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Title: Unsupervised Generative 3D Shape Learning from Natural Images

Paper content:
In this paper we present, to the best of our knowledge, the first method to learn a generative model of 3D shapes from natural images in a fully unsupervised way.
For example, we do not use any ground truth 3D or 2D annotations, stereo video, and ego-motion during the training.
Our approach follows the general strategy of Generative Adversarial Networks, where an image generator network learns to create image samples that are realistic enough to fool a discriminator network into believing that they are natural images.
In contrast, in our approach the image gen- eration is split into 2 stages.
In the first stage a generator network outputs 3D ob- jects.
In the second, a differentiable renderer produces an image of the 3D object from a random viewpoint.
The key observation is that a realistic 3D object should yield a realistic rendering from any plausible viewpoint.
Thus, by randomizing the choice of the viewpoint our proposed training forces the generator network to learn an interpretable 3D representation disentangled from the viewpoint.
In this work, a 3D representation consists of a triangle mesh and a texture map that is used to color the triangle surface by using the UV-mapping technique.
We provide analysis of our learning approach, expose its ambiguities and show how to over- come them.
Experimentally, we demonstrate that our method can learn realistic 3D shapes of faces by using only the natural images of the FFHQ dataset.
Generative Adversarial Nets (GAN) (see Goodfellow et al. (2014) ) have become the gold standard generative model over the years.
Their capability is demonstrated in many data sets of natural images.
These generative models can create sample images that are nearly indistinguishable from real ones.
GANs do not need to make assumptions on the data formation other than applying a neural network on a latent noise input.
They can also operate in a fully unsupervised way.
GANs have strong theoretical foundations since the beginning.
Goodfellow et al. (2014) demonstrate that, under suitable conditions, GANs learn to generate samples with the same probability distribution as samples in the training data set have.
However, there is one notable drawback of GANs compared to classical generative models like Gaussian Mixture Models by Xu & Jordan (1996) or Naive Bayes classifiers by McCallum et al. (1998) .
Classical models are interpretable, but GANs are not.
Interpretability is achieved in classical models by making strong assumptions on the data formation process and by using them on interpretable engineered features.
Our work combines the best of both worlds for 3D shape learning.
We keep the advantages of GANs: unsupervised training, applicability on real datasets without feature engineering, the theoretical guarantees and simplicity.
We also make our representation interpretable, as our generator network provides a 3D mesh as an output.
To make the learning possible, we make the assumption that natural images are formed by a differentiable renderer.
This renderer produces fake images by taking the 3D mesh, its texture, a background image and a viewpoint as its input.
During training a discriminator network is trained against the generator in an adversarial fashion.
It tries to classify whether its input is fake or comes from the training dataset.
The key observation is that a valid 3D object should look realistic from multiple viewpoints.
This image realism is thus enforced by the GAN training.
This idea was first applied for generating 3D shapes in Gadelha et al. (2017) .
Their pioneering work had several limitations though.
It was only applicable to synthetic images, where the background mask is available and produced only black and white images.
Our method works on natural images and we do not need silhouettes as supervision signal.
From a theoretical point of view, image realism means that the images lie inside the support of the probability distribution of natural images (see Ledig et al. (2017) , where image realism was used for super-resolution).
AmbientGAN of Bora et al. (2018) proves that one can recover an underlying latent data distribution, when a known image formation function is applied to the data, thus achieving data realism by enforcing image realism.
Our work and that of Gadelha et al. (2017) are special cases of AmbientGAN for 3D shape learning.
However this task in general does not satisfy all assumptions in Bora et al. (2018) , which results in ambiguities in the training.
We resolve these issues in our paper by using suitable priors.
We summarize our contributions below:
• For the first time, to the best of our knowledge, we provide a procedure to build a generative model that learns explicit 3D representations in an unsupervised way from natural images.
We achieve that using a generator network and a renderer trained against a discriminator in a GAN setting.
Samples from our model are shown in Figure 1 .
• We introduce a novel differentiable renderer, which is a fundamental component to obtain a high-quality generative model.
Notably, it is differentiable with respect to the 3D vertex coordinates.
The gradients are not approximated, they can be computed exactly even at the object boundaries and in the presence of self-occlusions.
• We analyze our learning setup in terms of the ambiguities in the learning task.
These ambiguities might derail the training to undesirable results.
We show that these problems can only be solved when one uses labels or prior knowledge on the data distribution.
Finally, we provide practical solutions to overcome the problems that originate from the ambiguities.

assistant: text: We train a generative 3D model of shapes from natural images in an fully unsupervised way.