Multi-mapping Image-to-Image Translation via Learning Disentanglement

09/17/2019 ∙ by Xiaoming Yu, et al. ∙ Tencent Peking University 23

Recent advances of image-to-image translation focus on learning the one-to-many mapping from two aspects: multi-modal translation and multi-domain translation. However, the existing methods only consider one of the two perspectives, which makes them unable to solve each other's problem. To address this issue, we propose a novel unified model, which bridges these two objectives. First, we disentangle the input images into the latent representations by an encoder-decoder architecture with a conditional adversarial training in the feature space. Then, we encourage the generator to learn multi-mappings by a random cross-domain translation. As a result, we can manipulate different parts of the latent representations to perform multi-modal and multi-domain translations simultaneously. Experiments demonstrate that our method outperforms state-of-the-art methods.



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1 Introduction

Image-to-image (I2I) translation is a broad concept that aims to translate images from one domain to another. Many computer vision and image processing problems can be handled in this framework,


 image colorization 

Isola et al. (2017)

, image inpainting 

Yang et al. (2019), style transfer Zhu et al. (2017a), etc.  Previous works Isola et al. (2017); Zhu et al. (2017a); Yi et al. (2017); Kim et al. (2017); Liu et al. (2017) present the impressive results on the task with deterministic one-to-one mapping, but suffer from mode collapse when the outputs correspond to multiple possibilities. For example, in the season transfer task, as shown in Fig.1, a summer image may correspond to multiple winter scenes with different styles of lighting, sky, and snow. To tackle this problem and generalize the applicable scenarios of I2I, recent studies focus on one-to-many translation and explore the problem from two perspectives: multi-domain translation Lample et al. (2017); Choi et al. (2018); Liu et al. (2018), and multi-modal translation Zhu et al. (2017b); Lee et al. (2018); Huang et al. (2018); Yu et al. (2018a); Yang et al. (2019).

The multi-domain translation aims to learn mappings between each domain and other domains. Under a single unified framework, recent works realize the translation among multiple domains. However, between the two domains, what these methods have learned are still deterministic one-to-one mappings, thus they fail to capture the multi-modal nature of the image distribution within the image domain. Another line of works is the multi-modal translation. BicycleGAN Zhu et al. (2017b) achieves the one-to-many mapping between the source domain and the target domain by combining the objective of cVAE-GAN Larsen et al. (2016) and cLR-GAN Chen et al. (2016); Donahue et al. (2016); Dumoulin et al. (2016). MUNIT Huang et al. (2018) and DRIT Lee et al. (2018) extend the method to learn two one-to-many mappings between the two image domains in an unsupervised setting, i.e., domain A to domain B and vice versa. While capable of generating diverse and realistic translation outputs, these methods are limited when there are multiple image domains to be translated. In order to adapt to the new task, the domain-specific encoder-decoder architecture in these methods needs to be duplicated to the number of image domains. Moreover, they assume that there is no correlation of the styles between domains, while we argue that they could be aligned as shown in Fig. 1. Besides, existing one-to-many mapping methods usually assume the state of the domain is finite and discrete, which limits their application scenarios.

In this paper, we focus on bridging the objectives of multi-domain translation and multi-modal translation with an unsupervised unified framework. For clarity, we refer our task to as multi-mapping translation. Simultaneous modeling for these two problems not only makes the framework more efficient but also encourages the model to learn efficient representations for diverse translations.

Figure 1: Multi-mapping image-to-image translation. The images with a black border are the input images, and other images are generated by our method. The images on the same column have the same style, which indicates that the styles between image domains could be aligned.

To instantiate the idea, as shown in Fig. 2(d), we assume that the images can be disentangled into two latent representation spaces: a content space and a style space , and propose an encoder-decoder architecture to learn the disentangled representations. Our assumption is developed by the shared latent space assumption Liu et al. (2017), but we disentangle the latent space into two separate parts to model the multi-modal distribution and to achieve cross-domain translation. Unlike partially-shared latent space assumption Huang et al. (2018); Lee et al. (2018), that treats style information as domain-specific, the styles between image domains are aligned in our assumption, as shown in Fig. 1

. Specifically, the style representations in this work are low-dimensional vectors which do not contain spatial information and hence can only control the global appearance of the outputs. By using a unified style encoder to learn style representations and thus fully utilizing samples of all image domains, the sample space of our style representation is denser than that learning from only one specific image domain. As for content representations, they are feature maps capturing the spatial structure information across domains. To mitigate the effects of distribution shift among domains, we eliminate domain-specific information in content representations via conditional adversarial learning. To achieve multi-mapping translation using a single unified decoder, we concatenate the disentangled style representations with the target domain label, then adopt the style-based injection method to render the content representations to our desired outputs. Through learning the inverse mapping of disentanglement, we can change the domain label to translate an image to the specific domain or modify the style representation to produce multi-modal outputs. Furthermore, we can extend our framework to a more challenging task of semantic image synthesis whose domains can be considered as an uncountable set and cannot be modeled by existing I2I approaches.

The contributions of this work are summarized as follows:

  • We introduce an unsupervised unified multi-mapping framework, which unites the objectives of multi-domain and multi-modal translations.

  • By aligning latent representations among image domains, our model is efficient in learning disentanglement and performing finer image translation.

  • Experimental results show our model is superior to the state-of-the-art methods.

Figure 2: Comparisons of unsupervised I2I translation methods. Denote as the k-th image domain. The solid lines and dashed lines represent the flow of encoder and generator respectively. The lines with the same color indicate they belong to the same module.
UNIT - - - - -
StarGAN - - - -
MUNIT - - - Partial
DRIT - - - Partial
SingleGAN - - - -
Table 1: Comparisons with recent works on unsupervised image-to-image translation

2 Related Work

Image-to-image translation. The problem of I2I is first defined by Isola et alIsola et al. (2017). Based on the generative adversarial networks Goodfellow et al. (2014); Mirza and Osindero (2014), they propose a general-purpose framework (pix2pix) to handle I2I. To get rid of the constraint of paired data in pix2pix, Zhu et al. (2017a); Yi et al. (2017); Kim et al. (2017) utilizes the cycle-consistency for the stability of training. UNIT Liu et al. (2017) assumes a shared latent space for two image domains. It achieves unsupervised translation by learning the bijection between latent and image spaces using two generators. However, these methods only learn the one-to-one mapping between two domains and thus produce deterministic output for an input image. Recent studies focus on multi-domain translation Lample et al. (2017); Choi et al. (2018); Yu et al. (2018a); Liu et al. (2018) and multi-modal translation Zhu et al. (2017b); Yang et al. (2019); Lee et al. (2018); Huang et al. (2018); Yang et al. (2019); Yu et al. (2018a); Press et al. (2019). Unfortunately, neither multi-modal translation nor multi-domain translation considers the other’s scenario, which makes them unable to solve the problem of each other. Table 1 shows a feature-by-feature comparison among various unsupervised I2I models. Different from the aforementioned methods, we explore a combination of these two problems rather than separation, which makes our model more efficient and general purpose.

Representation disentanglement. To achieve a finer manipulation in image generation, disentangling the factors of data variation has attracted a great amount of attention Kingma and Welling (2014); Higgins et al. (2017); Chen et al. (2016). Some previous works Lample et al. (2017); Liu et al. (2018) aim to learn domain-invariant representations from data across multiple domains, then generate different realistic versions of an input image by varying the domain labels. Others Lee et al. (2018); Huang et al. (2018); Gonzalez-Garcia et al. (2018) focus on disentangling the images into domain-invariant and domain-specific representations to facilitate learning diverse cross-domain mappings. Inspired by these works, we attempt to disentangle the images into solely independent parts: content and style. Moreover, we align these representations among image domains, which allows us to utilize rich content and style from different domains and manipulate the translation in finer detail.

Semantic image synthesis. The goal of semantic image synthesis is to generate an image to match the given text while retaining the irrelevant information from the input image. Dong et alDong et al. (2017) train a conditional GAN to synthesize a manipulated version of the image given an original image and a target text description. To preserve text-irrelevant contents of the original image, Paired-D GAN Minh Vo and Sugimoto (2018) proposes to model the foreground and background distribution with different discriminators. TAGAN Nam et al. (2018) introduces a text-adaptive discriminator to pay attention to the regions that correspond to the given text. In this work, we treat the image set with the same text description as an image domain. Thus the domains are countless and each domain contains very few images in the training set. Benefit from the unified framework and the representation alignment among different domains, we can tackle this problem in our unified multi-mapping framework.

3 Proposed Method

Let be an image set that contains all possible images of different domains. We assume that the images can be disentangled to two latent representations . is the set of contents excluded from the variation among domains and styles, and is the set of styles that is the rendering of the contents. Our goal is to train a unified model that learns multi-mappings among multiple domains and styles. To achieve this goal, we also define as a set of domain labels and treat as another disentangled representations of the images. Then we propose to learn mapping functions between images and disentangled representations .

As illustrated in Fig. 3(a), we introduce the content encoder that maps an input image to its content, and the encoder style that extracts the style of the input image. To unify the formulation, We also denote the determined mapping function between and as the domain label encoder which is organized as a dictionary111Since encoder has a deterministic mapping, it is no need for joint training with in our training stage. and extracts the domain label from the input image. The inversely disentangled mapping is formulated as the generator . As a result, with any desired style and domain label , we can translate an input image to the corresponding target

Figure 3: Overview. (a) The disentanglement path learns the bijective mapping between the disentangled representations and the input image. (b) The translation path encourages to generate diverse outputs with possible styles in different domains.

3.1 Network Architecture

Encoder. The content encoder is a fully convolutional network that encode the input image to the spatial feature map

. Since the small output stride used in

, retains rich spatial structure information of input image. The style encoder consists of several residual blocks followed by global average pooling and fully connected layers. By global average pooling, removes the structure information of input and extract the statistical characteristics to represent the input style Gatys et al. (2016). The final style representation are constructed as a low-dimensional vector by the reparameterization trick Kingma and Welling (2014).

Generator. Motivated by recent style-based methods Dumoulin et al. (2017); Huang and Belongie (2017); Karras et al. (2019); Huang et al. (2018); Yu et al. (2018a), we adopt a style-based generator to simultaneous model for multi-domain and multi-modal translations. Specifically, the generator consists of several residual blocks followed by several deconvolutional layers. Each convolution layer in residual blocks is equipped with CBIN Yu et al. (2018a, b) for information injection.

Discriminator. Unlike previous works Lee et al. (2018); Huang et al. (2018); Yu et al. (2018a) that apply different discriminators for different image domains, we propose to adopt a unified conditional discriminator for different domains. Since the large distribution shift between image domains in I2I, it is challenging to use a unified discriminator. Inspired by the style-based generator, we apply CBIN to the discriminator to extend the capacity of our model. For more details of our network, we refer the reader to our supplementary materials.

3.2 Learning Strategy

Our proposed method encourages the bijective mapping between the image and the latent representations while learning disentanglement. Fig. 3 presents an overview of our model, whose learning process can be separated into disentanglement path and translation path. The disentanglement path can be considered as an encoder-decoder architecture that uses conditional adversarial training on the latent space. Here we enforce the encoders to encode the image into the disentangled representations, which can be mapped back to the input image by the conditional generator. The translation path enforces the generator to capture the full distribution of possible outputs by a random cross-domain translation.

Disentanglement path. To disentangle the latent representations from image , we adopt cVAE Sohn et al. (2015) as the base structure. To align the style representations across visual domains and constrain the information of the styles Alemi et al. (2017), we encourage the distribution of styles of all domains to be as close as possible to a prior distribution.


To enable stochastic sampling at test time, we choose the prior distribution

to be a standard Gaussian distribution

. As for the content representations, we propose to perform conditional adversarial training in the content space to address the distribution shift issue of the contents among domains. This process encourages to exclude the information of the domain in content


the overall loss of the disentanglement path is


Translation path. The disentanglement path encourages the model to learn the content and the style with a prior distribution. But it leaves two issues to be solved: First, limited by the number of training data and the optimization of KL loss, the generator may sample only a subset of and generate the images with specific domain labels in the training stage Tian et al. (2018). It may lead to poor generations when sampling in the prior distribution and that does not match the test image, as discussed in Zhu et al. (2017b). Second, the above training process lacks efficient incentives for the use of styles, which would result in low diversity of the generated images. To overcome these issues and encourage our generator to capture a complete distribution of outputs, we first propose to randomly sample domain labels and styles in the prior distributions, in order to cover the whole sampling space at training time. Then we introduce the latent regression Chen et al. (2016); Zhu et al. (2017b) to force the generator to utilize the style vector. The regression can also be applied to the content to separate the style from . Thus the latent regression can be written as


To match the distribution of generated images to the real data with sampling domain labels and styles, we employ conditional adversarial training in the pixel space


Note that we also discriminate the pair of real image and mismatched target domain label , in order to encourage the generator to generate images that correspond to the given domain label. The final objective of the translation is


By combining both training paths, the full objective function of our model is


4 Experiments

We compare our approach against recent one-to-many mapping models in two tasks, including season transfer and semantic image synthesis. For brevity, we refer to our method, Disentanglement for Multi-mapping Image-to-Image Translation, as DMIT. In the supplementary material, we provide additional visual results and extend our model to facial attribute transfer Liu et al. (2015) and sketch-to-photo Yu and Grauman (2014).

4.1 Datasets

Yosemite summer winter. The unpaired dataset is provided by Zhu et alZhu et al. (2017a) for evaluating unsupervised I2I methods. We use the default image size 256256 and training set in all experiments. The domain label(summer/winter) is organized as a one-hot vector.
CUB. The Caltech-UCSD Birds (CUB) Wah et al. (2011) dataset contains 200 bird species with 11,788 images that each have 10 text captions Reed et al. (2016). We preprocess the CUB dataset according to the method in Xu et al. (2018). The captions are encoded as the domain labels by the pretrained text encoder proposed in Xu et al. (2018).

Figure 4: Qualitative comparison of season transfer. The first column shows the input image. Each of the remaining columns shows four outputs with the specified season from a method. Each image pair for the specified season reflects the diversity within the domain.

4.2 Season Transfer

Season transfer is a coarse-grained translation task that aims to learn the mapping between summer and winter. We compare our method against five baselines, including:

  • Multi-domain models: StarGAN Choi et al. (2018) and StarGAN that adds the noise vector into the generator to encourage the diverse outputs.

  • Multi-modal models: MUNIT Huang et al. (2018), DRIT Lee et al. (2018), and version-c of SingleGAN Yu et al. (2018a).

In the above models, MUNIT, DRIT and SingleGAN require a pair of GANs for summer winter and winter summer severally. StarGAN-based models and DMIT only use a unified structure to learn the bijection mapping between two domains. To better evaluate the performance of multi-domain and multi-modal mappings, we propose to test inter-domain and intra-domain translations separately.

As the qualitative comparison in Fig. 4 shows, the synthesis of StarGAN has significant artifacts and suffer from mode collapse caused by the assumption of deterministic cross-domain mapping. With the noise disturbance, the quality of generated images by StarGAN has improved, but the results are still lacking in diversity. All of the multi-modal models produce diverse results. However, without utilizing the style information between different domains, the generated images are monotonous and only differ in simple modes, such as global illumination. We observe that MUNIT is hard to converge and to produce realistic season transfer results due to the limited training data. DRIT and SingleGAN produce realistic results, but the images are not vivid enough. In contrast, our DMIT can use only one unified model to produce realistic images with diverse details for different image domains.

To quantify the performance, we first translate each test image to 10 targets by sampling styles from prior distributions. Then we adopt Fréchet Inception Distance (FID) Heusel et al. (2017) to evaluate the quality of generated images, and LPIPS (official version 0.1) Zhang et al. (2018) to measure the diversity Huang et al. (2018); Lee et al. (2018); Zhu et al. (2017b) of samples generated by same input image. The quantitative results shown in Table 2 further confirm our observations above. It is remarkable that our method achieves the best FID score while greatly surpassing the multi-domain and multi-modal models in LPIPS distance.

summerwinter summersummer wintersummer winterwinter
StarGAN 218.78 - 233.61 - 248.29 - 224.37 -
StarGAN 152.11 0.012 135.25 0.011 153.79 0.013 149.04 0.011
MUNIT 84.43 0.166 58.96 0.133 73.82 0.134 68.92 0.141
DRIT 58.70 0.205 49.58 0.166 53.79 0.192 57.11 0.179
SingleGAN 63.77 0.184 51.64 0.186 54.24 0.188 57.30 0.178
DMIT w/o T-Path 75.90 0.109 57.24 0.118 72.75 0.124 65.15 0.116
DMIT w/o D-Path 116.71 0.545 85.97 0.513 95.63 0.517 124.96 0.544
DMIT w/o 60.81 0.268 43.54 0.260 50.33 0.270 58.09 0.256
DMIT w/ VanillaD 63.34 0.259 44.73 0.239 50.79 0.255 60.10 0.242
DMIT w/ ProjectionD 66.50 0.289 46.92 0.301 52.4 0.293 65.66 0.299
DMIT 58.46 0.302 43.04 0.275 48.02 0.292 55.23 0.279
Table 2: Quantitative comparison of season transfer.
Ablation study.

To analyze the importance of different components in our model, we perform an ablation study with five variants of DMIT.

As for the training paths, we observe that both T-Path and D-Path are indispensable. Without T-Path, the model is difficult to perform cross-domain translation as we analyzed in Section 3. In contrast, without D-Path, the generated images are blurry and unrealistic and produce meaningless diversity by the artifacts. Combining these two paths result in a trade-off of quality and diversity of images.

As for the training incentive, we observe is influential for the diversity score. Without this incentive, the visual styles are similar in summer and winter. It suggests that encourages the model to eliminate the domain bias and to learn well-disentangled representations.

As for the architecture of discriminator, we evaluate two other conditional models with different information injection strategies, including vanilla conditional discriminator (VanillaD) Isola et al. (2017); Mirza and Osindero (2014) that concatenates input image and conditional information together, and projection discriminator (ProjectionD) Miyato and Koyama (2018); Miyato et al. (2018) that projects the conditional information to the hidden activation of image. The qualitative results in Table 2 indicate that the capacity of VanillaD is limited. The images generated of DMIT with ProjectionD are diverse, but prone to contain artifacts, which leads to its lower FID score. Our full DMIT, equipped style-based discriminator, gets the balance between diversity and quality.

4.3 Semantic Image Synthesis

To further verify the potential of DMIT in mixed-modality (text and image) translation, we study on the task of semantic image synthesis. The existing I2I approaches usually assume the state of the domain is discrete, which causes them to not be able to handle this task. We compare our model with the state-of-the-art models of semantic image synthesis: SISGAN Dong et al. (2017), Paired-D GAN Minh Vo and Sugimoto (2018), and TAGAN Nam et al. (2018).

Fig. 5 shows our qualitative comparison with the baselines. Although SISGAN can generate diverse images that match the text, it is difficult to generate high-quality images. The structure and background of the images are retained well by Paired-D GAN, but the results do not match the text well. Furthermore, it can be observed that Paired-D GAN cannot produce diversity for conditional input with different samples. TAGAN presents images with acceptable semantic matching results, but the quality is unsatisfactory. By encoding the style from the input image, DMIT can well preserve the original background of the input image and generate high-quality images that match the text descriptions. Meanwhile, DMIT can also produce diverse results by sampling other style representation.

Besides to calculate FID to qualify the performance, we perform a human perceptual study on Amazon Mechanical Turk (AMT) to measure the semantic matching score. We randomly sample images and mismatched texts for generating questions. For each comparison, five different workers are required to select which image looks more realistic and fits the given text. As shown in Table 3, DMIT gets the best of both image quality and semantic matching score. Since retaining the irrelevant information of the input image is important for semantic image synthesis, we also evaluate the reconstruction ability of different methods by transforming the input image with its corresponding text. The scores of PSNR and SSIM further demonstrate the capabilities of our method in learning efficient representations. It suggests that the disentangled representations enable our model to manipulate the translation in finer detail.

Figure 5: Qualitative comparison of semantic image synthesis. In each column, the first row is the input image and the remaining rows are the outputs according to the above text description. In each pair of images generated by DMIT, the images in the first column are generated by encoding the style from the input image and the second column are generated by random style.
SISGAN 67.24 15.3% 11.27 0.193
Paired-D GAN 27.62 25.2% 22.34 0.886
TAGAN 34.49 20.4% 19.01 0.736
DMIT 13.85 39.1% 25.49 0.934
Table 3: Quantitative comparison of semantic image synthesis.

4.4 Limitations

Although DMIT can perform multi-mapping translation, we observe that the style representations tend to model some global properties as discussed in Press et al. (2019). Besides, we observe that the convergence rates of different domains are generally different. Further exploration will allow this work to be a general-purpose solution for a variety of multi-mapping translation tasks.

5 Conclusion

In this paper, we present a novel model for multi-mapping image-to-image translation with unpaired data. By learning disentangled representations, it is able to use the advances of both multi-domain and multi-modal translations in a holistic manner. The integration of these two multi-mapping problems encourages our model to learn a more complete distribution of possible outputs, improving the performance of each task. Experiments in various multi-mapping tasks show that our model is superior to the existing methods in terms of quality and diversity.


This work was supported in part by Shenzhen Municipal Science and Technology Program (No. JCYJ20170818141146428), National Engineering Laboratory for Video Technology - Shenzhen Division, and National Natural Science Foundation of China and Guangdong Province Scientific Research on Big Data (No. U1611461). In addition, we would like to thank the anonymous reviewers for their helpful and constructive comments.


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Appendix A Implementation

a.1 Network Architecture

The architecture details are as follows, except that the domain label encoder is organized as a dictionary without learnable parameters. We follow the notations used in Choi et al. [2018]: and : height and width of the input image, and : dimensions of style and domain label

, N: the number of output channels, K: kernel size, S: stride size, P: padding size, FC: fully connected layer, IN: instance normalization, LN: layer normalization, CBN: central biasing normalization, LReLu: Leaky ReLu with a negative slope of 0.2.

Part Input Output Shape Layer Information
(h,w,3) (h,w,64) CONV-(N64, K7x7, S1, P3), IN, LReLU
Down-sampling (h,w,64) (,,128) CONV-(N128, K4x4, S2, P1), IN, LReLU
(,,128) (,,256) CONV-(N128, K4x4, S2, P1), IN, LReLU
Bottleneck (,,256) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), IN, LReLU
(,,256) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), IN, LReLU
(,,256) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), IN, LReLU
(,,256) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), IN, LReLU
Table 4: Architecture of content encoder
Part Input Output Shape Layer Information
Down-sampling (h,w,3) (,,64) CONV-(N64, K4x4, S2, P1)
(,,64) (,,128)
ResBlock: CONV-(N256, K3x3, S1, P1),
IN, LReLu, AvgPool-(K2x2, S2)
(,,128) (,,256)
ResBlock: CONV-(N256, K3x3, S1, P1),
IN, LReLu, AvgPool-(K2x2, S2)
(,,256) (,,256)
ResBlock: CONV-(N256, K3x3, S1, P1),
IN, LReLu, AvgPool-(K2x2, S2)
(,,256) (256) LReLu, GlobalAvgPool
Output Layer() (256) () FC-(256, )
Output Layer() (256) () FC-(256, )
Table 5: Architecture of style encoder
Part Input Output Shape Layer Information
Bottleneck (,,256)+(+) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), CBIN, ReLu
(,,256)+(+) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), CBIN, ReLu
(,,256)+(+) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), CBIN, ReLu
(,,256)+(+) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), CBIN, ReLu
(,,256)+(+) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), CBIN, ReLu
(,,256)+(+) (,,256) ResBlock: CONV-(N256, K3x3, S1, P1), CBIN, ReLu
Up-sampling (,,256) (,,128) DECONV-(N128, K4x4, S2, P1), LN, ReLu
(,,128) (h,w,64) DECONV-(N64, K4x4, S2, P1), LN, ReLu
(h,w,64) (h,w,3) CONV-(N3, K7x7, S1, P3), Tanh
Table 6: Architecture of generator . The style and the domain label are injected by central biasing normalization.
Part Input Output Shape Layer Information
Down-sampling (h,w,3) (,,64) CONV-(N64, K4x4, S2, P1)
(,,64)+() (,,128)
ResBlock: CONV-(N256, K3x3, S1, P1),
CBIN, LReLu, AvgPool-(K2x2, S2)
(,,128)+() (,,256)
ResBlock: CONV-(N256, K3x3, S1, P1),
CBIN, LReLu, AvgPool-(K2x2, S2)
Output Layer (,,256) (,,1) CONV-(N1, K1x1, S1, P0)
Table 7: Architecture of discriminator and . The domain label are injected by central biasing normalization.

a.2 Training Details

We train all our models with Adam optimizer Kinga and Adam [2015], setting the learning rate of 0.0001 and exponential decay rates . To keep each loss close in magnitude, the hyper-parameters are set as follows: , , . The batch size is set as one for season transfer and sketch-to-photo tasks, and eight for semantic image synthesis and facial attribute transfer. Besides, we adopt multi-scale strategy proposed by Zhu et alZhu et al. [2017b] to discriminate the real and fake images in different scales. Since the distributions of content are still changing, we use the objective of LSGAN Mao et al. [2017] to stabilize the training of . Besides, we replace the standard adversarial loss of with hinge version Miyato et al. [2018] to accelerate the convergence. All of the models are trained on a single NVIDIA TITAN V GPU.

Appendix B Additional Experiment Results

b.1 Season Transfer

Figure 6:

The interpolation on latent representations.

In this experiment, the content representation is extracted from the image of the first row and the style representations are extracted from the first and final columns. The images on the same row have the same style representation and images on the same column have the same domain label representation.
Figure 7: Example-guided image translation. In this experiment, the content representation is extracted from the image of the first column and the style representations are extracted from the first row. The images on the same row have the same content representation and images on the same column have the same style representation.

b.2 Semantic Image Synthesis

Figure 8: Semantic image synthesis on CUB Wah et al. [2011]. The first row shows the input images. Each of the remaining rows presents the translation results according to the text description.

b.3 Facial Attribute Transfer

we perform the facial attribute transfer on the The CelebFaces Attributes(CelebA) dataset Liu et al. [2015]. CelebA dataset contains a large number of celebrity images. We preprocess the CelebA dataset according to the method in Choi et al. [2018]. Twelve attributes are selected to construct the attribute vector: hair color (black, blond, brown, gray), gender (male/female), age (young/old), expression (with/without smile), and hairstyle (with/without bangs). The quantitative and qualitative comparisons are shown in Table 8 and Fig. 9, and more visual results are presented in Fig. 10.

StarGAN 51.20 -
StarGAN 48.16 0.001
DMIT 32.36 0.066
Real data - 0.473
Table 8: Quantitative comparison of facial attribute transfer.
Figure 9: Qualitative comparison of facial attribute transfer. The styles of StarGAN and DMIT are sampled from random noise.
Figure 10: Facial attribute transfer results of DMIT on CelebA (Input, Black hair, Blond hair, Brown hair, Gray hair, Gender, Age, Smile, Bangs). Since "Old" and "Gray hair" are generally present at the same time in CelebA, we thus set the hair to gray to generate a realistic old face.

b.4 Sketch-to-Photo

We use the dataset provided by Isola et alIsola et al. [2017] to perform Sketch-to-Shoe and Shoe-to-Shoe translations. Our model in this task is trained with unpaired data, and the qualitative results are shown in Fig. 11

Figure 11: Qualitative results of sketch-to-photo. The first two rows show the inter-domain translation results of our model, and the last two rows shows the intra-domain translation results.