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Title: Tranquil Clouds: Neural Networks for Learning Temporally Coherent Features in Point Clouds

Paper content:
Point clouds, as a form of Lagrangian representation, allow for powerful and flexible applications in a large number of computational disciplines.
We propose a novel deep-learning method to learn stable and temporally coherent feature spaces for points clouds that change over time.
We identify a set of inherent problems with these approaches: without knowledge of the time dimension, the inferred solutions can exhibit strong flickering, and easy solutions to suppress this flickering can result in undesirable local minima that manifest themselves as halo structures.
We propose a novel temporal loss function that takes into account higher time derivatives of the point positions, and encourages mingling, i.e., to prevent the aforementioned halos.
We combine these techniques in a super-resolution method with a truncation approach to flexibly adapt the size of the generated positions.
We show that our method works for large, deforming point sets from different sources to demonstrate the flexibility of our approach.
Deep learning methods have proven themselves as powerful computational tools in many disciplines, and within it a topic of strongly growing interest is deep learning for point-based data sets.
These Lagrangian representations are challenging for learning methods due to their unordered nature, but are highly useful in a variety of settings from geometry processing and 3D scanning to physical simulations, and since the seminal work of Qi Charles et al. (2017) , a range of powerful inference tasks can be achieved based on point sets.
Despite their success, interestingly, no works so far have taken into account time.
Our world, and the objects within it, naturally move and change over time, and as such it is crucial for flexible point-based inference to take the time dimension into account.
In this context, we propose a method to learn temporally stable representations for point-based data sets, and demonstrate its usefulness in the context of super-resolution.
An inherent difficulty of point-based data is their lack of ordering, which makes operations such as convolutions, which are easy to perform for Eulerian data, unexpectedly difficult.
Several powerful approaches for point-based convolutions have been proposed (Qi et al., 2017; Hermosilla et al., 2018; Hua et al., 2018) , and we leverage similar neural network architectures in conjunction with the permutation-invariant Earth Mover's Distance (EMD) to propose a first formulation of a loss for temporal coherence.
In addition, several works have recognized the importance of training point networks for localized patches, in order to avoid having the network to rely on a full view of the whole data-set for tasks that are inherently local, such as normal estimation (Qi Charles et al., 2017) , and super-resolution ).
This also makes it possible to flexibly process inputs of any size.
Later on we will demonstrate the importance of such a patch-based approach with sets of changing cardinality in our setting.
A general challenge here is to deal with varying input sizes, and for super-resolution tasks, also varying output sizes.
Thus, in summary we target an extremely challenging learning problem: we are facing permutation-invariant inputs and targets of varying size, that dynamically move and deform over time.
In order to enable deep learning approaches in this context, we make the following key contributions: Permutation invariant loss terms for temporally coherent point set generation; A Siamese training setup and generator architecture for point-based super-resolution with neural networks; Enabling improved output variance by allowing for dynamic adjustments of the output size; The identification of a specialized form of mode collapse for temporal point networks, together with a loss term to remove them.
We demonstrate that these contributions together make it possible to infer stable solutions for dynamically moving point clouds with millions of points.
More formally, we show that our learning approach can be used for generating a point set with an increased resolution from a given set of input points.
The generated points should provide an improved discretization of the underlying ground truth shape represented by the initial set of points.
For the increase, we will target a factor of two to three per spatial dimension.
Thus, the network has the task to estimate the underlying shape, and to generate suitable sampling positions as output.
This is generally difficult due to the lack of connectivity and ordering, and in our case, positions that move over time in combination with a changing number of input points.
Hence it is crucial that the network is able to establish a temporally stable latent space representation.
Although we assume that we know correspondences over time, i.e., we know which point at time t moved to a new location at time t + ∆t, the points can arbitrarily change their relative position and density over the course of their movement, leading to a substantially more difficult inference problem than for the static case.
We have proposed a first method to infer temporally coherent features for point clouds.
This is made possible by a combination of a novel loss function for temporal coherence in combination with enabling flexible truncation of the results.
In addition we have shown that it is crucial to prevent static patterns as easy-to-reach local minima for the network, which we avoid with the proposed a mingling loss term.
Our super-resolution results above demonstrate that our approach takes an important first step towards flexible deep learning methods for dynamic point clouds.
Looking ahead, our method could also be flexibly combined with other network architectures or could be adopted for other applications.
Specifically, a combination with PSGN (Fan et al., 2016) could be used to generate point clouds from image sequences instead of single images.
Other conceivable applications could employ methods like Dahnert et al. (2019) with our approach for generating animated meshes.
Due to the growing popularity and ubiquity of scanning devices it will, e.g., be interesting to investigate classification tasks of 3D scans over time as future work.
Apart from that, physical phenomena such as elastic bodies and fluids (Li et al., 2019) can likewise be represented in a Lagrangian manner, and pose interesting challenges and complex spatio-temporal changes.

assistant: text: We propose a generative neural network approach for temporally coherent point clouds.