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Title: Low Shot Learning with Untrained Neural Networks for Imaging Inverse Problems

Paper content:
Employing deep neural networks as natural image priors to solve inverse problems either requires large amounts of data to sufficiently train expressive generative models or can succeed with no data via untrained neural networks.
However, very few works have considered how to interpolate between these no- to high-data regimes.
In particular, how can one use the availability of a small amount of data (even 5-25 examples) to one's advantage in solving these inverse problems and can a system's performance increase as the amount of data increases as well?
In this work, we consider solving linear inverse problems when given a small number of examples of images that are drawn from the same distribution as the image of interest.
Comparing to untrained neural networks that use no data, we show how one can pre-train a neural network with a few given examples to improve reconstruction results in compressed sensing and semantic image recovery problems such as colorization.
Our approach leads to improved reconstruction as the amount of available data increases and is on par with fully trained generative models, while requiring less than 1% of the data needed to train a generative model.
We study the problem of recovering an image x x x 0 ∈ R n from m linear measurements of the form y y y 0 = A A Ax x x 0 + η η η ∈ R m where A A A ∈ R m×n is a known measurement operator and η η η ∈ R m denotes the noise in our system.
Problems of this form are ubiquitous in various domains ranging from image processing, machine learning, and computer vision.
Typically, the problem's difficulty is a result of its ill-posedness due to the underdetermined nature of the system.
To resolve this ambiguity, many approaches enforce that the image must obey a natural image model.
While traditional approaches typically use hand-crafted priors such as sparsity in the wavelet basis [5] , recent approaches inspired by deep learning to create such natural image model surrogates have shown to outperform these methods.
Deep Generative Priors: Advancements in generative modelling have allowed for deep neural networks to create highly realistic samples from a number of complex natural image classes.
Popular generative models to use as natural image priors are latent variable models such as Generative Adversarial Networks (GANs) [6] and Variational Autoencoders (VAEs) [18] .
This is in large part due to the fact that they provide a low-dimensional parameterization of the natural image manifold that can be directly exploited in inverse imaging tasks.
When enforced as a natural image prior, these models have shown to outperform traditional methods and provide theoretical guarantees in problems such as compressed sensing [4, 24, 11, 14, 20, 15] , phase retrieval [10, 21, 16] , and blind deconvolution/demodulation [2, 9] .
However, there are two main drawbacks of using deep generative models as natural image priors.
The first is that they require a large amount of data to train, e.g., hundreds of thousands of images to generate novel celebrity faces.
Additionally, they suffer from a non-trivial representation error due to the fact that they model the natural image manifold through a low-dimensional parameterization.
Untrained Neural Network Priors: On the opposite end of the data spectrum, recent works have shown that randomly initialized neural networks can act as natural image priors without any learning.
[22] first showed this to be the case by solving tasks such as denoising, inpainting, and super-resolution via optimizing over the parameters of a convolutional neural network to fit to a single image.
The results showed that the neural network exhibited a bias towards natural images, but due to the high overparameterization in the network, required early stopping to succeed.
A simpler model was later introduced in [13] which was, in fact, underparameterized and was able to both compress images while solving various linear inverse problems.
Both methods require no training data and do not suffer from the same representation error as generative models do.
Similar to generative models, they have shown to be successful image priors in a variety of inverse problems [13, 12, 23, 17] .
Based on these two approaches, we would like to investigate how can one interpolate between these data regimes in a way that improves upon work with untrained neural network priors and ultimately reaches or exceeds the success of generative priors.
More specifically, we would like to develop an algorithm that
1) performs just as well as untrained neural networks with no data and
2) improves performance as the amount of provided data increases.

assistant: text: We show how pre-training an untrained neural network with as few as 5-25 examples can improve reconstruction results in compressed sensing and semantic recovery problems like colorization.