Hierarchical Instance Mixing across Domains in Aerial Segmentation

We investigate the task of unsupervised domain adaptation in aerial semantic segmentation observing that there are some shortcomings in the class mixing strategies used by the recent state-of-the-art methods that tackle this task: (i) they do not account for the large disparity in the extension of the semantic categories that is common in the aerial setting, which causes a domain imbalance in the mixed image; (ii) they do not consider that aerial scenes have a weaker structural consistency in comparison to the driving scenes for which the mixing technique was originally proposed, which causes the mixed images to have elements placed out of their natural context; (iii) the source model used to generate the pseudo-labels may be susceptible to perturbations across domains, which causes inconsistent predictions on the target images and can jeopardize the mixing strategy. We address these shortcomings with a novel aerial semantic segmentation framework for UDA, named HIUDA, which is composed of two main technical novelties: (i) a new mixing strategy for aerial segmentation across domains called Hierarchical Instance Mixing (HIMix), which extracts a set of connected components from each semantic mask and mixes them according to a semantic hierarchy and, (ii) a twin-head architecture in which two separate segmentation heads are fed with variations of the same images in a contrastive fashion to produce finer segmentation maps. We conduct extensive experiments on the LoveDA benchmark, where our solution outperforms the current state-of-the-art.