#### Abstract

Image restoration problem is ill-posed, so most image restoration algorithms exploit sparse prior in gradient domain to regularize it to yield high-quality results, reconstructing an image with piecewise smooth characteristics. While sparse gradient prior has good performance in noise removal and edge preservation, it also tends to remove midfrequency component such as texture. In this paper, we introduce the sparse prior in fractional-order gradient domain as texture-preserving strategy to restore textured images degraded by blur and/or noise. And we solve the unknown variables in the proposed model using method based on half-quadratic splitting by minimizing the nonconvex energy functional. Numerical experiments show our algorithm's robust outperformance.

#### 1. Introduction

Mathematically, the image degradation is modeled as where is the original latent image and is an observed image degraded by blur and/or noise, which is produced by convolving with a blur point-spread-function (a.k.a. kernel) and adding zero mean Gaussian noise . Image restoration is recovering latent image from observed image.

Image restoration is ill-posed problem, so many methods introducing priors based on natural image statistics can regularize it. Total variation regularization is originally used for noise reduction [1, 2] and has also been used for image deblurring [3]. Chan and Wong [4] introduced total variational blind deconvolution method for motion blur kernel and out-of-focus kernel. Heavy-tailed natural image priors [5, 6] and hyper-Laplacian priors [7–10] were also extensively introduced. Numerous regularization approaches have been proposed too. Wang et al. [7] presented a fast total variation deconvolution algorithm to compute TV image deconvolution. Krishnan and Fergus [8] take a novel approach to the image restoration problem arising from the use of a hyper-Laplacian prior. Xu and Jia [11] developed a fast TV- deconvolution method based on half-quadratic splitting.

While image reconstructed by algorithms above suppresses noise and preserve edges, it has piecewise smooth characteristic that the mid-frequency components such as textures are removed too.

In digital images, the gray values between neighboring pixels have high correlation. This highly self-similar fractal information of image fractal information is usually represented by complex textural features, and the works in [12–18] showed that fractional-order gradient is more suitable to deal with fractal-like textures. It has been proved in [12] that the fractional-order derivative satisfies the lateral inhibition principle of biologic visual system better than the integer-order derivative. The fractional-order derivative operators have been used in texture enhancement [13], image denoising [14, 15], and image inpainting [16, 17]. Jun and Zhihui [14] replaced the first-order derivative in the regularized term of ROF model with the fractional-order derivative. Bai and Feng [15] designed fractional-order anisotropic diffusion equation to remove noise. Zhang et al. [16] exploited fractional-order TV sinogram inpainting model to reduce metal artifacts for X-ray computed tomography. In [18], fractional total variation method was introduced to restore textured image. This work shows that the fractional-order derivative not only nonlinearly preserves the textural details but also eliminates the staircase effect caused by low integral-order derivative in image processing. Different from work in [18], the sparse prior in fractional-order gradient domain is considered in our work, which is more suitable for the texture of image. It is explained clearly in Figures 2 and 3.

This paper presents fractional-order regularization for the restoration of textured image degraded by blur and/or additive noise. R. Tony uses the Laplacian prior in fractional-order gradient domain for to preserve the texture. According to our analysis in the next section, hyper-Laplacian image prior in fractional-order gradient domain for is more suitable to keep different texture for different texture image.

The outline of this paper is as follows. In Section 2, we analyze the reason why integral-order regularization fails to restore image texture. In Section 3, our fractional-order regularization model is proposed and based on half-quadratic splitting, we solve model using efficient alternating minimization method. Numerical experiments and comments are provided in Section 4 and the paper is concluded in Section 5.

#### 2. Motivation

The prior favors natural image, usually based on the observation that their heavy-tailed gradient distribution is sparse. For example, Figure 1 shows textured image and a histogram of its gradient magnitudes in* x*-direction and* y*-direction, respectively. The distribution shows that the image contains primarily small or zero gradients, but a few gradients have large magnitudes. A common measure [19] is
where and denote the horizontal and vertical derivatives at pixel (here, the simple and filters are used) and exponent value . leads to sparse prior and natural images usually correspond to in the range of [19]. and are Laplacian prior and Gaussian prior, respectively.

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The image restoration methods use the sparse prior term as a regularized term of variational energy functional [19], which is

The failure of restoring texture with the sparse gradient prior depends on the fact that the value of energy does not always decrease during restoration process, so the no-blur explanation is usually favored. To understand this, consider the 1D signals in Figure 2.

For sharp edge in Figure 2(a), while Gaussian prior favors the blurry explanation, the sparse prior favors the correct sharp explanation in Figure 2(b). The signal considered in Figure 2(c) illustrates that natural image contains a lot of medium contrast textures, which dominate the statistics more than step edges. As a result, blurring natural image reduces the overall contrast which cannot be restored by Gaussian prior or even sparse priors as in Figure 2(d).

The reason is that the gradient profile in fractal-like textures is close to Gaussian distribution and these small values are severely penalized by the sparse gradient prior.

A fractional-order gradient log distribution can be expressed as follows [18]: where and denote the horizontal and vertical fractional-order derivatives at pixel and is the fractional order . The exponent value is the same as value in (2).

Compared with result in Figure 3(b), the sharp explanation in Figure 3(c) is favored by sparse prior even by Gaussian prior in fractional-order gradient domain.

#### 3. The Proposed Model and Algorithm

The corresponding energy functional is as follows [18]: where is the pixel index and is the 2-dimensional convolution operator, and a weighting term controls the strength of the regularization. and denote the horizontal and vertical fractional-order derivatives at pixel defined by our coauthor as Tables 1 and 2 [16].

The coefficients of the operator in Tables 1 and 2 are
Equation (5) contains nonlinear penalties for regularization term, so we propose alternating minimization (AM) method, based on a half-quadratic splitting to solve it [18, 20]. We introduce auxiliary variables and at each pixel, so the energy functional in (5) can be modified as
where the first two terms are used to ensure the similarity between the measures and the corresponding auxiliary variables. As and the solution of (6) converges to that of (5). Equation (7) can be solved by AM method through fixing other variables to solve **,** and independently.

##### 3.1. Subsolution

Given fixed values of and from the previous iteration, (7) is quadratic in . So we compute by minimizing The optimal is where . According to Parseval’s theorem after the Fourier transform, (8) has the closed form solution in minimization, which enables us to find the optimal directly: where and denote the fast Fourier transform and inverse fast Fourier transform, respectively. is the complex conjugate operator.

##### 3.2. Subsolution

Here, and belong to different terms. They are not coupled with each other in the functional, so their optimization is independent. Given fixed value of , we compute by minimizing According to shrinkage formula [21], the optimal is

##### 3.3. Subsolution

We have the following:

For case, the subproblem about and is quadratic.

For case, the optimal solution for and can be derived by shrinkage formula too:

For the other case, setting the derivative of (13) with regard to and to zero gives Two special cases are discussed here.

For case, about , (15) becomes

Because is fixed and lies between and , we can replace with . Equation (17) can be rewritten as

So we can get the cubic polynomials about and :

The value of and is either or the root of cubic polynomial in (19).

For case, we can get the quartic polynomials about and :

The value of and is either or the root of cubic polynomial in (20).

Given the roots of cubic and quartic polynomials and zero solution, we need to determine which one corresponds to the global minima of (12), which can be confirmed by the following scheme.

Let be the nonzero real root. If value is the optimum solution of (13), for **,** this implies
We can use (15) to eliminate from (15) and (22), yielding the condition
since . So if is between and in the case or between and in the case. Otherwise, . The same scheme applies to .

For other cases, and can be computed by Newton method.

##### 3.4. Algorithm

and are positive values to enforce the similarity between the auxiliary variables and the respective terms. We empirically set , and , .

The algorithm of this fractional-order regularization model is shown in Algorithm 1.

#### 4. Numerical Experiments

We consider the restoration of a blur- and noise-contaminated test image represented by pixels. In order to compare the accuracy of FOR (fractional order regularization) and IOR (integer order regularization) more precisely, we list in Table 3 the peak signal-to-noise ratio (PSNR) and gray-scale structural similarity (SSIM) as quality metric. PSNR is most easily defined via the mean squared error (MSE). Given a noise-free by image and its noisy approximation , MSE is defined as PSNR is defined as and SSIM is defined as where and are different images, and are the average of and , respectively, and are the variance of and , respectively, and is the covariance of and . and are constant.

The desired blur- and noise-free image is depicted in Figure 4. The image is contaminated by motion blur generated by Matlab function (fspecial(“motion”,10,20)). The resulting image is displayed in Figure 4(f). In Table 3 the second column, with header PSNR and SSIM values for images that have been corrupted by motion blur, is characterized by and .

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The desired blur- and noise-contaminated image is depicted in Figure 5. The image is contaminated by motion blur, adding white Gaussian noise (its standard variance is 0.003). The resulting image is displayed in Figure 5(d). In Table 3 the third column, with header PSNR and SSIM values for images that have been corrupted by motion blur, is characterized by and .

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Figure 6 shows the result of deconvolving a real blurry image. We estimate the blur kernel using the algorithm in [9]. Again, textured regions are better reconstructed using our method in visual quality. Figure 6(b) is restored by total variation. Figure 6(c) is restored by fractional-order total variation. Figure 6(d) shows the details in Figure 6(b) and Figure 6(c).

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#### 5. Conclusion

By introducing sparse prior in fractional-order gradient domain, we propose a fractional-order regularization method for the restoration of textured image degraded by blur and/or noise. The regularizer is constructed by using fractional-order derivatives, where the choice of the fractional-order is driven by different textured image. This makes the proposed model an efficient tool to preserve texture well. Numerical results show that the proposed model yields better SSIM and PSNR value and visual effects than using integral-order regularization method.

Our following work is to use an automatic texture detection procedure for textured image restoration. Different parameters are applied for different textures.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgment

This work was supported by the National Natural Science Foundation of China (NSFC) (no. 61302028). This document is a collaborative effort.