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Title: Latent Convolutional Models

Paper content:
We present a new latent model of natural images that can be learned on large-scale datasets.
The learning process provides a latent embedding for every image in the training dataset, as well as a deep convolutional network that maps the latent space to the image space.
After training, the new model provides a strong and universal image prior for a variety of image restoration tasks such as large-hole inpainting, superresolution, and colorization.
To model high-resolution natural images, our approach uses latent spaces of very high dimensionality (one to two orders of magnitude higher than previous latent image models).
To tackle this high dimensionality, we use latent spaces with a special manifold structure (convolutional manifolds) parameterized by a ConvNet of a certain architecture.
In the experiments, we compare the learned latent models with latent models learned by autoencoders, advanced variants of generative adversarial networks, and a strong baseline system using simpler parameterization of the latent space.
Our model outperforms the competing approaches over a range of restoration tasks.
Learning good image priors is one of the core problems of computer vision and machine learning.
One promising approach to obtaining such priors is to learn a deep latent model, where the set of natural images is parameterized by a certain simple-structured set or probabilistic distribution, whereas the complexity of natural images is tackled by a deep ConvNet (often called a generator or a decoder) that maps from the latent space into the space of images.
The best known examples are generative adversarial networks (GANs) (Goodfellow et al., 2014) and autoencoders BID4 .Given
a good deep latent model, virtually any image restoration task can be solved by finding a latent representation that best corresponds to the image evidence (e.g. the known pixels of an occluded image or a low-resolution image). The attractiveness
of such approach is in the universality of the learned image prior. Indeed, applying the
model to a new restoration task can be performed by simply changing the likelihood objective. The same latent model
can therefore be reused for multiple tasks, and the learning process needs not to know the image degradation process in advance. This is in contrast to
task-specific approaches that usually train deep feed-forward ConvNets for individual tasks, and which have a limited ability to generalize across tasks (e.g. a feed-forward network trained for denoising cannot perform large-hole inpainting and vice versa).At the moment, such image
restoration approach based on latent models is limited to low-resolution images. E.g. BID16 showed how a latent
model trained with GAN can be used to perform inpainting of tightly-cropped 64 × 64 face images. Below, we show that such models
trained with GANs cannot generalize to higher resolution (eventhough GAN-based systems are now able to obtain high-quality samples at high resolutions BID9 ). We argue that it is the limited
dimensionality of the latent space in GANs and other existing latent models that precludes them from spanning the space of high-resolution natural images.To scale up latent modeling to high-resolution images, we consider latent models with tens of thousands of latent dimensions (as compared to few hundred latent dimensions in existing works). We show that training such latent
models is possible using direct optimization BID1 and that it leads to good image priors that can be used across a broad variety of reconstruction tasks. In previous models, the latent space
has a simple structure such as a sphere or a box in a Euclidean space, or a full Euclidean space with a Gaussian prior. Such choice, however, is not viable
in our Figure 1 : Restorations using the same Latent Convolutional Model (images 2,4,6) for different image degradations (images 1,3,5). At training time, our approach builds
a latent model of non-degraded images, and at test time the restoration process simply finds a latent representation that maximizes the likelihood of the corrupted image and outputs a corresponding non-degraded image as a restoration result.case, as vectors with tens of thousands of dimensions cannot be easily used as inputs to a generator. Therefore, we consider two alternative
parameterizations of a latent space. Firstly, as a baseline, we consider latent
spaces parameterized by image stacks (three-dimensional tensors), which allows to have "fully-convolutional" generators with reasonable number of parameters.Our full system uses a more sophisticated parameterization of the latent space, which we call a convolutional manifold, where the elements of the manifold correspond to the parameter vector of a separate ConvNet. Such indirect parameterization of images and
image stacks have recently been shown to impose a certain prior BID15 , which is beneficial for restoration of natural images. In our case, we show that a similar prior can
be used with success to parameterize high-dimensional latent spaces.To sum up, our contributions are as follows. Firstly, we consider the training of deep latent
image models with the latent dimensionality that is much higher than previous works, and demonstrate that the resulting models provide universal (w.r.t. restoration tasks) image priors. Secondly, we suggest and investigate the convolutional
parameterization for the latent spaces of such models, and show the benefits of such parameterization.Our experiments are performed on CelebA BID11 (128x128 resolution), SUN Bedrooms BID17 (256x256 resolution), CelebA-HQ BID9 ) (1024x1024 resolution) datasets, and we demonstrate that the latent models, once trained, can be applied to large hole inpainting, superresolution of very small images, and colorization tasks, outperforming other latent models in our comparisons. To the best of our knowledge, we are the first to demonstrate
how "direct" latent modeling of natural images without extra components can be used to solve image restoration problems at these resolutions (Figure 1 ).Other related work. Deep latent models follow a long line of works
on latent image models
that goes back at least to the eigenfaces approach BID14 . In terms of restoration, a competing and more popular approach are feed-forward
networks trained for specific restoration tasks, which have seen rapid progress recently. Our approach does not quite match the quality of e.g. BID6 , that is designed and
trained specifically for the inpainting task, or the quality of e.g. BID18 that is designed and trained specifically for the face superresolution task. Yet the models trained within our approach (like other latent models) are universal
, as they can handle degradations unanticipated at training time.Our work is also related to pre-deep learning ("shallow") methods that learn priors on (potentiallyoverlapping) image patches using maximum likelihood-type objectives such as BID12 BID8 BID21 . The use of multiple layers in our method allows to capture much longer correlations
. As a result, our method can be used successfully to handle restoration tasks that
require exploiting these correlations, such as large-hole inpainting.
The results in this work suggest that high-dimensional latent spaces are necessary to get good image reconstructions on desired hold-out sets.
Further, it shows that parametrizing these spaces using ConvNets imposes further structure on them that allow us to produce good image restorations from a wide variety of degradations and at relatively high resolutions.
More generally, this method can easily be extended to come up with more interesting parametrizations of the latent space, e.g. by interleaving the layers with image-specific and dataset-specific parameters.The proposed approach has several limitations.
First, when trained over very large datasets, the LCM model requires long time to be trained till convergence.
For instance, training an LCM on 150k samples of CelebA at 128 × 128 resolution takes about 14 GPU-days.
Note that the GLO model of the same latent dimensionality takes about 10 GPU-days.
On the other hand, the universality of the models means that they only need to be trained once for a certain image type, and can be applied to any degradations after that.
The second limitation is that both LCM and GLO model require storing their latent representations in memory, which for large datasets and large latent spaces may pose a problem.
Furthermore, we observe that even with the large latent dimensionalities that we use here, the models are not able to fit the training data perfectly suffering from such underfitting.
Our model also assumes that the (log)-likelihood corresponding to the degradation process can be modeled and can be differentiated.
Experiments suggests that however such modeling needs not be very accurate, e.g. simple quadratic log-likelihood can be used to restore JPEG-degraded images (Appendix H).
Finally, our model requires lengthy optimization in latent space, rather than a feedforward pass, at test time.
The number of iterations however can be drastically reduced using degradation-specific or universal feed-forward encoders from image-space to the latent space that may provide a reasonable starting point for optimization.

assistant: text: We present a new deep latent model of natural images that can be trained from unlabeled datasets and can be utilized to solve various image restoration tasks.