Applied Geometry

Large classes of observed data exhibit complex nonlinear structure. For example, human organs vary nonlinearly in shape, spherical images cannot be analyzed with classical convolutional neural networks due to the curvature of the spherical domain, and particles in a fluid flow interact nonlinearly. Nonlinearity makes geometry a central discipline for modeling dynamics, performing statistical analysis of complex data, and applying machine learning methods to learn from observed nonlinear data.

Riemannian Geometric Statistics in Medical Image Analysis cover

The research activities in the Applied Geometry group focus on using theory and techniques from the mathematical field of differential geometry to advance data analysis methodology. This happens at a foundational level where fundamental aspects of statistics of nonlinear data are investigated, in applied uses of geometry in e.g. shape analysis of medical and biological data, and on the machine learning side when defining models for neural networks that handles geometric data.

The book Riemannian Geometric Statistics in Medical Image Analysis (Elsevier, 2019) describes many of the complexities and challenges of working with geometric data with a focus on medical applications.

The group is connected to the Center for Computational Evolutionary Morphometry (CCEM) that focuses on shape analysis in evolutionary biology and phylogenetics.


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Shape Analysis

Shapes appear in various forms in medical image analysis and biology, e.g. as 2D surfaces of the human hippocampi or curves representing the outline of a butterfly wing. Analysing the progression of shapes during diseases such as Alzheimer’s disease allows organ shapes to be used as biomarkers for diagnosis and prognosis. Shape analysis also has import application in morphometrics, the study of animal shape changes in biology. More generally, shapes can be considered any object that can be deformed, thus also including complex images such as diffusion weighted MR images in the category of shapes. Mathematically, shape deformations happen by the action of diffeomorphism groups, and geometry of the diffeomorphism groups and its subgroups therefore play important roles in shape analysis. Shape spaces are generally infinite dimensional nonlinear Riemannian manifolds, and statistical shape analysis must thus handle both nonlinearity and infinite dimensionality simultaneously. Focus areas of the work performed in the group is stochastic models for shape evolution and subsequent statistical inference of the parameters governing the stochastically changing shapes. This includes phylogenetic inference using shape data.

1D outlines of human corpora callosa represented with two shape models – continuous curve and landmarks. In both cases, the diffeomorphism groups act on the shapes by deformation the underlying domains.

Geometric Statistics

Geometric statistics concerns the statistical analysis of nonlinear data, particularly data residing in geometric spaces such as manifolds. Examples include shapes and shape spaces, tensors as observed in diffusion imaging, protein angles, and directional data. Even the most classical constructions in statistics, e.g. the mean value, are difficult to define in the geometric setting (for the mean, the fact that shapes cannot be added or multiplied by scalars immediately rules out the classic mean estimator of summing observations divided by the number of observations). We focus on probabilistic interpretations of mean values and covariance, and asymptotics of manifold means (central limit theorems of diffusion means). This includes stochastic processes on Riemannian manifolds, and bridge sampling on manifolds and shape spaces.

Stochastic process on the torus T2

The software packages Jax Geometry and Theano Geometry showcases many of the geometric statistics constructions we work with.

Stochastic process on the rotation group SO(3)

sub-Riemannian geometry

We apply sub-Riemannian constructions for analysis of geometric data, particularly exploring how extremal paths for sub-Riemannian metrics in the frame bundle encode most likely transitions between mean and data points in the presence of non-trivial covariance; how PCA-like dimensionality reduction on manifolds can results in non-integrable distributions and hence allow modelling of higher dimensional data; and reduction theorems on Lie groups and homogeneous spaces for sub-Riemannian metrics.

sub-Riemannian frame bundle geodesics

sub-Riemannian geodesics in the frame bundle FM of a manifold M represented most probable paths between data mean and samples with different covariance structures.

Geometric Deep Learning

Convolutional neural networks classically take image on 2D domains as input. Geometric deep learning aims at extending the class of input data to allow curved domains or more general data with geometric structure. We tackle both foundational questions in geometric deep learning such as the definition of convolution operators on geometric domains and application of geometric deep learning methods in specific contexts such as for diffusion weighed images.

Bridge sampling techniques for defining manifold valued convolution operators.
























Current Research Projects

Stochastic Morphometry: Evolutionary Modeling of Whole Shape Data (2022-2025)
Villum Foundation
Stefan Sommer, Rasmus Nielsen, Mads Nielsen and Christy Hipsley

Probabilistic Shape Statistics (2019-2023)
Novo Nordisk Foundation 
Led by Stefan Sommer

Completed Research Projects

Daydreaming Algorithms (2019-2021)
VILLUM Foundation Experiment Programme
Led by Stefan Sommer















We participate in international networks and fora including








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