images. Besides, examplebased approaches are computationally inefficient and may, therefore, be unsuitable for realtime processing. A thorough discussion on singleframe superresolution methods is beyond the scope of our work. However, for a comprehensive review of this category of methods, we refer interested readers to the works in[7, 8, 14–20]
A multiframe superresolution approach—which this paper is based upon—attempts to generate a highresolution image by fusing some pieces of information from multiple degraded images of the same scene [21–24]. The fundamental premise of this approach is that pairs of similar images are likely to differ by rotations, translations, and/or affine transformations due to (1) shaking of the camera, (2) different capturing and exposure times, and (3) relative motion between camera and scene. We can exploit this additional information to produce a more resolute scene by computing the values of these attributes and using them to align the frames onto a common highresolution grid. The rest of this work discusses the multiframe superresolution methods.
The superresolution problem is highly illposed due to insufficient number of the observed lowresolution frames. A classical approach to address this problem is called regularization. Authors have, therefore, proposed different types of priors.
A classical and, perhaps, popular prior is based on the Tikhonov model [25, 26]. This prior introduces into the restoration problem a smoothness constraint that removes extraneous noise from an image. The weakness of the Tikhonov prior is that it tends to destroy edges—an effect that degrades images. Therefore, the prior has captured the interest of many researchers to develop models that simultaneously suppress noise and preserve critical image features: Huber Markov random field (HuberRMF) [27, 28], edgeadaptive RMF [29, 30], sparse directional [31, 32], and total variation (TV) [33–35].
Of the aforementioned priors, TV has attracted more attention of researchers as it generates results with pleasing objective and subjective qualities. The major weaknesses of the TV model are blocking and staircasing effects, falseedge generation near edges, and nondifferentiability property at zero—a situation that makes the numerical implementation rather challenging. The TV model was initially applied in image denoising [36]. Later, the model was adapted to other applications: superresolution [33], MRI medical image reconstruction [37], inpainting [38], and deblurring [39]. In this work, we explore some classical TVbased approaches to address the superresolution problem.
In 2008, Marquina et al. proposed a convolutional model for a superresolution problem based on the constrained TV framework [33]. In their work, the authors introduced the Bregman algorithm as an iterative refinement step to enhance the spatial resolution. The results demonstrate that the method generates detailed and sharper images, but blocking and staircasing artifacts are still evident.
In [35], Farsiu and colleagues proposed a bilateral total variation (BTV) prior, which is based on the L ^{1} norm minimization and the bilateral filter regularization functional, for a multiframe superresolution problem. Their method is computationally inexpensive and robust against errors caused by motion and blur estimations and generates images that are convincingly sharper. However, bilateral filters that the method derives its strengths from are known to introduce artifacts like staircasing and gradient reversal. Additionally, the BTV inadequately addresses the partial smoothness of an image [40]. Besides, the numerical implementation of the L ^{1} norm component is challenging as it masks the superresolution data term.
In [34], Ng et al. applied the TV prior to address the following issues in the superresolution video reconstruction: noise, blurring, missing regions, compression artifacts, and motion estimation errors. The authors demonstrated the efficacy of their method in several cases of motions and degradations and provided the experimental results that outperform some other classical superresolution methods.
Ren et al. proposed a superresolution method, which is based on the fractional order TV regularization, with a focus to handle fine details, such as textures, in the image [41]. Results show that their approach addresses to some extent the weaknesses of the traditional TV.
In [21], Li et al. attempted to address the drawbacks of the global TV by proposing two regularizing functionals, namely locally adaptive TV and consistency of gradients, to ensure that edges are sharper and flat regions are smoother. The method heavily depends on the gradient details of an image, a feature that may produce pseudoedges in noisy homogeneous regions. Note that both noise and edges are image features with highgradient (or highintensity) values. As Li’s method is gradientdependent, it may equally treat both noise and edges, and this may generate unwanted artifacts.
Yuan et al. proposed a spatially weighted TV model for the multiframe superresolution problem [42]. Their model incorporates a spatial information indicator (difference curvature) that locally identifies the spatial properties of the individual pixels, thus providing the necessary level of regularization. The authors employed the majorizationminimization algorithm to optimize their formulation. Results show that the Yuan et al. method overcomes some challenges of the original TV model (discourages piecewise constant solutions and is less sensitive to regularization parameters) and outperforms the Li et al. method. But under severe noise conditions, the Yuan et al. approach fails as it is pixelunitbased [40].
Recently, Weili et al. proposed an adaptive TVdriven superresolution method that provides convincing results compared with those generated by the standard TV model [40]. The authors incorporated into the classical superresolution formulation a featuresensitive prior that approximates the L ^{1} norm near edges, and this helps to efficiently highlight these critical features. In flat regions, the prior approximates the L ^{2} norm, thus providing noise removal.
Inspired by the weaknesses of the standard TV and its variants, this work proposes an alternative framework based on nonlinear diffusion processes—which have yielded promising results in imagedenoising applications [36, 43–47]—to address the superresolution illposedness. Our diffusiondriven prior includes an adaptive kernel that sensitively and dynamically updates its value in accordance with the scanned local image features. That is, the kernel is linear isotropic in flat regions and nonlinear anisotropic near edges. Being locally adaptive, the model generates more resolute scenes and avoids blocking artifacts inherent in the conventional TV.
A classical multiframe image degradation model
Imaging devices, such as cameras, endure unavoidable hardware limitations and influence of external noise. Therefore, an image signal that passes through the imaging system usually undergoes degradations before reaching its destination (Fig. 1).
Let us consider that u is the unknown highresolution image. In the image degradation model (Fig. 1), u is first acted upon by the warping operator, W _{ k }, which rotates and translates it. Then, the degraded version of u proceeds to the next stage, where it is blurred by the blurring operator, B _{ k }. In the next stage, the dimension (width × height) of the degraded (warped and blurred) u is decreased by the decimation operator, D _{ k }. Finally, noise (assumed to be additive white Gaussian), η _{ k }, is added as a further image degradation agent. Consequently, a sequence of lowresolution images, {y _{ k }}, where k=1,2,3,…,M is generated. As the model is multiframebased, the degradation process incorporates a sequence of composite operators— W _{1} B _{1} D _{1},…,W _{ k } B _{ k } D _{ k },…,W _{ M } B _{ M } D _{ M }—which are applied to u to produce y _{1},…,y _{ k },…,y _{ M }, respectively.
Therefore, we intuitively represent the multiframe image degradation model by the equation
Treating η _{ k } as energy, our objective is to minimize it using the L ^{2} norm that is known for its ability to suppress noise. Therefore, the minimization problem becomes
where Ω is the supporting domain of u. Applying the EulerLagrange optimization approach, and embedding the resulting solution into a dynamical system, we get
Multiframe superresolution process
Background
The fundamental aim of the superresolution process is to improve the spatial resolution of an image while suppressing unwanted artifacts. A wellknown approach to achieve this goal is to incorporate a regularizing potential into the superresolution energy functional. Therefore, we can reformulate (2) as a variational problem
where the terms from left are defined as follows: superresolution or data, regularization potential, and fidelity, respectively. From (4), the coefficient function, ψ(s), simultaneously detects edges and penalizes the norm of the image gradient, and the fidelity term, which contains λ as the regularizing parameter, establishes a tradeoff between the evolving image, u, and the initial guess, f.
The parameterized (timedependent) EulerLagrange form of (4) is
In [36], Rudin et al. proposed ψ(s)=1, and plugging this value into (5) yields
which is the multiframe superresolution model based on the RudinOsherFatemi (ROF) regularizing functional. Although the ROF model suppresses noise and effectively recovers edges, it has some limitations, as noted by Ogada and colleagues [43]. Firstly, the formulation favors piecewiseconstant solutions that result into staircasing effects or even generation of false edges. Secondly, the ROF model tends to reduce contrast in homogeneous or noisefree image regions. Thirdly, the TV diffusion model—despite its anisotropic property—produces a process that only diffuses in a direction that is orthogonal to the gradients of the image contours. This has a consequence of producing blockiness in the results. And lastly, the ROF evolution system contains a \(\frac {1}{\nabla u}\) component that runs into a spike when ∇u=0 in flat regions. To solve this challenge, the numerical implementations of the model usually incorporate a lifting parameter 0<ε<1, where ε is made too small. That is, the diffusion coefficient in the divergence part of (6) is replaced by \(\frac {1}{\nabla u+\epsilon }\). In our view, this modification limits the accuracy of the results and may even cause instabilities.
Motivated by the weaknesses of the ROF model, Ogada et al. proposed an alternative method for image denoising that focuses on selecting an appropriate value of ψ(s). Their approach provides a criteria for deciding the nature of ψ(s) in terms of linearity, sublinearity, and superlinearity. This ensures that the resulting regularizing functional is strictly convex and grows linearly. In imagedenoising problems, linear growth and convex functionals are known to generate appealing results. Additionally, the authors’ diffusion equation contains the denominator that never collapses to zero even in smooth regions (where ∇u=0), thus avoiding the singularity risk like that encountered in the TV problem.
Therefore, Ogada et al. selected a sublinear growth functional coefficient
Plugging ψ(s) from (7) into (5) gives
Decomposing (7) into \(u_{_{\textit {TT}}}\) (tangential) and \(u_{_{\textit {NN}}}\) (normal) components yields
which possesses the diffusion mechanisms in both tangential and normal directions to the isophote lines. Furthermore, Eq. 9 promotes varying degrees of diffusions depending upon the local image structures, particularly edges and contours.
Therefore, in order to reap the benefits of the model by Ogada et al. and, indeed, the edgepreserving capability and sensitivity of the model to the finer local image structures, we have exponentiated the denominator of the diffusivity of (8) by an adaptive term. Our goal was to make the smoothing functional adapt several models: isotropic diffusion, ROF, and Ogada et al., and to ensure global minimum energy that guarantees uniqueness in the results.
Proposed model
2.2.1 Model formulation
The smoothing functional proposed by Ogada et al. [43] produces superior results compared with the PeronaMalik (PM) [44], D αPM [45], and total variation [36]. The model has been used for denoising applications. In this work, we have modified the model by integrating into its diffusivity an edgeprobing variable exponent that is robust against noise. Furthermore, the modified formulation was encapsulated into the classical multiframe superresolution model.
Now, we propose a nonvariational problem
where β is a shapedefining parameter and
is an adaptive featuredependent variable exponent with σ>0 and \(G_{\sigma }(x)=\frac {1}{4\pi \sigma }\exp \left (\frac {x^{2}}{4\sigma ^{2}}\right)\) (Fig. 2). The authors in [46] found that σ=0.50 and 0.0025<κ<0.025 produced promising restoration results, and indeed, their findings worked well for our case. Equation 10 is usually solved by embedding it into a dynamical system, which is then evolved until steady state conditions are attained. Therefore, parameterizing the equation in time yields
which can be implemented in the computer using the appropriate numerical schemes. In this paper, we used the fourpoint neighborhood explicit scheme to implement (12), as detailed in the later sections.
Figure 3 shows the block diagram that implements our model in (12). From the illustration, a sequence of M frames, y _{1},y _{2},…,y _{ k },…,y _{ M }, is first captured by an imaging device. Then, the values of the motion parameters (rotations and translations) between {y _{ k+1}}_{ k=1,2,…,M } and y _{1} (reference frame) are computed using the Keren et al. algorithm—which is chosen for its reliability, robustness, accuracy, and computational efficiency. Next, the frames are aligned using the computed motion values and projected onto the highresolution grid. Next, the Zomet et al. method is used to robustly detect outliers in the data. The actual reconstruction is done in the following step, and we used the steepest descent approach for this purpose. Lastly, the proposed regularizing functional is incorporated to address the superresolution illposedness and also to address noise issues in the image. The program’s iteration exit point is determined using the L ^{2} error norm
where 0<ε<1 is a tuning constant that determines the final error of the results.
2.2.2 Physical significance and roles of α(x)
The variable, α∈[1,2], sweeps values between one and two according to the local features of an image (Fig. 4). From (11), we observe the following cases:

1.
In flat regions (∇(G _{ σ }∗f)→0), α=1. Substituting this value of α into (12) yields
$$ \begin{aligned} \frac{\partial u}{\partial t}=&\frac{1}{M}\sum_{k=1}^{M}W_{k}^{\prime}B_{k}^{\prime}D_{k}^{\prime}(W_{k}B_{k}D_{k}uy_{k})\\ &+\text{div}\left(\frac{2+\frac{\nabla u}{\beta}}{1+\frac{\nabla u}{\beta}}\nabla u\right)\\&\lambda(uf). \end{aligned} $$Now, expanding the divergence part of this equation produces
$$ \begin{aligned} \frac{\partial u}{\partial t}=&\frac{1}{M}\sum_{k=1}^{M}W_{k}^{\prime}B_{k}^{\prime}D_{k}^{\prime}(W_{k}B_{k}D_{k}uy_{k})\\ &+\text{div}\left(\frac{1}{1+\frac{\nabla u}{\beta}}\nabla u\right)+\triangle u, \end{aligned} $$((14))which combines two regularizing models, namely ROF and isotropic diffusion. Therefore, the formulation can isotropically remove noise in these (flat) regions. Additionally, if β is carefully tuned, we may preserve weak edges due to the presence of a regularizing component—middle term of (14)—that is similar to that of TV.

2.
Near edges (∇(G _{ σ }∗f)→∞), α=2. Thus, Eq. 12 becomes
$$\begin{aligned} \frac{\partial u}{\partial t}=&\frac{1}{M}\sum_{k=1}^{M}W_{k}^{\prime}B_{k}^{\prime}D_{k}^{\prime}(W_{k}B_{k}D_{k}uy_{k})\\&+\text{div}\left(\frac{2+\frac{\nabla u}{\beta}}{\left(1+\frac{\nabla u}{\beta}\right)^{2}}\nabla u\right)\\ &\lambda(uf), \end{aligned} $$which contains a regularizing part proposed by Ogada et al. [43]. This formulation helps to suppress noise, avoid diffusion of edges, and enhance the spatial resolution of an image.

3.
From the above two cases, I and II, we see that α(x) plays another role of segmenting an image into two subregions, namely Ω _{1} (flat regions; α=1) and Ω _{2} (edges and contours; α=2). Hence, α is an edgedefining variable. An important aspect of α is that it contains a convolution operator between the Gaussian kernel and the image, G _{ σ }∗f, which helps to suppress noise and other unwanted artifacts—thus detecting useful image features robustly, even under harsh imaging conditions (Figs. 5 and 6).
2.2.3 Properties of the model
To comprehensively understand the (mechanical) properties of our model, consider two orthogonal vectors, T(x)=(u _{ x },u _{ y })/∇u and N(x)=(−u _{ y },u _{ x })/∇u, in an image, u, where u _{ x } and u _{ y } are the firstorder partial derivatives of u in the x and ydirection, respectively, and ∇u≠0. Additionally, let \(u_{_{\text {TT}}}\) (tangential) and \(u_{_{\text {NN}}}\) (normal) be the secondorder partial derivatives of u, representing diffusions, in the directions of T and N, respectively. Defining \(u_{_{{\text {TT}}}}\) and \(u_{_{\text {NN}}}\) as
where \(T^{\prime }=(1/\nabla u) \left (\begin {array}{c}u_{y}\\ u_{x}\end {array}\right), N^{\prime }=(1/\nabla u) \left (\begin {array}{c}u_{x}\\ u_{y}\end {array} \right),\)and \(\nabla ^{2}u=\left (\begin {array}{cc}u_{\textit {xx}} & u_{\textit {xy}}\\ u_{\textit {xy}} & u_{\textit {yy}} \end {array}\right)\) is the Hessian matrix; Eq. 12 can compactly be rewritten in terms of \(u_{_{\text {TT}}}\) and \(u_{_{\text {NN}}}\) as
Equation 15 possesses some interesting properties worth noting: in flat regions (∇u→0 and α→1), the equation reduces to
(C is a constant value) which contains an isotropic diffusion component (\(\triangle u=u_{_{\text {TT}}}+u_{_{\text {NN}}}\) is the pure heat equation) that removes noise uniformly over the regions. Also, near edges (∇u→∞ and α→2), the coefficient of \(u_{_{\text {NN}}}\)—which contains the denominator larger than that of \(u_{_{\text {TT}}}\)—vanishes faster. Consequently, the \(u_{_{\text {TT}}}\) component that is responsible to preserve edges dominates.
Numerical implementation
The desire to implement our method using an explicit scheme is attributed to the following reasons: computational efficiency, ability to produce more accurate and appealing results, intuitiveness to be understood and analyzed mathematically, and stability over the time interval defined by the CourantFriedrichsLewy criterion (0<τ≤0.25) [48]. A significant drawback, however, of explicit schemes is that they are susceptible to instabilities for larger iteration steps.
Now, consider a fivepoint discretized space with four directions: north, south, east, and west (Fig. 7). The gradients of u _{ i,j } along these directions are, respectively, \(\triangle _{i,j}^{N}=u_{i,j+1}u_{i,j}, \triangle _{i,j}^{S}=u_{i,j1}u_{i,j}, \triangle _{i,j}^{E}=u_{i+1,j}u_{i,j}, \text {~and~} \triangle _{i,j}^{W}=u_{i1,j}u_{i,j}\), and the corresponding discrete conduction coefficients from the divergence part of (12) are
where 0≤i≤P and 0≤j≤Q; P and Q are, respectively, the horizontal and vertical dimensions of u _{ i,j }. Following the explicit scheme, we discretize the divergence component of our formulation as
From the given discretizations, therefore, the steepest descent implementation of the regularized superresolution method in (12) becomes
where μ is a tuning constant and
is a superresolution result at the nth iteration. The boundary conditions of (17) are as follows: \(u_{i,j}^{(0)}=(y_{i,j})_{1}=y_{1}(ih,jh), u_{i,0}^{(n)}=u_{i,1}^{(n)},~u_{0,j}^{(n)}=u_{1,j}^{(n)},~u_{P,j}^{(n)}=u_{P1,j}^{(n)},~\text {and} ~u_{i,Q}^{(n)}=u_{i,Q1}^{(n)}\)
Performance evaluation of the superresolution models
To quantitatively evaluate the performance of different superresolution models, we have used three metrics: peaksignaltonoise ratio (PSNR) [49, 50], edge similarity (ESIM) [45], and mean structure similarity (MSSIM) [51]. The PSNR measures signal strength relative to noise in the image and is defined by the equation
Though widely applied in many imageprocessing disciplines, PSNR fails to explain the quality of edges in the restored scenes. Guo et al. addressed this limitation by proposing the metric
where EM(u) and EM(f) are, respectively, the edge maps of u (restored image) and f (initial ideal image), and
is as proposed in [52]. Moreover, both PSNR and ESIM inadequately address the perceptual qualities of scenes. Wang and colleagues proposed a metric (MSSIM) that explains the statistical interdependency of pixels in scenes, thus quantifying their visual appeals [51]. The MSSIM is governed by the equation
where the variables, respectively defined for u and f are as follows: μ _{ u } and μ _{ f }, mean; \({\sigma _{u}^{2}}\) and \({\sigma _{f}^{2}}\), variance; and σ _{ uf }, covariance. And c _{1} and c _{2} are stabilizing constants.
Experiments
We conducted experiments from both simulated and real environments to test and compare the performance of the proposed method and some classical superresolution methods, namely (Total variation) [33], (Spatially weighted total variation) [42], (Fractional order total variation) [41], and (Adaptive total variation) [40]. In the first experiment, we degraded each of the original highresolution images of Boat, Bridge, Building, Fish, Goldhill, Lena, Mandrill, and Wheel (http://decsai.ugr.es/cvg/CG/base.htm) (Fig. 8) to generate the corresponding sequence of ten lowresolution images. Next, we applied TVSR, SWTVSR, FTVSR, ATVSR, and the new method to restore the respective original versions of the degraded image sequences (Fig. 9). These procedures were repeated in the second experiment but with the input images being the degraded lowresolution video frames of EIA (https://users.soe.ucsc.edu/~milanfar/software/srimages.html) (Fig. 10). The last experiment tested the efficacy of our method for two conditions of the variable exponent, α(x): fixed (α=1 and 2) and adaptive (Fig. 11).
Results and discussions
The visual results demonstrate that the proposed method outperforms in several cases compared with some stateoftheart classical methods (Figs. 9 and 10). Both simulated and real experiments show that the new approach generates appealing images that are sharper and detailed. Other methods, such as TVSR, SWTVSR, and FTVSR, reveal obvious artifacts—ringing, blocking, and staircasing (Fig. 9). The last experiment proves that the adaptive nature of our formulation helps to preserve useful image features and suppress noise; setting constant the edgelocating variable exponent, α(x), in the proposed model lowers its performance (Fig. 11). We observed earlier in Section 2.2.2—“Physical significance and roles of α(x)”—that fixing α to 1 (α=1), for example, makes the superresolution problem regularized by both TV and isotropic diffusion, and this promotes edge recovery and noise removal, respectively. The results in the third experiment prove this mathematical intuition but also reveal some artifacts that are probably due to fixing α(x), as depicted by Fig. 11 c. In Fig. 11 e, we observe that adaptively updating α(x) makes the results appear attractive.
Quantitative results further confirm that the proposed model is superior as it achieves higher quality values. Table 1 shows that the new method achieves promising PSNR values in most cases, which implies that the method retains higher signal contents in the restored results. In terms of the edge recovery capabilities, our method performs better as it shows larger values of ESIM compared with those from the other methods (Table 2). This finding concurs with the subjective assessments that the proposed method generates images with sharper and stronger edges, which raises the value of the ESIM. In Table 3, the results demonstrate that the new method produces MSSIM values that are larger than those produced by TVSR, SWTVSR, FTVSR, and ATVSR. This quantitative quality assessment justifies the observation that our results are visually appealing (Figs. 9 and 10). Recall that MSSIM favors the human visual system. Furthermore, Table 4 demonstrates that our method offers lower computational times—thus making it a suitable choice in tasks that demands parallel computing.
The performance of ATVSR closely follows that of our method. Subjectively, the results of these two methods are hard to distinguish (Fig. 9; images of Building and Lena; and Fig. 10). However, numerical results indicate that the proposed approach outperforms in several cases (Tables 1, 2, and 3). For the “Wheel” image, however, Table 1 shows that the ATVSR outperforms our method by a small amount. Also, the ATVSR is slightly faster than the proposed method (Table 4), but the deviation is too small that we may assume the two methods perform equally.
Despite the promising performance, the proposed method suffers from one weakness—it tends to slightly blur the output images. This is probably caused by the lowpass filtering operation of the regularizing functional or inappropriate estimation of the blur function. More research is thus needed to address the limitation. It is worth noting that the superresolution results are, however, not only limited to human consumption but also to industrial applications, such as control and automation [53, 54], object detection, and feature extraction. With the proposed method generating promising objective results, we hope that it may as well suit these other disciplines.
Conclusions
In this paper, we have presented an adaptive multiframe superresolution model that sufficiently restores fine image details. The new method incorporates a spatially varying regularizing term that updates its value according to the local image features—linear isotropic in flat regions and nonlinear anisotropic near edges. This flexibility and adaptability makes the model generate promising results, objectively and quantitatively. Also, the proposed adaptive term includes a convolution operation with the Gaussian filter, and this allows the model to robustly emphasize critical and meaningful features. Experimental results visually demonstrate the strength of the new method that it reveals more information in the reconstructed images compared with other methods. Objectively, we have shown that the method generates promising values of the quality metrics (PSNR, ESIM, and MSSIM).
In the future, we are contemplating the possibilities of extending our method to other fields, such as in medical imaging. For example, doctors in sonography require highquality ultrasound images to provide accurate treatments to patients. The current instruments produce lowquality images that are heavily degraded by multiplicative noise. The problem can be approached in a variety of ways. In the context of the new method, three important processes that may help to address the problem are (1) modifying the prior to cover multiplicative noise, (2) transforming the model into the threedimensional space to comprehensively treat the ultrasound images, and (3) implementing the model using more accurate and fast numerical schemes that support realtime parallel computing.
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Maiseli, B.J., Elisha, O.A. & Gao, H. A multiframe superresolution method based on the variableexponent nonlinear diffusion regularizer. J Image Video Proc. 2015, 22 (2015). https://doi.org/10.1186/s1364001500772
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Keywords
 Superresolution
 Regularization
 Image reconstruction
 Diffusion