Multi-task Learning Using Uncertainty to Weigh Losses for Scene Geometry and Semantics
Alex KendallYarin GalRoberto Cipolla
Proposes a principled multi-task learning method that automatically balances classification and regression losses using task-dependent homoscedastic uncertainty, eliminating manual weight tuning and outperforming individually trained models on joint scene understanding.
The paper addresses the challenge of jointly learning multiple vision tasks—semantic segmentation, instance segmentation, and per-pixel depth regression—from a single monocular image. Performance in such multi-task settings depends heavily on how the individual task losses are weighted, yet manual or uniform weighting is expensive to tune and often yields suboptimal results, especially for tasks with different units and noise characteristics. This limits practical use in real-time systems such as robotics, where separate models for each task would be computationally prohibitive.
The work sets out to derive and validate a principled loss function that automatically learns relative task weights by treating homoscedastic uncertainty as a learnable parameter for each output. The authors start from a maximum-likelihood formulation: for regression outputs they model a Gaussian likelihood whose variance acts as a task weight, and for classification outputs they scale the softmax logits by an analogous uncertainty term. The resulting objective is differentiable, requires no additional hyper-parameter search, and is applied to a shared ResNet-101 encoder with task-specific decoders on the Cityscapes dataset.
Experiments on both down-sampled validation data and the full-resolution benchmark show that the learned weighting consistently outperforms single-task baselines and naive multi-task sums. On the reduced-resolution set the combined model raises semantic-segmentation IoU from 59.4 % to 63.4 % while also lowering depth and instance errors; on the full test set it reaches 78.5 % class IoU, 21.6 % instance AP, and 2.92 px mean depth error with one forward pass. The final learned loss ratio is approximately 43 : 1 : 0.16 for semantic segmentation, depth regression, and instance segmentation, confirming that the network discovers non-uniform, time-varying task importances.
These gains matter because they simultaneously improve accuracy and cut inference cost, enabling richer scene understanding on embedded platforms. The approach also demonstrates that complementary cues across geometry and semantics regularize one another, an effect that separate models cannot exploit. Because the weighting emerges from the data rather than exhaustive search, the method scales more readily to additional tasks.
Further work is needed to determine the optimal depth at which the shared encoder should branch into separate decoders and to quantify how task relationships evolve with network capacity. The main limitations are that most ablation studies used down-sampled images and that some benchmark comparisons involve models trained on additional external data; nevertheless, the core finding—that homoscedastic uncertainty provides a robust, automatic weighting mechanism—holds across both regimes and initialization choices.
- Paper: What Uncertainties Do We Need in Bayesian Deep Learning for Computer Vision?, Alex Kendall et al. (2017). This foundational paper formally introduces epistemic and aleatoric uncertainty in deep learning, providing the theoretical basis that the source paper builds upon for multi-task loss weighting.
- Paper: An Overview of Multi-Task Learning in Deep Neural Networks, Sebastian Ruder (2017). This comprehensive survey of multi-task learning offers crucial context on parameter sharing and loss balancing strategies that precede the principled uncertainty weighting approach developed in the source.
- Paper: Dropout as a Bayesian Approximation: Representing Model Uncertainty in Deep Learning, Yarin Gal et al. (2016). This pivotal work establishes Monte Carlo dropout as a practical Bayesian approximation for modeling uncertainty, directly enabling the epistemic uncertainty estimation used in the source paper.
- Paper: Modeling Task Relationships in Multi-task Learning with Multi-gate Mixture-of-Experts, Jiaqi Ma et al. (2018). This work extends multi-task learning architectures by proposing Mixture-of-Experts to dynamically learn task relationships, following the foundational loss-weighting concepts established in the source.
- Paper: DSelect-k: Differentiable Selection in the Mixture of Experts with Applications to Multi-Task Learning, Hussein Hazimeh et al. (2021). This paper builds on multi-task learning paradigms by introducing differentiable expert selection mechanisms to resolve task conflicts, directly extending the multi-objective optimization goals of the source.
