Methods, systems, and apparatus, including steps of obtaining the source image having a plurality of modules, where a module corresponds to a smallest element that can be encoded in the source image; obtaining input data to be encoded; and encoding the input data in one or more modules of the plurality of modules of the source image. For each respective module of the one or more modules, the encoding includes determining, based on a corresponding value of the input data to be encoded in the respective module, a particular intensity level of multiple intensity levels associated with a particular color, where a number of the multiple intensity levels is greater than two; and encoding the particular color having the particular intensity level in the respective module of the source image.