Generated target → embed → XOR delta → compress → encrypt. Encode, decode, and paint masks.
Load a Piculum PNG and recover the message with just the file + password.
Load or start blank, paint the mask (white = carries data), then save the image + mask pair for mask mode.
JSON that maps named layers to bit planes of the mask. Each definition gives a plane a label and a purpose: piculum (encoded data) or human-readable (visible text drawn for a person to read). The default is one piculum layer, matching today's behavior.