Cox [2012] proposed a complicated algorithm to embed a binary
image into a QR code during the data encoding stage of generating
the code. He carefully investigated the internal structure of
QR code and the logic behind data encoding, and designed an algorithm
to encode image content as redundant numeric strings appended
to the original data as shown in Figure 2(b). However, this
technique works only for URL type data string and the quality of
embedded image is limited by the length of encoded URL. Similarly,
Duda [2012] attempted to embed halftone images into QR
codes using a constrained coding technique. While a sophisticated
analysis of freedom bits is proposed for embedding image pixels,
it currently works for images of monotonic shapes and produces
mediocre results. An application of extending to delicate halftone
images is not addressed.