A Convex Approach for Image Hallucination

In this paper we propose a global convex approach for image hallucination. Altering the idea of classical multi image super resolution (SU) systems to single image SU, we incorporate aligned images to hallucinate the output. Our work is based on the paper of Tappen et al. where they use a non-convex model for image hallucination. In comparison we formulate a convex primal optimization problem and derive a fast converging primal-dual algorithm with a global optimal solution. We use a database with face images to incorporate high-frequency details to the high-resolution output. We show that we can achieve state-of-the-art results by using a convex approach.


Introduction
Single image Super Resolution (SU) systems yield to estimate a high-resolution (HR) image from low-resolution (LR) input. This is clearly an ill-posed problem due to the fact that important high frequency information is lost in a downsampling process.
A common constraint to nearly all SU systems is the reconstruction constraint, which says that the HR result down-sampled should be the same as the LR input. However, this constraint is weak and the space of possible solutions is large. A generic smoothness prior, like the Total Variation (TV), can improve this constraint but no lost information is infered. More advanced systems model edge statistics which can produce HR images with sharp edges while leaving other regions smooth [12]. This approach has its advantages in creating sharp edges with minimal jaggy or squary artifacts. But their performance will decrease as the resolution of the input decreases because the perceptual important edges will vanish. Additionally such systems cannot introduce novel high frequency details which were lost in the down-sampling process.
Backer and Kanade have shown in [1] that systems which only rely on the reconstruction constraint (and possible altered with a smoothness prior) cannot create high frequency image content. They propose the technique of image hallucination where HR image details and there LR correspondences are learned on a patch basis to synthesise HR images. Such systems like [9] can introduce new details which are not present in the low resolution image. However, the patchselection process remains a key problem in such systems and the mathematical models make it difficult to control artefacts in the output. A state-of-the-art enhancement of such a system is the work of Sun et al. [13] which incorporates their so called textual context bridging the gap between image hallucination and texture synthesis.
While these systems perform well on general images, domain based SU system where the content of the image is known have shown improvements of the results again. An example of such an approach is the work of Liu et al. face Figure 1: System-Overview: We use SiftFlow [10] to find similar appearing images based on the alignment energy. These Canididates are warped to match the input. The LR input and the aligned candidates are incorporated to form the estimate.
hallucination [8]. Their system inferes regularities of face-appearances to hallucinate details that a general image model can't create. However, the system of [8] is limited to frontal face images and can't handle large pose and viewpoint changes.
Our work is based on the paper of Tappen el al. [14]. This approach uses aligned face images prior to the hallucination process and therefore incorporates the ideas of the classic multi-image SU-systems. Tappen et al. uses Patch Match [2] to quickly search for similar face images in a large database. The best matches are called candidates. These candidates are densely aligned using the SIFTflow algorithm [10]. Their system incorporates a edge focusing image prior, a global likelihood function (the reconstruction constraint) and an examplebased non-convex hallucination model within a Baysian framework. Tappen et al. pointed out that if their system can't find good candidates, the performance decreases fast and the results get blurry. These limitations could be compensated by falling back to an edge-based system. Our system tries to improve this behaviour by using a hallucination model more robust to outliers.
We proposes a similar work flow, but in comparison all our models are convex and we omit PatchMatch. We search the database with SiftFlow utilizing the SiftFlow energy and warp the search results with the same algorithm. We omit PatchMatch because we think it is more important to have good aligned candidates rather than similar appearing images. Our convex optimization problem joins an total variation based image prior, the reconstruction constraint and a hallucination model robust to outliers. Starting with the primal minimization problem we derive a generic saddle-point problem and solve it with a fast converging primal-dual algorithm proposed by Chambolle et al. [5]. Figure 1 shows our system overview.
This paper is organized as followed. After presenting a convex approach for image hallucination in section 2 we derive a generic saddle-point problem and solve it with a so called primal-dual algorithm in section 3. In section 4 we describe the experiments made and we conclude in section 5.

A Convex Approach for Image Hallucination
As pointed out in the introduction, our work alters the model of Tappen et al. [14] to a convex approach. Solving a convex minimization problem has nice advantages. Convexity guarantees an existing, unique solution [4] and fast convergence can be achieved. However, choosing the image models and energy minimization terms are crucial for a perceptual good solution. The minimization problem of (1) combines 3 different image models and constraints where u is our HR estimate, f the LR input and g C are the aligned candidate images.
The first term is a general smoothness prior equipped with the TV-norm. This model preserves sharp edges while staying smooth in other regions. The TV is defined as T V (u) = Ω |∇u|dx, where ∇ = ∂ ∂x , ∂ ∂y is the gradient operator and the · is the L1-norm.
The second term of (1) models the reconstruction constraint. The constraint ensures that the down-sampled HR image yield to the LR input. In other words, the HR estimate down-sampled should be the same as the input. The matrices DB composes of a Gaussian blurring or anti-alias filter B and a downsampling matrix D. The reconstruction constraint implies a linear model where the observed image f is a linear combination of the undistorted image u added by noise f = Au+n. If we model the noise as Gaussian, we end up by equipping this term with a quadratic norm minimizing the noise. The factor λ controls how strong the constraint is imposed.
The third term of (1) represents a non-parametric image model here referred as the hallucination term. Having found similar candidate images from a database and aligned them to our input, high-frequency details can be introduced from these candidates. The term minimizes the difference between the HR result u and the candidate images g C after applying a high-pass filter H. We apply the high-pass filter H to infer only high-frequency information from the candidates. Thats because the low-frequency details are still present in the LR input and can be omitted. Equipping this function with the L1-norm makes it robust to outliers which is the case if no good candidates where found or if the alignment fails.
Note that there exists a strong relation between the blurring matrix B, the high-pass filter H and the scaling factor. In fact the high-pass filter kernel equals a all-pass kernel δ subtracted by the blurring kernel. So we incorporate just frequencies we lost in the down-sampling process. Moreover the blurring kernel depends on the scaling factor [15]. We choose the standard derivation for the blurring kernel as σ = 1 4 ξ 2 − 1, where ξ is the scaling factor.

Deriving the Primal-Dual Algorithm
In this section we will derive the first order primal-dual form of (1). We will solve this generic saddle-point problem with a variational approach, the primal-dual algorithm of Chambolle et al. [5].
The goal is to transform the primal minimization problem (1) into a convexconcave saddle-point problem of the type: with a continuous linear operator K, and G(x) and F (x) being convex functions.
In a first step one has to apply the Legendre-Fenchel transformation also refered as the conjugate of a function [4]. We derive the conjugate of the total variation (TV) ∇u 2,1 and of the hallucination term γ i H(u − g i ) 1 introducing the dual variables p and r i respectively. Additionally the primal variables w i is introduced as a lagrange-multipliere of the hallucination term leading to: with the structure of K, x and y as: The term δ p ∞≤1 (p) denotes the indicator function and p ∞ the maximum norm. Note that we don't apply the Legendre-Fenchel transformation to the reconstruction constraint λ DBu − f 2 2 . Instead we solve this sub-problem using the conjugate-gradient method (CG) [3] in an subroutine. Because the reconstruction constraint imply a linear model and thus consists of a linear system of equations, it is reasonable to use a fast-converging solver specialized on such systems. We refer to 3.2 for further details.

Algorithm
We use the first order primal-dual algorithm proposed in [5], there referred as "Algorithm 1". The idea is to perform a gradient ascent/decent step on the unconstraint objective function and sequentially reproject the variables according to the constraints. The gradient step-size σ and τ are crucial for convergence and have to satisfy τ λL 2 < 1 with L = K the operator norm of K. Within an iteration we perform a gradient decent in the primal variable x and a gradient ascent in the dual variable y followed by the reprojection utilizing the proxoperators. Additionally we perform a linear extrapolation of the dual variable based on the current and the previous iterates with θ = 1. This can be seen as an approximate extragradient step and offers fast convergence.

The prox-operators
Proximity Operators are a powerful generalization of projection operators. Their importance is attached by splitting the subject to be minimized into simpler functions that can be handled individually [6]. The proximity operator then assures to "resolve" the sub-gradient ∂G of any function G even if G is nonsmooth. We assume that F and G are simple so that one can compute their proximity operator in a closed-form. The operator is defined as: In order to apply the algorithm we have to compute the prox-operator for (I + σ∂F * ) −1 and (I + τ ∂G) −1 . In (3) we see that and The first term in F * (y) is the indicator function of a convex set and the proxor resolvent operator reduces to a pointwise Euclidean projection onto L 2 balls. The function −Hg i , r i poses an inner product and the prox operator of reduces to an affine function.
For w i 1 the resolvent operator poses a soft-threshold shrinkage function. The prox-operator of the reconstruction constraint poses again a linear problem: Note that the CG-method expects a symmetric positive definite matrix A new which is clearly the case. We apply the CG with a so called "hot-start" where the previous iterate of u is used for initialization. The hot-start initialization achieves faster convergence of the CG-method.

Experiments
In our experiments we use the PubFig83 database presented in [11]. The database consists of over 14,000 images of public figures, cropped to include just the faces and resized to the identical resolution of 100 × 100 pixels.
All results are produced in the same manner. First we down-sample the input by a factor of 4 using bicubic interpolation, followed by a bicubic upsampling by the same factor. We use the resampled image as an input to the SiftFlow [10] and search for candidates with the least SiftFlow energy. Figure 1  demonstrates this process. We just search in the set of pictures from the same individual as the input. We imply that the person of the input has already been identified by a face-recognition system and pictures of this person are available. Having found the best 6 candidate images, we aligned them to our input using again Siftflow. With this aligned candidates we run the primal-dual algorithm. Note that the input image f of the algorithm is still the bicubic down-sampled 25 × 25 image, while the candidates g i and the result u * are 100 × 100. On the output we calculate the Signal-to-Noise ratio (PSNR) and the SSIM index. Figure 2 shows some results of our algorithm.
In our experiments we discovered that a strong reconstruction constraint is needed and therefore the λ-value was set to λ = 5 · 10 4 , which has proven a high PSNR. The hallucination parameter γ was set to γ = 20 so that smoothing by the TV-regulatization is still applied. To treat the color images in optimization, we did a so called channel-by-channel optimization. A more comprehensive color treatment was proposed in [7] called vectorial total variation. This advanced TV-regulatization should be included in future work.
We ran our algorithm on all 14,000 images and got a mean PSNR of 24,13dB. This result outperforms the work of Tappen et al. which got a mean PSNR of 24.05dB. Table 1 shows a comparison with different algorithms and the achieved PSNR and SSIM index. The table was partly taken from Tappen et al. and we refer to [14] for further information. In figure 3 we show a comparison between the results of Tappen et al. and our approach. The percetual differences on these results are quite low which is not extraordinary because all these examples achieve a high PSNR and SSIM compared to the average.

Conclusion
We presented a convex and global approach for image hallucination. This implies a fast converging algorithm with a unique solution. By incorporating high-frequency information from similar images we get perceptually good solutions. Especially if the alignment of the candidates image works well, the results can be nearly perfect. A crucial part in our system poses the SiftFlow algorithm, first because we use it as a searching tool, and second and more important we use it for the alignment of the images. If SiftFlow is able to align the images, the results are superior to those where the alignment fails. Tracking failed alignments and replacing such candidates should achieve improvements in future work. We think that it is not so important to take images from the same person rather than having good alignments. For future work we propose to build a bag of visual words taken from face images and to apply the same algorithm so that no face-recognition system is needed. Due to the fine modeling of the down-sampling, blurring and highpass filter and the robust hallucination model our convex approach achieves good performance and state-of-the-art results.