We provided a reference docker container to all participants that was done by Frauke Wilm (FAU, Germany). Since the approach seems harder to beat than we expected, we prepared a two-pager on arxiv.org to let you know how exactly she did it. Of course the approach was not optimized on the test set or the preliminary test set in any way.
In principle, the approach is a vanilla RetinaNet approach with a domain adversarial branch added:
You can find all the details in the paper:
Wilm, F., Breininger, K., Aubreville, M: Domain Adversarial RetinaNet as a Reference Algorithm for the MItosis DOmain Generalization (MIDOG) Challenge, https://arxiv.org/abs/2108.11269