A decorative medallion, from "Principles of Decorative Design, Fourth Edition" (1873), by Christopher Dresser, via Project Gutenberg which claims public domain status in the USA. The area inside the design is transparent, as is the background.
裝飾獎章 decorative、line、medallion 裝飾、線、獎章 資料來源:openclipart ( https://openclipart.org/ )