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 JCC  Vol.9 No.10 , October 2021
Domain Adaptation for Synthesis of Hazy Images
Abstract: Most existing image dehazing methods based learning are less able to perform well to real hazy images. An important reason is that they are trained on synthetic hazy images whose distribution is different from real hazy images. To relieve this issue, this paper proposes a new hazy scene generation model based on domain adaptation, which uses a variational autoencoder to encode the synthetic hazy image pairs and the real hazy images into the latent space to adapt. The synthetic hazy image pairs guide the model to learn the mapping of clear images to hazy images, the real hazy images are used to adapt the synthetic hazy images’ latent space to real hazy images through generative adversarial loss, so as to make the generative hazy images’ distribution as close to the real hazy images’ distribution as possible. By comparing the results of the model with traditional physical scattering models and Adobe Lightroom CC software, the hazy images generated in this paper is more realistic. Our end-to-end domain adaptation model is also very convenient to synthesize hazy images without depth map. Using traditional method to dehaze the synthetic hazy images generated by this paper, both SSIM and PSNR have been improved, proved that the effectiveness of our method. The non-reference haze density evaluation algorithm and other quantitative evaluation also illustrate the advantages of our method in synthetic hazy images.

1. Introduction

Single image dehazing is an important and difficult task in computer vision, so people usually only pay attention to how to dehaze, and seldom pay attention to how to synthesize hazy images and how to synthesize more realistic hazy images. This will cause some problems today with the rapid development of deep learning. Having a large amount of high-quality data can make the deep learning model perform better, however, most of the datasets in single image dehazing based learning are artificially synthesized. These synthetic datasets cannot fully simulate the hazy images in the real world. Therefore, it is difficult for the model based learning using such data sets to perform well on the real hazy images.

Currently, there are three main methods for hazy image synthesis. The first is synthesis based on the physical scattering models [1] [2]. It need to measure or estimate the depth map corresponding to the image, and then synthesize the hazy images [3] [4] with different parameters of the physical scattering model. Because the depth map inevitably has errors, and the physical scattering model is difficult to fully simulate the real environment, the synthesized hazy images have certain defects. The second is synthesis based learning, which uses paired data to train to generate hazy images. Since this method also requires large amounts of paired data which is mostly synthetic, there may be deviation between the generated hazy images and the real. The third is some others, such as synthetic software, rendering and so on. However, the quality of the hazy images generated by these methods [5] is low, or these hazy images synthesized from virtual environment [6], and there may be still a deviation from the real hazy images.

In order to alleviate the above problems, this paper proposes a domain adaptive hazy image synthesis model. The model is based on variational autoencoder (VAE) and generative adversarial network (GAN), which adapts the synthetic hazy images to the real hazy images by generative adversarial loss, thereby generating a more realistic hazy images.

The main contributions of this paper are as follows:

1) Propose a new domain adaptive model to generate a more realistic hazy images which have various densities.

2) Propose to use generative adversarial loss to adapt to the real hazy images’ domain.

2. Related Works

2.1. Atmospheric Scattering Model

Haze is produced by the scattering of fine substances suspended in the atmosphere, which will reduce the quality of the image and affect subsequent analysis. In computer vision and computer graphics, the following equation is widely used to describe the generation of a hazy image [1] [2].

I ( x ) = J ( x ) t ( x ) + A ( 1 t ( x ) ) (1)

where I is the hazy image, J is the scene radiance representing the haze-free image, A is the atmospheric light, t is the medium transmission, β is the scattering coefficient of the atmosphere and d is the depth of scene, t can be calculated with the following equation.

t ( x ) = e β d ( x ) Math_5# (2)

Image dehazing can restore the lost details of the image and facilitate in-depth analysis. Therefore, this task has always been paid attention in computer vision and progressing rapidly. Traditional methods in single image dehazing based on physical priors [7] [8] do not require paired data to train, but often have certain requirements on the environment to perform well. Methods based learning [9] [10] require a lot of data for training,because it is difficult to obtain the paired images, the hazy images in the training set are usually artificially synthesized by the atmospheric model. There is a certain deviation between the synthesized hazy images and the real hazy images, which causes some models to perform poorly on the real hazy images.

2.2. Hazy Image Synthesis

2.2.1. Traditional Methods

The traditional method of hazy image synthesis mainly adopts Equation (1) and Equation (2), and the synthesis process is shown in Figure 1. The available datasets for indoor hazy image synthesis include NYUv2 dataset [11], Middlebury Stereo [12], etc. The source data of NYUv2 is obtained through Microsoft Kinect, and there are 1499 pictures after the filling of the depth missing value and the post-processing of marking. In general, these data can be used for tasks such as depth estimation and semantic segmentation. The available data for outdoor hazy image synthesis include HazeRD dataset [4], Cityscapes [13], KITTI [14], et al.

Adobe Lightroom CC can synthesize hazy images, it contains a dehazing function that can make the picture clear. When the value is adjusted to a negative value (−70 in this paper), a hazy image will be generated. The dehazing function of this software is used to synthesize hazy images for control experiments. Hazy images can also be generated in some 3D games or rendering software.

2.2.2. Deep Learning Method

The generative adversarial network (GAN) [15] has been receiving widespread attention and developing rapidly [16] [17] [18], it can be used to synthesize hazy

Figure 1. Haze image synthesis based on atmospheric scattering model.

images. The image translation model pix2pix [16] is composed of a multi-reso- lution generator and a multi-scale discriminator. The discriminator which uses the Patch GAN structure can perform multi-scale discrimination on the generated result. The basic structure of the generator is U-Net [19], it can generate high-resolution and clear images. But this method is supervised learning, if used for synthesizing hazy images, the training data that can be used is artificially synthesized hazy images, there may be a certain difference between the generated result and the real hazy image.

The well-known unsupervised image conversion method Cycle-GAN [17] has been proposed to provide an effective loss function named cycle consistency loss for unpaired training data, and the real hazy images can be used for synthesizing hazy images. But this method without absolute mapping will cause the result to appear undesired conversion style.

3. Method

3.1. Domain Adaptation and Image to Image Translation

Domain adaptation aims to reduce the differences between different domains, and it is mainly divided into feature level or pixel level. The feature-level adaptive method aims to align the feature distribution between the source domain and the target domain by minimizing the maximum mean difference (MMD) or applying adversarial learning strategies on the feature space [20]. Another study focused on pixel-level adaptation [21] [22]. These methods deal with the domain transfer problem by applying image-to-image translation.

Image-to-image translation [16] [23] refers to the translation of images from one domain to the other through learning. Image translation of supervised learning requires paired data sets, and the corresponding image translation algorithms include pix2pix [16] model, pix2pixHD [23] model, etc. Unsupervised learning image translation [17] [24] does not require paired datasets.

Figure 2. Overview of the proposed domain adaptation for synthesis of hazy images network.

3.2. Synthetic Hazy Images

The overview model structure is shown in Figure 2. It is trained on paired data and real hazy images, paired data contains synthetic hazy images and clear images.

The synthetic hazy images domain and the real hazy image domain are respectively recorded as D X , D R . The clear image domain corresponding to the synthesized fog image domain is denoted as D Y . Synthetic hazy images, real hazy images, and clear images are recorded as x D X , r D R , y D Y . The latent space of x, y, r is recorded as Z x , Z y , Z r . VAE1 which is trained on x, r includes encoder E D X , E D R and generator G D X , G D R . VAE2 includes encoder E D Y and decoder G D Y . Inspired by [25], our model first encodes x and r into the latent space, and then adapts their domains; there is a big difference between the latent space Z x and Z y , we use mapping net (denote it as T Z ) to connect them.

The VAE1 losses for the real hazy images (r) are as follows:

L V A E 1 ( r ) = K L ( E D R , D X ( Z r | | N ( 0 , I ) ) ) + α E Z r ~ E D R , D X ( Z r | r ) [ | | G D R , D X ( r D R D R | Z r ) r | | 1 ] + L V A E 1 , G A N ( r ) (4)

z r Z D R is the Hidden code of real hazy images (r), r D R D R is generated by G D R , KL stands for KL divergence. The second term is L1 loss, in order to reconstruct the image. The third term is the least-square loss [26], to make the reconstructed image realistic.

To adapt the domain from synthetic hazy images to realistic hazy images, we use adversarial loss as follows.

L V A E 1 , G A N l a t e n t ( r , x ) = E x ~ D X [ 1 D D R , D X ( E D R , D X ( x ) ) 2 ] + E r ~ D R [ D D R , D X ( E D R , D X ( r ) ) 2 ] (5)

So the total loss of VAE1 is as follows.

min E D R , D X , G D R , D X max D D R , D X L V A E 1 ( r ) + L V A E 1 ( x ) + L V A E 1 , G A N l a t e n t ( r , x ) (6)

The mapping net learns the connection between the VAE1 and VAE2, it’s loss is as follows.

L T Z ( x , y ) = δ 1 L T Z + L T Z , G A N + δ 2 L p e r c e p t u a l (7)

where L T Z is stand for the L1 loss between T Z ( Z x ) mapped by T Z and Z y ,

L T Z = E | | T Z ( Z x ) Z y | | 1 (8)

L T Z , G A N is the least-square loss(LSGAN), L p e r c e p t u a l is perceptual loss, as shown in Equation (9).

L p e r c e p t u a l = E [ i 1 n D T i | | ϕ D T i ( x D X D Y ) ϕ D T i ( y D Y D Y ) | | 1 ] + i 1 n V G G i | | ϕ V G G i ( x D X D Y ) ϕ V G G i ( y D Y D Y ) | | 1 ] (9)

where ϕ V G G i ( x ) is the i t h layer feature map of the discriminator (VGG network), and n V G G i indicates the number of activations in that layer. D T is the same as VGG.

4. Experiment

4.1. Implementation

The experiment environment uses the deep learning framework Pytorch1.6.0. The graphics card is NVIDIA RTX 3090Ti, the graphics memory is 32 GB.

The 8970 pairs of images selected from the RESIDE‑OTS [3] and 2864 real hazy images selected from RESIDE-beta [29] are used to train our model. We choose some images from HazeRD [4], SOTS (indoor and outdoor) to test the model’s ability on synthesizing hazy images.

4.2. Comparisons

We compare the model proposed qualitatively and quantitatively in this part. AuthESI [27], PSNR and SSIM is used to evaluate the quality of images. Density [28] of images is also measured.

As shown in Figure 3, some photos selected from the SOTS are used to synthesize hazy images. It can be seen from the second row of the figure that in some cases, the overall color shift will occur on the hazy images synthesized by Adobe Lightroom CC. And our model doesn’t have this kind of problem and generates hazy images successfully.

Table 1 is a quantitative test of the corresponding image of Figure 3. It can be

Figure 3. Qualitative comparison of Adobe Lightroom CC and our method on the SOTS outdoor testset.

seen from the table that the quality of the hazy images synthesized by our model is quite high, and this is in the case of similar hazy images’ density.

As shown in Figure 4, these images are from HazeRD dataset, and some are synthesized by Lightroom CC and our method. It can be seen from the second column of the figure that there are some abnormalities around the plant, because the nearby depth has been wrongly measured, and our model performs well.

The quantitative results of Figure 3 are shown in Table 2. Similarly, our method performs well, indicating that our method also has a certain degree of generalization on various types of images.

To further improve the effective of our model, we use the DCP [7] to dehaze the iamges and measure the PSNR and SSIM. Since the DCP [7] is invalid for the sky, we choose the indoor pictures in Figure 3 for testing. First, test the PSNR and SSIM of the hazy image synthesized by our model, and then use DCP [7] to dehaze and then test PSNR and SSIM again. The results are as shown in Table 3. It can be seen that after DCP [7] processing our synthetic hazy images’ SSIM and PSNR are improved, which proves the effectiveness of our model.

Running our model repeatedly can obtain hazy images with different densities, as shown in Figure 5.

Table 1. Compare our method with Adobe Lightroom CC software on the SOTS outdoor dataset.

Table 2. Compare our method with Adobe Lightroom CC software on the HazeRD dataset.

Table 3. Changes of SSIM and PSNR before and after using DCP to remove haze.

Figure 4. Qualitative comparison of Adobe Lightroom and our method on the HazRD dataset.

Figure 5. Hazy images of different densities generated by our model.

5. Conclusion

In order to alleviate the problem that the dehazing dataset deviates from reality and deep learning image hazing methods usually perform poorly on real hazy images, we propose a model for synthesizing hazy image based on domain adaptation, which can generate hazy images without depth map. Both qualitative and quantitative experiments show the effectiveness of our method.

Cite this paper: Sun, H. , Zheng, Y. and Lang, Q. (2021) Domain Adaptation for Synthesis of Hazy Images. Journal of Computer and Communications, 9, 142-151. doi: 10.4236/jcc.2021.910010.
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