Stella Grasshof

Reconstructing 3D trajectories from video is often cumbersome and expensive, relying on complex or multi-camera setups. This paper proposes SynthNet, an end-to-end pipeline for monocular reconstruction of 3D tennis ball trajectories. The pipeline consists of two parts: Hit and bounce detection and 3D trajectory reconstruction. The hit and bounce detection is performed by a GRU-based model, which segments the videos into individual shots. Next, a fully connected neural network reconstructs the 3D trajectory through a novel physics-based training approach relying on purely synthetic training data. Instability in the training loop caused by relying on Euler-time integration and camera projections is circumvented by our synthetic approach, which directly calculates loss from estimated initial conditions, improving stability and performance. In experiments, SynthNet is compared to an existing reconstruction baseline on a number of conventional and customized metrics defined to validate our synthetic approach. SynthNet outperforms the baseline based on our own proposed metrics and in a qualitative inspection of the reconstructed 3D trajectories.

This paper presents, RallyTemPose, a transformer encoder-decoder model for predicting future badminton strokes based on previous rally actions. The model uses court position, skeleton poses, and player-specific embeddings to learn stroke and player-specific latent representations in a spatiotemporal encoder module. The representations are then used to condition the subsequent strokes in a decoder module through rally-aware fusion blocks, which provide additional relevant strategic and technical considerations to make more informed predictions. RallyTemPose shows improved forecasting accuracy compared to traditional sequential methods on two real-world badminton datasets. The performance boost can also be attributed to the inclusion of improved stroke embeddings extracted from the latent representation of a pre-trained large-language model subjected to detailed text descriptions of stroke descriptions. In the discussion, the latent representations learned by the encoder module show useful properties regarding player analysis and comparisons.

Denoising Diffusion Models (DDMs) have emerged as a strong competitor to Generative Adversarial Networks (GANs). However, despite their widespread use in image synthesis and editing applications, their latent space is still not as well understood. Recently, a semantic latent space for DDMs, coined ‘h-space’, was shown to facilitate semantic image editing in a way reminiscent of GANs. The h-space is comprised of the bottleneck activations in the DDM’s denoiser across all timesteps of the diffusion process. In this paper, we explore the properties of h-space and propose several novel methods for finding meaningful semantic directions within it. We start by studying unsupervised methods for revealing interpretable semantic directions in pretrained DDMs. Specifically, we show that interpretable directions emerge as the principal components in the latent space. Additionally, we provide a novel method for discovering image-specific semantic directions by spectral analysis of the Jacobian of the denoiser w.r.t. the latent code. Next, we extend the analysis by finding directions in a supervised fashion in unconditional DDMs. We demonstrate how such directions can be found by annotating generated samples with a domain-specific attribute classifier. We further show how to semantically disentangle the found directions by simple linear projection. Our approaches are applicable without requiring any architectural modifications, text-based guidance, CLIP-based optimization, or model fine-tuning.

This paper presents TemPose, a novel skeleton-based transformer model designed for fine-grained motion recognition to improve understanding of the detailed player actions in badminton. The model utilizes multiple temporal and interaction layers to capture variable-length multi-person human actions while minimizing reliance on non-human visual context. TemPose is evaluated on two fine-grained badminton datasets, where it significantly outperforms other baseline models by incorporating additional input streams, such as the shuttlecock position, into the temporal transformer layers of the model. Additionally, TemPose demonstrates great versatility by achieving competitive results compared to other state-of-the-art skeleton-based models on the large-scale action recognition benchmark NTU RGB+D. Experiments are conducted to explore how different model parameter configurations affect Tem-Pose’s performance. Additionally, a qualitative analysis of the temporal attention maps suggests that the model learns to prioritize frames of specific poses relevant to different actions while formulating an intuition of each individual’s importance in the sequences. Overall, TemPose is an intuitive and versatile architecture that has the potential to be further developed and incorporated into other methods for managing human motion in sports with state-of-the-art results.

In this paper, we present an approach for combining non-rigid structure-from-motion (NRSfM) with deep generative models, and propose an efficient framework for discovering trajectories in the latent space of 2D GANs corresponding to changes in 3D geometry. Our approach uses recent advances in NRSfM and enables editing of the camera and non-rigid shape information associated with the latent codes without needing to retrain the generator. This formulation provides an implicit dense 3D reconstruction as it enables the image synthesis of novel shapes from arbitrary view angles and non-rigid structure. The method is built upon a sparse backbone, where a neural regressor is first trained to regress parameters describing the cameras and sparse non-rigid structure directly from the latent codes. The latent trajectories associated with changes in the camera and structure parameters are then identified by estimating the local inverse of the regressor in the neighborhood of a given latent code. The experiments show that our approach provides a versatile, systematic way to model, analyze, and edit the geometry and non-rigid structures of faces.

In this work, we present a new versatile 3D multilinear statistical face model, based on a tensor factorisation of 3D face scans, that decomposes the shapes into person and expression subspaces. Investigation of the expression subspace reveals an inherent low-dimensional substructure, and further, a star-shaped structure. This is due to two novel findings. (1) Increasing the strength of one emotion approximately forms a linear trajectory in the subspace. (2) All these trajectories intersect at a single point – not at the neutral expression as assumed by almost all prior works—but at an apathetic expression. We utilise these structural findings by reparameterising the expression subspace by the fourth-order moment tensor centred at the point of apathy. We propose a 3D face reconstruction method from single or multiple 2D projections by assuming an uncalibrated projective camera model. The non-linearity caused by the perspective projection can be neatly included into the model. The proposed algorithm separates person and expression subspaces convincingly, and enables flexible, natural modelling of expressions for a wide variety of human faces. Applying the method on independent faces showed that morphing between different persons and expressions can be performed without strong deformations.