2D code patterns, such as digital watermark patterns, are formed on plastic objects by injection molding. In some implementations, a marked cell of the code pattern is not formed by a single mark on the mold surface, but by multiple discrete marks. Such marks can be exceedingly small (e.g., 50 microns or less—smaller than the width of a human hair), yet the resulting code pattern on the molded object is still readable from a distance. The small scale of the marks assures that the code pattern does not detract from object aesthetics, while also speeding the mold-marking process. Style transfer networks are employed in some implementations. The detailed technologies facilitate digital marking and identification of a great number of consumer plastic objects, thereby aiding recovery of such objects for recycling. Many other features and arrangements are also detailed.