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Title: RPGAN: random paths as a latent space for GAN interpretability

Paper content:
In this paper, we introduce Random Path Generative Adversarial Network (RPGAN) --- an alternative scheme of GANs that can serve as a tool for generative model analysis.
While the latent space of a typical GAN consists of input vectors, randomly sampled from the standard Gaussian distribution, the latent space of RPGAN consists of random paths in a generator network.
As we show, this design allows to associate different layers of the generator with different regions of the latent space, providing their natural interpretability.
With experiments on standard benchmarks, we demonstrate that RPGAN reveals several interesting insights about roles that different layers play in the image generation process.
Aside from interpretability, the RPGAN model also provides competitive generation quality and allows efficient incremental learning on new data.
Nowadays, deep generative models are an active research direction in the machine learning community.
The dominant methods for generative modeling, such as Generative Adversarial Networks (GANs), are currently able to produce diverse photorealistic images (Brock et al., 2019; Karras et al., 2019) .
These methods are not only popular among academicians, but are also a crucial component in a wide range of applications, including image editing (Isola et al., 2017; , super-resolution (Ledig et al., 2017) , video generation (Wang et al., 2018) and many others.
Along with practical importance, a key benefit of accurate generative models is a more complete understanding of the internal structure of the data.
Insights about the data generation process can result both in the development of new machine learning techniques as well as advances in industrial applications.
However, most state-of-the-art generative models employ deep multi-layer architectures, which are difficult to interpret or explain.
While many works investigate interpretability of discriminative models (Zeiler & Fergus, 2014; Simonyan et al., 2013; Mahendran & Vedaldi, 2015) , only a few (Chen et al., 2016; Bau et al., 2019) address the understanding of generative ones.
In this work, we propose the Random Path GAN (RPGAN) -an alternative design of GANs that allows natural interpretability of the generator network.
In traditional GAN generators, the stochastic component that influences individual samples is a noisy input vector, typically sampled from the standard Gaussian distribution.
In contrast, RPGAN generators instead use stochastic routing during the forward pass as their source of stochasticity.
In a nutshell, the RPGAN generator contains several instances of the corresponding layer.
For each sample, only one random instance of each layer is activated during generation.
The training of the RPGAN can then be performed in the same adversarial manner as in traditional GANs.
In the sections below, we show how RPGAN allows to understand the factors of variation captured by the particular layer and reveals several interesting findings about the image generation process, e.g. that different layers are "responsible for" coloring or objection location.
As a practical advantage, RPGANs can be efficiently updated to new data via the simple addition of new instances to the bucket, avoiding re-training the full model from scratch.
Finally, we observe that RPGANs allow the construction of generative models without nonlinearities, which can significantly speed up the generation process for fully-connected layers.
In summary, the main contributions of our paper are the following:
• We introduce RPGAN -GAN with an alternative source of stochasticity, based on random routing.
While being close to traditional GANs in terms of generation quality, RPGAN allows natural interpretability and efficient model updates with new data.
• With extensive experiments on standard benchmarks we reveal several insights about the image generation process.
Many of our insights confirm and extend recent findings from Bau et al. (2019) .
Note, that our scheme is more general compared to the technique from Bau et al. (2019) as RPGAN does not require labeled datasets or pretrained segmentation models.
• We open-source the PyTorch implementation of RPGAN with common generator architectures 1 .
The rest of this paper is organized as follows.
In Section 2 we review relevant ideas from prior art.
The proposed Random Path GAN design is described in Section 3 and experimentally evaluated in Section 4.
Section 5 concludes the paper and discusses possible directions for future work.
In this paper, we address the interpretability of generative models.
In particular, we have introduced RPGAN, an alternative design of generative adversarial networks, which allows natural interpretation of different generator layers via using random routing as a source of stochasticity.
With experiments on several datasets, we provide evidence that different layers are responsible for the different factors of variation in generated images, which is consistent with findings from previous work.
As a possible direction of future research, one can use the RPGAN analysis to construct efficient models, e.g., via identification of redundant parts of the generator for pruning or inference speedup.
If the number of blocks is too low, the resulting latent space appears to have insufficient cardinality to cover the dataset.
On the other hand, a too high number of blocks results in a difficult training procedure and also fails.

assistant: text: We introduce an alternative GAN design based on random routes in generator, which can serve as a tool for generative models interpretability.