  Text-to-image generation has seen an explosion of interest since 2021. Today,
beautiful and intriguing digital images and artworks can be synthesized from
textual inputs ("prompts") with deep generative models. Online communities
around text-to-image generation and AI generated art have quickly emerged. This
paper identifies six types of prompt modifiers used by practitioners in the
online community based on a 3-month ethnographic study. The novel taxonomy of
prompt modifiers provides researchers a conceptual starting point for
investigating the practice of text-to-image generation, but may also help
practitioners of AI generated art improve their images. We further outline how
prompt modifiers are applied in the practice of "prompt engineering." We
discuss research opportunities of this novel creative practice in the field of
Human-Computer Interaction (HCI). The paper concludes with a discussion of
broader implications of prompt engineering from the perspective of Human-AI
Interaction (HAI) in future applications beyond the use case of text-to-image
generation and AI generated art.
