  In safety-critical systems (e.g., autonomous vehicles and robots), Deep
Neural Networks (DNNs) are becoming a key component for computer vision tasks,
particularly semantic segmentation. Further, since the DNN behavior cannot be
assessed through code inspection and analysis, test automation has become an
essential activity to gain confidence in the reliability of DNNs.
Unfortunately, state-of-the-art automated testing solutions largely rely on
simulators, whose fidelity is always imperfect, thus affecting the validity of
test results. To address such limitations, we propose to combine meta-heuristic
search, used to explore the input space using simulators, with Generative
Adversarial Networks (GANs), to transform the data generated by simulators into
realistic input images. Such images can be used both to assess the DNN
performance and to retrain the DNN more effectively. We applied our approach to
a state-of-the-art DNN performing semantic segmentation and demonstrated that
it outperforms a state-of-the-art GAN-based testing solution and several
baselines. Specifically, it leads to the largest number of diverse images
leading to the worst DNN performance. Further, the images generated with our
approach, lead to the highest improvement in DNN performance when used for
retraining. In conclusion, we suggest to always integrate GAN components when
performing search-driven, simulator-based testing.
