FontCode is an ingenious system of tiny font adaptations to hide messages in printed (or screen-text) documents. It would be perfect for watermarking, for example, while the video demonstration uses it as an elegant (though undetectable?!) alternative to QR codes.
by Chang Xiao, Cheng Zhang , Changxi Zheng (Columbia University) ACM Transactions on Graphics (TOG), Presented at SIGGRAPH 2018 http://www.cs.columbia.edu/cg/fontcode/ We introduce FontCode, an information embedding technique for text documents. Provided a text document with specific fonts, our method embeds user-specified information in the text by perturbing the glyphs of text characters while preserving the text content.
from Wired.