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Paper title:
Data Instance Prior for Transfer Learning in GANs

Paper body:
1 INTRODUCTION . Generative Adversarial Networks ( GANs ) are at the forefront of modern high-quality image synthesis in recent years ( Brock et al. , 2018 ; Karras et al. , 2020b ; 2019 ) . GANs have also demonstrated excellent performance on many related computer vision tasks such as image manipulation ( Zhu et al. , 2017 ; Isola et al. , 2017 ) , image editing ( Plumerault et al. , 2020 ; Shen et al. , 2020 ; Jahanian et al. , 2020 ) and compression ( Tschannen et al. , 2018 ) . Despite the success in large-scale image synthesis , GAN training suffers from a number of drawbacks that arise in practice , such as training instability and mode collapse ( Goodfellow et al. , 2016 ; Arora et al. , 2017 ) . It has been observed that the issue of unstable training can be mitigated to an extent by using conditional GANs . However , this is expected as learning the conditional model for each class is easier than learning the joint distribution . The disadvantages of GAN training have prompted research in several non-adversarial generative models ( Hoshen et al. , 2019 ; Bojanowski et al. , 2018 ; Li & Malik , 2018 ; Kingma & Welling , 2014 ) . These techniques are implicitly designed to overcome the mode collapse problem , however , the quality of generated samples are still not on par with GANs . Current state-of-the-art deep generative models require a large volume of data and computation resources . The collection of large datasets of images suitable for training - especially labeled data in case of conditional GANs - can easily become a daunting task due to issues such as copyright , image quality and also the training time required to get state-of-the-art image generation performance . To curb these limitations , researchers have recently proposed techniques inspired by transfer learning ( Noguchi & Harada , 2019 ; Wang et al. , 2018 ; Mo et al. , 2020 ) and data augmentation methods ( Karras et al. , 2020a ; Zhao et al. , 2020b ; Zhang et al. , 2019 ) . Advancements in data and computation efficiency for image synthesis can enable its applications in data-deficient fields such as medicine ( Yi et al. , 2019 ) where labeled data generation can be difficult to obtain . Transfer learning is a promising area of research ( Oquab et al. , 2014 ; Pan & Yang , 2009 ) that leverages prior information acquired from large datasets to help in training models on a target dataset under limited data and resource constraints . There has been extensive exploration of transfer learning in classification problems that have shown excellent performance on various downstream data-deficient domains . Similar extensions of reusing pre-trained networks for transfer learning ( i.e . fine-tuning a subset of pre-trained network weights from a data-rich domain ) have also been recently employed for image synthesis in GANs ( Wang et al. , 2018 ; Noguchi & Harada , 2019 ; Mo et al. , 2020 ; Wang et al. , 2020 ; Zhao et al. , 2020a ) in the limited data regime . However , these approaches are still prone to overfitting on the sparse target data , and hence suffer from degraded image quality and diversity . In this work , we propose a simple yet effective way of transferring prior knowledge in unsupervised image generation given a small sample size ( ∼ 100-2000 ) of the target data distribution . Our approach is motivated by the formulation of the IMLE technique ( Li & Malik , 2018 ) that seeks to obtain mode coverage of target data distribution by learning a mapping between latent and target distributions using a maximum likelihood criterion . We instead propose the use of data priors in GANs to match the representation of the generated samples to real modes of data . In contrast to ( Li & Malik , 2018 ) , we use the images generated using data priors to find the nearest neighbor match to real modes in the generator ’ s learned distribution . In particular , we show that using an informative data instance prior in limited and large-scale unsupervised image generation substantially improves the performance in image synthesis . We show that these data priors can be derived from commonly used computer vision pre-trained networks ( Simonyan & Zisserman , 2014 ; Zhang et al. , 2018 ; Noguchi & Harada , 2019 ; Hoshen et al. , 2019 ) or self-supervised data representations ( Chen et al. , 2020 ) ( without any violation of the target setting ’ s requirements , i.e . ensuring that the pre-trained network has not been trained on few-shot classes in the few-shot learning setting , for instance ) . In case of sparse training data , our approach of using data instance priors leverages a model pre-trained on a rich source domain to the learn the target distribution . Different from previous works ( Noguchi & Harada , 2019 ; Wang et al. , 2020 ; 2018 ) which rely on fine-tuning models trained on a data-rich domain , we propose to leverage the feature representations of our source model as data instance priors , to distill knowledge ( Romero et al. , 2015 ; Hinton et al. , 2015 ) into our target generative model . We note that our technique of using data instance priors for transfer learning becomes fully unsupervised in case the data priors are extracted from self-supervised pre-trained networks . Furthermore , in addition to image generation in low data domain , we also achieve state-of-the-art Fréchet inception distance ( FID ) score ( Heusel et al. , 2017 ) on large-scale unsupervised image generation and also show how this framework of transfer learning supports several image editing tasks . We summarize our main contributions as follows : • We propose Data Instance Prior ( DIP ) , a novel transfer learning technique for GAN image synthesis in low-data regime . We show that employing DIP in conjunction with existing few-shot image generation methods outperforms state-of-the-art results . We show with as little as 100 images our approach DIP results in generation of diverse and high quality images ( see Figure 3 ) . • We demonstrate the utility of our approach in large-scale unsupervised GANs ( Miyato et al. , 2018 ; Brock et al. , 2018 ) achieving the new state-of-the-art in terms of image quality ( Heusel et al. , 2017 ) and diversity ( Sajjadi et al. , 2018 ; Metz et al. , 2017 ) . • We show how our framework of DIP by construction enables inversion of images and common image editing tasks ( such as cutmix , in-painting , image translation ) in GANs . We call our method a data instance prior ( and not just data prior ) , since it uses representations of instances as a prior , and not a data distribution itself . 2 RELATED WORK . Deep Generative Models In recent years , there has been a surge in the research of deep generative models . Some of the popular approaches include variational auto-encoders ( VAEs ) ( Rezende et al. , 2014 ; Kingma & Welling , 2014 ) , auto-regressive ( AR ) models ( Van Oord et al. , 2016 ; Van den Oord et al. , 2016 ) and GANs ( Goodfellow et al. , 2014 ) . VAE models learn by maximizing the variational lower bound of likelihood of generating data from a given distribution . Auto-regressive approaches model the data distribution as a product of the conditional probabilities to sequentially generate data . GANs comprise of two networks , a generator and a discriminator that train in a min-max optimization . Specifically , the generator aims to generate samples to fool the discriminator , while the discriminator learns distinguish these generated samples from the real samples . Several research efforts in GANs have focused around improving the performance ( Karras et al. , 2018 ; Denton et al. , 2015 ; Radford et al. , 2016 ; Karras et al. , 2020b ; 2019 ; Brock et al. , 2018 ; Zhang et al. , 2019 ) and training stability ( Salimans et al. , 2016b ; Gulrajani et al. , 2017 ; Arjovsky et al. , 2017 ; Miyato et al. , 2018 ; Mao et al. , 2017 ; Chen et al. , 2019 ) . Recently , the areas of latent space manipulation for semantic editing ( Shen et al. , 2020 ; Jahanian et al. , 2020 ; Zhu et al. , 2020 ; Plumerault et al. , 2020 ) and few-shot image generation ( Wang et al. , 2020 ; Mo et al. , 2020 ; Noguchi & Harada , 2019 ) have gained traction in an effort to mitigate the practical challenges while deploying GANs . Several other non-adversarial training approaches such as ( Hoshen et al. , 2019 ; Bojanowski et al. , 2018 ; Li & Malik , 2018 ) have also been explored for generative modeling , which leverage supervised learning along with perceptual loss ( Zhang et al. , 2018 ) for training such models . Transfer Learning in GANs While there has been extensive research in the area of transfer learning for classification models ( Yosinski et al. , 2014 ; Oquab et al. , 2014 ; Tzeng et al. , 2015 ; Pan & Yang , 2009 ; Donahue et al. , 2014 ) , relatively fewer efforts have explored this on the task of data generation ( Wang et al. , 2018 ; 2020 ; Noguchi & Harada , 2019 ; Zhao et al. , 2020a ; Mo et al. , 2020 ) . ( Wang et al. , 2018 ) proposed to fine-tune a pre-trained GAN model ( often having millions of parameters ) from a data-rich source to adapt to the target domain with limited samples . This approach , however , often suffers from overfitting as the final model parameters are updated using only few samples of the target domain . To counter overfitting , the work of ( Noguchi & Harada , 2019 ) proposes to update only the batch normalization parameters of the pre-trained GAN model . In this approach , however , the generator is not adversarially trained and uses supervised L1 pixel distance loss and perceptual loss ( Johnson et al. , 2016 ; Zhang et al. , 2018 ) which often leads to generation of blurry images in the target domain . Based on the assumption that source and target domain support sets are similar , ( Wang et al. , 2020 ) recently proposed to learn an additional mapping network that transforms the latent code suitable for generating images of target domain while keeping the other parameters frozen . We compare against all leading baselines including ( Noguchi & Harada , 2019 ; Wang et al. , 2020 ) on their respective tasks , and show that our method DIP outperforms them substantially , while being simple to implement . A related line of recent research aims to improve large-scale unsupervised image generation in GANs by employing self-supervision - in particular , an auxiliary task of rotation prediction ( Chen et al. , 2019 ) or using one-hot labels obtained by clustering in the discriminator ’ s ( Liu et al. , 2020 ) or ImageNet classifier feature space ( Sage et al. , 2018 ) . In contrast , our method utilizes data instance priors derived from the feature activations of self-supervised/supervised pre-trained networks to improve unsupervised few-shot and large-scale image generation , leading to simpler formulation and higher performance as shown in our experiments in Section 5.3 and Appendix A . Recently , some methods ( Karras et al. , 2020a ; Zhao et al. , 2020b ; Zhang et al. , 2019 ; Zhao et al. , 2020c ) have leveraged data augmentation to effectively increase the number of samples and prevent overfitting in GAN training . However , data augmentation techniques often times alter the true data distribution and there is a leakage of these augmentations to the generated image , as shown in ( Zhao et al. , 2020c ; b ) . To overcome this , ( Zhao et al. , 2020b ) recently proposed to use differential augmentation and ( Karras et al. , 2020a ) leveraged an adaptive discriminator augmentation mechanism . We instead focus on leveraging informative data instance priors , and in fact , show how our DIP method can be used in conjunction with ( Zhao et al. , 2020b ) to improve performance .

4-sentence paper summary:
This paper illustrates how they train GANs with small sample sizes with the help of Transfer Learning. The paper tackled a very specific problem: what should we do with a small sample training size if we want to train a GAN. The authors have supported their arguments by a proof in Data In Prior and experiment results. They illustrated well in both aspects.