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Title: Convolutional Neural Networks on Non-uniform Geometrical Signals Using Euclidean Spectral Transformation

Paper content:
Convolutional Neural Networks (CNN) have been successful in processing data signals that are uniformly sampled in the spatial domain (e.g., images).
However, most data signals do not natively exist on a grid, and in the process of being sampled onto a uniform physical grid suffer significant aliasing error and information loss.
Moreover, signals can exist in different topological structures as, for example, points, lines, surfaces and volumes.
It has been challenging to analyze signals with mixed topologies (for example, point cloud with surface mesh).
To this end, we develop mathematical formulations for Non-Uniform Fourier Transforms (NUFT) to directly, and optimally, sample nonuniform data signals of different topologies defined on a simplex mesh into the spectral domain with no spatial sampling error.
The spectral transform is performed in the Euclidean space, which removes the translation ambiguity from works on the graph spectrum.
Our representation has four distinct advantages: (1) the process causes no spatial sampling error during initial sampling, (2) the generality of this approach provides a unified framework for using CNNs to analyze signals of mixed topologies, (3) it allows us to leverage state-of-the-art backbone CNN architectures for effective learning without having to design a particular architecture for a particular data structure in an ad-hoc fashion, and (4) the representation allows weighted meshes where each element has a different weight (i.e., texture) indicating local properties.
We achieve good results on-par with state-of-the-art for 3D shape retrieval task, and new state-of-the-art for point cloud to surface reconstruction task.
We present a unifying and novel geometry representation for utilizing Convolutional Neural Networks (CNNs) on geometries represented on weighted simplex meshes (including textured point clouds, line meshes, polygonal meshes, and tetrahedral meshes) which preserve maximal shape information based on the Fourier transformation.
Most methods that leverage CNNs for shape learning preprocess these shapes into uniform-grid based 2D images (rendered multiview images) or 3D images (binary voxel or Signed Distance Function (SDF)).
However, rendered 2D images do not preserve the 3D topologies of the original shapes due to occlusions and the loss of the third spatial dimension.
Binary voxels and SDF representations under low resolution suffer big aliasing errors and under high resolution become memory inefficient.
Loss of information in the input bottlenecks the effectiveness of the downstream learning process.
Moreover, it is not clear how a weighted mesh where each element is weighted by a different scalar or vector (i.e., texture) can be represented by binary voxels and SDF.
Mesh and graph based CNNs perform learning on the manifold physical space or graph spectrum, but generality across topologies remains challenging.In contrast to methods that operate on uniform sampling based representations such as voxel-based and view-based models, which suffer significant representational errors, we use analytical integration to precisely sample in the spectral domain to avoid sample aliasing errors.
Unlike graph spectrum based methods, our method naturally generalize across input data structures of varied topologies.
Using our representation, CNNs can be directly applied in the corresponding physical domain obtainable by inverse Fast Fourier Transform (FFT) due to the equivalence of the spectral and physical domains.
This allows for the use of powerful uniform Cartesian grid based CNN backbone architectures (such as DLA BID40 , ResNet (He et al., 2016) ) for the learning task on arbitrary geometrical signals.
Although the signal is defined on a simplex mesh, it is treated as a signal in the Euclidean space instead of on a graph, differentiating our framework from graph-based spectral learning techniques which have significant difficulties generalizing across topologies and unable to utilize state-of-the-art Cartesian CNNs.We evaluate the effectiveness of our shape representation for deep learning tasks with three experiments: a controlled MNIST toy example, the 3D shape retrieval task, and a more challenging 3D point cloud to surface reconstruction task.
In a series of evaluations on different tasks, we show the unique advantages of this representation, and good potential for its application in a wider range of shape learning problems.
We achieve state-of-the-art performance among non-pre-trained models for the shape retrieval task, and beat state-of-the-art models for the surface reconstruction task.The key contributions of our work are as follows:• We develop mathematical formulations for performing Fourier Transforms of signals defined on a simplex mesh, which generalizes and extends to all geometries in all dimensions.
(Sec.3)• We analytically show that our approach computes the frequency domain representation precisely, leading to much lower overall representational errors.
(Sec. 3)• We empirically show that our representation preserves maximal shape information compared to commonly used binary voxel and SDF representations.
(Sec. 4.1)• We show that deep learning models using CNNs in conjunction with our shape representation achieves state-of-the-art performance across a range of shape-learning tasks including shape retrieval (Sec. 4.2) and point to surface reconstruction (Sec. 4.3) DISPLAYFORM0 Index of the n-th element among a total of N elements Ω j n Domain of n-th element of order j x Cartesian space coordinate vector.
DISPLAYFORM1 Imaginary number unit Shape learning involves the learning of a mapping from input geometrical signals to desired output quantities.
The representation of geometrical signals is key to the learning process, since on the one hand the representation determines the learning architectures, and, on the other hand, the richness of information preserved by the representation acts as a bottleneck to the downstream learning process.
While data representation has not been an open issue for 2D image learning, it is far from being agreed upon in the existing literature for 3D shape learning.
The varied shape representations used in 3D machine learning are generally classified as multiview images BID28 BID26 BID2 , volumetric voxels BID38 BID14 BID37 BID0 , point clouds BID19 BID24 BID36 , polygonal meshes BID3 BID34 BID15 BID12 , shape primitives BID42 BID39 , and hybrid representations (Dai & Nießner, 2018) .Our
proposed representation is closest to volumetric voxel representation, since the inverse Fourier Transform of the spectral signal in physical domain is a uniform grid implicit representation of the shape. However
, binary voxel representation suffers from significant aliasing errors during the uniform sampling step in the Cartesian space BID16 . Using
boolean values for de facto floating point numbers during CNN training is a waste of information processing power. Also,
the primitive-in-cell test for binarization requires arbitrary grouping in cases such as having multiple points or planes in the same cell BID32 . Signed
Distance Function (SDF) or Truncated Signed Distance Function (TSDF) Canelhas, 2017) provides localization for the shape boundary, but is still constrained to linear surface localization due to the linear interpolation process for recovering surfaces from grids. Our proposed
representation under Fourier basis can find nonlinear surface boundaries, achieving subgrid-scale accuracy (See FIG1 ).
We present a general representation for multidimensional signals defined on simplicial complexes that is versatile across geometrical deep learning tasks and maximizes the preservation of shape information.
We develop a set of mathematical formulations and algorithmic tools to perform the transformations efficiently.
Last but not least, we illustrate the effectiveness of the NUFT representation with a well-controlled example (MNIST polygon), a classic 3D task (shape retrieval) and a difficult and mostly unexplored task by deep learning (point to surface reconstruction), achieving new state-of-the-art performance in the last task.
In conclusion, we offer an alternative representation for performing CNN based learning on geometrical signals that shows great potential in various 3D tasks, especially tasks involving mixed-topology signals.

assistant: text: We use non-Euclidean Fourier Transformation of shapes defined by a simplicial complex for deep learning, achieving significantly better results than point-based sampling techiques used in current 3D learning literature.