Weakly-supervised instance segmentation (WSIS) based on either box-, point-, or image-level annotations has been proposed to overcome the labor-intensive annotation process in fully-supervised instance segmentation. Despite decent results, most WSIS methods only focus on a single annotation level, ignoring that some objects are easy to spot from point annotations, while others require box or even mask annotations. Therefore, we propose the Mixed-Supervised Instance Segmentation (MixSIS) task, which allows a flexible mix of annotation levels (image, point, box, and mask) within a dataset. MixSIS has a high practical relevance, as it allows optimizing the annotation process by annotating challenging objects in more detail, while saving time on simpler instances with less detailed annotations. To address MixSIS, we propose AnnotationsMixer (AnnsMixer) as a baseline. AnnsMixer is a truly mixed-supervised instance segmentation system, which generates pseudo mask annotations from all annotation levels, even mixed within a single image. We evaluate AnnsMixer on both WSIS and MixSIS tasks, largely outperforming state-of-the-art methods. In three case studies, we also demonstrate how the unique features of MixSIS and AnnsMixer can substantially reduce the annotation time (-90%) or improve the results (+34%) in instance segmentation with a simple annotation rule or leveraging heterogeneous data. Our code is available at https://github.com/chwilms/MixSIS

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