Few-Shot Unsupervised Image-to-Image Translation about papernotes

[howardyclo]

Abstract

• Current unsupervised/unpaired image-to-image translation (UIT) methods (see ref) typically requires many images in both source and target classes, which greatly limits their use.
• This paper proposes novel framework that only needs a few examples (few-shot) and can work on unseen target classes.
• The proposed framework can also be applied to few-shot image classification and outperform a SoTA method based on feature hallucination.

Method Overview

• Motivation: Human can imagine the unseen target classes (e.g., seeing a standing tiger for the first time and imagine it lying down) by past visual experiences (e.g., seeing another animal standing and lying down before).
  • Past visual experience: Learn on images of many different classes.
  • Imagine unseen classes: Translate images from source class to target class with few examples of target class.
• Data: Source class images: Many source classes with each contain many images (e.g., species of animals).
• Training: Use source class images to train a multi-class UIT model (the target class is still from source classes).
• Inference: Few seen/unseen target class images only accessible during inference.

Model

• x¯ = G(x, {y_1, ..., y_K}): A conditional few-shot image generator (translator) takes a content image x and 1-way (class) K-shot images {y_1, ... y_K} as input and generates the output image x¯.
  • z_x = E_x(x): A content encoder maps content image x to content latent code z_x.
  • z_y = E_y({y_1, ..., y_K}): A class (style) encoder maps {y_1, ... y_K} to latent vectors individually and averages them into a class latent code z_y.
  • x¯ = F_x(z_x, z_y): A decoder consisted of several adaptive instance normalization (AdaIN) residual blocks followed by several upscale conv layers.
  • By feeding z_y to the decoder via the AdaIN layers, we let the class images control the global look (style), while maintaining the local structure (content).
  • The generalization capability depends on the number of source classes during training (more is better).
• D: A multi-task adversarial discriminator.

Training

• |S|: Number of source classes.
• For D, each task determines whether an input image is real or fake of the source class. As there are |S| source classes, we have |S| binary outputs for D.
• Input an real image x of a source class c_x, penalize D if its c_x-th output is fake. However, no penalization for outputting fake for other (|S|-1) source classes.
• Input an fake image x¯ of a source class c_x, penalize D if its c_x-th output is real. Otherwise, penalize G.

Losses

• Overall loss

• GAN loss: As described above.

• Reconstruction loss encourages content similar to image of source class.

• Feature matching loss encourages style similar to images of target class.
• D_f is the feature extractor of the discriminator D without the last layer.

UIT methods with Different Constraints (Enforce translation to preserve certain properties)

• Pixel values
  • Learning from simulated and unsupervised images through adversarial training. CVPR 2017.
• Pixel gradients
  • Unsupervised pixel-level domain adaptation with generative adversarial networks. CVPR 2017.
• Semantic features
  • Unsupervised cross-domain image generation. ICLR 2017.
• Class labels
  • Unsupervised pixel-level domain adaptation with generative adversarial networks. CVPR 2017.
• Pairwise sample distances
  • One-sided unsupervised domain mapping. NIPS 2017.
• Cycle consistency
  • Dualgan: Unsupervised dual learning for image-to-image translation. ICCV 2017.
  • Unpaired image-to-image translation using cycle-consistent adversarial networks. ICCV 2017.
  • Learning to discover cross-domain relations with generative adversarial networks. ICML 2017.
  • Augmented cyclegan: Learning many-to-many mappings from unpaired data. ICML 2018.
  • Toward multimodal image-to-image translation. NIPS 2017.
• Shared latent space assumption
  • Coupled generative adversarial networks. NIPS 2016.
  • Unsupervised image-to-image translation networks. NIPS 2017.
• Partially-shared latent space assumption
  • Multimodal unsupervised image-to-image translation (MUNIT). ECCV 2018.
  • Diverse image-to-image translation via disentangled representation. ECCV 2018.
  • This work.

Related work

• One-shot unsupervised cross domain translation. NIPS 2018: Assume one source class image but many target class images.