  StyleGAN is one of the state-of-the-art image generators which is well-known
for synthesizing high-resolution and hyper-realistic face images. Though images
generated by vanilla StyleGAN model are visually appealing, they sometimes
contain prominent circular artifacts which severely degrade the quality of
generated images. In this work, we provide a systematic investigation on how
those circular artifacts are formed by studying the functionalities of
different stages of vanilla StyleGAN architecture, with both mechanism analysis
and extensive experiments. The key modules of vanilla StyleGAN that promote
such undesired artifacts are highlighted. Our investigation also explains why
the artifacts are usually circular, relatively small and rarely split into 2 or
more parts. Besides, we propose a simple yet effective solution to remove the
prominent circular artifacts for vanilla StyleGAN, by applying a novel
pixel-instance normalization (PIN) layer.
